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What is the most accurate measurement?
Hello askscience, I am a chemistry undergrad in an analytical/instrumental chem course, and just learned about one of my professor's research topics that is a method with detection limits theoretically in the parts per quadrillion, which got me thinking. I was wondering: what is the most accurate measurement technique out there? In terms of detection limit, or measurement uncertainty, or whatever. I realize that I'm asking a question that could be taken several ways, as you can have accuracy in looking for a concentration, a distance, or a whole host of other things, but I feel like it would be really cool to hear about things from a variety of fields. Thanks all!
The quantity that is often cited as being the best measured is the magnetic moment of an electron (related to how an electron responds to magnetic fields). It is very close to two, and half the deviation from 2 has been measured as 0.00115965218073(28), where the brackets indicate uncertainty on the final digits. This means the magnetic moment has been measured to a part per trillion. The theoretical value, based on quantum electrodynamics, is .001159652180{85(76)}, where I put curly brackets where it deviates from the experiment (within the uncertainties). This is the most accurate verified prediction in science. |
Are there any studies or can anyone explain to me in the most objective way possible why some people can smoke all their lives and never get any kind of cancer?
My girlfriend's grandmother has been smoking UNFILTERED pall malls since she was in her early 20's/late teens, and she is 80 now and is still healthy. WTF?
There is hardly anything that has a 100% guaranteed chance to cause cancer. Cancer development is an extremely complicated process which requires successive mutations in cell populations over your lifetime. Smoking increases your RISK of cancer. According to the CDC lifetime smoking increases your risk of having lung cancer by a factor of 10-20. |
What keeps an object in orbit at the same distance instead of being pulled toward the source of the gravity?
Eg what keeps an satellite at the same distance from earth? and why is it not pulled in? What keeps earth from being pulled into the sun?
We being pulled towards the major source of gravity in our Solar System (the Sun), but the reason why we don't fall into the star is because our planet is moving at a pretty high rate of speed relative to it, with the vector of our travel being roughly 90 degrees tangent to the Sun's center of mass - our planet's tendency to maintain the same direction of travel offsets this force - per Newtonian law and also due to the conservation of angular momentum, which originated from the proto-planetary disc which formed before fusion even commenced in the Sun billions of years ago. Anyway, this creates a scenario in which the force that is keeping our planet in orbit around the Sun also happens to maintain our relative velocity while travelling around it, and also our distance from it rather steady. Try playing around with this simple online orbital simulator and you'll begin to understand how it all works. |
This may be a dumb question, but if losing weight requires simply eating less calories than your body burns, what are the biggest differences from eating healthy or poorly?
If I understand correctly, if your body burned, say, 2500 calories a day, and you ate 2000 calories of fast food, you'd lose weight. If you ate 3000 calories of extremely healthy food, you'd gain weight. Obviously eating healthier food is better, but why exactly?
This is complicated because your question isn't entirely clear. If by "health", you mean weight gain/loss, it is very hard to overeat truly healthy food. have you ever tried eating 3000 calories of meat and vegetation? your stomach and satiety hormones simply won't let you. healthier food also tends to impact insulin levels and sensitivity, controlling blood sugar and thus, hunger. If "health" for you goes beyond just weight control, there are many vitamins and minerals that we must eat to maintain proper health, and junk food tends to be extremely low in these, as well as plant-based compounds that are very good for you (think fruit phytochemicals and leafy greens). while it is true that the sheer number of calories matters for weight loss, it is clearly easier and healthier to eat higher-quality foods for these calories than lower-quality ones. |
When our epidermis grows with our size, does the number of nerve endings increase to maintain a constant density, or are they simply spaced further apart?
And is the phenomenon the same or different between adolescent body growth/adult weight gain? EDIT: Thank you for the responses! Looks like my question has been answered quite thoroughly. This is why I love , I'd been wondering about this for ages, and may have gone one wondering if you guys hadn't explained it. Great work!
the number of sensory nerves innervating your skin is determined by the number of neuronal cell bodies in the dorsal root ganglia. During gestation this number increases through division (some die off) and reaches a stable number. Neurons are 'post-mitotic' and do not divid further. The neuronal cell bodies by then have extended their peripheral terminals out to the skin. However, the branching of sensory nerve terminals in the target organs (e.g. skin) continues to be plastic throughout life. Thus you have the same number of nerves, but the branching or arborization of the terminal can adjust to your size. |
Will light be refracted by any glass like object? Can it be treated to decrease the amount of refraction?
I'd like to think I remember this topic from my classes but I have been asked this question in such a way that it made me question my knowledge. As far as I know any glass like object will refract light that passes through it to a certain degree. Can this effect be controlled by the manufacturing process of said object? Here is the actual situation I'm dealing with. I am a customer service rep handling warranty claims on motorcycle helmets. I have a customer who is complaining to me that the visor on his helmet refracts light and distracts him and could pose a safety risk. I had thought that any glass like material will do this. Can this effect be lessened by the way it is manufactured? Thanks for your time guys.
Light is refracted whenever the light encounters a medium with a different refractive index at any angle other than normal incidence (anything other than perpendicular to the surface). Typically this means the surface is curved like a lens or tilted, etc. but it can also happen when the light is coming into, or going out of focus. This is very much dependent on design on the material (glass, plastic, crystal, liquid, gas... pretty much anything can refract as long as its at least partially transparent). The amount the light is refracted is very wavelength dependent, but most optical materials used for applications like helmets are specifically chosen to have small changes in the refractive index over the visible wavelengths (dispersion). Manufacturing processes can the refractive index of most optical materials, specifically if the material is injection or compression molded (both of which require the material to be slightly fluid, then rapidly cool it down to a manageable temperature). If you take some plastic safety goggles and put them between two crossed linear polarizers, you can see the effects of how the stress in the glasses is distributed, and that can give you an idea of how the refractive index also changes. But that being said, we still dont notice that level of a difference in refractive index very readily. For your warranty issue...what do you mean (in this case) by 'refracts the light'? Is the visor causing a lensing (magnification) effect, or is it acting like a prism, where the colors are being separated? |
Why doesn't an alpha decay release a huge amount of energy like nuclear fission?
I've heard the word "tunneling" used in this context, but don't really understand what's going on...
The idea of interest for this is Nuclear binding energy . The total energy of the nucleus is the mass of all of the nuclei the binding energy of all the nuclei. As you increase the number of protons and neutrons (A) in the nucleus, the amount of binding energy per nucleon (proton or neutron) holding the nucleus together (on average) until you hit Iron-56. That is the most strongly bound nucleus (in this sense) in the universe. As you add more nucleons, the binding energy per nucleon drops. It's like they stick more and more tightly until you pass Iron-56, then the whole thing gets less sticky each time you add another nucleon. This is important: as you keep going, this effect gets worse and worse (a nucleus with 200 nucleons is stuck together much more poorly than a nucleus with 100 nucleons) For these heavy elements, then, the total energy can be lowered if they can nucleons. Alpha decay decreases the total number of nucleons by 4. The binding energy per nucleon goes , so everything is stuck together better and more stable. And as I noted above, since the binding energy goes up, the total energy goes down and you've released energy. On the other hand, fission will split the nucleus into two nuclei with about half (ok, not really, but close enough for our purposes) of the nucleons. So you've decreased the number of nucleons by a LOT more than 4. So you've increased the binding energy per nucleon by a LOT, thus getting more energy out. |
Any chemists in the house? Can fluoride really cause hypothyroidism?
