Ask HN: What scientific phenomenon do you wish someone would explain better? - <AUTHOR> I've been studying viruses lately, and have found that the line between virus/exosome/self is much more blurry than I realized. But, given the niche interest in the subject, most articles are not written with an overview in mind.<p>What sorts of topics make you feel this way? <comment><USER5> Quantum Computers. Not like I'm five, but like I'm a software engineer who has a pretty decent understanding of how a classical turing machine works. I can't tell you how many times I've heard someone say "qubits are like bits except they don't have to be just 1 or 0" without providing any coherent explanation of how that's useful. I've also heard that they can try every possible solution to a problem. What I don't understand is how a programmer is supposed to determine the correct solution when their computer is out in some crazy multiverse. I guess what I want is some pseudo code for quantum software.

<comment><USER6> Since no one has listed it yet, please check out [https://quantum.country/](https://quantum.country/)

It's by Andy Matuschak and Michael Nielsen, and it is excellent. Have fun!

<comment><USER7> Yes! How come this isn't higher up in the list? This is one of the best pieces of education I've ever seen. Absolutely wonderful.

</comment></comment></comment><comment><USER8> Macroeconomics. Central banks are "creating" a trillion here, a trillion there, like nobody's business. But what are the consequences? What is the thought process that central bankers have gone through to make these decisions?

Also why, exactly, are they buying the exact assets that they are buying (govt. debt, high-yield bonds, etc..) and why not others (e.g. stocks or put money into startups)? And then, what happens if a debtor pays back its debt? Is that money consequently getting "erased" again (just like it's been created)? What happens if a debtor defaults on its debt? Does that money then just stay in the economy, impossible to drain out? What is the general expectation of the central banks? What percentage of the debt is expected to default and how much is expected to be paid back?

And specifically in the case of central banks buying govt. debt: Are central banks considered "easier" creditors than the public? What would happen if a country defaults on a loan given by a central bank? Would the central bank then go ahead and seize and liquidate assets of the country under a bankruptcy procedure to pay off the debt (like it would be standard procedure for individuals and companies)?

<comment><USER9> One approach is reductio ad absurdum:

If Central Banks can create money without negative effects, then

\- why tax people?

\- why even work? Can't we just print enough money for everyone and live happily ever after?

I realize these questions are quite provocative and their answering only explains _if_ it will work but not how or when it will fail.

<comment><USER10> > \- why tax people?

Printing money is actually more or less equivalent to a tax, because it reduces the value of the existing money supply.

> \- Can't we just print enough money for everyone and live happily ever > after?

No, because printing money redistributes wealth, it doesn't create it.

<comment><USER11> Not just any tax, but a wealth tax affecting anybody who has US dollars, regardless of their citizenship or location.

<comment><USER12> That's a really good point and I never thought about it this way. It just makes me think how powerful is for US the world relying on dollars as an international currency.

</comment></comment></comment></comment></comment><comment><USER13> I would like to understand how cellular biology processes actually work. Like, how do all the right modules and proteins line up in the right orientation every time? Every time I watch animations, it seems like the proteins and such just magically appear when needed and disappear when not needed [0]. Sometimes it's an ultra-complex looking protein and it just magically flys over to the DNA, attaches to the correct spot, does it's thing, detaches, and flies away. Yeah right! As if the protein is being flown by a pilot. How does it really work?

[0] [https://youtu.be/5VefaI0LrgE](https://youtu.be/5VefaI0LrgE)

<comment><USER14> They don't. This is a pet-peeve of mine, and it's reinforced by animation after animation.

Everything is being jostled around randomly. The molecules don't have brains or seeker warheads. They can't "decide" to home in on a target.

The only mechanisms for guidance are: diffusion due to concentration gradients, movement of charged molecules due to electric fields, and molecules actually grabbing other molecules.

It's all probabilities. This conformation makes it more likely that this thing will stick to this other thing. You may have heard that genes can be turned on or off. How? DNA is literally wound on molecular spools in your cell nuclei. When the DNA is loosely wound other molecules can bump into it and transcribe it -- the gene is ON. When the DNA is tightly spooled, other molecules can't get in there and the gene is OFF for transcription. There's no binary switch, just likelihoods.

Everything is probabilistic, but the probabilities have been tuned by evolution through natural selection to deliver a system that works well enough.

<comment><USER15> Even diffusion isn't some magical force guiding chemicals through the medium. It's just random movement that statistically results in the chemical being spread out. This is the same principle that the 2nd law of thermodynamics is based upon. There's nothing magic to it, it's just the statistically likely end result over many particles.

<comment><USER14> Yes. It's interesting how powerful and clarifying this model of its-all-just- atoms-bumping-into-atoms is. It's interesting how many people take science courses, but don't really _get_ this.

In the context of Covid19, I see so many people wearing PPE, but failing to act as though they understand that the actual goal is to prevent this tiny virion dust from entering your orifices. Like wearing gloves and a mask, but then picking up unclean item in store then using now unclean gloves to adjust mask and make it unclean.

People seem to think of things as having essences or talismanic effects. Like gloves give you +2 against covid and a mask gives you +5 when it's really all about preventing those virus things from bumping into your cell things.

<comment><USER16> Masks are for keeping your own particles from spreading far, not the other way around.

<comment><USER17> > Masks are for keeping your own particles from spreading far, not the other > way around.

Masks are for keeping your own particles from spreading far _AND_ for lowering the probability of virions found in the environment from entering your respiratory system.

Masks lower the probability when all other variables are held constant. If someone thinks wearing a mask grants invincibility and in turn chooses to increase their exposure to high viral load individuals or environments, they're putting themselves at risk.

<comment><USER18> > _Masks are for keeping your own particles from spreading far AND for > lowering the probability of virions found in the environment from entering > your respiratory system._

Both of you may be correct. I think the person you responded to may not have been precise in their framing.

I suspect that you had _N95 masks_ in mind when you wrote masks, which doesn't negate the point of the person you responded to, if they had _surgical masks_ in mind when they wrote masks. Surgical masks are far more common than N95 masks since they are cheaper and do not provide protection against viral particles for the wearer.

<comment><USER19> Surgical masks do provide some level of protection against virus droplets and aerosol for the wearer they just are not as effective as N95. Even a teacloth or a scarf wrapped around your face will provide some level of protection to the wearer from virus particles entering their mucus membranes.

</comment></comment></comment></comment></comment></comment></comment></comment><comment><USER20> I find most explanations of the Equivalence Principle that lies at the foundation of General Relativity to be very lax.

To wit, the idea is that you cannot distinguish whether you are in an accelerated frame or in a gravitational field; alternatively stated, if you're floating around in an elevator you don't know whether you're freefalling to your doom or in deep sideral space far from any gravitational source (though of course, since you're in an elevator car and apparently freefalling... I think we'd all agree on what's most likely, but I digress).

Anyway, what irks me that this is _most definitely not true_ at the "thought experiment" level of theoretical thinking: if you had two baseballs with you in that freefalling lift, you could suspend them in front of you. If you were in deep space, they'd stay equidistant; if you were freefalling down a shaft, you'd see them move closer because of tidal effects dictated by the fact that they're each falling towards the earth's centre of gravity, and therefore at (very slightly) different angles.

Of course, they'd be moving slightly toward each other in both cases (because they attract gravitationally) but the tidal effect presents is additional and present in only one scenario, allowing one to (theoretically) distinguish, apparently violating the bedrock Equivalence Principle.

I never see this point raised anywhere and I find it quite distressing, because I'm sure there's a very simple explanation and that General Relativity is sound under such trivial constructions, but I haven't been able to find a decent explanation.

<comment><USER21> You're right that this is glossed over in popular explanations, but the point you make is exactly the starting point for all formal courses and textbooks.

The first part of the argument is that for single _point_ particles falling, the effect of gravity is the same for all particles. This suggests that we should model gravity as _something_ intrinsic to spacetime itself, rather than as a field living on top of spacetime, which could couple to different particles with different strengths.

The second part of the argument, which is what you point out, is that gravity can have nontrivial tidal effects. (This had better be true, because if _all_ gravitational effects were just equivalent to a trivial uniform acceleration, then it would be so boring that we wouldn't need a theory of gravity at all!) This suggests that whatever property of spacetime we use to model gravity, it should reduce in the Newtonian limit to something that looks like a tidal effect, i.e. a _gradient_ of the Newtonian gravitational field. That leads directly to the idea of describing gravity as the curvature of spacetime.

So both parts of the argument give important information (both historically and pedagogically). Both parts are typically presented in good courses, but only the first half makes it to the popular explanations, probably out of simplification.

<comment><USER22> > it should reduce in the Newtonian limit to something that looks like a tidal > effect, i.e. a gradient of the Newtonian gravitational field.

Can you please explain to me how you went from"looks like a tidal effect in the Newtonian limit" to "a gradient of the Newtonian Graviational field"?

<comment><USER21> "Tidal effects" are defined in terms of having different gravitational fields in one place than another (i.e. the tidal bulge near to the moon occurs because the moon's field is stronger there).

<comment><USER23> That's not quite true, as illustrated by the tidal bulge opposite the moon.

Tidal forces occur much more due to the difference in the direction of gravity than due to the difference in magnitude.

</comment></comment></comment></comment></comment><comment><USER24> Sort of meta, but I always shudder when someone says that science has "proven" something.

What sets science apart from most other methods of seeking answers is its focus on _disproof_. Your goal as a scientist is to devise experiments that can _disprove_ a claim about the natural world.

This misconception rears its head most prominently in discussions at the intersection between science and public policy. Climate change. How to handle a pandemic. Evolution. Abortion. But I've even talked to scientists themselves who from time to time get confused about what science can and can't do.

The problem with believing that science proves things is that it blinds its adherents to new evidence paving the way to better explanations. It also leads to the absurd conclusion that a scientific question can ever really be "settled."

<comment><USER25> Not to be rude, but given current daily attacks on science and the scientific method, I can't let this stand - I think your meta intuition represents a fundamental misunderstanding of how science works.

It is simply wrong to think that scientific questions can never be definitively settled. Clearly there are some hypotheses that have been difficult (and may be impossible) to prove, for example, Darwin's idea that natural selection is the basis of evolution. There's ample correlative evidence in support of natural selection, but little of the causal data necessary for "proof" (until perhaps recently). In the case of evolution the experiments required to prove that natural selection could lead to systematic genetic change were technically challenging for a variety of reasons.

In the case of climate change, the problem again is that the evidence is correlative and not causal. Demonstrating a causal link between human behavior or CO2 levels and climate change (the gold standard for "proof") is technically challenging, so we are forced to rely on correlations, which is the next best thing. But, you are right, it is not "proof".

Establishing causality can be difficult but not impossible - the standard is "necessary and sufficient". You must show necessity: CO2 increase (for example) is necessary for global warming; if CO2 remains constant, no matter what else happens to the system global temperatures remain constant. And you must also demonstrate sufficiency: temperatures will increase if you increase CO2 while holding everything else constant. Those are experiments that can't be done. As a result, we are forced to rely on correlation - the historical correlation between CO2 and temperature change is compelling evidence that CO2 increases cause global warming, but it is not proof. It then becomes a statistical argument, giving room for some to argue the question remains "unsettled".

My point is that there are plenty of examples in science where things have been proven -- DNA carries genetic information, DNA (usually) has a double stranded helical structure, V=IR, F=Ma, etc. And there are things that are highly likely, but not "proven", e.g., human activity causes of climate change.

While some of the issues you bring are remain unproven, what's really absurd is to think that no scientific questions can be settled.

<comment><USER26> > Establishing causality can be difficult but not impossible - the standard is > "necessary and sufficient". You must show necessity: CO2 increase (for > example) is necessary for global warming; if CO2 remains constant, no matter > what else happens to the system global temperatures remain constant. And you > must also demonstrate sufficiency: temperatures will increase if you > increase CO2 while holding everything else constant. Those are experiments > that can't be done.

No. What is the basis for these claims?

They're both wrong.

It's not true that CO2 increase is necessary for global warming. If the sun got a lot hotter, global temperatures would rise. If non-CO2 GHGs increased, global temperatures would rise. If the overall albedo of the planet changes, global temperatures can rise. There are literally thousands of things that could cause the temperature to rise.

It's also not true that CO2 increase, holding everything else constant, would lead to long term or even medium term warning. We have no idea what the ecosystem will do for any given change in CO2 levels, since there are countless species both who are net producers and net consumers of atmospheric CO2, all of whom have exponential growth and feedback loops.

Even still, even since both of those claims are wrong, CO2 increase _may still_ cause global warming.

Furthermore, the things you claim are proven, are not proven, they are true by definition. All molecules carry information, and the fact that DNA carries genetic information is a direct consequence of the fact that it is DNA. V=IR by definition. F=ma by definition. There's no such thing as a "force" or "mass" or "acceleration" entity per se, these are metrics that are by definition equal in a given physical framework.

There is no way to 'technically' prove _anything_ in science, and the reasons are simple:

(1) The past is gone - you can't access it

(2) You can't see the future

(3) Your knowledge of the present is extremely limited and inaccurate

These are the limitations of the real world, and science does its best to provide utility within that. It only focuses on making future predictions using the observed past as evidence, because you only _can_ do that. You can't check your model in the present, because you can't instantaneously observe anywhere you aren't already observing. Checking your model on the past relies on what you _think_ happened, i.e. what _allegedly_ happened, but there is absolutely no way to truly know.

You can't even really prove anything 'novel' in mathematics, which is the only place where you _can_ actually prove anything, but even there all proofs are effectively just framing something that was already implied axiomatically in a way that allows our limited human minds to see the relevant/useful patterns that aren't immediately obvious to us.

My point is, acting as though you can _truly_ prove anything in science,

> what's really absurd is to think that no scientific questions can be settled

is not only wrong, but in my opinion is a distraction from what science is actually for. It's not about settling questions. Science is never settled, and that's part of what's beautiful about it. It's about reducing our own ignorance and proving our past selves wrong, discovering patterns and models that equip us with the knowledge to build a better world for ourselves and the rest of humanity.

Why lie about being a great soccer player when you're already great at basketball? Let's focus on the beauty of science as a great journey of growth and exploration that accelerates the progress of humanity, instead of trying to make it do something that isn't possible in the real world.

<comment><USER27> > No. What is the basis for these claims?

"Science", as it is represented in the media, and in turn repeated _and enforced_ (not unlike religion, interestingly) on social media and in social circles.

As opposed, of course, to _actual science_.

 _" Perception is reality."_ \- Lee Atwater, Republican political strategist.

[https://www.cbs46.com/news/perception-is- reality/article_835...](https://www.cbs46.com/news/perception-is- reality/article_83572d66-9e74-53ec-b0b2-60610fda4e75.html)

[https://en.wikipedia.org/wiki/Lee_Atwater](https://en.wikipedia.org/wiki/Lee_Atwater)

 _" Sauron, enemy of the free peoples of Middle-Earth, was defeated. The Ring passed to Isildur, who had this one chance to destroy evil forever, but the hearts of men are easily corrupted. And the ring of power has a will of its own. It betrayed Isildur, to his death."_

 _" And some things that should not have been forgotten were lost. History became legend. Legend became myth. And for two and a half thousand years, the ring passed out of all knowledge._"

[https://www.edgestudio.com/node/86110](https://www.edgestudio.com/node/86110)

Threads like this one, and _many_ others like it, well demonstrate the precarious situation we are in _at this level_. Imagine the state of affairs around the average dinner table. Although, it's not too infrequent to hear the common man admit (which is preceded by _realization_ ) that they don't know something. As one moves up the modern day general intelligence curve, this _capability_ seems to diminish. What the exact cause of this is a bit of a mystery (24 hour cable propaganda and the complex dynamics of social media is my best guess) - hopefully someone has noticed it and is doing some research, although I've yet to hear it mentioned anywhere. Rather, it seems we are all content to attribute any misunderstanding that exists in modern society to Fox News, Russia, QAnon, or the alt-right. I'm a bit concerned that this approach may not be the wisest, but I imagine we will find out who's right soon enough.

</comment></comment></comment></comment><comment><USER28> I don't know if this would be my "one question" if I could ask the most brilliant minds in science, but something that always bothered me:

When I took physics they basically said "at first scientists were disturbed by the fact that magnets imply that two objects are interacting without any physical contact, but then Faraday came along and said 'the magnets are actually connected by invisible magnetic field lines' and that resolved everything."

How does saying "but what if there's invisible lines connecting them" resolve anything? To be clear, I'm not objecting to any of the actual electromagnetic laws or using field lines to visualize magnetic fields. It's just that I don't get how invoking invisible lines actually explains anything about how objects are able to react without physical contact.

(Also, it is not lost on me I that this question boils down to "fraking magnets, how do they work?")

<comment><USER29> I'm a physicist specifically working with magnetic systems, but I have very little pre-graduate teaching experience, so take this attempt to answer the question with a grain of salt.

The reason some people regard Faraday's original explanation of the eponymous law (it is worth noting that at the time it was widely regarded as inadequate and handwavy) as illuminating is because Faraday visualized his "lines of force" as literal chains of polarized particles in a dielectric medium, thereby providing a seemingly mechanistic local explanation of the observed phenomena. Not much of this mindset survived Maxwell's theoretical program and it has very little to do with how we regard magnetism today. Instead, the unification of electricity and magnetism naturally arises from special relativity, whereas the microscopic basis of magnetism requires quantum mechanics. There isn't really any place for naive contact mechanics in the modern picture of physics, so in that sense I would regard Faraday's view as misleading.

Finally, I can't end any "explanation" of magnetism without linking the famous Feynman interview snippet [1] where he's specifically asked about magnetism. It doesn't answer your question directly, but it's worth watching all the more because of it.

[1] [https://www.youtube.com/watch?v=MO0r930Sn_8](https://www.youtube.com/watch?v=MO0r930Sn_8)

<comment><USER14> That interview is so good! What a dick, but what a teacher!

<comment><USER30> What I see at the beginning of that video is somebody who doesn't want to spend the energy answering a complex question. Then, in the process of dismissing the question he gets drawn in and can't help himself from really getting into it.

I don't know anything about Feynman beyond vaguely associating his name with science, but watching this makes me want to seek out more from him.

<comment><USER31> You're in for quite a treat then. It sounds like you might have more of an interest in his technical work and scientific contributions and teaching materials (of which there is plenty, and of high quality), but personally I quite enjoyed this book of his as well: [https://en.wikipedia.org/wiki/Surely_You%27re_Joking,_Mr._Fe...](https://en.wikipedia.org/wiki/Surely_You%27re_Joking,_Mr._Feynman)!

</comment></comment></comment></comment></comment><comment><USER32> Crypto and practical security. I get tired of the circular "don't roll your own crypto unless you're qualified". How does one become qualified? I don't feel like I know how to evaluate many of the arguments people make for or against technologies people argue about on HN, such as Signal or different password managers. I feel like "security through obscurity" is a bad thing, and "layers of security" are a good thing, but isn't all security obscuring something, and how does one evaluate whether a layer is adequate? "Just use bcrypt" - okay, help me understand!

<comment><USER33> The reason people say not to roll your own crypto is that there is no secret answer to making things secure, we just have smart and creative people bash their heads against a crypto protocol/implement for a long time and hope we found all the problems.

So unless you have a good reason to do something else, and the budget to pay experienced people to bash their heads against it, you should stick to an implementation that has had this effort expended on it.

If you want an intro about common problems in custom cryptosystems, go look at cryptopals or something, but don't get too cocky that you know everything.

<comment><USER34> It's also easy to dramatically underestimate the order of magnitude of effort involved in "the budget to pay experienced people to bash their heads against it".

</comment></comment></comment><comment><USER35> Has someone that thought they were taking LSD _ever_ turned into a permanent schizophrenic zombie or in a mental institution, or is it all urban legend. If someone that didn't know they were predisposed to mental illness, is it applicable to dismiss their experience in order to maintain how safe LSD is?

If any of this is true, are there any sources aside from "my friend's friend's brother took too much and now he is....", and what is the scientific explanation and do we know enough about the mind at all?

I feel like LSD has a lot of contradictory information out there, and the proponents feel the need to hand waive concerns away because it is 'completely harmless and leaves your system in 10 hours'. But when nobody knows what they're actually getting because it doesn't exist in a legal framework, then it muddies the whole experience.

People say certain doses can't do more effect than lower doses after a certain threshold. It seems like the same people say "omg man 1000ug you are going to fry your brain!"

What is the truth? If it "just" had an FDA warning like "people with a family history of schizophrenia should not take it", that would be wildly better than what we have today.

Please no explanation about shrooms. Just LSD the 'research chems' distributed as LSD.

<comment><USER36> Permanent schizophrenic zombie, maybe a bit extreme, but severe and traumatic long-lasting psychological damage is a not-uncommon phenomena.

I had a fling with psychedelics in my teens, and everything was great until the one time it wasn't. I was taking psychedelics pretty much every weekend, and by my count have tried over a dozen of them.

Had an experience with LSD which completely shook me to my core and gave me such severe PTSD and trauma that every night I started to have massive panic attacks and needed medical help. My entire worldview and perception of reality was shattered, I wasn't able to "anchor" myself anymore and it all felt like a sham. I was completely dissociated. I also got HPPD: to this day, everything has a sharpened oil-painting type texture to it that increases based on my anxiety level, and I'm sensitive to visual + aural stimuli (loud, brightly- colored places are unpleasant). If I get too anxious, I start to dissociate.

It took ~2 years for the PTSD to subside for the most part, but still if I am under a lot of stress I am liable to have a panic attack and get flashbacks and need to go find somewhere quiet to sit somewhere alone to try to work through it.

LSD being the particular substance has nothing to do with it, in my opinion. I was young, dumb, reckless, and played with fire then got burned. It could have happened with any of the other dozen psychedelics I took, but it just so happened to be LSD the one time that it did.

But I want to add, that while giving me the most nightmarish, traumatizing experience of my life, the best/most positively-profound experience has also been on the same substance. I grew up in a pretty abusive household and didn't do well forming relationships growing up, and had a lot of anger and resentment in my worldview. After taking psychedelics (LSD, 2C-B, Shrooms) and MDMA with the right group of people a few times, my entire perspective shifted. For the first time in my life, it felt like I understand how it felt to be loved, and what _" love"_ was, and how we're "all in this together" so we may as well be good to each other while we're here.

It's been a long time since I've touched any of that stuff and I'm not sure I ever will again, but I don't think it's inherently bad or good. Psychedelics are like knives, they're neutral - can be used as a tool or cut the hell out of you if you're reckless.

\---

Footnote: For context, this was probably due to life circumstances/psyche at the time. I was in a relationship with a pretty toxic partner, and my mental state wasn't the greatest. In hindsight, it seems like I was almost begging for a "slap in the face" if you will.

<comment><USER37> If you don't mind me asking (and this is clearly a sensitive topic so feel free not to reply): What do you mean by PTSD and flashbacks? As in, was the trip so bad that remembering it creates anxiety, or are you reliving unrelated traumatic memories that weren't an issue to live with before using the drug?

<comment><USER36> It literally feels as if I'm being transported back to that same night, starting to relive it all over again. It's entirely illogical but if you knew what happened it might make more sense (happy to elaborate here and give a brief description of what happened/why it messed me up so bad, I'm perfectly okay to talk about it now).

<comment><USER38> I'd be interested to hear your story. I've never used psychedelic drugs, but I find their effects fascinating.

<comment><USER36> I took 300ug of LSD recklessly on a particularly bad day for me, in a particularly uncomfortable setting.

Well, that night went bad. Really, really, life-alteringly bad. For the first time, I had a bad trip. And not like, some mildly uncomfortable thoughts. I got a bad feeling in my stomach from the moment I dosed, and I knew something was going to be different this time.

As I started to come up, the bad feeling and a dark presence grew, and I pulled out my phone. I started a timer, and I watched as the time slowed to a point where it completely stopped. I started looping, I would get up off the couch, walk a few feet, and be teleported back. Over and over.

I realized that I had gotten so high, that time was no longer moving. And if time was not moving, I could maybe never come down. I was stuck here forever. And then the hellish nightmare started.

I felt like I was losing control of myself, like something else was trying to take over, and whoever won the battle, that is the consciousness that would exist. The more I fought, the more painful things got. Pain the likes of which I no one can physically imagine.

Went upstairs and laid down in my bed, began going out of body. I started dying over and over in unimaginable ways in my head, trapped in loops. Pain beyond anything I've ever felt in reality, there was no limit. It was tied to my breath, I realized that it had been so long since I had breathed, I kept forgetting who I was and what was going on, and then I would catch a slight glimpse and remember and fight so hard to take another breath. And there was so much pain in fighting to "survive" and hold on to who I was.

Eventually, the pain/struggle became too much, and I "gave in" and said "okay, I give up, you win, I can't take it anymore, I'd rather die." And that's when it's stopped. There appeared this giant shape of light/energy that was every color at once, and colors we don't have words for, and it "touched me" (could have been me moving towards it, or it towards me, there wasn't really a concept of this).

When it "touched" me, what it "showed" me was something I later learned is called an "Ouroboros", the snake eating it's tail. It showed me what "infinity" really meant, and that was too much to handle and shattered my psyche.

In that moment my body/mind/soul felt like it was obliterated to pieces by some energy beam in the most excruciating, searing pain, and I woke up in my bed having just pissed myself.

It took a long time to piece myself back together after that one.

\---

There are a lot of details I've omitted for brevity's sake, but this captures the gist of it.

The majority of my trauma has to do with anything related to loops: think Nietzsche's Eternal Return, general time-loops, fear of time-stopping, etc.

When I have panic attacks I have to stop myself from starting a stopwatch on my phone to make sure time is still moving because it'll cause a feedback loop and ratchet-up the panic, causing the time-dilation to increase in a vicious cycle.

<comment><USER39> Holy. That sounds intense, to say the least. A part of me likes to experience this, even though you made it abundantly clear, that it did not have a positive impact on your life.

<comment><USER35> "ego death" is a common aspect of acid trips and the experience seems to come down to your willingness to relinquish control. this reads like what was described. if you were to look up that term you'll see others that will feature similar features - with or without pain, with or without worry.

not having a reliable way to know exactly what you took can amplify the anxiety, when your brain starts filling up with seratonin and whites everything out just like people on their deathbeds report, are you supposed to let go? when your sense of self has been obliterated and the next moment you are in the body of another mammal lost and confused in the forest for an entire lifetime before being transported back into your body and only a minute has gone by - but your trip is to last another 9 hours, should you fight it? Distinct neural networks in your mind that never communicate are now connected, vestigial components of the mind are now being expressed, are you being replaced in a firmware dump and flash?

a lot of people have a friend with them to guide them through an acid trip because trips can be steered with sounds and words, simple chimes, melodies.

would it have helped? very hard to say. but as the author wrote, the bad day and uncomfortable setting did not help. It is similar to a dream state (just radically more intense), where the things on your mind and also happening around you can affect the direction of your dreams.

<comment><USER36> Yeah, I think it entirely had to do with my inability to relinquish control and "just let go". Although in this context, that was literally what felt like the fight to survive, instead of "being chill". Ego death commonly is either the most horrendous or most nirvanic thing depending on how readily someone gives in.

> when your sense of self has been obliterated and the next moment you are in > the body of another mammal lost and confused in the forest for an entire > lifetime before being transported back into your body and only a minute has > gone by - but your trip is to last another 9 hours, should you fight it?

There was a lot of this, during that out-of-body-period. I existed in multiple places/points in time at once as different people of various ages/genders/nationalities and then occasionally as animals, and lived entire simultaneous lifetimes. At one "time", in places + times A, B, C, D as different living things. Really does a number on your sense of self for a bit, heh.

<comment><USER35> and that had never happened to you in your other trips?

<comment><USER36> No, was really strange, I was pretty experienced by then too. Was never the same after.

<comment><USER35> Did you take 300ug before?

Sorry for the questions, we can talk about it somewhere else, just add an email or protonmail account to your hackernews account I'll mail you there

</comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment></comment><comment><USER40> If I buy a stock, does the price at which I agreed to buy it become the new share price on the stock exchange?

Every article on "Where do stock prices come from?" seems to just talk at a high level about supply and demand.

But where does the price come from at a nitty-gritty level? Is it an average of all existing offers or something?

Do different exchanges and stock-ticker websites have different formula for calculating share price?

If a very low-volume stock is listed at $4, and then I offer to buy a share for $100, does the NYSE suddenly start listing its price at $100?

<comment><USER41> Disclaimer: I used to work in HFT

Each exchange is basically its own world, with the exception of Reg NMS, which I'll get to in a sec.

Let's task about order books. Each stock has its own order book. This might be an example of the book for AAPL:

* SELLING 100 shares @ $10.02 * SELLING 200 shares @ $10.01 * SELLING 100 shares @ $10.00 * BUYING 100 shares @ $9.99 * BUYING 200 shares @ $9.98 * BUYING 100 shares @ $9.97

So if you want to buy some AAPL, you will want to go grab the cheapest shares you can see, which here is the fellow selling 100 shares at $10.00. You submit a limit order to buy at 10.00 and are matched with that guy. The book now looks like this:

* SELLING 100 shares @ $10.02 * SELLING 200 shares @ $10.01 * BUYING 100 shares @ $9.99 * BUYING 200 shares @ $9.98 * BUYING 100 shares @ $9.97

Now let's suppose a market maker decides they think the price is going to follow, so they go and fill in the hole by submitting an order to BUY 100 shares at 10.00. There's no more shares to buy at $10.00, so their order rests on the book.

Now we have:

* SELLING 100 shares @ $10.02 * SELLING 200 shares @ $10.01 * SELLING 100 shares @ $10.00 * BUYING 100 shares @ $10.00 * BUYING 100 shares @ $9.99 * BUYING 200 shares @ $9.98 * BUYING 100 shares @ $9.97

Now that we've played out this scenario, let's go back to your original question. What is the price of AAPL at any point in here? Well, it depends. At the start, if you wanted to buy, you could say the price is $10.00. But if you wanted to sell, the best you'd get is 9.99. So, hard to say.

It's worth noting that the prices you see in the book are only there because people _aren 't_ agreeing on the prices. If they did agree, a trade would happen, and the prices wouldn't be on the book. So, with that in mind, you could say that really, the price of a stock is the last price people agreed at: the last trade price. That's better, we're at least down to just one price to think about.

That could be quite different from what the best bid/offer are right now, though (some stocks don't trade very often) so even if (let's say) you last saw AAPL trade at 9.50 before our example, obviously that price is long gone. So even the last trade price is potentially not "the price of the stock".

So, in short, there's really no such thing as "the price of a stock". It'll all depend on how sophisticated you want to be about the price at which you buy your shares.

When people talk generally about the price of a stock, it's usually just up to whatever site people are looking at, and usually markets are liquid enough and trade enough that all the kinds of prices we just talked about are usually only a penny different, so when people are just at the watercooler saying "Did you see the price of AAPL?" they don't care about the pennies, and by the time they've managed to say that question, the price has moved anyway, probably lots of times. So it all gets a little hand-wave-y.

I want to mention two other things that might interest you. Reg NMS is what ties all the exchanges together, so to speak. Let's say you want to buy AAPL and NYSE has shares selling at $10.00 each, but NASDAQ has them for $9.99 each. It's actually illegal (against Reg NMS) to trade with that guy at $10.00 at NYSE because NASDAQ has the "NBBO" (national best bid/offer) right now. Extra caveat: if you sent a special order to NYSE that says "I promise you, I've also sent an order to NASDAQ to buy the shares for $9.99 and I've determined you're the next best price at $10.00, let me buy them", it'll let you. It's called an ISO (Intermarket Sweep Order) and if you lie about them or mistakenly lie about them, you get fined. A lot.

The other interesting thing: Your last question was "If a very low-volume stock is listed at $4, and then I offer to buy a share for $100, does the NYSE suddenly start listing its price at $100?" There's actually a lot to unpack here. Let's go through it.

If you're a registered broker-dealer and are connected directly to NYSE, and you send a limit order for XYZ @ $100/share, what's going to happen is you're going to get "price improvement" and you'll end up getting the shares at $4. If you send an order for LOTS of shares at $100, you'll clear out a bunch of price levels in one go. Ex:

Let's say this is the book for XYZ:

* [...] * SELLING 200 shares @ $110.00 * SELLING 1000 shares @ $5.00 * SELLING 200 shares @ $4.02 * SELLING 100 shares @ $4.01 * SELLING 500 shares @ $4.00 * BUYING 100 shares @ $3.99 * [...]

Usually when you get away from the middle of the book, liquidity dries up fast and the prices get further apart. So let's say you send an order for 10000 shares at $100. You're going to get 500 at $4, 100 at $4.01, 200 at $4.02, 1000 at $5.00. Now the next price is 110, but your limit price is 100. So your order will now actually rest partially-filled on the book. So now this is the book:

* [...] * SELLING 200 shares @ $110.00 * BUYING 8200 shares @ $100.00 * BUYING 100 shares @ $3.99 * [...]

Neat, huh? That was a lot of price movement. So yes, if you can send for enough shares and are willing to pay through a lot of price levels, you can move the price of the stock. Remember Reg NMS though - if more stock exchanges existed in our example, you'd also likely need to go get shares at them if they have a better price than the exchange you just moved the price at.

But let's now suppose you're NOT a registered broker-dealer, but are instead Joe A. Schmoe, a client of Charles Schwab Brokerage. You enter your order in your web browser and hit trade. Schwab has a legal obligation to fill your order, if possible, only at the NBBO. They could route your order right to an exchange, but instead, they will send your order to their friend, Citadel, who will have the opportunity to trade against your flow before it gets routed to the stock exchanges. Generally, this is good for you: they might decide your order represents good information and they want your shares. They could decide to fill your order themselves and sell you all 10000 shares you want. They're constrained by the NBBO though, so you get all 10000 shares at $4. For being the source of this order, Citadel pays Schwab some money. Usually practically a pittance, pennies, if that. Order flow is dirt cheap nowadays.

This is called "selling order flow" and lots of people find it scary, because it's not really super intuitive why someone would want to buy or sell the actual flow of orders. But it's actually pretty boring and more about high- level statistics than anything actually interesting to Joe Schmoe, who would get bored when he realized he's not really getting ripped off.

Sorry, I got a bit off-topic. But I love finance, so please forgive me.

<comment><USER42> Love the detailed explanation. Thanks. Once I saw Boeing at $120. I thought to myself. That's dirt cheap and should buy. So I hit buy at market and lo behold I bought for $130. Wait! Whaaa! Did the exchange lie to me ? That day I learned a very hard lesson that there are infact two prices. Ask price and bid price. I wasn't paying attention.

Now I try to always put limit orders. I put sell for Boeing at $150 with "good till cancelled" option. One morning I wake up to see they've been fulfilled. Wohoo! But the price had dropped down to $140. So I cashed in on the spike.

The market is crazy. I still don't understand if. P/E ratios for some companies are through the roof (100+), why are people still investing in them like crazy? We don't have a cov2 vaccine, millions of people don't have jobs, why did the marker recover half it's losses already? Shopify, Amzn, Zoom. WTF! Their charts seem hyped. Or may be I'm just plain wrong and don't understand the fundamentals.