Is this legit? If so, what does this have to do with the autoimmune disease Hashimoto's thyroiditis (which causes low thyroid levels because of an attack by antibodies on TSH and TPO receptors on the thyroid gland). It would seem that this is an entirely different path to hypothyroidism. I looked for epidemiological evidence of thyroid disease on the rise, but because of changes in testing procedures and disagreement on proper lab ranges, it's hard to say how many people have thyroid disease now, much less how many had it 60 years ago before the time of ubiquitous fluoride...
There is a LOT of pseudo-science around fluoride use in drinking water, and I'm sure you've found it by conducting Google searches. Many of them ignore basic concepts (dose-dependence is a big one i.e. taking the results of a high dose study and implying a low dose has the same effect) or cherry-pick research without giving the full story or noting the limitations of the research. All peer-reviewed studies are not created equal, and should not carry equal weight, and in the case of anything medical is nature, one study does not allow for a general conclusion. Here's a reference put out by the ADA: QUESTION 30.
Does the ingestion of optimally fluoridated water adversely affect the thyroid gland or its function? Answer. There is no scientific basis that shows fluoridated water has an adverse effect on the thyroid gland or its
function. p. 34 of http://www.ada.org/sections/newsAndEvents/pdfs/fluoridation_facts.pdf The document contains references for making this statement. Also from the CDC( http://www.cdc.gov/fluoridation/pdf/pollick.pdf): In the literature reviewed, doses appropriate for caries reduction were not shown to negatively impact thyroid function. Studies in which humans received doses significantly higher than the optimum fluoride intake for long periods of time showed no negative impact on thyroid function. Claiming that the ADA and the CDC are involved in a giant conspiracy to cover up the health effects of fluoride is frankly laughable, these are serious professionals who do their jobs ethically and responsibly. People who claim a conspiracy are basically telling you they don't have a real argument or data to present. Also, contrary to a lot of the claims of the anti-fluoride websites, it is quite easy to conduct studies comparing fluoridated water to non, there are a lot of communities that do not fluoridate their water in the USA, but for a more basic reason: they don't want to spend the money on it. Edit: here is a page from the CDC summarizing Fluoride research over the years by the National Research Council
http://www.cdc.gov/fluoridation/safety/nas.htm |
Does wiping off your sweat prevent the body from cooling down?
Does the cooling effect come from the sweat absorbing the heat on its way out or should you leave it and not wipe it to lower your temperature more efficiently?
Yes, sweating is based on evaporative cooling, where the phase change from liquid to gas absorbs heat energy from its surroundings (including your skin and the warm blood carried by the superficial veins from deeper tissues. So yes, wiping off sweat actively hinders your body's natural cooling mechanism, however, it's not the secretion of sweat itself that does anything, but the subsequent evaporation. So taking a shower or just rinsing your skin with water, and letting it dry naturally, has largely the same effect, and won't dehydrate you so fast as a bonus. |
How do we know the structure of molecules?
This has been asked a few times, so you may want to do a search. In short, chemists use a variety of tools to determine the structure of molecules. These techniques include: A chemist looking to determine the structure of an unknown molecule would use a combination of these tools to come to a conclusion. |
How long do you have to be exposed to the sun to get the same dose of vitamin D as your daily supplements?
It will depend on the strength of sunlight, but this study found >1hr per day was sufficient in their study population in India. However, it seems that time of day and latitude would have a significant impact on duration needed to achieve healthy levels of vitamin D production. |
Would science in general (I mean in academia in practical terms) benefit from discarding all prior assumptions in every field and trying to formulate fresh sets of hypotheses based solely on observations to date?
This isalready practiced in science, to the benefit of scientifical progress. My favourite example of this is Newton's law of universal gravitation, which is known to be imprecise in some regards, but still accurate enough to describe most cases. |
Electronics on Airplanes
Bit of your own medicine, Doctor. From the first link in your search: It is not, contrary to popular opinion, to force you to pay attention to the safety briefings. If that was the case, you wouldn't be allowed to have books out, and you couldn't sleep - neither of which is prohibited. http://en.m.wikipedia.org/wiki/Mobile_phones_on_aircraft#section_1 [ABC came to conclusion] the primary reason for the ban on cell phone use in flight was that neither the FAA nor the FCC were willing to spend the money to perform conclusive safety tests. Most people who do an overview of the situation come to similar conclusions. The culprit is "regulatory inertia", not passenger safety. |
How can computers calculate sines so accurately?
I've been learning about rewriting a function as a series in my Calculus class, and my professor mentioned that computers need to use series to approximate the values for functions such as sine. To calculate a sine accurately, you need a good number of terms, and even with that, the error from using a series with a finite number of terms increases by quite a lot as the sines argument (for lack of a better word) distances from zero. I wrote a rather simple calculator program to compare the time of executing the Taylor series with <0.01% error to the built in program, and found that the built in program is much faster, especially as the input numbers got larger. How does a computer calculate these functions so quickly and accurately? Is it even using a Taylor series, or some other approach?
First of all, you can (and really should) always arrange the argument of your sine or cosine to be between 0 and π/4 by using periodicity and identities. In this small interval the error in the series expansion is much smaller, and controlled. However, computer implementations (like C built-in functions) do not usually use the series expansion. They tend (there's actually a different function for each architecture) to use an algorithm called CORDIC . CORDIC is better on hardware because it reduces mostly to multiplications and divisions by powers of 2, which can be done with bitshifts and so are faster than floating point operations. |
does the vitamin c from my lemon tea breaks down when i'm boiling it?
i heard that vitamins are destroyed when cooked so i googled it but haven't found any thing that satisfied my curiosity. i found the melting and boiling points for vitamin c but i have no idea what to make of them so im asking for your counsel. p.s. sorry for any grammar mistakes and punctuation errors
Vitamin C doesn't decompose until around 370°F/190°C so boiling water alone won't destroy it. It does, however, react with oxygen in the air. The oxygen in the water combined with the high temperature will probably lead to some of the ascorbic acid being oxidized but unless you're leaving it out boiling for a very long time I think it's safe to say any loss of vitamin C would be rather minuscule. One thing worth noting is that because vitamin C is soluble in water, after boiling most of the vitamin C will stay in the water instead of in the lemon. If you're drinking the tea this ends up being a benefit, but if you were cooking vegetables and throwing out the water afterward you would be losing it with the water. |
Why do your arms float up after pressing outwards on a door frame with locked elbows?
I make educational science videos and I wanted to make one about this phenomenon a while ago, but I was unable to find any satisfactory explanations. My understanding is that it's a generalized muscle effect - you can induce something similar by making a fist and squeezing really hard for a while, then gently trying to open your hand. It will feel 'locked up'. I saw various sources claim different effects were responsible: that excess calcium buildup was stimulating the muscle fibers, that muscle spindles were somehow getting confused and setting a false equilibrium point for muscle lengths, or that the opposing set of muscles were getting tired trying to oppose the anticipated movements leading to mismatched muscle tone. Can anyone shed some light on the accuracy of any of these claims? I'd love to make a video about it if I can get a well-supported explanation. Thanks for your help!
EDITS: Here is how I Think it happens, and here is a picture for reference : http://imgur.com/PaYsC When you contract the triceps (extrafusal muscle), an intrafusal muscle also contracts, wich slows down signaling along the 1a sensory neuron. A gamma motor neuron then activates to stretch out the intrafusal muscle fiber (IF) again. This alows the brain to tell if there is further contraction. If you release your arms, the signal to the gamma motor neuron will also stop. This causes the IF to shrink to its normal size slowly, which tells the brain that your arms are raising. However, now its contracting on its own, without the triceps actually contracting. The sensory sytem sends collaterals to the motor system, telling it the arm is raising, which in turn makes your arms raise. The brain wants sensory and motor systems to be in synch, which is why unless you try to stop it, if you feel your arms rising, they will rise. It can't be muscle memory as it is a form of procedural memory that involves consolidating a specific motor task into memory through repetition. This works first time. It also can't be fatigue, as your arms travel the SAME direction you were pressing. You were using the Extensor (triceps) to push the door jam, and they continue to contract afterwards. This is not like a reflex, or sensory adaptation. It is a result of the IF muscle fibers resetting after extended periods of load. |
Would everything on an extrasolar planet be dark to human eyes?
Other suns might have their peaks at completely different frequencies Yup, it depends on the temperature of the star (and a couple of other things because they aren't perfect black bodies , but that's not really important here) and not much light of the kind that we are used to seeing. Not necessarily so - whilst a hotter object has a higher peak frequency, it will emit more light at wavelength that a cooler object ( pretty graph time! ). Conversely, a cooler object has a lower peak frequency emits less at every wavelength that a hotter object. It would only be in the case of very cool stars (which emit mostly in the infrared) or planets very distant from their host stars (for obvious reasons!) that you would really be lacking in visible light. |
What is happening physiologically when we feel so happy our heart feels like it will burst?
Sometimes when I am extremely happy, my breath catches and my chest feels so full. Alternatively, when I feel very emotional or sad, my chest actually aches. What is going on in my body? |
When flying away from earth at the speed of light, why does "my" time stop passing and not the earths?
Since there is no magical static grid covering the universe that tells us how fast we travel, speed can only be measured between two observable objects i.e. inertial systems. So if I fly away from earth at the speed of light, there is a relative speed c between me and earth. Couldn't one say that earth is flying away from me at the speed of light instead of vice versa? What is the difference between me and the earth that makes "my" time stop?
Because when you come back to earth, you change your frame of reference while the earth remains in the same frame of reference. You change your direction of travel. This causes the symmetry to stop working as before. |
Would we be better at mathematics if we weren't bought up using a base-10 number system?
Decimal may be an easy-to-grasp concept because of the whole ten fingers thing, but would we be at mathematics if, for example, we were taught from a young age to use hexadecimal (base-16) instead?
Very little mathematics has anything to do with the base for decimal representations of numbers. (For that matter, a lot of mathematics isn't really about numbers at all!) |
How do fish and other marine animals survive the crushing pressure of the abyss?
I was reading on the recent discovery of the Airbus that had crashed off of Brazil a few years ago, I think it's about 2 1/2 miles down -I thought wow, that's a lot of pressure. Then I began to wonder how ocean life can survive such crushing depths...any ideas? relevant link:
Well the pressure inside them matches the pressure outside.. They're kinda equillibriated with their surroundings, and hence they should be fine. The structural properties maybe very slightly different but I don't see any reason why any biological process should happen in a different manner under very high pressure as long as the pressure is equal everywhere. Thats why its almost impossible to actually bring these animals back up, since the pressure difference will be so great they will just disintegrate! |
Does dissolution affect volume differently than just addition?
Say I have two glasses of x amount of water. If I add y amount of salt to one and y amount rocks to the other, will both volumes be x+y
Yes. Well, sometimes. Take alcohol, and add water. The total volume won't be x+y as the tiny water molecules "nest" themselves in the spaces in between the way bigger alcohol molecules, taking up less space in the end. This isn't always true though |
Are people with high reward dependence more affected by low expectancy?
Since people high in reward dependence ( ) are finely tuned to rewards, I would think that low expectancy ( ) would cause them to procrastinate more than people with low reward dependence, all other things being equal. Do you agree?
Perhaps I don't understand your question, but I don't really see them as related. "Expectancy" is the level of perceived behavior control or self-efficacy in relation to a particular behavior (see Theory of Planned Behavior, Icek Ajzen 1985). We approach each behavior with a certain subjective belief that we will or will not be able to do that certain thing, both accounting for our own perceived abilities as well as any environmental factors which may hinder or assist us. Are you saying that if people expect to fail, then they are consistently rewarded for their expectation? Or that with low expectancy, any small success is a reward? |
How did astronomers discover the expansion of the universe was accelerating, given the huge uncertainty the Hubble constant?
According to my physics teacher, depending on who you ask, they would say the Hubble constant is anywhere from 60-80km s MPc . But if that's the rate of expansion of the universe, how could they prove it's increasing if they don't know what the actual value of it is?
Having some uncertainty doesn't mean we have no idea what's going on. If we say it's from 60-80 km/s/Mpc, then we really do mean it. If we said the Hubble constant is somewhere from -50 to 200 km/s/Mpc, then it would be okay to say that there is quite possibly no trend. But if we're saying it's 60-80 km/s/Mpc it means there's some scatter in the trend, but there is a trend. This is Hubble's original data. You can see there's clearly an upwards trend, there's just some scatter in it. The top plot here is a more modern one. There's still a little bit of wiggle room, but you can't look at that data and say it doesn't look like a straight line. |
How can energy be quantized if both speed and wave length are both continuous quantities? How discrete is energy?
We all know that energy of electromagnetic radiation is equal to Planck's constant times the frequency. But both wave length and speed are continuous quantities that can take on any value, so how can it still be discrete? Thanks!
The range of energy of electromagnetic radiation is not discrete. EM radiation does come in discrete packets, though (photons). Maybe you were confusing these two things? |
How does stress actually make you ill?
I've been told, and read many times that stress makes you ill, and I don't doubt it; but how does it happen, biologically? It's always puzzled me as to how what is essentially an emotion or a feeling can make you physically ill.
The answer here is a hormone called cortisol, as other posters have pointed out. Whenever you have stress, you will get a hormone from your adrenal gland (your adrenal gland looks like a beanie hat that sits atop each of your kidneys) called cortisol. This hormone was designed with real stress (fight-or-flight, being chased by a lion) in mind; it raises your blood sugar and increases delivery to the brain, increases the breakdown of fat, protein, and carbohydrates to provide fuel to your muscles, and also suppresses the immune system as an anti-inflammatory. In fact, when you have an infection your immune system uses cortisol to ensure that it keeps itself in check too. The problem is if you have a lot of cortisol around all the time, you'll continue to suppress your immune system and with your immune system dampened you will be less able to fight off pathogens and more likely to get sick. Cortisol was designed to be a short-term hormone-- in people with chronic elevations, it can be really bad (i.e. people with pathologic high levels of cortisol all the time from certain tumors, etc, known as Cushing syndrome). |
How do I stop crystalization in a non-hygroscopic sweetner?