<comment><USER18> > _Shopify, Amzn, Zoom. WTF! Their charts seem hyped. Or may be I'm just plain > wrong and don't understand the fundamentals._

Since the outbreak of COVID-19, demand for the kind of services offered by those 3 Internet businesses have in fact skyrocketed. Increasing demand imply those businesses still have room to grow revenue. Shopify [1] for instance is now seeing huge Black Friday-like traffic during the shelter-in-place and a lot of these small businesses are first-timers on their platform who will likely stick around after the pandemic.

1: [https://mobile.twitter.com/jmwind/status/1250816681024331777](https://mobile.twitter.com/jmwind/status/1250816681024331777)

</comment></comment></comment></comment><comment><USER43> Quantum spin. Electrons aren't really spinning, right? But why do we call it spin? I know it has something to do with angular momentum. What are the possible values? Is it a magnitude or a vector? Is there a reason we call it "spin" instead of "taste" or some other arbitrary name? How do you change it? What happens to it when particles interact?

<comment><USER44> > Electrons aren't really spinning, right?

Correct.

> But why do we call it spin?

Because it is a physical quantity whose units are those of angular momentum, and we have to call it _something_.

> What are the possible values?

+/\- h/4pi where h is Planck's constant. (It is usually written has h-bar/2 where h-bar is h/2pi.)

> Is it a magnitude or a vector?

It's a vector that always points in a direction corresponding to the orientation of the apparatus you use to measure it.

> Is there a reason we call it "spin" instead of "taste" or some other > arbitrary name?

Yes. See above.

> How do you change it?

You can change an electron spin by measuring it along a different axis than the last time you measured it. The result you get will be one of two possible values. You can't control which one you get.

> What happens to it when particles interact?

Their spins become entangled.

<comment><USER45> > It's a vector

It's not exactly a vector...

<comment><USER26> It only has direction + magnitude right? (±h/4π)e_i for some unit vector e_i.

So it can be written as a vector? No?

</comment></comment></comment></comment><comment><USER32> Law. How much of it rests on technicalities, and how much do judges care about the essence of the facts of a case? You only hear about the weird outcomes on the news. For example, I was once working with an attorney because my landlord didn't supply heat in the apartment. I started keeping a temperature log, but how would a judge know that the log was accurate? Do I need to prove that my thermometer is accurate... etc. In practice, no, but how can I better reason about what is "likely" vs not?

<comment><USER46> I've begun to grok so much about how law _practically_ works by listening to Popehat's All the President's Lawyers podcast.

Basically you get an educated layperson asking a veteran criminal lawyer questions, usually around the First Amendment, always related to current events. The lawyer (Ken White) explains in practical terms what is likely to happen and why.

</comment></comment><comment><USER47> I consider that there are 4 levels of scientific writing.

1) news articles/lay press - basically terrible and typically get things wrong

2) scientific lay press (scientific american, discover, science news) - <USER2> things right, but generally no data/citations or nuance

3) journal summaries - get things right, citations and data for everything. Good summary of the latest scientific thought on a topic. Tend to push a point of view, which generally will be right, but that educated people can debate. Dont always show the data, but at least refer to it. These help you to get up to speed with the primary experiments that were used to establish current thinking.

4) first source articles - typically make claims too broad for the actual results. But has all data. Often times the claims don't follow from the data at all. Generally have to work in the field to understand strengths and weaknesses of methods and you cant just take the conclusions at face value.

As a PhD student, I used #3 a lot to get centered on a space. To understand 4, I typically had to learn directly from my research advisor or other grad students that specialized in an area.

My point here is that you can find these summary articles in journals (microbiology, immunology,virology etc). They are published infrequently so can be hard to find, but they exist and you should look for them.

<comment><USER48> I was shoulder deep in primary source material (#4) on real-time fuzzy search and wasn't able to make heads or tails of where everything fit. Until I found a #3 overview that referenced most of the primary sources and put them in context. That paper was worth its weight in gold!

Do you have any tips for how to quickly find these #3 materials in other spaces?

<comment><USER49> The name for what you and the parent described is a review paper. I can't speak for Math or CS, but in life sciences the following techniques would work:

1\. Search X and sort by citation count. High quality review papers get cited A LOT, typically in introduction sections of primary research papers. Alternatively, google "[X] review" or "best review paper on X".

2\. Look for review journals. Many fields will have journals who only publish reviews. Nature has several such publications for example.

3\. Look for the top journals in the space (start by sorting by impact factor) and see if they have review sections. If they do, try to search those sections. Most journals will reach out to top labs in a space and request that they write a review on a subject if the journal editors feel one is needed.

4\. Ask someone in the field. Any researcher should be able to immediately point you to canonical reviews in their space.

</comment></comment></comment><comment><USER50> Subreddit /r/askscience does a good job at explaining science in plain words. I usually google "site:reddit.com/r/askscience/ __QUESTION__".

The StackExchange sites have less coverage and answers tend to be more technical.

University websites return reliable answers, but often neither short nor accessible.

</comment><comment><USER51> Bell's theorem. It somehow proves that quantum physics is incompatible with local hidden variables, but I could never see an understandable explanation (for me at least) of just how it works.

<comment><USER52> Yudkowsky's explanation[1] is the first one that worked for me. I later found Quantum mysteries for anyone[2] helpful. The latter has less soap-boxing.

1: [https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s- the...](https://www.lesswrong.com/posts/AnHJX42C6r6deohTG/bell-s-theorem-no- epr-reality)

2: [https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/...](https://kantin.sabanciuniv.edu/sites/kantin.sabanciuniv.edu/files/makale/mermin.pdf)

<comment><USER53> I started to read the first one but his insistence that Many Worlds is true was too frustrating. Many Worlds Theorem seems specifically useful at saying "the variables aren't hidden because everything before wavefunction collapses actually plays out in different worlds.

But, we specifically have no way of proving that theory. So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables?

<comment><USER54> Well, I'd recommend to read the whole series. It's not so bad as it sounds. There are so many steps from where you are to appreciating the utter _weirdness_ of Bell's experimental result. Not the weirdness of any theory (or an interpretation, which Many Worlds actually is) but of the basic experimental result.

If you _are_ properly amazed by it, rejecting MWI or any crazy-ish borderline- conspiracy theory seems suddenly a lot harder.

I feel the whole Yudkowsky's QM series in fact served to deliver that one post.

<comment><USER55> Why isn't the MWI another form of hidden variables (a supremely non- parsimonious one at that), where the hidden variable is which of the many worlds you happen to inhabit?

<comment><USER56> I think you can make an argument for viewing it that way, depending on exactly what you mean by "you".

But IIUC, one of the remarkable things about MWI is that it would be a _local_ hidden variable theory!

This is a very important property to have because the principle of locality is deeply ingrained in the way the Universe behaves. Note that (almost?) no other quantum interpretation is both realist and local at the same time.

Maybe you wonder, how is it possible that MWI can be considered a local hidden variable theory if Bell's theorem precisely shows that local hidden variable theories are not possible?

I think that it was Bell himself who said that the theorem is only valid if you assume that there is only one outcome every time you run the experiment, which is not the case in MWI.

This means that MWI is one of the few (the only?) interpretation we have that can explain how we observe Bell's theorem while still being a local, deterministic, realist, hidden variable theory.

<comment><USER55> For it to be local (causality does not propagate faster than light), it must be superdeterministic (all the many worlds that ever will be, already are). For it not to be superdeterministic (many worlds decohere at the moment of experimentation), it is also not local (the decoherence happens faster than the speed of light, across the universe).

<comment><USER56> I'm sorry but I don't follow.

If you take the Bell test experiment where Alice and Bob perform their measurements at approximately the same time but very far apart, I think you and I both agree that when Alice does a measurement and observes an outcome, she will have locally decohered from the world where she observes the other outcome.

But I don't see why the decoherence necessarily has to happen faster than the speed of the light.

It makes sense that even if Alice decoheres from the world where she observes the other outcome, the outcomes of Bob's measurement are still in a superposition with respect to each Alice (and vice-versa).

And that only when Alices' and Bobs' light cones intersect each other will the Alices decohere from the Bobs in such a way that the resulting worlds will observe the expected correlations (due to how they were entangled or maybe even due to the worlds interfering with each other when their light cones intersect, like what happens in general with the wave function).

I admit I'm not an expert in this area, but is this not possible?

</comment></comment></comment></comment></comment></comment></comment></comment><comment><USER57> Abiogenesis. I understand that this is not considered understood but I'd be interested in hearing a qualified scientist talk about how we get from "dumb" matter to self-replicating, goal-driven matter. The closest I've ever heard anyone get (in a personal conversation) is that chemistry is about transformation, so "dumb" matter isn't really dumb in the sense that it's static. Still lots of hand waving to get from baking soda and vinegar volcanoes to me typing this question, however.

What's the playing field look like for proto-life? How "smart" are the simplest molecular interactions? What does almost-replication look like? Could we use a computational model for this?

Not sure how much of this is known, but I'd love to hear an expert paint a picture of their mental model of the subject.

<comment><USER58> This question interests me as well, and I've done some thinking about it over the years. In particular, what I'm interested in is a hypothetically simplest object that can reproduce in a solution of simpler components, along with some differentiating characteristic. For example, maybe you have toroids that pick up particules, grow the torus until its too big, and then splits - and the ends of both halves click together, forming a total of two toroids. Another characteristic that very simple life must have (I believe) is some level of circularity in the sense that the element is an "accumulation of experience" \- we might say a reduction of its environment. In the same way Schordinger was interested in life thermodynamically[1] I am interested in speculating about the simplest possible mechanisms in the beginning. (NB I'd expect _none_ of these very simple machines to survive to present day - in fact, I'd imagine there to be several generations of early life, each obliterating/consuming/sublimating the ones before.)

1 - [https://en.wikipedia.org/wiki/What_Is_Life%3F](https://en.wikipedia.org/wiki/What_Is_Life%3F)

<comment><USER57> > NB I'd expect none of these very simple machines to survive to present day - > in fact, I'd imagine there to be several generations of early life, each > obliterating/consuming/sublimating the ones before.

That's another really interesting axis of this topic -- why was early Earth special?

There may be no environments on Earth today that are like early Earth, but we could probably recreate them. Wouldn't we then be able to witness abiogenesis?

Or, early Earth wasn't special and abiogenesis happens today. If so, where do we look?

<comment><USER58> _> Wouldn't we then be able to witness abiogenesis?_

It may happen quite infrequently, and only because it happened in a large, lifeless (but 'nutrient' rich bath) did it have a chance to amplify. What's interesting to me is that it only has to happen once.

Yes, Earth's specialness is interesting, too, and counts for what I believe are the best reasons to believe in God. Earth has so many amazing qualities: it is a cozy distance from the Sun (temp), tilted quite a bit (seasons), with a molten core (cosmic ray protection) and a huge moon (tides, nocturnal light). All of these may be necessary conditions for life to arise, and they are all, as far as we know, quite rare individually, and astronomically unlikely in combination.

</comment></comment></comment></comment><comment><USER59> Fourier Transforms. I'd wish I had a intuitive understanding of how they work. Until then I'm stuck with just believing that the magic works out.

<comment><USER60> The best way to understand the Fourier transformations is to think of them as change-of-basis operations, like we do in linear algebra. Specifically a change from the "time basis" (normal functions) to the "frequency basis" (consisting of a family of orthonormal functions).

Here is the chapter on Fourier transforms from my linear algebra book that goes into more details: [https://minireference.com/static/excerpts/fourier_transforma...](https://minireference.com/static/excerpts/fourier_transformations.pdf)

As for the math, there really is no other way to convince yourself that sin(x) and sin(2x) are orthogonal with respect to the product int(f,g,[0,2 _pi]) other than to try it out[https://live.sympy.org/?evaluate=integrate(%20sin(x)*sin(2*x...](https://live.sympy.org/?evaluate=integrate\(%20sin\(x\)*sin\(2*x\)%2C%20\(x%2C0%2C2*pi\)%20\)%0A%23--%0A) Try also with sin(3_x) etc. and cos(n*x) etc.

<comment><USER61> > As for the math, there really is no other way to convince yourself that > sin(x) and sin(2x) are orthogonal with respect to the product > int(f,g,[0,2pi]) other than to try it out > [https://live.sympy.org/?evaluate=integrate(%20sin(x)*sin(2*x...](https://live.sympy.org/?evaluate=integrate\(%20sin\(x\)*sin\(2*x..). > Try also with sin(3x) etc. and cos(n*x) etc.

I disagree with that. It's pretty easy to prove it in general by calculating \int_0^{2\pi} sin(mx)sin(nx) dx etc. for m ≠ n.

<comment><USER60> I would count an analytic solution as in the "trying out" category (actually the best kind of trying out!).

The "no other way..." was referring to me not having an _intuitive_ explanation to offer about why an sin(x) and sin(2x) are orthogonal.

</comment></comment></comment></comment><comment><USER62> • Magnetism. There are plenty of videos out there calling it the result of a relativistic charge imbalance. But I've never been able to use this point-of- view to practical use cases like understanding how permanent magnets work or how increasing the number of windings in inductors boosts the magnetic field strength. There were more situations I tried to put this POV into use but I can't remember them off the top of my head.

• Qualia. What is this subjective experience that I know as consciousness? I've gone through Wiki, SEP and a fair number of books on philosophy and a few on neuroscience but I still don't understand what it is that I experience as the color "red" when in reality it's just a bunch of electric fields (photons). Why can't I get the same experience — i.e., color — when I look at UV or IR photons? These too are the very same electric fields as the red, blue, green I see all the time.

• Photographic composition. I'm a designer. I know them. I use them. But only empirically. I just do not understand them at a neuroscientific level. Why does rule-of-thirds feel pleasing? Is the golden ration bullshit? My gut says yes but I'm unable to come up with a watertight rebuttal. Why do anamorphic ultra-widescreen shoots feel so dramatic/cinematic? Yet to see an online exposition on the fundamental reasons underlying the experience. Any questions to artists are deflected with the standard "It's art, not science" reply.

• Wave-Particle duality. "It's a probability wave that determines when a particle will pop into existence out of nothingness." okay, where exactly does this particle come from? If enough energy accumulates in a region of empty space, a particle pops into existence? What is this "energy"? What is it made of? What even is an electron, really? I've followed quite a few rabbit holes and come out none the wiser for it.

• Convolution. It's disappointing how little I understand it given how wide its applications are. Convolution of two gaussians is a gaussian? Convolution in time domain is multiplication in frequency domain and vice-versa? How do these come out of the definition which is "convolution is sliding a flipped kernel over a signal"?

<comment><USER63> The issue with wave-particle duality is that most of us think about it backwards.

The universe is actually made of quantized fields. Both particles and waves are imprecise models/approximations. There's no such thing as a particle, instead there are just excitations of this field which we cannot measure with complete accuracy.

<comment><USER64> > Which we cannot measure with complete accuracy

I very much dislike this phrasing, because it suggests that it is just us that are not capable of building an apparatus to enable us to do so.

Imagine a gear transmission or a lever: You can transform distance into force and vice versa. It is up to your choosing if you want to go with more speed or more force by changing the point along the lever, where your transmission happens. It is not possible to build a transmission, which gives you the most distance and the most force simultaneously. In this system of transmission, one is the other, just a different perspective.

And it is the same with the location and impulse of a quantum. You can choose to have more information in the shape of location or more in the shape of impulse by changing your measurement (like the point along the lever). But you can't have both, because there is only a constant amount of information which is represented in a combination of location and impulse.

Actually, the uncertainty part of heisenberg uncertainty principle is a purely mathematical limitation (called Gabor limit) and only the Planck constant makes it physical.

Gabor limit: a • b >= 1/(4•PI)

Heisenberg's uncertainty principle: a • b >= h/(4•PI)

So the Planck constant is kind of the maximal sampling resolution of the fields / signals in our universe.

<comment><USER63> Good point

</comment></comment></comment></comment><comment><USER65> Lot's of quantum related phenomenon here, but what keeps bothering me is that while I get it that light is both a wave and a particle, but I have no clue what that means. I mean, a wave of sound, is made from air particles, a wave of ripples in a pond is made of movement of water molecules. In the double slit experiment, it's explained that the single photon has to be "interfering with itself", so I don't get it if by being a "wave" it means that the single photon is basically a bunch of "magic" photon ghosts that behave like a wave, but once it is measured or any other reason to "collapse" these ghosts "disappear". I just don't get what the "wave" of light/radio wave is. Is it just an abstract concept of something that behaves like a wave but not the same as sound waves / ripples since we simply don't know? Or is it just a wave of these "not yet collapsed" probabilities of the photons locations that are interfering with each other right until we ask them to choose a location, then they just collapse magically into a single "real" photon. Another thing I don't get is in the double slit experiment, a LOT of the measure before, measure after, etc, are told to be thought experiments, but it's also claimed that someone managed to actually replicate them. Why isn't there a video showing it? I obviously believe they happened, and understand why more or less (e.g. in the one photon at a time experiment, it's spooky that over time you get the same pattern that indicates interference as if you shoot many) but the more spooky result is that thought experiment, that if you measure which slit the photon actually traveled through, you'll see 2 slits on the screen vs the famous pattern. e.g. you'll cause the wave to collapse back to particles. So any video of reproducing of that thought experiment or explanation why it's so hard to reproduce, will be super helpful.

<comment><USER66> > while I get it that light is both a wave and a particle, but I have no clue > what that means.

Because it's wrong. It's a quantum of the electromagnetic field. It's _neither_ a wave _nor_ a particle. It just happens to have some properties of both.

<comment><USER67> I've done plenty of layman reading into QFT, and I'm truly good with this. All particles are really waves. There are no particles. There's self-interaction, and all other kinds of weird stuff. (it's important to qualify that stable orbits and molecules and stuff exist)

But for the duality, there's something bigger that the responses always seem to blow past. Is wave-like nature for explaining behavior (wavy double-slit intensity pattern), or is it something to have a mathematical mapping to measured probabilities?