How do I prevent Erythritol from recrystalizing in drinks, candies, baked goods, etc? I've asked this in askcullinary and nobody could help me. It dawned on me it may be more of a science question. My diabetic father is eating himself to death. I am desperate to make some low carb deserts that will fool him and hopefully convince him to switch to some of my recipes i'm trying to work on. (Wikipedia) is the best candidate so far, there is absolutely no aftertaste. I'll spare you all a description, you can click the link for more info. Please correct me if I'm asking the wrong questions. Unlike sugar the erythritol does not readily absorb water, it repels it. That seems to cause it to easily crystalize. For example, when adding them to a cheesecake it will be fantastic at first. Twenty-four hours later and the sugar has recrystalized and the treat now feels as though it has sand inside it. Gummy-Bears were awesome for about 10 minutes, then opaque speckles apeared which started growing till the whole candy became crunchy. I've heard of mixing sugar with corn syrup and the long molecular chains disrupt the sugar recrystalizing. This (even if it worked) isn't an option because of the carbs. I've heard cream of tartar can help prevent sugar from crystalizing. Is this because of the acidity? I'm willing to do tests, but I was hoping I could get some direction instead of baking $20-$30 failures every day.
Amateur candy-maker/chemist here. The usual MO is to add glucose/corn syrup/inverted sugar to a sucrose mixture to stop crystalization. Corn syrup is glucose, mostly. The similar-but-not-quite-the-same structure of sucrose to glucose or other sugars means the crystals can't form efficiently. Even with sucrose being hygroscopic there will still be sugar crystals if you don't have the correct recipe and technique (which is tricky to do at home). Cream of tartar is an acid, as you noted, and the hydrolysis of sucrose to glucose and fructose is acid-catalyzed. So basically by adding cream of tartar you make your own invert sugar out of sucrose instead of adding it. It will do nothing useful to erythritol. Swapping out one chemical for another is always going to cause problems, as you're finding out. But you could try to mix a few sugar alcohols together to keep any single one from crystallizing out. Or stick to confections like hard candy or fudge where you want crystals. |
Interresting Physic-experiments with light bulbs?
Hi there, . Some of my buddies, and I are doing some experiments with light bulbs, for a physic-assignment. Do you guys know any interesting/radical/fun experiments, involving light bulbs? Preferably experiments where you can calculate a specific result based upon the experiement, and/or its result(s). I want to thank you all in advance, for taking the time to read this.
If you have something to measure light intensity and some filters:
Apply some voltage over the bulb and measure the light intensity at different wavelengths (using filters). Plot the intensity as a function of wavelength and compare to a blackbody radiation (and estimate the temperature). Repeat with different voltages and plot the bulb temperature as a function of voltage and compare to something like P = V *R (you have to convert temperature to power before doing so). |
Are there any substances the Liver or Kidneys can't remove? Therefore in our blood forever.
The liver can’t directly remove substances. It can, however, chemically break down many substances, rendering them harmless.
The kidneys work like inverse filters, wih the Nephron being the functional subunit.
At the first step, everything under a certain size (approx. 60000 dalton) gets thrown out of the blood, including water, salts, smaller proteins etc.
This happens at the basal membrane inside the glomerulus. This ‘primary urine’ then enters the decending limb followed by the loop of Henle, then the ascending limb. Cells making up these structures then reclaim everything the body still needs. Unknown substances are neither necessary nor recognizable so they just get thrown out.
Molecules larger than 60000d can’t be handled in this fashion but then it is hard to see how such large molecules would enter the body other than trauma. In those cases the Glomeruli can get ‘clogged’ and the kidneys can fail. |
Why does the parsec exist?
Why does the parsec exist next to the lightyear? The difference between the two (a parsec is just over 3 lightyears) means that using one or the other doesn't make much of a difference, does it? Most of the time I see distances to other planets expressed, it's in lightyears, too.
The parsec is named for the allax of one arc ond. Now parallax deals with perspectives, and how things closer to you seem to move more quickly than things that are further away, so the observed, or apparant shift with regards to more distant objects,and arcseconds are a measure of angle.
Now the parsec is used to measure large distances, and if you imagine a right angled triangle, if the angle is 1 arcsecond, and the opposite is 1 astronomical unit (distance between the Earth and the Sun) then the adjacent will be 1 parsec. This gives us a unit for long distance that can be related to known values.
Now we use the parallax as we determine distances in space by the change in apparent angles to bodies in space. Once we have determined the angle, we can then use trigonometric small angle approximations to calculate the distance between the body in space, and the Sun, as that us the reciprocal of the angle in arcseconds. Wikipedia's diagram |
Does light affect our organs?
Since our organs are always in the dark, presumably one of the only circumstances they get exposed to light is during some sort of surgery. Does this sudden exposure to light have any effect on our insides?
evolved (or were created with depending upon your theory of the origin of life) Please refrain from making the implication that evolutionary theory and creationism are on the same footing here. Evolution is well substantiated and supported by thorough evidence, while creationism is an unsupported religious teaching. There's no need for you to try to play safe by including that line. I trust we are in a place where science denial is uncommon. |
Why doesn't the Alcubierre drive violate causality?
With the understanding that don't exist yet, but are theoretically possible if we ever discover the requisite type of exotic matter. However, I was under the that any faster than light communication could result in a causality violation. So why are Alcubierre drives theoretically OK?
It does. Anything moving faster than light can be used to communicate with the past, regardless of how it functions. There are other reasons why such a thing can't exist, see here . |
Pick a (uniformly) random integer 0 <= a1 < 1000, then pick another 0 <= a2 < a1, and so on. What is the expected number of steps to reach 0?
Hmm I should be packing but you caught my attention. With problems like these it's often helpful to start smaller to build the intuition. Perhaps we can generalize the problem to E_x(Y), where x is the starting number and Y is the number of steps to reduce it to 0. We can find out that E_0(Y) = 0 and E_1(Y) = 1 fairly easily. From here on I'm going to drop the (Y) from the notation because I'm lazy. Now we can look at this as a recursion problem. E_2 = 1 + (1/2)E_0 + (1/2)E_1. This is true because you will use one step to reduce the problem to E_0 or E_1, each with equal probability. Since we already know both these smaller values, we can compute that E_2 = 3/2. We can generalize this formula: E_n = 1 + (1/n)SUM(E_i) over 0 <= i < n
But since each E_i can be rewritten as previous E_i's, why don't we try to get this strictly in terms of the previous one? Recall that E_2 = 1 + (1/2)(E_0 + E_1). This means that E_0 + E_1 = 2(E_2 - 1). We can substitute this into the formula for E_3 to get E_3 = 1 + (1/3)[2(E_2-1) + E_2] = 1 + (1/3)[3E_2 - 3] = E_2 + 1/3. Now, in general terms, E_n = 1 + (1/n)[E_(n-1) + SUM(E_i)], where our sum is from 0 to n-2. Also, E_(n-1) = 1 + [1/(n-1)]SUM(E_i) where the sum is also from 0 to n-2. We can combine these by solving the second for the sum, SUM(E_i) = (n-1)(E_(n-1) - 1). Putting this back into our expression for E_n, we get E_n = 1 + (1/n)[E_(n-1)+(n-1)(E_(n-1) - 1)] = 1 + (1/n)[nE_(n-1)-(n-1) = E_(n-1) + 1 - (n-1)/n = E_(n-1) + 1/n. Long story short, E_n = E_(n-1) + 1/n and E_0 = 0. This recursion is solved with the harmonic sequence , or E_n = SUM(1/i) where i ranges from 1 to n. This means that your question has no closed form answer, but it does act like ln(n). Your experimental value of 2.66 is fairly close to e, so this makes sense. Now if only I were writing this in LaTeX so you could actually follow it... But thank you for the interesting question! I thoroughly enjoyed solving it this morning. |
How do we have accurate records of hurricanes before satellites?
For example, the 1899 San Ciriaco hurricane. We have a track for it, and even a time frame for intensification and weakening.
Reports from different places at the same time and different times in the same place, from ships at sea as well as land-based observers. Put the data together like pieces in a puzzle. Fill in the holes with educated inference. |
It is a better strategy to attempt to appeal to the logical or emotional nature of a jury (or equivalent person(s)), given similar/equal strengths of each argument?
My intention is to ignore any counterargument by an outside party in this case (i.e. considering only the target party's reaction to your argument). Stated more generally, is a person (or group of people) more likely to be susceptible to holes in an argument of logical vs. emotional nature? Also, are they more likely to be swayed by equivalently compelling arguments of one type vs. another?
This is a difficult question to answer because the line between the emotional and the logical is not clear cut. I am going to try and answer you by separating out affect and cognition based on their deliberative components. Think of affect and cognition as two separate sources of input driving us towards an attitude or decision. Consider affect as the sum of the jury's intuitions, automatic associations, and general feelings toward the question they are considering. Consider cognition as the mental processes that the members of the jury actively and deliberately engage when trying to sort out the information that they have been given. The way these two processes are going to interact and the ultimate answer to your question is going to come down to the decision making strategy employed by the members of the jury. I suggest that individuals in the situation that you have described are likely to be engaging in deliberate cognitive processes and that these processes have a tendency to bias people against affective information even when that information is useful and relevant to the judgment that they are being asked to make. The reason for this bias is debated but generally it is attributed to the difficulty associated with verbalizing many types of affective information and the general consensus among many western thinkers regarding the usefulness/accuracy (or lack thereof) of affective information. Worded another way: when individuals are motivated to consciously consider their reasons for an attitude or decision there is strong evidence that they are particularly biased towards immediately salient and easily verbalizable information, often at the expense of other types of useful information. My short answer to your question would be that your best bet is to rely on logical holes because people in this situation may be biased against using emotional information in their decision making even if that emotional information is relevant or important. Here are a few references if you are interested, also I research the interplay between affect and cognition and the specific way that the generation of reasons can alter attitude formation and maintenance. Reasons generation has been shown to decrease accuracy in some situations. Also, people who make decisions based on reasons that they have generated have been shown to be less happy with those choices at a later date. I think the key is realizing that both sources of information have a place in our decision making and attitude formation. The last reference is a fairly long paper and its from 1977, but its still worth a read if you have the time and interest. Feel free to send me a message if you want to talk more about these effects. References: 1) Dijksterhuis, A., & Nordgren, L. F. (2006). A theory of unconscious thought. Perspectives on Psychological Science,
1(2), 95–109. 2) Halberstadt, J., & Hooton, K. (2008). The affect disruption hypothesis: The effect of analytic thought on the flu-
ency and appeal of art. Cognition and Emotion, 22, 964–976. 3) Halberstadt, J. B., & Levine, G. L. (1999). Effects of reasons analysis on the accuracy of predicting basketball games.
Journal of Applied Social Psychology, 29, 517–530. 4) Wilson, T. D., Kraft, D., & Dunn, D. S. (1989b). The disruptive effects of explaining attitudes: The moderating effect of knowledge about the attitude object. Journal of Experimental Social Psychology, 25, 379–400. 5) Wilson, T. D., Lisle, D. J., Schooler, J. W., Hodges, S. D., Klaaren, K. J., & LaFleur, S. J. (1993). Introspecting about reasons can reduce post-choice satisfaction. Personality & Social Psychology Bulletin, 19(3), 331–339. 6) Nisbett, R. E., & Wilson, T. D. (1977). Telling more than we can know: Verbal reports on mental processes. Psy-
chological Review, 84, 231–259. Edit: changed 'out' to 'our' in the last line of my second to last paragraph before references. |
Helicase - how does it work?
How does this enzyme decide to move its arms around to cut bits of dna up to make two copies of dna? This is what happens in your cells - but what motivates huge molecules to move around and do the things they do?
I can't speak to they do it, but helicase hydrolyzes ATP to create the energy needed to do it's task. Contrary to what you said, Helicase only unwinds the two annealed strands of DNA. If you think of DNA as two pieces of string twisted together, Helicase moves along the two strands, and separates them into two separate strands. You may also want to think of it as the zip on a zipper (except it only unzips). The energy needed for this task is provided by ATP. ATP, Adenosine Tri-Phosphate, is essentially the base unit of energy used in our bodies. It's a long task to explain how it's created (read more here) , but in short, all the food we eat gets catabolized into ATP for use in energy-requiring tasks. The thing that actually does the duplicating is DNA Polymerase . After the two halves are separated, DNA polymerase comes along, and starts adding on the DNA base pairs. The actual biochemical mechanism behind base-pair attaching and detaching, and the kinetics behind the motion of helicase I am unsure. |
Why do cymbals/hi-hats still sound so horrible in high bit-rate mp3's?
They sound like they're vibrating against a bunch of sand, or something. Are the standard codecs not suited to something unique about cymbals' spectrum of tones? If so, what?
Cymbals are a " canary in the mine " indicator of low-bitrate mp3's. A crash cymbal's decay, for example, sounds quite different as you drop the bitrate while they generally sound fine at 320kbps and up. And yes, the standard codecs require a higher bitrate to encode such sharp, wideband sounds. What bitrate are you using? |
Why do batteries have internal resistance?
College student here with some questions after my basic EMF physics lecture. The prof brought up a lot of things but can't go over them due to time constraints so I'd ask here. 1) Why do batteries have internal resistance? 2) How does a defibrillator return the heart to a normal beating rate when such a current through the body would stop it in the first place? 3) Why has the conventional "positive charges flow" not been changed now we know it is electrons that move through the circuit? Thank you everyone for your time, I'm sure to be back with more questions in the future.