Quantum stories always seem so backwards. The root phenomenon is some sort of irreducible probability. But then the mechanical part (inference in double- slit) goes a totally different direction. Instead of just turning the situation into a probability of one-or-the-other slit, it STAYS as a wave.

Okay, now you have a new hole in the story. If the photon refuses to choose just 1 slit to go through, why does it choose 1 spot on the photo paper to land on?

Why do we not still have to consider interference in outcomes after the photon makes its mark on the paper? Why does there appear to be like a limit on entanglement, such that it goes away beyond a certain scale? Why are quantum computers hard?

<comment><USER63> > If the photon refuses to choose just 1 slit to go through, why does it > choose 1 spot on the photo paper to land on?

The photon (as a field excitation) goes through both slits, but is quantized so only has enough energy to trigger a mark at 1 spot on the photo paper.

> Why do we not still have to consider interference in outcomes after the > photon makes its mark on the paper?

If we want to be completely accurate, we should. However so many interactions happen so quickly that the law of large numbers quickly takes over and obfuscates the quantum reality. Technical term for this is decoherence.

> Why are quantum computers hard?

Exactly because of this decoherence. It is very difficult to keep the qubit state isolated from the environment throughout the computation.

</comment></comment></comment></comment><comment><USER68> Flight. Apparently "air flows faster on the top side of the wing, lowering the pressure" is an incomplete explanation; I even heard we don't completely understand why it works (?!?).

<comment><USER69> I've been informed that I'm part of the problem. Comment removed, sorry for the trouble, folks!

<comment><USER70> >The top of a wing is curved, making it longer than the bottom of the wing. This means that air takes longer to go over it, meaning it has to spread out further to go the same distance as the air under the wing. As a result, the air going over the top of the wing is less dense, (aka lower pressure). The wing tries to equalize the pressure by moving in the direction of the low pressure, which is Up. We call this Lift.

100% completely false.

Imagine you have two particles of air, and they are immediately adjacent to each other. Suppose now that one goes above the wing, and one goes underneath. In your example, the particle going upward goes further in the same amount of time.

But ask yourself this: Why do the particles of air have to arrive at the same time? What mechanism from physics requires that they meet up again at the far end of the wing?

Then ask yourself this: If what you described is true, then how do aircraft fly upside down?

<comment><USER71> For years I thought I was crazy or stupid, whenever I heard this story of "air has to go faster" I was like "but how does air know?? It's not like it has a Google maps plan telling it it needs to reach the other end of the wing at a precise time!"

By chance, in the last few years I've started reading more and more comments debunking this absurd explanation. Not that I understand perfectly now, but at least I know I'm not crazy.

<comment><USER70> In fact that "air has to go faster" silliness only works if you completely neglect air friction entirely, because then the air can be said to part around the wing like butter around a hot knife.

But of course anyone who's seen snow billowing off the back of a car knows that air doesn't just close up behind the object like a ziplock bag: it's messy and turbulent and gets all over your windows while you're tailgating.

</comment></comment></comment></comment></comment><comment><USER72> Why does time slow down/go faster with movement compared to another object.

The well known example that if you travel into space you'd gain let's say 5 years and people on earth 25 in the same time or so.

I just don't get it and I can't find any logic explanation.

For instance: Two twins who came to live exactly at the same moment in the year 2000 and both die on their 75th birthday at the same time. One travels into space, the other stays on earth. Earth-brother dies on earthyear 2075,space-brother dies in earthyear 3050 or so...

I know its Einstein's point but that just doesn't instantly make it correct to me.

<comment> opinion-is-bad All things in the universe have four measures of velocity. Three of these are easily observable to us in terms of x, y, and z movement. To understand time dilution we need to realize that we are also physically traveling through time.

The total velocity of our <x, y, z, t> vector will always be equal to the speed of light constant, c. You can think of something that has no physical movements as moving forward in time at the speed of light. As x, y, or z increases the magnitude of t will decrease so that the speed of light constant is always achieved.

Why this link has to hold is more complex and I cannot explain it well, but hopefully this gives some insight into time slowing as velocity increases.

<comment><USER73> > The total velocity of our <x, y, z, t> vector will always be equal to the > speed of light constant, c. You can think of something that has no physical > movements as moving forward in time at the speed of light. As x, y, or z > increases the magnitude of t will decrease so that the speed of light > constant is always achieved.

I've gone decades without hearing it explained that clearly and simply. Thank you (sincerely).

</comment></comment></comment><comment><USER74> What happens when you actually fall inside a black hole and what is the singularity.

I never really understood what happened really when the guy fell inside it in Interstellar and how come he started seeing all those photos. I just accepted it as Hollywood bs.

I know my question is based on a movie but would still like to know what will someone witness (assuming of course they somehow live)

<comment><USER75> Singularity in AI?

<comment><USER76> No, [https://en.m.wikipedia.org/wiki/Gravitational_singularity](https://en.m.wikipedia.org/wiki/Gravitational_singularity)

</comment></comment></comment><comment><USER77> Avogadro constant - I can accept the number, if someone can show me how, over a hundred years ago, without having every really seen a molecule, this number could be derived. Obviously no one ever sat there and counted them but I find it hard to believe, they could have decided this without being able to see it.

<comment><USER78> Here's how to estimate the size of an oil molecule just using water and an oil drop: [https://spark.iop.org/estimating-size-molecule-using-oil- fil...](https://spark.iop.org/estimating-size-molecule-using-oil-film)

(They use some other stuff, but you get the idea)

You can back out Avogadro constant starting with this experiment.

<comment><USER77> Thank you. I do get the idea. And now I can go back and finish High School chemistry. I appreciate you help in understanding this.

</comment></comment></comment><comment><USER79> Why tardigrades are so hardy, how their biology is so different?

How immune system and medications work.

Why some plastics are recyclable and others are not.

<comment><USER80> I've been mulling making a youtube channel with ten minute videos on immunology; what would be a good starter video that might interest someone like you? I thought I'll do something about antibodies as drugs!

<comment><USER79> Thank you and please please do it and post a link here or send me an email (in profile)! For me the most interesting is the recognition/pattern matching aspect: how antibodies find what to attack and what to leave alone.

<comment><USER80> Most definitely one of the hardest questions to answer :) I'll take it up as a challenge!

</comment></comment></comment></comment><comment><USER81> Non-interactive zero knowledge proofs.

ZK proofs have a number of good explainers, mostly using graph colorings. Non- interactive versions, however, require quite a bit more than that explanation allows - and despite asking experts, I still haven't found a good, basic explanation.

<comment><USER82> Maybe the blog of Prof. Matthew Green of JHU is of use. Specifically, the two- part series about zero-knowledge proofs. Part II discusses non-interactive ZK proofs. Part I is really required to grasp the extension to non-interactive ZKP's, so you may need to read that first. [https://blog.cryptographyengineering.com/2017/01/21/zero- kno...](https://blog.cryptographyengineering.com/2017/01/21/zero-knowledge- proofs-an-illustrated-primer-part-2/)

</comment></comment><comment><USER83> A non handwavy explanation of how does evolution result in such complicated and carefully orchestrated mechanisms, way beyond our engineering capability. The computer analogy of genetic algorithms certainly doesn't explain much, they are not very effective, and if they were, we should be able to generate marvels of engineering with GAs and current computational power.

<comment><USER77> A whole LOT of time, a whole lot of space.

So estimate 1 minor good random mutation per 10,000 population. Assume 1 major mutation per 1000 years, per 10,000 population.

<comment><USER83> That's essentially the same kind of explanation as 'god did it'.

<comment><USER84> I dont know if you are being serious, but if you are, Dawkins books are pretty good (The blind watchmaker,or the Ancestors tale).

<comment><USER83> I will have to check them out, but I remember being unimpressed with the table of contents. Looks like it will be more handwaving.

<comment><USER84> Ah OK, you are not being serious, fair enough. Magic god did it all.

<comment><USER83> I am being serious. I just purchased Dawkin's book "Blind Watchmaker". My point is no explanation should amount to "magic X did it" whether X is god, evolution, the earth spirit, aliens, etc. All the above are bad explanations.

It is not scientific to substitute one bad explanation for another. The scientific approach is to say we don't know, and then look for a good explanation.

</comment></comment></comment></comment></comment></comment></comment><comment><USER85> Mach's principle. Why is there a "preferred" rotational frame of reference in the universe? Or as stated in this Wikipedia article,

"You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?"

[https://en.wikipedia.org/wiki/Mach%27s_principle](https://en.wikipedia.org/wiki/Mach%27s_principle)

<comment><USER86> The unsatisfying mathematical answer is that it is impossible to have a uniform distribution of rotational speeds, therefore there must be a preferred one.

It's the same reason the universe has an average speed (unlike what you might expect from special relativity), although it is unclear if this is true for the _entire_ universe or just the portion we can see. We can measure how fast we're moving w.r.t the cosmic microwave background radiation though (it is red-/blue-shifted in a particular direction).

<comment><USER87> This is an interesting argument! Wouldn't it also work for positions, though? That is, either the universe is finite, or, since there can't be a uniform distribution over an infinite space of positions, there must be some preferred "center" of the universe?

<comment><USER86> You'd think, but of course we know that not to be the case. It's hard to pinpoint the exact reason though. Sure we know time and space are rather special, but its hard to say exactly why.

In the end though I reckon the most obvious reason is that speed is a property that directly corresponds to energy, therefore for each region of space to have a well defined energy (which is required for e.g. general relativity) every region of space needs to have a well defined distribution of speeds.

I suppose this does leave open a small loophole, as you can easily correlate speed with position in order to get a distribution that is uniform in both (but correlated). But this goes against our assumption that the universe is uniform everywhere (which might turn out to be false, but so far it's holding up well).

</comment></comment></comment></comment><comment><USER88> Asynchronous programming

With the addition of async to django core, I felt its time to finally learn the concept. I first took interest in async early last year when I re-read a medium post on Japronto; an async python web framework that claims to be faster than Go and Node.

Since then, I've been on the lookout for introductory posts about async but all I see is snippets from the docs with little or no modifications and a lame (or maybe I'm too dumb) attempt at explaining it.

I picked up multi threaded programming few weeks ago and I understand (correct me if I'm wrong) it does have similarities with asynchronous programming, but I just don't see where async fits in the puzzle.

<comment><USER89> The first couple of paragraphs of the documentation for asyncore, the module in Python's standard library that implemented the machinery for async IO all the way back in 2000, has a great description of what async programming is all about. Here it is:

[https://python.readthedocs.io/en/latest/library/asyncore.htm...](https://python.readthedocs.io/en/latest/library/asyncore.html)

'There are only two ways to have a program on a single processor do "more than one thing at a time." Multi-threaded programming is the simplest and most popular way to do it, but there is another very different technique, that lets you have nearly all the advantages of multi-threading, without actually using multiple threads. It's really only practical if your program is largely I/O bound. If your program is processor bound, then pre-emptive scheduled threads are probably what you really need. Network servers are rarely processor bound, however.'

'If your operating system supports the select() system call in its I/O library (and nearly all do), then you can use it to juggle multiple communication channels at once; doing other work while your I/O is taking place in the "background." ...'

</comment></comment><comment><USER90> [Sorry I have misinterpreted the subject, I thought it could be some phenomenon still not well understood by science.]

Placebo. There are biological bases of it (I don't believe in soul). Find these bases, study them, make a model of them. Then use proxy variables to measure it instead of trying to eliminate it statistically. Predict it in studies to avoid the need of placebo groups (and possibly of double blind methodology). Also, after it is completely measurable and its mechanisms are understood, if (very hypothetical) it has a really substantial effect, just use it to help treat patients.

</comment><comment><USER91> What actually happens to the money that gets put into the stock market? I mean, I understand that there are people holding stocks and then people offer to buy/sell them at a given price; but, how does the company plan to benefit if the stock just keeps moving back and forth multiple third parties? Doesn't a company go public to raise money? If so, how does the company benefit from the daily changes in the stock and when someone buys stock in the company? Does the company have a fixed number of stock that they trade as well on the market?

<comment><USER92> Company raises money from market by giving away part of the company.

If company shares are cheaper than other source of money, then company can buy back them, or other potential owner can buy them. It also sends strong signal to owners and investors about future of the company.

If company shares are costing more than other sources of money, then company can sell more of them to return costly money back or to expand business.

For example, bank rent rate is 10% per year. Company worth is $1 million. Company created 1 million of shares. 1 share is $1 of company worth. Company pure profit is 10% per year. Company needs $1 million in circulation to operate.

If company will rent money from bank, it will have 10%-10%=0% of profit. If company will sell 100% of shares, it will have 10%-0%=10% of profit, but this profit will go to someone else. If company will sell 50% of shares and will rent $0.5 million from bank, it will have 10%-5%=5% of profit, 2.5% will go to company, 2.5% will go to other owners. By reinvesting this profit, company can reduce bank money or expand company to worth more, increasing company profit share.

</comment></comment><comment><USER93> The one-electron universe is always a personal favorite. Though more a far- fetched theory than a proper "scientific phenomena", I'd be eager to learn more about it in layman's terms

[https://en.wikipedia.org/wiki/One- electron_universe](https://en.wikipedia.org/wiki/One-electron_universe)

[https://www.youtube.com/watch?v=9dqtW9MslFk](https://www.youtube.com/watch?v=9dqtW9MslFk)

</comment><comment><USER94> vertical alignment in CSS

<comment><USER95> this made me laugh!

</comment></comment><comment><USER96> Automatic differentiation. It's useful to so much computational work, but most people only get a cursory introduction to the topic (a rough intro to the minimum they need to know), whereas really understanding it seems to open up a lot of research.

<comment><USER97> Oh man, I read a super cool article about that about a year ago. It provided an algorithm for automatic differentiation using an imaginary number such that it times itself equaled 0, but wasn't equal to zero itself. I'll try to find the link.

I don't know if this was it, but an explanation nonetheless [https://medium.com/@omaraflak/automatic- differentiation-4d26...](https://medium.com/@omaraflak/automatic- differentiation-4d26d03b7508)

</comment></comment><comment><USER98> I'm sure there are great explanations out there but I haven't had time to read up on them, but a few of the space things that always bother me when I watch pop-sci space tv:

\- "as soon as iron starts to be produced in the core of a star it instantly collapses" \- I get that fusing iron costs energy rather than produces it and this causes a collapse.. but can it really be that quick? There are other fusion reactions that are still producing energy, right?

\- dark matter / energy - I understand we have observations that indicate there is some type of matter we can't see but it feels a lot like saying "magic" or "the ether".

\- how different size stars form - if there is a critical mass where a star "ignites" and after igniting starts pushing away from itself with the energy being produced, how do we get stars of such varying masses? Like, why didn't this 100x solar mass star start fusing and pushing the gases away before they were caught in its gravity? Do the more massive stars ignite on the same schedule but continue to suck in additional matter anyway, gravity overcoming the solar wind?

<comment><USER99> Not an expert, so take these with a grain of salt:

The core of the star is the hottest and most dense part. Greater heat and density make it easier for fusion reactions to run. If suddenly the core is made mostly of iron, then the amount of energy it produces rapidly drops. Even if there are nice, easily fusible hydrogen atoms farther out from the core, they will not be fusing at a very high rate, because the temperature and pressure is lower where they are. Also, the more easily fusible atoms remaining outside the core can't diffuse into the core fast enough to refuel it. The only possible outcome is collapse.

In some sense "dark matter" and "dark energy" are just placeholder words for "whatever thing is causing all this weird stuff to happen". This is actually very analogous to how "the ether" was a placeholder term for "whatever thing that radio waves are waves in". (Now we refer to it as "the electromagnetic field". The "ether" terminology was associated with some incorrect assumptions, such as a privileged reference frame, which is why people sometimes say it was an incorrect hypothesis. But the electromagnetic field is certainly real, it just didn't turn out to work like some people thought it did.) Scientists have observed so far the dark matter seems to behave pretty much like ordinary matter, except that it just happens to ignore the electromagnetic and strong nuclear forces. Not only does it hold galaxies together, but its gravity also bends the paths of light rays, just as we expect of anything massive. So calling it "matter" isn't too much of a stretch. It's still very mysterious, though.

Radiation pressure actually does limit the mass of stars, to something on the order of 100 to 200 solar masses, see this stack exchange question: [https://astronomy.stackexchange.com/questions/328/is- there-a...](https://astronomy.stackexchange.com/questions/328/is-there-a- theoretical-maximum-size-limit-for-a-star) That doesn't stop smaller clouds of gas from collapsing to form smaller stars, though.

<comment><USER98> Thank you for even trying to answer my rambles! :-)

I think my contention with the iron is the tipping point and how quickly it goes. Pop-sci tv makes it seem like you fused a single iron atom and bam. Maybe it is you fused an iron atom and it is like a day, a year or a thousand years and that adds up; still bam in terms of cosmic timelines but it is not what I hear when I listen and hear "instant collapse".

Thank you for the thoughts on dark matter and energy as well, and the link on radiation pressure, I will read it.

</comment></comment></comment><comment><USER100> When I hear explanations like "space is expanding like the surface of a balloon" it's always confusing. Because a surface is an object, separate from anything on it, but space is the thing we're all embedded in, so we're like drawings on the balloon.

If space is expanding why aren't the radii of fundamental particles and their orbits and molecules also expanding? And if that were the case we couldn't notice space expanding.

<comment><USER21> > If space is expanding why aren't the radii of fundamental particles and > their orbits and molecules also expanding? And if that we're the case we > couldn't notice space expanding.

> Does space only expand somewhere else? Only between me and the Andromeda > galaxy, and not _within_ me and the Andromeda galaxy? How would it know to > do that?