1) Why do batteries have internal resistance? Well, imagine you're an ion heading through the electrolyte to be oxidized/reduced at the anode/cathode. You're going to be bumping into other molecules, giving off heat. 2) It wouldn't necessarily kill someone, but anyway, the mechanism is that it depolarizes the heart muscle, resetting the action potential ion gradient. 3) The is that charge flows from positive to negative in an electrical circuit. There is no reason to change that because A) It's an established convention and the underlying physics of the charge carriers is irrelevant at the circuit design level. B) It's simply not the case that electrons are the only charge carriers. Batteries have positive ions moving in one direction and negative ones moving in the opposite direction. Even in a metal you have virtual positive particles ( holes ) moving in the opposite direction. C) Because of A and B, the concept of the direction of an electrical current is and always will be a separate thing from the direction of the charge carriers anyway. All you'd be doing is changing so that the direction of current matched the direction of charge carriers . |
Why aren't viruses considered alive?
I get that we have a list of criteria for something being alive (i.e. it has to grow, reproduce, evolve, respond to stimuli, etc.) and that viruses fail several of these criteria. Here is my issue though, isn't this list arbitrary? Like, why draw the line at a place that excludes viruses. They sure alive by many standards. They even have genetic material, reproduce, and evolve. It has been proposed before that our definitions of life could exclude life on other planets that evolved differently from Earth. Doesn't that mean we are just arbitrarily choosing to exclude some from Earth itself too? Is there a reason or a benefit to exclude viruses from the tree of life?
It's semantics, someone drew a line in the sand and here we are. Definitions get fuzzy in biology sometimes. There isn't a great way to determine how different two organisms need to be to be two different species, and biologist will often fall into two different camps on how to do it (called lumpers and splitters ). Your genome is filled with sequences that are basically retroviruses that have lost the ability to escape the cell (called LTR Retrotransposons ) but still replicate and move around your genome. It's even possible that some viruses evolved from transposons. If you look hard enough you can even find LTR Retrotransposons that still have parts of the genes they needed to burst out of a cell (called Endogenous Retroviruses ). Where are you going to draw the line here? Is your genome filled with living organisms? Or has it been part of the genome so long (all eukaryotes have transposons to some extent) that it is just part of who you are? Dichotomies are useful for discussing biology, but the reality is that things are rarely black and white. |
Why does light differ from everything else?
What caught me dumbfounded is that why is light/photons (including the visible and not visible spectrum and microwaves, radio waves etc.) the only thing that can pass through objects transparent or not(light specifically for transparent and the other waves for both transparent and opaque). I know they are like waves but a light can pass through a window while water cannot so to speak. ELI5 if you can EDIT: I know there are other things that can pass through objects aside from light BUT what I want to know is why can they while others cannot.
First of all, light is not the only thing that can pass through matter. For example, neutrinos are constantly passing through even the most solid objects. To understand why water can't pass through glass but light can, you have to understand electromagnetism. Matter that interacts with electromagnetism can interact with other matter that interacts with electromagnetism. So when the two objects get really close, they get repelled from each other and can't pass through each other. Photons are electromagnetic excitations, and objects that allow them to pass through like glass allows the light to be absorbed remitted in the same direction as it came in. There is no repulsive effect like with matter. So that was a very simple explanation but I hoped it help. It comes down to the fact that photons are like waves on the ocean (excitations) while other matter is more like boats (can interact with photons, but not excitations). |
How does a strong acid or base burn your skin?
So I've tried to google this, and my bio/micro teacher doesn't have an answer for me. My money is on rapid denaturation of proteins in human cells, but all I can find from searching is "it causes a chemical burn." Thanks for any information you can provide!
A strong acid is a strong dehydrating agent and what it does is take up all water from skin cells in an exothermic reaction, effectively "burning" them. |
How is genetic information arranged across chromosomes?
We all learn in school that (nearly) all animal cells contain DNA. We also learn that humans have 46 chromosomes, arranged in pairs. But that's where the details seem to end unless we go study this stuff on our own. Therefore, my questions: Basically I'm trying to understand why we have two kinds of DNA and how our genetic instructions are arranged. I've been studying neurology and neuroscience (you know, for fun); and it's making me start thinking about also studying gene expression.
These are all great questions to be asking about DNA so I'll try to give you good answers to them. In non-meiotic cells that are not actively going through the cell cycle there are exactly 46 mostly contiguous DNA molecules. Our cells are, of course, always in flux whether it be transcription, repair or new replication but outside of replication there are statically 2 copies of each Gene. The copies are not identical as one is from each parent and these will have sequences that are slightly variant, which is good because some mutations only cause disease if you have 2 bad copies. There are examples of single chromosome eukaryotes but as far as humans are concerned our genome is spread across distinct molecules which is why the distinction between the 23 chromosomes is important. Again, this is evolutionarily advantageous because sometimes terrible things can happen to genetic material and the impact of serious mishaps is somewhat reigned in by spreading the information around. Mitochondrial DNA has been sequenced, although it is not typically considered when discussing the human genome from a broader perspective - it is not one of the 23 chromosomes. Unlike the rest of our DNA, mtDNA exists in a circular chromosome just like most prokaryotes and it codes for not just the machinery used for cellular respiration but also it's own large and small subunit of the ribosome and 22 different tRNAs. Ok this part is incredibly complex and we truthfully don't understand the whole picture still but I'll give it a shot. There are many factors that go into Gene expression but a few include promoters and repressors. These are sequences that recruit different proteins that interact with the replisome machinery differently, either encouraging or discouraging expression. There are also transcription factors that get involved with these players and even deeper there is the actual structure of DNA and epigenetics. DNA is involved, as you may know, in binding to histone protein complexes - textbooks like calling it "beads on a string." Well, depending on how tightly bound the DNA is, it may be inaccessible to the replication machinery. This is modulated by different chemical modifications on the tails of the histone protein, loosening or tightening the DNA on the protein. This is known as epigenetics and helps explain how a Gene can be turned on or off on a larger scale. Finally, another important concept that factors into expression is the domainization of the nucleus. This is a newer idea, but it has been shown that certain parts of chromosomes occupy specific territories of the nucleus and they don't really move around. How this plays into expression is largely unknown still but it is thought to be relevant to the bigger picture. Overall, having 2 copies of everything is very beneficial as it allows us to be more resistant to mutations which could be injurious. There are repair mechanisms that depend on that other "good" copy to fix a bad one, not to mention the benefits of genetic diversity. I hope I've answered your questions but if you have any more I'm happy to try to answer them! |
With regards to the "black hole ripping apart a red dwarf", is the red dwarf exerting any attraction/gravity thats noticeable towards the BH?
This:
It's there in the article: Because the star is the lighter object, it lies further from this point and has to travel around its larger orbit at a breakneck speed of two million kilometres per hour – it is the fastest moving star ever seen in an X-ray binary system. On the other hand, the black hole orbits at ‘only’ 150 000 km/h. Elsewhere in the article we find that the black hole is 15 times the mass of the red dwarf, and the red dwarf is 1/5. By contrast, the Earth is 82 times the mass of the Moon, and they co-orbit a point about 1,000 miles below the surface of the Earth. Without knowing the separation between the black hole and the red dwarf, it's hard to say where the c of g is exactly, except that it's 1/15th of the distance between them. edit: Hang on, it's not that hard. The red dwarf is going 2 million km/h and the system has a period of 2.4 hours. So that means it's going 2*pi*r km in 2.4 hours, which works out to a radius of 7.5 million km from the c of g, with the black hole orbiting at 570,000 km from the c of g. |
How do we know that mass extinctions happened in the past ?