If you start with expanding space in general relativity, and then carefully take the limit where you get back to Newtonian gravity, then it just corresponds to a classical force, specifically a very tiny force that weakly pulls everything apart, growing with distance.

This doesn't expand small objects, because they're rigid. It's the same reason that I can't make my laptop get bigger by gently pulling on the ends. On the other hand, it would pull apart two independent, noninteracting objects (such as the Milky Way and Andromeda).

<comment><USER101> On top of that, FLRW spacetime is a large-scale approximation: More realistic models should probably follow the 'swiss-cheese' approach, where local conditions can look rather different.

</comment></comment></comment><comment><USER102> Antenna Design/RF Theory. I would love a simple text/YouTube series to learn this stuff...would definitely be good learning for the summer :)

<comment><USER103> I second this. I once worked with an assistant professor to do some of that, and in the end a lot of what we did boiled down to trial and error: do a design, run simulations in some software, and if it doesn't look good try again. It makes me sad that I didn't really encounter anyone capable of explaining things beyond trial-and-error.

</comment></comment><comment><USER104> * Lie algebras and Lie groups - I still don't understand this, what they're used for or how to use them in any practical sense.

* Galois Theory - I have a basic understanding of abstract algebra but for some reason Galois theory confounds me, especially as it relates to the inability of radical solutions to fifth and higher degree polynomials

* "State-of-the-art" Quantum Entanglement experiments and their purported success in closing all loopholes

* Babai's proof on graph isomorphism being (almost/effectively) in P - specifically how it might relate to other areas of group actions etc.

* Low density parity checks and other algorithms for reaching the Shannon entropy limit for communication over noisy channels

* Hash functions and their success as one-way(ish)/trapdoor(ish) functions - is SHA-2 believed to be secure because a lot of people threw stuff at the wall to see what stuck or is there a theoretical backpinning that allows people to design these hashes with some degree of certainty that they are irreversible?

<comment><USER105> Probably the best place to get started with Lie groups is to wrap your head around SO(n), which has both a nice geometric interpretation as rotations of n-dimensional space as well as a concrete representation as the space of orthogonal (determinant 1) nxn matrices. With a little matrix calculus you can work out what the tangent directions are at the identity matrix: they're precisely the skew-symmetric matrices. This is the Lie algebra so(n). Where the Lie group consists of rotations, the Lie algebra consists of directions in which you can rotate something, or really velocities of rotation. This is why classical angular momentum is actually an element of a Lie algebra.

<comment><USER106> I'm sorry but you used so many big words so close together, I could find no purchase with the things I know. Perhaps you could tell me what was the problem that Lie groups are supposed to solve. I read something about an infinitely symmetric transformation- the rotation of a circle is a Lie group. But then what are all the letters about- the A,B,C etc all the way up to E 8. And why did it take 77 hours on the Sage supercomputer to solve E8.

</comment></comment></comment><comment><USER107> Light, a mixture of why a thing is the colour that it is? As well as why reflection would be angle of incidence is angle of exit.

My current understanding of colour is that the colour of an object is defined by the ability of the electrons in the compound jump different energy levels. I don't know if that in itself is enough to result in all the colour we see.

My current understanding of reflection is that because of wavyness of light when lots of light gets absorbed (to my understanding a single photon exciting a single electron to jump some amount) and reemited (the electron falling back down) together the light ends up forming that angle pattern. Under than understanding single photons don't bounce in the same way rays of light do?

I don't know how correct either of those understandings are, but my understanding has been put together from so many places and I've never heard any source explain either like that so I don't trust they are correct.

<comment><USER108> You are mixing a lot of different things:

* The reflection angle laws are due to the laws of conservation, see [https://en.wikipedia.org/wiki/Snell%27s_law](https://en.wikipedia.org/wiki/Snell%27s_law)

* For a pure colour, the colour is simply the energy of the photons. Atoms have discrete stable electron orbits, and electrons moving between these levels will absorb or emit discrete levels of energy in the form of photons, which is why we have spectral lines. Reality is more complicated because part of the energy may be converted to vibrations of the atom itself (phonons).

* Another factor is the perception of colour. In physics to characterize light one measures its spectra, the intensity of the light versus its wavelength (wavelength = speed of light in vacuum / frequency). The perception of colour of these distributions isn't always always what one would expect.

</comment></comment><comment><USER109> The connection between elementary particles and group representations. I get the math, I just don't see in what way it corresponds to reality (i.e. in addition to being a bijection).

</comment><comment><USER110> Wormhole time machines.

The idea is, you can transform a normal [0] wormhole that isn't a time machine into one which is by:

1) accelerating one end to high speed relative to the other

2) keeping on end in a lower gravitational potential than the other

Why are either of these considered meaningful statements, never mind _correct_?

In the case of 2 in particular, isn't GR supposed to require smooth values? So any time dilation effect would be almost identical on a pair of points +δ and -δ from the throat? Making it similar to the case of a gravitational potential without a wormhole?

And in the case of 1, the more I think about it, the less I understand the concept. What is being moved? An imaginary clock that would've been in the part of the wormhole at the far end? The apparent speed as measured going through the throat will be zero regardless of the apparent speed of the same as measured when going the long way around.

[0] yes, I know

<comment><USER99> I'll answer 2. You are correct that clocks just to either side of the throat will stay synchronized. But it's important to remember that time dilation isn't some kind of absolute effect. The easiest way to talk about it is in terms of clocks following paths.

Imagine you have two spaceships in London. At the fire of a starting gun, they both take off, and fly around, each one taking a different path. Each ship has a clock that records the time elapsed since it took off. Eventually, they both return to the same landing pad in New York, landing at the same time. Thanks to time dilation, the readings on the clocks of the two ships might be different, even though they started and landed at the same places and times. Imagine we start both ships from a space station in deep space. One ship doesn't leave the station at all, it just stays in its docking bay, with its clock ticking along. The other flies down to the surface of Earth, sits there for a few years, and then flies back up to the space station. Thanks to the gravitational field of Earth, the ship that stayed home has more time elapsed on its clock than the ship that went to Earth.

Now suppose that each ship is carrying one end of a wormhole. The clocks on either end of the wormhole must stay synchronized. Someone sitting in the middle of the wormhole would be able to see the inside of one ship by looking to their left, and the inside of other ship by looking to their right. The clocks start out synchronized. As you point out, no matter how the ships move about, this does not change as the ships fly around. Anyone standing in the middle of the wormhole always sees that the clocks on the wall of each ship are in sync.

So: Entering the wormhole from ship A when ship A's clock reads X means you exit at ship B, at the time when ship B's clock reads X. And vice-versa for going from B to A. Now thanks to time dilation, ship B might arrive back at the station when its clock reads X, while ship A, which stayed behind, has a clock reading of (X + 20 minutes). If you are the station master, you can go into each ship to look at the clocks, and you will find that ship A's clock is always 20 minutes ahead of ship B's. But suppose that instead of walking between the ships through the station, you use the handy wormhole that connects them directly.

Suppose you enter ship B when its clock reads Y, and walk through the wormhole. You exit at ship A when its clock also reads Y. Then you step out of ship A, and walk through the station to ship B. It's clock reads (Y - <USER3> minutes), since according to people on the station, ship A's clock is still 20 minutes ahead of ship B's. When you originally entered ship B, its clock read Y. It now reads (Y - 20 minutes). Time travel. By retracing your path in the opposite direction, you can also travel 20 minutes into the future.

<comment><USER110> That helps, thanks.

It doesn't fully solve my confusion, but I suspect I need to study more before I can even ask the right next question. :)

</comment></comment></comment><comment><USER111> How can metric expansion of space redshift light? Doesn't that violate conservation of momentum, since the redder light has lower momentum?

And adding on to that: Will light inside a box redshift? If I weigh the box (i.e. weigh the light inside the box), then wait a bit for the light to redshift, then weigh the box again?

<comment><USER112> You are correct! Cosmological redshift does violate conservation of momentum (and energy as well [1]). But conservation of energy and momentum does not actually apply if spacetime itself is globally changing.

The underlying reason for this is that Noether's theorem tells us that every physical symmetry implies a conservation law for some physical quantity. Conservation of energy and momentum comes from the fact that the physical laws are the same throughout time and space. However, cosmological expansion violates that assumption, so there is no reason that energy and momentum should still be conserved. [2]

[1]: One side note here is that relativistically, energy and momentum are not really separate physical quantities, but instead two components of the same underlying physical quantity. Unfortuantely, this quantity does not really have a good name (despite Taylor & Wheeler's attempt to call it "momenergy"). It ends up being called the momentum 4-vector, but the temporal component of this 4-vector is energy.

[2]: This is only true _globally_. Locally, the laws are approximately the same from one moment to the next, so conservation of energy and momentum hold for small distances and short times.

</comment></comment><comment><USER113> Flight. How can a plane fly when it's thrust to weight ratio is less than one? It's like, if you can produce 10 pounds of thrust, who would look at that and say "ah ha, we can use this to keep a 100 pound machine miles in the air indefinitely"?

I understand flight from a mathematical point of view. I've actually read a few books on the subject, and I could explain how flight works to someone. However, I'm still fishing for an explanation that "feels" more satisfying though. Per the question, I still want it explained better.

EDIT: There's already a thread about flight. I asked the same question there, but phrased a bit differently: [https://news.ycombinator.com/item?id=22993460](https://news.ycombinator.com/item?id=22993460)

<comment><USER114> The 10 pounds trust is used to overcome the airodynamic friction, when the plane is in high speed. Because the "way" around the wing is longer on one side compared to the other this create a lower pressure on one side, and higher on the other. This pressure both lift and push the plane up. If there was zero friction you would hold the plane up with zero thrust ;)

I build this at school, using the same principle: [https://no.wikipedia.org/wiki/Sivilingeni%C3%B8r#/media/Fil:...](https://no.wikipedia.org/wiki/Sivilingeni%C3%B8r#/media/Fil:Wind_turbine_from_student_project_at_NTNU.jpg)

<comment><USER63> The problem with this description is that you don't need a fancy shaped wing to fly, a flat board will work. Aerofoils provide better efficiency but are not required.

<comment><USER114> Don't agree, unless you only did read the first 1/4 of my comment?

<comment><USER63> I did, but if I use an extremely thin wing with zero aerofoil (a wing made out of credit-cards or flat balsa wood for example), one "way" around the wing is not longer. Why would pressure be different on the top vs. bottom of this flat wing?

</comment></comment></comment></comment></comment><comment><USER115> I wish I could get a clear explanation of why faster than light travel breaks causality. I have seen the math but intuitively I am having trouble grasping it especially when you have only two reference frames (most explanations will use three reference frames to show the violation)

<comment><USER97> The simplest answer is that events that happen further apart in space than in time can be observed in different orders depending on the frame of reference. If an effect can happen before its cause, causality is violated.

The three-reference frame example is the easiest, because you can start with a frame where two events, A and B, happen simulateously. A reference frame (say, a spaceship), flying along a line in the A to B direction will observe A happen, then B. A ship flying the opposite direction will experience the opposite, B then A.

So whose observations were correct? All them are perfectly valid. The problem is if we allow A to cause B, in which case the B then A frame has the effect happen before the cause.

<comment><USER115> Isn't cause and effect mediated by forces that travel at the speed of light? Simultaneous events cannot be in a causal relationship, no?

</comment></comment></comment><comment><USER116> Absorption spectra baffle me and I've never seen an explanation that helps me understand.

It seems conceptually simple, except the requirement that the energy of an photon _exactly match_ some required energy in order to be absorbed seems really unlikely, since photon energy not a discrete quantity, and varies according to doppler effects and other things.

It seems like the vast majority of photons would just fly through the universe without interacting with anything, unless there are other ways for photons to interact with matter besides being absorbed. (If there are other ways, they are seemingly never mentioned as a potential alternative fate for the photon).

<comment><USER117> > It seems like the vast majority of photons would just fly through the > universe without interacting with anything, unless there are other ways for > photons to interact with matter besides being absorbed.

Maybe someone who actually knows will chime in, but afaik:

\- Most light doesn't have a fixed frequency. If it did, it would have a fixed momentum, but then you would have no idea where it is! Instead it is some superposition of many frequencies. That could be part of the story.

\- Light could be stopped by something other than absorption into an individual atom. Metals don't have discrete spectra.

</comment></comment><comment><AUTHOR> Another frustrating one - what is heredity? If it's possible to inherit something due to a shift in behavior (i.e. it's a cultural change that leads to a biochemical change), how does that connect neatly to mendalian inheritance?

<comment><USER118> Are you talking epigenetics? It's relatively closely related to mendelian inheritance. At a high level view, if genes are present in your genome that aren't "on" in a previous generation, but due to changes in behavior become more active, then in a future generation that gene could be expressed more. It's not that the genes have changed their code, they are just flipped on or off.

<comment><AUTHOR> No, more things that are modifications without any genetic basis - <USER4> myopia. Doesn't seem to have any genetic/epigenetic basis. It's purely a physical use case, where the eye matures improperly if it's only exposed to near-field work.

<comment><USER118> ah, I see what you're saying. Luckily, I've looked into this specific topic!

Myopia is definitely hereditary, especially the pathologic variants that can lead to retinal tears and the like.

That being said, there is the process of refractive development that occurs early on in life. The eye develops at a frighteningly fast pace, and you achieve near-adult globe size after about 18 months. The genes that drive this refractive development could be hereditary, if that is what you're trying to figure out.

Now, we can make a claim that this adaptation during infancy could eventually affect our genome, but I have not delved into the epigenetic literature to determine if that has been borne out or not.

<comment><AUTHOR> 90% saturation in a few generations suggests that it is something other than purely hereditary mutations accumulating: [https://www.nature.com/news/the- myopia-boom-1.17120](https://www.nature.com/news/the-myopia-boom-1.17120)

<comment><USER118> I know, I'm not saying that these are hereditary mutations. I'm saying that genes in all of us are involved in refractive development, and thus _have_ to be inherited if mutations exist. I'm saying that our environment of myopic environments is feedback that forces our eyes to develop to support better focus at those distances.

</comment></comment></comment></comment></comment></comment><comment><USER119> Quantum Mechanics, and why we need interpretations of it.

<comment><AUTHOR> Like.... why are sub-atomic phenomena important in general?

<comment><USER120> To add to all this, the double-slit experiment. What exactly does it mean that light moves as a wave or a particle?

<comment><USER121> >What exactly does it mean that light moves as a wave or a particle?

One idea is known as the Copenhagen interpretation.

It basically says that the wave-like effects we associate with matter is merely a wave of probabilities. Or in terms of the double-slit experiment and in other words, light behaves like a particle, but the wave-like effects you see is just the result of probabilities where the particles end up. Dark areas are areas of low probability, and lighter areas high probability.

One might imagine the light particles streaming through the slit end up having slight variation in trajectory from one particle to another (for various reasons such as interference with other particles), which results in areas where most particles end up and others where few end up... representing a wave.

</comment></comment></comment></comment><comment><USER122> Wave function collapse. As far as I can tell there is no discernable difference between a particle whose wave function collapsed (perhaps via measurement with another entangled one) and one that hasn't.

<comment><USER123> This is super naive but I just think of it as you (or whatever other particle is interacting with the one of interest) have now become part of its system. Like a causal event cone reaching you. Nothing about the particle changed, just your relationship with it (hence locking down the uncertain parameters).

I'm sure you or another physicist could point out the flaws in my mental model.

</comment></comment><comment><USER124> Sort of trying to piggy-back on this thread in hopes someone with knowledge will be able to share it.

I've been having conversations about viruses recently and in those conversations / thought experiments I keep coming back to a point someone made to me.

Someone this person knows, with extensive medical expertise, explained that the "membrane" of the cell contains a ridiculously large number of unique types of proteins.

Understanding, in vague terms, how viruses penetrate cells the question I pose is "is this true because each of those proteins has a unique and distinct function in the cell membrane? Or is it more a matter of scale and utility?" In other words, does the observation simply indicate that our bodies are not as perfect as we'd like to think they are and the body's process for creating / repairing cells is more a utilitarian function where the "rules" of cell construction are extremely flexible such that these molecules are constructed in various ways where our cells are using materials available to them at the time?

If this is the case it starts to make a lot of sense to me at a molecular level why certain people tend to be more susceptible to contracting certain diseases. Could a lot of it really just come down to diet, along with probably a hint (or more) of DNA's interaction with those proteins we're providing to our bodies? And to what extent does each of those play a role? DNA and the proteins.

</comment><comment><USER125> Time dilation with regards to the "no absolute rest frame" in physics.

The famous twin thought experiment where one gets in a spaceship, accelerates away from the planet, turns around, and comes back.

The twin that stayed on earth is old and the traveling twin is young still.

On one hand, I know that time will "pass differently" for each twin....but why is it the twin in the spaceship that ages less? Why isn't it true that the entire universe accelerated away from the spaceship and then returned, leaving the entire earth young?

<comment><USER31> You're in luck my friend, the Wikipedia article does a rather nice job of describing just this "paradox": [https://en.wikipedia.org/wiki/Twin_paradox](https://en.wikipedia.org/wiki/Twin_paradox)

> In physics, the twin paradox is a thought experiment in special relativity > involving identical twins, one of whom makes a journey into space in a high- > speed rocket and returns home to find that the twin who remained on Earth > has aged more. This result appears puzzling because each twin sees the other > twin as moving, and so, according to an incorrect[1][2] and naive[3][4] > application of time dilation and the principle of relativity, each should > paradoxically find the other to have aged less. However, this scenario can > be resolved within the standard framework of special relativity: the > travelling twin's trajectory involves two different inertial frames, one for > the outbound journey and one for the inbound journey, and so there is no > symmetry between the spacetime paths of the twins. Therefore, the twin > paradox is not a paradox in the sense of a logical contradiction.

There's multiple explanations included to resolve the "paradox" from different lines of argument; I particularly like this one: [https://en.wikipedia.org/wiki/Twin_paradox#A_non_space- time_...](https://en.wikipedia.org/wiki/Twin_paradox#A_non_space- time_approach)

<comment><USER125> Hahah 2 separate sources each for why I'm incorrect and naive. Thanks!

</comment></comment></comment><comment><USER126> The rising cost and decreasing productivity of scientific research.

</comment><comment><USER127> When two particles get closer, their mutual gravitational attraction increases. As the distance approaches zero, the force approaches infinity. In the limit of d -> 0, the _energy released_ -> infinity. Obviously at some scale the notion of a point mass breaks down, but even quantum theory would be problematic if we think of a wave function as describing a _probability_ distribution, wouldn't it? What's the "official" story on this?

<comment><USER128> What follows is very hand-wavy, and the renormalization sibling post may touch on it.

An answer is that the d->0 approaches infinity presumes a nice, continuous analytic function. If d->epsilon, you can't get to that singularity.

There was an equivalent problem in the E/M space with "The Ultraviolet Catastrophe" [1], which turned out to go away if you assumed quantization.

I'm not going to claim this is a perfect analog to the gravity problem, only that a lot of physics doesn't quite work right when you assume continuity. (The Dirac delta is a humorous exception that proves the rule here, in that doing the mathematically weird thing actually is closer to how physics works, and it required "distribution theory" as a discipline to prove it sound.)

[1] [https://en.wikipedia.org/wiki/Ultraviolet_catastrophe](https://en.wikipedia.org/wiki/Ultraviolet_catastrophe)

<comment><USER21> This is definitely related, though not the whole story. Quantization did get rid of some infinities, but as GP kind of states, it also introduced others. My comment focuses on what we do for those.

<comment><USER128> > In the limit of d -> 0, the energy released -> infinity.

I believe the poster's general premise to be false. While renormalization may be useful in resolving infinities in general, I don't think it's necessary for this one.

You can't commute the dp*dx of a Hamiltonian to be zero in a quantized world, so if gravity has quantum properties, you don't need to worry about what happens when d -> 0\. There is no "0" distance.

<comment><USER21> I guess, but it depends on how one parses the poster's setup. You're correct that for particles interacting under a 1/r^2 force, the energy turns out finite in quantum mechanics. My comment was referring to the fact that once you quantize the field that gives rise to that force, the infinities return, but for a different reason.

</comment></comment></comment></comment></comment><comment><USER129> I like that for a full decade, people discussed measurements at reputable labs indicating that certain radioactive species decayed at rates that varied >0.1% depending on the season, and explored possible neutrino flux influences.

The measurements were finally shown to be effects of the immediate environment on the measurement apparatus.

That detectors used in labs may vary with time by >0.1%, unknown to their users, seems pretty important. How did everybody involved not know?

<comment><USER130> Because people live in a busy world, where knowledge is not transferred with enough love and integrity. And also people are afraid to say "I don't know" and what little they know they tend not to share.

To make things more specific, those labs had uncertainty budget with something like 20 terms for the things they measured. Each of those terms had associated probability distribution etc. They had uncertainty budgets for all the methods they did etc., and some of those where probably dated, done by someone else, etc. etc. Who checks that? Is the check rigorous enough? Are some assumptions made that don't hold to scrutiny?

So it is actually very easy for error to creep in, I would say actually very likely.

</comment></comment><comment><USER46> Why does my bread last for a while and then go bad? Or is it constantly going bad and I just haven't hit critical mass of mold for me to notice?

<comment><USER78> Correct. Mold will grow where there is food for it to grow, like bread. It will divide and grow exponentially, up to a point. That fact of exponents where they seem slow at first and then really rocket away? That's why mold seems to just appear overnight.

<comment><USER46> So it might be a fair statement to say that when I eat fresh bread there's already a molecular amount of mold already on it?

<comment><USER131> There's a molecular amount of mold already on _everything._ Mold spores are ubiquitous.

</comment></comment></comment></comment><comment><USER132> Why bicycles stay upright.

For every authoritative-sounding, in-depth explanation, there is an equally plausible, yet conflicting and contradictory alternative.

<comment><USER133> Two conditions are necessary for a bicycle to remain upright (at least with most riders.)

One is gyroscopic forces. Ever picked up a spinning hard drive? Notice that it feels strangely hard to turn in some directions? Same idea.

The other is the feedback loop consisting of the bicycle and its rider.

If the bike is stationary, it's hard to keep it upright because you have no assistance from gyroscopic forces. At low speeds, you have some assistance but not enough. At higher speeds, the bike wants to maintain its current orientation, and it's easy to feed in the slight corrective forces needed to keep it that way. Hop off the bike and it will keep going until something causes it to veer off course.

You can throw a ton of math at it, as in the paper mentioned elsewhere in the thread, but at the end of the day, gyroscopic forces and negative feedback are all that's necessary. The Schwab paper appears to show that the gyroscopic forces aren't necessary, but no bicycle in the real world is ever going to work that way except in rare corner cases, e.g., if you're one of those riders who can stay upright at a standstill.

<comment><USER132> That is indeed one of the authoritative explanations, and in a few seconds of ad-hoc search we may find at least two refutations from authoritative sources.

For example, gyroscopic-forces-as-stabiliser don't need the Schwab paper's "ton of math" to be undermined. A simple counter-rotating wheel was used empirically at Cambridge to show as much, alongside notes that gyroscopic forces _are_ relevant to the dynamics of a loaded bicycle, but misconstrued; far from assisting to hold it upright whilst ridden, they induce _instability_ at the beginning of a change in direction, and more so at speed, a phenomenon (counter-steering) familiar to cyclists and relied upon by motorcyclists.

Then there's a simpler observation that can be made: people have to learn to ride a bicycle. The fact it stays upright when rolling unloaded, but not when loaded, is indicative of how _small_ the gyroscopic effect is, not how significant it is. Ergo, that argument would suggest, it is tiny shifts body position that contribute all of the stability.

And then others, proposing further explanations, etc etc ad nauseum.

I have come around to the view that in fact they don't stay upright, and they are almost always falling over, but in a many-branched configuration of the universe our observer effect sends us preferentially down the vanishingly unlikely path where they didn't, and there are an uncountably many alternatives of Me that have nothing but knee scars to show for it.

<comment><USER133> Sorry, I don't buy anything beyond what I stated. Those papers are exercises in violating Occam's Razor. Your quantum model makes more sense. :)

When you learn to ride a bike, you're simply training the feedback mechanism. Much like a PID controller, your brain has to keep track of the amount of error and null it out with proportional and integral terms (at least). Once those constants are dialed in, it's "just like riding a bike" \-- they're yours for life.

Then there's the matter of learning which way to lean so that the gyroscopic instability inherent in turning doesn't send you into the nearest ditch...

<comment><USER132> Occam's razor is for hypotheticals. You're rejecting an actual physical demonstration, by a reader of engineering dynamics and vibration at Cambridge.

The feedback mechanism you've described is likely correct, and also a complete furphy, since the central nervous system is not part of the bicycle.

All of which is par for the course and rather confirms the point, viz. that people will happily hold forth on any explanation they care to latch on to, secure in the knowledge that the total absence of consensus makes it impossible to say, definitively, "that is wrong"

<comment><USER82> Great discussion, I agree with inopinatus who, if I am not mistaken, supports my position that the science is not quite out on this topic yet. Many explanations exist, none seem to be all-encompassing.

That being said, it doesn't answer the question posed in this forum: "What scientific phenomenon do you wish someone would explain better?"

Instead, it answers the question: "What scientific phenomenon do you wish someone could explain?"

<comment><USER133> I guess my question on this specific topic would be, "What aspects of bicycle operation are _not_ adequately explained by gyroscopic action under the control of a human rider?" That would give us a good basis for inquiry into other possible principles.

Sure, a bike could work some other way, but my point is, it doesn't need to. Anyone who has ever picked up a hard drive should understand how a bicycle remains upright. What else is there to know? It's not like an airplane wing, where the "obvious" conventional wisdom is inadequate, misleading, or incomplete.

<comment><USER132> Well, apparently it is.

</comment></comment></comment></comment></comment></comment></comment></comment><comment><USER43> Entropy. Sometimes you read that it's a measure of randomness; sometimes, information. Aren't randomness and information opposites?

<comment><USER82> Let me give an analogy and then a solution to your paradox.

Temperature. Sometimes you read that it's a measure of warmth; sometimes cold. Aren't hot and cold opposites?

Yes, hot and cold are opposites, but in a way they give the same kind of information. That's also true for information and randomness. Specifically, little randomness means more (certain) information.

</comment></comment><comment><USER134> Why are clouds flat on the bottom?

<comment><USER135> Because that's (roughly) the Lifted Condensation Level: [https://en.wikipedia.org/wiki/Lifted_condensation_level](https://en.wikipedia.org/wiki/Lifted_condensation_level)

Water vapor around the LCL starts condensing and turning from a gas into liquid cloud droplets. This process happens considerably faster once it begins for a variety of reasons, so once you can have cloud droplets, you get a ton of cloud droplets - not a gradual transition from water vapor to cloud. It's almost like a light switch.

Most air masses are relatively homogenous anyways, so unless there are underlying processes causing things like undulatus asperatus, it will certainly appear very, very flat over a large area.

</comment></comment><comment><USER136> Gravity wells. I only realised in my 20s that the only reason satellites can orbit the Earth without crashing into the ground is by going sideways really, really fast. So as they inch closer to the ground, they also travel parallel to the ground fast enough so that they stay approximately the same height from the ground.

<comment><USER137> This is Newton's Cannonball. Honestly, I've found the best way to learn more about orbital mechanics is with a simulator - Kerbal Space Program is a fun version.

<comment><USER138> What other subjects would simulation enlighten people on?

<comment><USER133> Turing equivalence comes to mind.

</comment></comment></comment></comment><comment><USER139> Human Biology, particularly the "bigger scale" things like joints & ligaments.

I have arthrtic knees, and I'd like a better understanding of how joints work, and where the various clicks, pops and swellings come from.

It's easy to find really simple things, but harder to understand "how things go wrong".

<comment><USER140> This won't help with everything you asked for but there is neat piece of software called Complete Anatomy that helped me with this sort of thing.

It renders an interactive model of the human body and you can toggle different layers, from bones to nerves, to various layers of muscles and ligaments. It also contains animations of treatment exercises/stretches, surgeries and highly detailed models of various biological components.

It helped me understand my injury and why certain exercises help. It's paid software but it comes with a free trial.

</comment></comment><comment><USER133> I've always had trouble following Searle's Chinese Room argument as it applies to the nature and identification of intelligent action. I've never understood how the Chinese Room shows what its adherents say it shows, so that would be one topic I'd like to see more perspectives on.

<comment><USER53> "Searle understands nothing of Chinese, and yet, by following the program for manipulating symbols and numerals just as a computer does, he sends appropriate strings of Chinese characters back out under the door, and this leads those outside to mistakenly suppose there is a Chinese speaker in the room. "

I have trouble with this too. I think it's actually incorrect, or at least misleading. I think what it's _trying_ to say is that even if an entity can perform a complex task doesn't mean it can understand a complex task.

I think the more important result of this argument is that certain complex tasks can be "pre-baked" into rulesets _by an existing intelligence_. To me this just means that intelligent entities can sort of copy parts of their intelligence into other entities which are not intelligent i.e. computer programming.

I think with this argument they're trying to say "a series of sufficiently complex if statements isn't necessarily intelligent" by choosing something we know computers are good at - string manipulations and applying it to something we consider intelligence - language translation.

The argument holds that the computer is obviously not intelligent because it's just a function that takes a character and outputs another character.

But it needs to be a convincing translation, right? The computer would then be able to spit out not just accurate translations but also properly converted cultural idioms and new combinations of words where one didn't exist in the other language. That requires context of surrounding characters, memory of common language use, statistical analysis and creativity.

One implication that arises from this argument is actually about humans. How do we know that we aren't all just incredibly detailed rulesets ourselves without any actual understanding?

Well, first off - we technically can't prove it for anyone other than ourselves. More pragmatically, it's obvious that we, unlike the computer translator, can probe ourselves and be probed by others on whether or not we understand the subject. It's not like we're a bunch of Boltzmann's Brains that just happened into existence. We evolved intelligence in order to survive, not to "trick" other intelligent beings into thinking we're more intelligent than we are. There's no need for that. There's no one smarter around that we need to "trick".

</comment></comment><comment><USER141> I have wondered if heat and photons impart a partial charge on atoms and molecules which causes several phenomena. Faster Brownian motion due to the increased repelling action of stronger charges which cause pressure/volume changes in gasses.

Also are these charges responsible for some weather effects such as the jet stream. In a tornado is the negative charges on the dry side of the dry line interacting with the moist air on the wet side really just a local intense acceleration of the dry air trying to "get to" the oppositely charged moist air?

Are the rotation of low and high pressure systems basically due to the same condition? Is lightning also just basically a flood situation of the charges?

<comment><USER97> Charge can be pretty easily shown to be a conserved, quantized value. So chargeless particals imaprting a partial charge would be a big no-no.

<comment><USER141> I didn't mean it was equivalent to the charge of an electron. What I was wondering is how do heat and light impart more kinetic energy to physical objects? Light may be the quantity of a photon but heat seems to be more of an analog property. And what are some ramifications of these energized particles in small spaces like enclosed gasses or large unenclosed spaces with effects like the weather.

</comment></comment></comment><comment><USER142> Spin aka intrinsic angular momentum

<comment><USER97> Basocally, we can think of momentum in two parts: the momentum of an object orbitting another, and the momentum of the object spinning on an axis.

At the subatomic level, we observe that electrons have some extra angular momentum, beyond what we'd expect from their "orbits". We call that spin, because it's intrinsic, like the spinning of a macroscale object.

</comment></comment><comment><USER143> What's the point of defining summation methods for divergent series? Do they have any connections to any other area of mathematics or physics? Is analytic continuation of complex analysis relevant to these things? How about p-adics?

<comment><USER104> This is not an answer as this is something I've only just started wondering myself, but, if I understand correctly, perturbation theory [1] uses divergent series (and _wants_ divergent series in certain conditions over convergent series(?)) in it's methodology.

I've started but haven't finished the physics lectures by Card Bender on mathematical physics, where he features perturbation theory prominently [2].

If someone could chime in on this, I would also be appreciative. Also if someone has better resources to learn about perturbation theory, I would also be appreciative.

[1] [https://en.wikipedia.org/wiki/Perturbation_theory](https://en.wikipedia.org/wiki/Perturbation_theory)

[2] [https://youtu.be/LYNOGk3ZjFM](https://youtu.be/LYNOGk3ZjFM)

</comment></comment><comment><USER144> Consciousness.

How is it possible that the thread is up 5 hours and ctrl-f consciousness returns nothing?

<comment><USER14> Because no one knows what it is or even agrees on a definition. Consciousness is a pre-scientific concept at this point.

There are attempts to rigorously define it. I'm currently reading this paper, but not really convinced: [https://journals.plos.org/ploscompbiol/article?id=10.1371/jo...](https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1003588)

</comment></comment><comment><USER145> Evolution.

How does large scale randomness result in such complicated and intelligent systems, while after decades of research and all the computing power we have today, we still struggle to model and reproduce the intelligence of an insect.

<comment><USER146> "The Selfish Gene" was the book that finally made this click for me.

<comment><USER145> Unfortunately, The Selfish Gene leaves it an open question of how the brain developed its capacity to represent and model the world, which is the very notion of intelligence.

From the book:

> The evolution of the capacity to simulate seems to have culminated in > subjective consciousness. Why this should have happened is, to me, the most > profound mystery facing modern biology. There is no reason to suppose that > electronic computers are conscious when they simulate, although we have to > admit that in the future they may become so. Perhaps consciousness arises > when the brain's simulation of the world becomes so complete that it must > include a model of itself.

</comment></comment></comment><comment><USER147> Low, medium, and high-frequency radio waves (below 30MHz, wavelengths longer than 10 meters) are a form of EM radiation. Which means that an antenna is 'transmitting' low-energy, long-wavelength photons. (Which gets us into the whole wavicle question.) But the signal from a radio station appears to be continuous.

So: how to picture this? Is the signal made of discrete 'photons' overlapping, or combined somehow? Or is it that the 'wave-like' aspect of these photons is so predominant at these frequencies? (I've grappled with this one for a long time.)

<comment><USER148> A radio station in the 1MHz range is transmitting in the 10s of kW range. The energy of a single photon is tremendously small. So if you divide out the large transmitting power by the energy of a single photon, you'll see that an unimaginable number of photons is being released, giving the impression of a continuous process.

More detail:

Say you have an AM station that transmits at frequency f = 650kHz and uses power P = 50kW.

The equation for a photon's energy is E = hf, where E is the energy, h is Planck's constant and f is the frequency. Here h = 6.63 _10^(-34) Js and f = 6.5_ 10^5 Hz. Thus the photon's energy is E = 4.31 _10^(-28) J. This is very tiny number.

The number of photons per second is n = P/E = 1.16_10^32.

Let's try to visualize this. Avogadro's number is 6.022*10^23 for each mol of something, so if we divide it out from n, we see that there are almost 200 million mol of photons being released every second!

Water is 18g/mol, which takes up about 16 cm^3. 200 million moles of water is about a million gallons. If a photon was like a water molecule, a "water AM station" would be releasing about a million gallons of water per second.