How can we know that ? or even know how many extinctions happened ?
Its easy to see in the fossil record. You can find many fossil species in one strate (lets say end of Permian) while the next one (Begining of Triasic) is very empty of species and most of the groups that you could find before have disappeared. In the case of the Permian extinction it affected 96% of marine species and like 70% of vertebrates, so it is a very big difference between both fossil records. |
Why is it so hard to wake up from an induced coma? Example of F1 pilot Michael Schumacher
*For those who don't know, Schumacher was skiing off-piste, he fell and hit his head on a rock, sustaining a head injury despite wearing a ski helmet. Towards the end of January, doctors at Grenoble University Hospital began trying to gradually bring Schumacher out of his induced coma but he just won't wake up.* What exactly goes on in the brain or rest of the body that prohibits one from waking up? Why is it harder to wake up the more time you continue in that coma?
When someone is put into an induced coma, it's generally because they are in very dire straits. They may have severe brain swelling (likely the case with Schumacher), or they may have intractable seizures. The purpose of the coma is to make the brain calm and slow down its metabolism, so that it uses less blood, lowering intracranial pressure (to prevent the brain crushing itself against the skull or even herniating) and hopefully preventing the brain from harming itself with dysfunctional overactivity as in a seizure. Because of how bad-off all patients who are put into an induced coma are to begin with, it's really tough to tell whether the difficulty in waking up, or even the long-term neurological sequelae (assuming they do successfully wake up), are related to the drugs used to maintain the coma, or simply the inevitable result of the brain injury that prompted the induction in the first place. Obviously, putting a healthy person into a coma for a few weeks to a few months to verify what the long term effects of the drugs are at these dosage levels is not ethically defensible, and I doubt many people would volunteer. All the studies done on this are either aiming to verify that an induced coma does at least as good or better than the conventional therapy, or trying to understand the mechanism by which it helps. Damage to the reticular activating system (brainstem), the thalamus, or widespread injury to the cerebral cortex (or some combination of the three) generally underlie permanent unconsciousness/inability to wake. As for why it seems like people that stay in an induced coma longer have a harder time coming out of it, that's probably sampling bias. Doctors are constantly checking to see if they can begin the weaning process while someone is in an induced coma, similar to how they really don't want people staying on a ventilator any longer than they absolutely have to. So, the people who are required to be in an induced coma for a long time (due to not showing encouraging signs of recovery), are also the people who are most likely to have had a severe injury capable impairing consciousness all on it's own. TL;DR If you're in an induced coma, you're not the sort of patient that is expected to come out of it at the drop of a hat, no worse for the wear. |
Since a focal point can theoratically be infinitely small and photons have an impulse, does this mean that we could theoratically create an "impulse-singularity"?
In physics class my prof told me that a focal point can theoratically be 1-dimensional. I don't know too much about photon-impulse, but shouldn't that mean that we can focus a certain impulse on an infinitely small "area", meaning 1 point? Would this have any interesting consequences?
In theory, photons have energy and they curve spacetime just like mass so a high enough density of photons can produce a black hole. Such a thing is called a kugelblitz. And again, in theory, we often assume lenses are perfect geometric objects whose focal points are 1 dimensional, so on paper your idea isn't crazy. In reality, any lens will have imperfections that will cause the focal point to not be an exact point. Additionally, diffraction will cause your waves to spread a tiny bit so I doubt you'll find it easy to focus any light to an point. To give you an example of just impractical this is, suppose we want to make a black hole using a lens and a really powerful laser. How many photons do we need? We can approximate the focal point as a sphere with a radius of 1 mm, which all the photons leaving the lens pass through. Taking the equation for the Schwarzchild radius: R = 2 G M / c^2
We can solve for an equivalent mass for the photons in this volume, nE = mc implies that m = nE/c where n is the number of photons. If we use green photons (~500 nm) then we'd need approximately 10 photons. For perspective, our sun puts out approximately 10 photons per second. That means you'd have to focus several hundred million years worth of sunlight into a box the size of a grain of sand in order to produce a black hole. |
What is the effect of numerous atmospheric nuclear tests on global warming?
After watching a video on youtube about all the nuclear blasts from 1944-1998 i am curious what effect that radiation would have on global warming and/or any other short/long term effects on the planet?
An nuclear explosion is impressive-looking, but ultimately the energy-content is pitiful, compared to that of the global circulation. The thermal energy from a single mature hurricane equates to a 10 megaton nuclear explosion every 20 minutes . The impact of a single hurricane on the global climate is incredibly small, so you can imagine how small the impact is from nuclear tests. |
What inductance and resistance do I need for 500v system with 30a and power factor of 90%.
The final inductor value will be frequency-dependent, but the system impedance can be calculated without knowing frequency. PF of .9 means the phase angle is ~25.78deg. 500v@30a at the input means the system has to have an equivalent real impedance of 16.66ohm. This is all speculative as I'm working from memory and haven't had to do AC power stuff in a long while. As always one should verify all calculations via phasors / whatever method wasn't used to make sure it works. e: I'm assuming sinusoidal waveforms. Not entirely sure if the power factor stuff is waveform-dependent, but as this is very likely homework related I can't imagine they would use anything else. e2: Nope I borked this calculation up good. The total system impedance has to have magnitude 16.66ohm, not the resistor you'd use to make it.
Sin[25.78 deg] = (reactive component) / 16.66 => reactive component = 7.24 ohm
Cos[25.78 deg] = (real component) / 16.66 => real component = 15.00 ohm |
What does it mean for a discovery to be "7 sigma"?
According to just posted on , the Higgs boson is close to the "7 sigma" level. I know it's some sort of measure of certainty that the finding is accurate, but can someone give me a more exact definition?
You are correct in that it is a measure of certainty. I'll refer you to this article that gives a more detailed explanation. Essentially the "sigma" for an event like the discovery of a particle can be converted into the probability that the evidence for the particle arose due to random chance. Higher sigma values mean that the discovery is less and less likely to be accidentally a mistake. |
Why do arctic climates often have days where night is warmer than day?
I just looked at the weather for McMurdo Station in Antarctica and Longyearbyen in Svalbard, and this upcoming week has many consecutive days where it is 3C warmer at night. How is this possible? My guess is that the sun loses much of its influence on the daily weather cycles, and that wind and sea currents become more influential.
As close to the poles as these places are, “day” and “night” are rather meaningless - at McMurdo Sound it’s currently perpetual daylight and at Svalbard the sun doesn’t rise for a few months. Basically it’s weather and local topography which matter. |
Why is a frozen and thawed banana so much sweeter, and how does this change its nutritional value?