<comment><USER147> Thanks, Vlasev, helpful ... easy to scale down to my hometown's measly 2,000 gallon AM station. I guess I'll need to go back to the radio physics math and puzzle it out more (directional arrays, groundwaves, and the like.) The idea of a photon with such a miniscule energy yet a wavelength of a thousand meters is a tad non-intuitive to me.

</comment></comment></comment><comment><USER149> I had a very mathy explanation of Spinodal Decomposition in my graduate work. I wonder if there's a more intuitive explanation than just "that's how the energy landscape works".

<comment><USER150> Hmmm, you might look up how dendrites form. They're roughly analogous in how the thermodynamics are favorable to forming more complex structures, IIRC. But it's easier/more intuit to see how dendrites form. Dendrite formation is also a huge problem in many fields, like electronics manufacturing (eg tin whiskers).

</comment></comment><comment><USER151> Quantum entanglement. How do they know it's happening?

<comment><USER152> Have you studied Bell's Inequalities at all? There are various YT videos:

[https://www.youtube.com/watch?v=zcqZHYo7ONs](https://www.youtube.com/watch?v=zcqZHYo7ONs)

[https://www.youtube.com/watch?v=f72whGQ31Wg](https://www.youtube.com/watch?v=f72whGQ31Wg)

[https://www.youtube.com/watch?v=qd- tKr0LJTM](https://www.youtube.com/watch?v=qd-tKr0LJTM)

</comment></comment><comment><USER153> The periodic table.

Can the properties of the elements be computed from the first principles of particle physics, or do you need to observe the atoms in real life to figure them out? For example, some isotopes are stable and others have a finite half- life. Can you know beforehand or you have to observe the decay? Can you compute exactly the mass of each atom without measuring it? Can you know compute its electronegativity? Etc.

</comment><comment><USER19> Basic probability. How come when I flip a coin it is just as likely to come up heads or tails even if it has just been heads 5 times in a row?

I understand the physical properties of the coin make it so it is an independent event but if I were to run the experiment multiple times the number of times it would be heads after 5 heads would not be an even probability, it would be unlikely since 6 heads in a row is a rare event.

<comment><USER109> Yeah, but 5 heads followed by 1 tail is also a rare event. Basically, after drawing 5 heads you have a choice between two rare events which are equally probable. The same after drawing anything else, not just 5 heads.

</comment></comment><comment><USER154> The mathematics of training a neural network. I understand how they work once trained, but that you can train them almost seems too good to be true.

<comment><USER155> Also, for me, how all these advances in ML/computing are alleged to be on the horizon, when I hear that A/D/C(NN)s are actually so slow in learning. How can something that has to be trained with 1M of <xyz> be smart? What's the next thing?

<comment><USER97> You and I probably took 4 or 5 years to recognize the alphabet. Cut the machines some slack.

</comment></comment></comment><comment><USER156> Lagrangian and Hamiltonian mechanics. That is usually presented in a very mysterious manner. Also I think most engineers do not get taught these.

</comment><comment><USER157> I didn't see good explanation of Einstein's twin paradox. Have you seen one? I have seen only clear explanation based on fact that observer will feel acceleration when he will come back and this is what breaks symmetry, but it appears that explanation based on acceleration are wrong, because it seems that acceleration is not necessary to explain this paradox.

</comment><comment><USER158> Double slit experiment. There's a lot out there detailing what happens but relatively few explanations go into _why_ observing the state of a photon results in it "choosing" one slit over the other. I suspect many come away with the impression that it's magic despite there being a reasonably simple explanation for what's going on.

<comment><USER159> This video explains the double slit experiment using probabilities and it's the only one that made sense to me without bringing in quantum mystery: [https://www.youtube.com/watch?v=iyN27R7UDnI&feature=share](https://www.youtube.com/watch?v=iyN27R7UDnI&feature=share)

It also goes on to explain the delayed choice quantum eraser experiment but I don't think that's quite convincing.

</comment></comment><comment><USER160> Electricity. What is it? Is it related to electrons? I thought it was? If the plug in my house is AC then why is one wire "hot" and one "neutral?" (on plugs in North America at least) if there is a ground wire then why are plugs polarized? Why do some things come with a three prong plug and others don't?

</comment><comment><USER161> Plasma ball, and how to create one. I've seen two of those with my own eyes: One in nature after a lightening strike to a lone maple tree in a pasture and one accidentally man-made via 50,000 VDC. And none of the literature shows how how such a plasma ball can travel in a fairly straight line at steady level from the ground.

</comment><comment><USER162> Morphological development of organisms. Mainly, how do cells form the shape of the organism and how do they specialize to different type of tissues? How do cells know where they are? What is the mechanism that puts the constraints on the shape of the body? I can understand mitosis but the rest from there is magic.

</comment><comment><USER163> The cosmic inflation. Yes, the universe is smooth but how is that enough to justify this unimaginable expansion? There seem to be no proof or possible experiment yet the scientific community apparently accepted this theory with its abrupt start and abrupt end, only secondary to the big bang itself in its scale and energy.

</comment><comment><USER164> BlackHoles - where does the 'extra' mass/gravity come from?

I've been watching a lot of documentaries lately, and I can't figure out how a star that _radiates_ light, collapses and suddenly light can't escape? Doesn't that mean the blackhole has more mass/gravity then the star that created it?

<comment><USER165> What teachers tell you in school is that as you get close to a uniform sphere the gravitational force increases with the square of the distance to the center of such sphere.

What they don't tell is that once inside the sphere the force decreases linearly, because the planetary mass ahead of you is partially balanced by the mass behind you.

With this you can see that the gravitational pull of a planet/start/spheroid is largest at its surface. So, if something happens to make a start shrink by some factor, the gravity pull at its surface is increased by the square of this factor, even if the mass of the star remains the same.

Actually I believe starts eject a lot of mass when they become black holes, and this is just a Newtonian argument for a phenomenon intrinsically relativistic, but I hope you get the gist of it.

<comment><USER164> Yeah - a lot of mass gets 'blown away' which really made it even stranger that gravity increased. While I found your explanation illuminating, I still don't see how you can end up with 'more' gravity then you started with? Mass determines gravity right ? not volume ?

</comment></comment></comment><comment><USER163> How a single hormone can regulate so many different things. Looking at Wikipedia, a single hormone like progesterone which is apparently one molecule regulates a plethora of effects. How is this implemented? As a coder, if I had to use one variable to simultaneously regulate 10 different things I would go crazy.

</comment><comment><USER166> Fermat's theorem: a^n + b^n = c^n For n > 2, why are there no integers a, b, and c that satisfy the equality?

<comment><USER167> One of my advisors in college said "The proof takes about 10 years of graduate mathematics to understand".

<comment><USER168> Is there really no intuitive way to communicate the answer to that question without needing 10 years of grad math? I find that to be somewhat hard to believe

<comment><USER82> No, there is no intuitive way to communicate it. The theorem has taken >350 years to prove, which makes it clear that the proof is not some intuition that was somehow missed by hundreds of people for centuries.

Fermat's Last Theorem (book) by Simon Singh is the source to check out if you're interested in the details of how it eluded mathematicians and a general idea of how the problem was solved, without getting too technical. It's a great story well told.

<comment><USER168> Thanks for the recommendation good sir!

</comment></comment></comment></comment></comment><comment><USER169> the fundamentals of computer science. Unlike many who comment here regularly, I am not a programmer or developer and though it might seem silly, the way in which a bunch of code written in a programming language of any sort translates to a computer physically or electrically causing things to "happen" because of it has always been a bit fuzzy. I know that electronic systems and machines translate all instructions to binary code but still, from there how does the rest happen? such as an OS on my laptops working the way it does, or more specialized: an unmanned spacecraft autonomously navigating its way through the solar system and doing complex physical tasks. Anyone have a suggestion for a good starting point on learning through these fundamentals and on upwards?

<comment><USER170> "Code: The Hidden Language of Computer Hardware and Software" by Charles Petzold should be what you are looking for. Author describes computing from the very ground up, and in clean, approachable manner. [https://en.wikipedia.org/wiki/Code:_The_Hidden_Language_of_C...](https://en.wikipedia.org/wiki/Code:_The_Hidden_Language_of_Computer_Hardware_and_Software)

</comment></comment><comment><USER171> Protein Folding

<comment><USER172> Isn't protein folding considered one of the biggest problems ever?

<comment><AUTHOR> sure is.. but somehow biology manages it with few issues. It seems like it has a lot to do with the presence of chaperone proteins that babysit the protein as it's coming off the ribosome, preventing pieces of it from sticking together that shouldn't touch, etc.

[https://en.wikipedia.org/wiki/Protein#/media/File:Chaperonin...](https://en.wikipedia.org/wiki/Protein#/media/File:Chaperonin_1AON.png)

I'm working on making a model of this chaperone complex relative to a folded protein to get a sense of how it might be interacting with the amino acid chain before it becomes globular

</comment></comment></comment><comment><USER173> Genetics. I still have no intuition for how combining my wife's and my DNA has resulted in children with traits from both of us. My brain always tries to imagine interleaving two binaries and hoping for a resulting program that works a bit like the two sources, which it of course wouldn't.

<comment><USER174> Don't think if it as a monolith. It's more like thousands of microservices and thousands of different binaries all interacting in a whole bunch of different and convoluted ways. Sexual reproduction is jumbling together two different people's thousands of microservices and seeing what happens. Or, each program is relying one library, imagine just randomly mixing around different libraries. Genes and proteins are discrete parts that can be swapped out and in.

</comment></comment><comment><USER175> Hawking Radiation. I know just enough to know that the story of "anti-particle of virtual pairs happens to always fall in the hole" is a Lie Told To Children, but the explanations seem to go straight from there to rather dense math and I've never wrapped my head around it.

<comment><USER76> I hope this helps: [https://www.youtube.com/watch?v=qPKj0YnKANw](https://www.youtube.com/watch?v=qPKj0YnKANw)

<comment><USER175> Actually... yes, which just makes it worse that something about that presenter's style really rubs me the wrong way. Do they have their content in text format somewhere? Anyway, thanks!

<comment><USER76> I'm sure somone's made transcripts but I have't come across them yet

</comment></comment></comment></comment><comment><USER176> Water pressure (like PSI) and how it relates to water flow (like GPM).

It's difficult to relate the two together and even after hearing every heuristic and every cutesy analogy, I still can't quite wrap my head around what happens to one when I increase the other (and so on).

</comment><comment><USER177> Interesting that you mentioned exosome, and no one else did. That in itself should give a hint on why people cannot do a better job at explaining stuff. It's complicated, , not well understood, extremely messy and does not fit well in a textbook.

</comment><comment><USER178> I'd love to see shows like 'Nature' and 'Planet Earth' but focused exclusively on single celled organisms; I find that whole world very interesting. Journey to the Microcosmos on youtube is the closest thing I've found.

<comment><USER179> Cells at work! [https://en.m.wikipedia.org/wiki/Cells_at_Work](https://en.m.wikipedia.org/wiki/Cells_at_Work)!

It's an animated series that takes place inside the human body. I've been meaning to watch it myself. It's supposed to be pretty accurate.

</comment></comment><comment><USER180> Relativity.

I just can't grok it.

I can't understand how time would flow differently depending of your speed.

I don't get why C is a constant no matter the referential, for any other object the speed is relative to your referential. I just don't see how those 2 are compatible.

</comment><comment><USER146> Frame dragging. How does space know a body is rotating if the body is "smooth"?

<comment><USER181> "Know" in which way, what kind of interaction? What about magnetic field due to poles rotation?

</comment></comment><comment><USER182> The Coandă effect [1] and how it applies to plane wings and sailboats.

[1] [https://en.wikipedia.org/wiki/Coand%C4%83_effect](https://en.wikipedia.org/wiki/Coand%C4%83_effect)

</comment><comment> ur-whale Why does quantum mechanics need complex numbers ?

Every time I read an introductory QM book / article, the complex numbers just come out of nowhere and no one bothers to explain how that makes any kind of physical sense.

<comment><USER97> Imaginary numbers, pretty much everywhere in physics, represent an extra dimension. e^ix results in a unit vector spinning around.

<comment> ur-whale Thanks for your answer, but that doesn't really cut it for me. For example, if complex numbers are so "natural" at modeling rotations, why don't they show up e.g. in Maxwell's equations ?

</comment></comment></comment><comment><USER183> The unification of electo-magnetism. I know the classical equation well, but never formally studied physics and have never been able to get my head around Maxwell's equations.

</comment><comment><USER184> The LIGO detector. I've never heard a logical explanation for it. If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. If your explanation doesn't involve higher spatial dimensions (not time) then you don't understand it. If you haven't even _considered_ higher spatial dimensions when explaining LIGO then you shouldn't even try for the incorrect explanation above because you don't have any of the pieces of the puzzle.

<comment><USER21> > If your explanation is that gravitational waves stretch and squash spacetime > so light takes different amount of time to bounce from an emitter back to a > measurer, then you don't have even a slight understanding of how it works.

Well, I'll bite. I'm a physicist and I understand LIGO. What's your alternative explanation?

<comment><USER184> I have no idea how it works, I don't have an explanation, I just know the one spouted by people doesn't make sense

<comment><USER21> Well, if you're uncomfortable with LIGO, you're in good company. Once a whole decade went by in physics where people couldn't agree if it would work in principle or not. And it is true that a lot of common explanations are bad (e.g. "it's just a ruler" is not complete by itself because "why doesn't the light get stretched too?"). Nonetheless, today we have a variety of independent explanations.

Maybe you'll find this paper helpful: [https://aapt.scitation.org/doi/10.1119/1.18578](https://aapt.scitation.org/doi/10.1119/1.18578)

<comment><USER185> Would you say it's accurate to say LIGO is a big interferometer measuring changes in lengths of the arms due to gravitational waves, with some extra analysis to figure why it still works even though the light waves get stretched? Or is OP correct that such an explanation means I "don't have even a slight understanding of how it works" and a correct explanation must "involve higher spatial dimensions (not time)?"

<comment><USER21> I'd say that's a perfectly fine explanation. And in fact the light _does_ get stretched. For sufficiently low frequencies, that's fine because there's enough time for _new_ (unstretched) light to enter and exit the apparatus. At higher frequencies this causes the sensitivity of LIGO to fall.

Part of the popular confusion around how LIGO works is the freedom in coordinates: there are different, perfectly good definitions of space and time you can use, and the explanation sounds different in each one. So people can get them mixed up. For example, my previous paragraph makes sense in "transverse traceless gauge", but not in others.

I'm not sure what GP was referring to with "higher spatial dimensions".

</comment></comment></comment></comment></comment></comment><comment><USER186> I'd really love an explanation of what specific impulse is. I've looked it up a few times but the units seconds^-1 confuses me, what does this represent?

<comment><USER187> I think the unit is just one of those weird dimensional equivalences that pop up from time to time. E.g. fuel efficiency in cars is measured as distance / volume (of fuel consumed), so it's dimensionally equivalent to area^-2. But we don't use this because "1 mile per gallon" makes a lot more sense than "42.5 cm^-2".

</comment></comment><comment><USER42> Quantum computers. I still don't get it. What are they useful for, what are it's computing bounds and why it's a big deal. When will the future actually come ?

</comment><comment><USER188> How to design compact antenna without those math voodoo.

</comment><comment><USER189> We aren't short of interesting problems.

To pick a couple fo them randomly, understanding amyotrophic lateral sclerosis or alzheimer would be terrific starts.

</comment><comment><USER190> Superconductive flux pinning. Specifically, I'm curious what behavior it'd exhibit if used as a core for superconducting inductors.

</comment><comment><USER75> Quantum Physics. Still can't wrap my mind around multiverse being a simpler explanation then the future holds data we won't have in the now.

<comment><USER60> If you remember your linear algebra basics, you could easily pick up QM --- the parts that deal with finite size systems. Here is an excerpt from my book I posted a while back in another thread, and refreshed today since you asked: [https://minireference.com/static/excerpts/noBSLA_quantum_cha...](https://minireference.com/static/excerpts/noBSLA_quantum_chapter.pdf)

As for the multiverse, I don't know enough to talk about it. I just know it's one of the possible interpretations of quantum mechanics. Note that the various interpretations are generally considered more philosophy than science, and have no (or very little) practical implications. I would suggest ignoring all analogies and not looking too deeply for interpretations, and instead focus on basic concepts like "What is a quantum state?" and "How do I compute measurement outcomes?" which are super well understood and the same under all interpretations.

You can think of the various interpretations of QM as different software methodologies, scrum, agile, waterfall, etc. just stuff people like talk about endlessly, but ultimately irrelevant to the code that will run in the end.

</comment></comment><comment><USER191> The Oberth Effect. I've seen a bunch of awful attempts at the "why" and some "well it's just in the math, so there" but nothing satisfying.

<comment><USER21> I answered this here: [https://physics.stackexchange.com/questions/287101/where- doe...](https://physics.stackexchange.com/questions/287101/where-does-the- extra-kinetic-energy-of-the-rocket-come-from/287102#287102)

The basic answer is that the extra energy that goes to the rocket comes from harvesting the kinetic energy that _the fuel itself_ had by virtue of being in the moving rocket.

</comment></comment><comment><USER192> Why is it that the smartest people in various fields constantly need reference materials, but medical doctors never look anything up?

<comment><USER78> They look things up _constantly_ , you just don't see it because it's typically very boring and not a good use of time for you or the MD. They get symptoms from you and then will go off to research. Also, a broken arm or strep isn't very difficult to figure out. ~80% of visitations are fairly simple.

The site most MDs use is here: [https://www.uptodate.com/home](https://www.uptodate.com/home)

<comment><USER192> thanks for the reference. I'll taking a peek.

</comment></comment></comment><comment><USER193> First principle reasoning in real life examples.

</comment><comment><USER109> I think we should have this thread more often.

</comment><comment><USER194> \- How computers translate bits into electrical signals

\- What does it mean the universe is expanding

\- Bayesian statistics

\- How information is stored in magnetic tapes

<comment><USER97> First one's easy. You've got a bunch of electronically controlled switches (transistors), and you encode a higher voltage as 1, and something near ground as 0.

You can build logic gates out of 2 or 3 transistors, and combine those logic gates into more complex gates until you've got a computer.

But how does a transistor work? Basically, you've got semiconducting materials of two types (Phosphorus or Boron doped silicon), one of which wants a few more electrons to be a conductor, and the other wants a few less electrons to be a conductor. If you stick the two types next to each other, the electron- wanting (N) one snatches up the electrons from the electron-offering (P) one, and you get an Electric field going from the P to N. Now, that alone makes a diode. Already cool, nonlinear electronics. But what if we go N-P-N? Now we've got two electric fields, going opposite directions. With three leads, you can adjust the strength of those electric fields with one, creating a variable resistor, a transistor.

</comment></comment><comment><USER195> Electricity.

Electricity is always explained by its effects, but never by its actual nature. I'd like better explanation :-)

</comment><comment><USER163> How did Mendeleev and his peers knew something was the final indivisible element and not just another molecule?

<comment><USER97> It was pretty wild wild west, really.

The idea that some some compounds didn't just contain some fire was still common when the first list of elements was put together. A big leap forward was realizing air had two principle components, burn-y air and not burn-y air.

They figured out water wasn't an element when the burn-y air and some mystery gas burned to make it.

Basically, everything was maybe an element until they either broke it into pieces, or made it out of other stuff.

</comment></comment><comment><USER154> The twin paradox. All explanations seem to be just "something something one twin has to accelerate"

<comment><USER196> The amount of time you experience is directly related to the length of your worldline--the path you trace out in 4D spacetime as you move through space and simultaneously move forward in time.

It should be easy to see that a non-accelerating object (or person) will trace out a straight worldline. If you ever change your velocity, though, either through smooth acceleration or instantaneous rotation of your velocity vector, you will trace out some non-straight curve in spacetime. If you leave your friend behind and then, at some later time, meet back up again, if you did not undergo exactly the same amount of acceleration throughout your journeys (i.e., because one of you stayed behind and hardly accelerated at all, tracing out a boring straight line path), then you will have different world-line lengths (different "path integrals") between the starting and ending points, and thus will have experienced different amounts of subjective time.

Now, in a Euclidean spacetime, the traveling twin would end up older, because a straight line is the shortest distance between two points. But our spacetime is not Euclidean--it is a Minkowski space, in which acceleration is equivalent to a hyperbolic rather than Euclidean rotation of your velocity vector, so it turns out that straight line is actually the _longest_ distance between any two points, and the twin who leaves and comes back will have a shorter worldline, and thus will have aged less.

</comment></comment><comment><USER197> Can anyone explain disc loading and solidity in a propeller and if there are any equations governing them?

<comment><USER70> Disc Loading is just the force on the actuator disc (such as a rotor, in which case the force is the weight of the vehicle the rotor holds aloft) divided by the swept area (pi times r squared).

It's used when discussing propulsive efficiency, as it's a proxy measurement for how much "work" each blade is doing. Because propeller/rotor blades are just high-aspect wings, if you have high disc loading your blades are at a high lift coefficient which means they'll be incurring lots of lift-induced drag which increases your power requirements.

Solidity in the same context refers to the amount of volume within an actuator disk that's occupied by actual solid material. If you have a 4-bladed rotor and you move to a 5-bladed rotor, all else equal, you've increased your solidity.

There are many many equations, and as most things in fluid mechanics you can get as deep into the weeds as you want. As a starting point, have a look at the wiki article for Blade Momentum Theory[0]

[0] - [https://en.wikipedia.org/wiki/Momentum_theory](https://en.wikipedia.org/wiki/Momentum_theory)

<comment><USER197> Thanks na85

Let's take a contrived example. I have a 4 engine high wing airplane with 2 bladed 55 inch props on each engine with 100kg force (1 kg-f is equal to 9.8 N) thrust per engine.

Now, I need to make that a low wing airplane so need to change to 22 inch blades with the same thrust and I don't want to change engines. So I want to add more prop blades. How many more blades do I need to add?

<comment><USER70> Couple of points:

1) 100kg is a measure of mass, not force. Thrust is a force, not a mass. But let's say you have 100N of thrust per engine.

2) Why would changing from a high-wing to a low wing monoplane require you to add prop blades?

<comment><USER197> Sorry, edited question

<comment><USER70> So, your question is a little off, because propeller thrust changes dramatically with airspeed. Propeller aircraft are typically referred to as being "power producers", i.e. we talk about power when we analyze props, not about thrust, because the mechanics and mathematics behind propeller thrust are annoying and complex.

But to get to the heart of your question: You want to reduce the prop diameter (ground clearance perhaps? The engineer in me asks why you don't just make the landing gear taller) and not change the engine. You don't actually have to add blades (maybe). You could also just make the existing blades fatter, or change the airfoil, or increase the propeller RPM. Lots of ways to attack that.

But, playing along that adding blades is the only way:

1) Take your existing high-wing plane and calculate power required for all phases of flight: Take-off, climb, cruise, etc.

2) Take your new low-wing plane with its smaller prop diameter and work backwards to ensure you can actually meet the power requirements to stay aloft through your envelope. Adding blades reduces efficiency because the blades' wakes interfere with each other so you'll have to dig into some experimental data based on the prop of your choice. Much depends on blade pitch and washout.

Very likely you will need to increase the RPM (if your engine can deliver enough power) or change engines to a more powerful model because your props are now less efficient.

Such is the nature of aircraft design - almost nothing can be changed without affecting something else.

</comment></comment></comment></comment></comment></comment><comment><USER198> Wave-particle duality (of e.g. light)

<comment><USER199> The usual misunderstanding is that light is sometimes a wave and sometimes a particle, but that is wrong. Light is always a weird thing, that you have never seen before in macroscopic objects, and you need to use some math to describe.

In some experiments the weird mathematical thing can be _approximated_ as an almost classical particle. That approximation simplifies the calculation a lot, and sometime you can get some result intuitively. But it is never true, it is only a very good approximation.

In some experiments the weird mathematical thing can be _approximated_ as an almost classical wave. That approximation simplifies the calculation a lot, and sometime you can get some result intuitively. But it is never true, it is only a very good approximation.

Try to read again everything you have read about the subject, but every time the text says "here light is a wave/particle" use a red marker to rewrite that sentence as "here light can be approximated as a wave/particle".

</comment></comment><comment><USER200> Non-linear optic explanation from quantum standpoint (classical explanation is quite clear).

</comment><comment><USER201> Kalman filters. The explanations always start easy and then get too confusing.

</comment><comment><USER202> Einstein's (summarized) quote, "Time is a persistent illusion"

</comment><comment><USER203> \- Hegel. I haven't found any resource that can explain it clearly.

<comment><USER204> I'd suggest Kojeve's Introduction to the Reading of Hegel (a classic expository work on the Phenomenology of Spirit)

</comment></comment><comment><USER205> How depression, autism and gender dysphoria work on a molecular level.

<comment><USER206> As for autism, it's related with a kind of general reduction of gene expression: [https://www.researchgate.net/publication/328734204_A_theory_...](https://www.researchgate.net/publication/328734204_A_theory_of_evolution- biased_reduction_of_gene- expression_manifesting_as_autism_the_antiinnatia_theory_Improvedupdated_presentation_of_A_theory_of_general_impairment_of_gene- expression_manifesting_as_)

</comment></comment><comment><USER207> All the credible theories of why there are 3 generations of particles.

</comment><comment><USER208> How EM radiations propagate through the air and what is an EM field.

</comment><comment><USER209> Why is there no evidence of black matter annihilation in astronomy?

<comment><USER21> You mean dark matter annihilation? There are some hints: an excess of gamma rays detected by Fermi-LAT, an excess of X-rays detected by Chandra, excess ionization detected by EDGES, and excess positrons detected by AMS-02. However, all of these anomalies are contentious and could well be explained by systematic effects or unknown astrophysical sources.

If none of these hold up, it could be that dark matter just doesn't annihilate. That's not weird or anything -- I mean, your room is full of matter right now, and it's not annihilating itself either.

</comment></comment><comment><USER210> Eddy currents. Why does a magnet move slowly down a copper tube?

<comment><USER97> A changing magnetic field induces a current in a material, which creates an opposing magnetic field which slows the moving magnet.

<comment><USER131> The kinetic energy turns into heat due to resistance to the induced current?

</comment></comment></comment><comment><USER211> Agile, an empirical survey of whatever the hell it actually is.

<comment><USER212> Agile is a short-lived V cycle repeated for ever.

Legacy V-cycles (needs - spec - code - test - integration - product) were such that everything was written down and planned in advance for months/years. So, if the customer had made an error, his/her needs had changed... you were basically screwed.

Agile advocates for Short V cycles while getting often user feedback. But __it 's __a V cycle.

\- PO speaks to the customer = get needs \- PO writes tickets, UX design something = specification \- And then it follows the classical cycle : develop, test, integrate, deliver.

What's remain around agile (ceremonies & co) feel much more like bullshit to me, and people follow it religiously without understanding the core idea of agile as they think V cycle is an insult.

</comment></comment><comment><USER156> Wavelets: Usually given explanations are hard to understand.

</comment><comment><USER213> Amperes, Voltage, Watts, without using a liquid analogy.

</comment><comment><USER214> ionization. The resources I can find on it are mostly conspiracy theory types or the same basics explained over and over again.

</comment><comment><USER215> Axial flux motors

</comment><comment><USER216> Gauge theory.

</comment><comment><USER217> Quantum stuff

</comment><comment><USER218> Why a flat earth is impossible.

<comment><USER101> Gravity. Gather enough stuff, and you end up with balls (unless you spin things really fast).

</comment></comment><comment><USER219> How airplane wing really works.

<comment><USER220> I found this article useful: [https://www.scientificamerican.com/article/no- one-can-explai...](https://www.scientificamerican.com/article/no-one-can- explain-why-planes-stay-in-the-air/)

<comment><USER219> thanks!

</comment></comment></comment><comment><USER69> Reactance.

<comment><USER82> If you have a specific question about electrical reactance, I can help. If not electrical, I probably can't help since I don't know what you are referring to.

</comment></comment><comment><USER221> why does blender at times become slower for no fucking apparent reason

</comment><comment><USER222> Why the earth is a sphere.

<comment><USER97> Systems tend towards low-energy states (because small perturbations keep things from getting stuck at local minima or saddle points. The lowest potential energy configuration for a bunch of particles pulled together by gravity is a sphere, because every point is as close to to the middle as possible.

</comment></comment><comment><USER223> Human counsciousness.

</comment><comment><USER224> 1\. Carbon dating. Sure, I get that carbon decays over time and this changes the proportion of isotopes. But why does this give you any information? That carbon didn't come into existence just to be in that bone, it was made in the sun billions of years before that, so why does the age of the carbon tell us anything about organic matter? The key fact, which I think is not emphasized enough, is that the ratio of isotopes in atmospheric carbon is kept at a constant equilibrium by cosmic rays. So you can use carbon dating to tell roughly when the carbon was pulled out of the atmosphere. Without this additional fact, the concept of carbon dating makes absolutely no sense.

2\. The tides. The explanation I was given is roughly something like "the tides happen because the moon's gravity pulls the water toward it, so you have high tide facing the moon. There's also a high tide on the opposite side of the earth, for subtle reasons that are too complicated for you to understand right now and I don't have time to get into that."

The first problem with this explanation is this: gravitational acceleration affects everything equally right? So it's not just pulling on the water, it's also pulling on the earth. So why does the water pull away from the earth? Shouldn't everything be accelerating at the same rate and staying in the same relative positions?

The second problem is that, when viewed correctly, the explanation for why there is a high tide on the opposite side of the earth as the moon is equally simple to why there is a high tide on the same side as the moon.

The resolution to both these problem is this: tides aren't actually caused by the pull of the moon's gravity per se, but are actually caused by the difference in the strength of the pull of the moon's gravity between near and far sides of the earth, since the strength of the moon's gravitational pull decreases with distance from the moon. The pull on the near water is stronger than the average pull on the earth, which again is stronger than the pull on the far water. So everything becomes stretched out along the earth-moon axis.

3\. This one isn't so much a problem with the explanation itself, more about how it's framed. I remember hearing about why the sky is blue, and wondering, "ok, more blue light bounces off it than other colours. But isn't that essentially the same reason why any other blue thing is blue? Why are we making such a big fuss about the sky in particular? " A much superior motivating question is "why is the sky blue during midday, but red at sunrise / sunset"? I was relieved when I saw this XKCD that I'm not the only one who felt this way:

[https://xkcd.com/1818/](https://xkcd.com/1818/)

<comment><USER82> Carbon dating can't say anything about dinosaurs (in the colloquial definition) because dinosaurs went extinct >>50k years ago and CD doesn't work after such long time spans.

<comment><USER224> Ok fair enough. I admit I regret this comment and think I posted it a bit hastily, I tried to delete it a few minutes ago but it appears to be too late now. I edited out the part about dinosaurs though.

</comment></comment></comment><comment><USER225> Coroutines in C++.

<comment><USER129> Coroutines are most easily understood as a way to write a state machine in a way that looks like a function. I.e., it's just a notational trick to make one function do different things according to when it's called.

To see it, imagine you have a struct with a data member for each local variable of your function, and replace your function with a member function that has no local variables, but uses "this" to get at what was local data.

Add one more data member, a number that is set differently right before each place the function returns.

Finally, insert some code at the start of the function that, according to the number, jumps to just after the last return statement executed.

Then, each time you call the function, what happens depends on what happened last time.

There are more details, but that is the gist.

You can write that yourself in C++98, with the body of the function inside a switch statement. Getting it past code review would be the real challenge.

<comment><USER225> I think I get coroutines in theory. It's the C++ specific complexity that is the hardest barrier. Thanks for the explanation though

<comment><USER129> Yes, a lot of extra junk is needed to make them actually work, but the extra junk goes a long way toward obscuring what you actually need to know.

Ultimately, though, you are right that you have to understand it all, once, even if you can't remember it all a month later. The explanations I find online are not good at presenting just the details you need when you need them, and building up to the full picture.

</comment></comment></comment></comment><comment><USER226> The Coriolis Force

<comment><USER101> Drag a marker with a constant radial velocity (from your perspective) across a spinning disk. It'll trace out a curved line, so from the perspective of an observer on the disk, an acceleration must have been present.

The full 3d Coriolis force is more complicated than that (eg accounting for the Eötvös effect): The spinning disk example only gets you to the -2vω term (where v denotes radial velocity and ω angular velocity).

</comment></comment><comment><USER227> celullar replication and virus code insertion

</comment><comment><USER228> Orbital mechanics

<comment><USER229> Have you heard of / played the game Kerbal Space Program?

I believe even folks at NASA have even said it helped cement their mathematical knowledge with a better intuitive understanding.

</comment></comment><comment><USER230> Escape velocity

<comment><USER77> My dad was an aero space engineer, and I always had trouble with this. It is the speed you need to be going at, to not be pulled back by gravity. As a kid, I always thought, well big deal, if I am in a plane and just keep going up, eventually I will be out. The key I always missed was the velocity at that point was with no added power. So when the engine cuts off, what is your velocity? Is it enough to escape? Or do you need more power. Of course there is a good chance I still do not understand it.

</comment></comment><comment><USER231> Measure theory.

<comment><USER60> This is a very good playlist: [https://www.youtube.com/playlist?list=PLBh2i93oe2qvMVqAzsX1K...](https://www.youtube.com/playlist?list=PLBh2i93oe2qvMVqAzsX1Kuv6-4fjazZ8j)

Starts from basic concepts and builds up a nice overview.

</comment></comment><comment><USER232> Conciousness.

</comment><comment><USER233> Yoneda lemma

<comment><USER105> As with most things in category theory, the way to really understand the Yoneda lemma is to sit down and prove it yourself. Break the statement down into the underlying definitions, draw lots of diagrams, and convince yourself that it's true.

The other thing you can do is think about what it means for particular types of categories. For a posetal category, it says that an element of a poset is uniquely determined by the set of all elements that come before it in the ordering. For a group, it says that every element is uniquely determined by its action on the group. (This is basically Cayley's theorem.) See this MSE post for more intuition: [https://math.stackexchange.com/questions/37165/can- someone-e...](https://math.stackexchange.com/questions/37165/can-someone- explain-the-yoneda-lemma-to-an-applied-mathematician)

</comment></comment><comment><USER234> Women!

</comment><comment><USER235> Math!

</comment><comment><USER236> electromagnetic waves

</comment><comment><USER237> Haskell's Monads

</comment><comment><USER238> coriolis force

</comment><comment><USER239> string theory

</comment><comment><USER221> why does blender become slower at times for no apparent reason

</comment><comment><USER77> Reminds me of a story:

A guy was walking along the beach and found a lamp. Of course he rubs the lamp, and sure enough a genie appears.

Genie: master of the lamp I can grant you a wish, you may wish for anything.

Guy: Wait, isn't it supposed to be 3 wishes?

Genie: One or nothing, and do not wish for more wishes.

Guy thinks for a while ....

You know I have pretty much everything I need. But I have always wanted to travel to Hawaii. But I get sea sick and am afraid to fly.

Genie: Very well I will take you there.

Guy: No no, if you take me there, I wont be able to come back. And what about next year? Since I only get one wish, I want a bridge built to Hawaii.

Genie: That does not make any sense. Please make a different wish. One that does not involve so much construction.

Guy: hmmmm you know I know, can you explain women?

Genie: So do you want the bridge to be a suspension bridge or truss? and how many lanes ....</comment>