Putting together the information here One of the main processes in bananas (and all fruit) ripening is the amylase dependent conversion of savoury or flavourless starches in to sugar (specifically glucose). Amylase is a common enzyme (also present in your saliva) which converts starch to sugar and is an important part of your digestion. There are essentially 2 ways something will taste sweeter. 1) There is more sugar present or 2) Your tastebuds can access the sugar more rapidly Freezing and then defrosting fruit essentially lets both of these things happen. Freezing causes water in the fruit cells to crystallise and expand. This destroys the cell walls and is the principal reason defrosted fruit is soggy and limp. However it also means that the cell contents (all those sugars) are now in the juices that are running off the fruit and if you taste the juice you'll find it is very sweet. You can experience this at the most extreme if compare the difference in sensation between holding a mouthful of orange juice in your mouth or holding a slice of orange (without chewing). In the case of a banana there isn't much excess of liquid to run off so those exposed cell contents will largely stay within the fruit pulp/body rather than running off. The other thing that happens while the fruit is defrosting is that all the amylase and starches in the cells are now able to diffuse (a little) through the defrosting fruit pulp. The amylase is no longer confined to the cell it started in, where it may have completed its starch converting job, and is free to find any remaining starch that may have come out of other nearby cells. This means that some of the remain starches will be converted to some extra sugars. |
What determines the rate of action potential propagation?
I'm going to ask what I think is the same question three times in three different ways because I can't quite pin down what I'm trying to say. Hopefully it makes sense... Assuming no myelination, what is actually responsible for mediating the change in charge along an axon during an AP? It's not the diffusion of Na+, is it? My understanding is that Na+ flows into the axon, creates a localized positive relative potential, which then activates nearby channels and repeats this process. If that's the case, then this should be slower than diffusion alone, yes? Since it's local diffusion plus the time it takes to open the next set of sodium channels? Myelination speeds signal conduction, but what is actually being conducted if there are no sodium channels to open under the sheath? None of this is instantaneous, but then what factors influence the time it takes for one node to activate the next (aside from distance?)? Na+ isn't diffusing down the covered regions of axons is it? Assuming myelination, does the influx of sodium across the membrane produce a local electric field which is then felt at the next node of Ranvier faster than diffusion alone could explain? If so, what mediates this? What is the ultimate determinant of signal propagation velocity? Thanks!
The Na+ is diffusing down the covered regions of the axon. Time is saved by not having to open successive ion channels. The signal propagation velocity is limited by the diffusion rate of Na+. |
How can you figure out the age of water? Thermohaline circulation related.
I was talking to a gentlemen from the UK a few months back, and we were talking about the age of water. He was telling me that you cannot date water unless it's pre-WWII water/post WWII water because of tritium found in water. I was trying to do some searching on how long it takes for a cycle in thermohaline circulation to take place, but can't find anything on it. I keep reading that it takes 2,000 years, but how do they know that? How do they know that the water they are drinking started as glacier melt 2,000 years ago? Thanks in advance!
Ocean water incorporates atmospheric carbon, some of which has been converted to the radioactive species (C-14) through reaction of cosmic rays with nitrogen atoms in the upper atmosphere. We can thus date oceanic waters using carbon 14. Here are two summaries of the results from such work, this one and this other one . |
What's the earliest animal that plays games?
Anyone with a cat or a dog knows animals like to play games. How about evolutionarily simpler animals? Do turtles or frogs play? What is the earliest animal that plays? Is playing an emergent trait that evolved separately in different places and times?
Playing might have evolved as a way for young animals to develop motor function as well as to hone stalking/hunting skills. At least this is true for dogs and cats and other carnivorous animals. There is no way to know for sure, but it could be that it simply came about that the cubs/pups/whatever who played ended up more successful hunters than their lazy counterparts. Of course, this is speculation. I don't know if simpler animals play as youngsters. Piranhas hunt, but don't exactly play as spawnlings. |
Would a flower maintain its color in a room totally void of light?
Color is a psychological, not a physical property. Objects have surface reflectance properties which do not depend on whether there is or isn't light any more than their mass does. |
Why is blood group O+ so common?
Would it not be a rare blood type as the allele for blood group O (I is recessive to the other alleles?
Whether an allele (type of gene) is dominant or recessive, doesn't really determine if it is common or not. Just think of an island of mostly red-heads and a couple brunettes. If mating isn't biased by hair color, the number of red hair alleles and the number of brunette alleles wont change. In fact if a number of assumptions are made, the number of red-heads and brunettes will reach an equilibrium known as Hardy-Weinberg equilibrium. Basically, allele frequency wont change between generations unless some force acts upon them (nonrandom mating, mutation, natural selection, gene flow, genetic drift). Without something driving change, the alleles are simply "reshuffled" every generation. I can't say O is a common allele. Perhaps it was advantageous in our past/right now. Perhaps it became more common by chance. However, the fact that is is recessive, doesn't impact its frequency. EDIT: I feel I should add that the O allele is present in some type-A and type-B people, as it is recessive. A person with type-A blood can be AA (two "A" alleles") or AO (an "A" and "O" allele). A person with type-B blood can be BB (two "B" alleles") or BO (an "B" and "O" allele). If an "AO" and a "BO" have children, there is a 25% change of having an blood type-O ("OO") child. If an "AO" or a "BO" have a child with an "OO", there is a 50% chance of having an "OO" child. No other combination of parents will result in an "OO" child expect for two type-O parents, but sometimes the "O" allele will still be passed on to the next generation. EDIT EDIT: The Rh factor (the + or - of the blood type) is inherited rather simply with Rh+ being dominant to Rh-. For whatever reason, the Rh+ allele is more common than the Rh- allele. So Rh+ blood-types are more common than Rh- blood types. This link explains Rh in inheritance. This link explains ABO inheritance and has a nice table of blood-type frequencies. Notice that for any given ABO bloody type there are more Rh+ than Rh-. This is not because Rh+ is dominant but simply because it is a more common allele. |
Is it possible for animals to have mental disorders?
If so, are human mental disorders common in animals (A dog with schizophrenia)? Are there unique disorders specific to animals?
Although there are certainly objective physical changes in some disorders, much of what we deem psychiatric disorders are based on subjective states--the person depressed, the person is hearing voices, etc. Since we can't ask animals how they feel, we can't know much about the subjective internal states of animals. We can, however, note behavioural changes that seem to match up with the behaviours noticed in humans with mental problems. For example, this classic experiment (don't read if you're a huge animal lover! http://people.whitman.edu/~herbrawt/classes/390/Seligman.pdf ) established a paradigm known as "learned helplessness"--you basically put an animal in a situation where bad things keep happening to them and they have no control over them. Soon enough, they start acting a bit like people do when they're depressed. This paradigm was applied to humans for a long time: depression happens when people feel like they're losing control over their environment and nothing they do will change it. They actually have used this to test anti-depressant treatments and whatnot--basically, if you do this to a rat then give it prozac, it gets better. (This view is now viewed as overly simplistic, btw. It's still used as the "closest we've got" in animals, but they realize there's a lot more complexity in human depression. Here's a good wiki summary: http://en.wikipedia.org/wiki/Learned_helplessness#Attributional_reformulation ) Another way to look at it is that many mental disorders can be considered extreme versions of normal behaviour that causes problems with the person's functioning in society. In almost all DSM-IV diagnoses, it mentions that the disorder must actually be causing problems for the person in their personal/professional life. If somebody is a bit ADD but they're happy, successful, and it's never really affected them negatively, a good psychiatrist would never diagnose them with having a problem. If we think about it that way, we certainly see maladaptive patterns of normal behaviour in animals. |
Morals and ethics aside, could a developed nation feasibly develop and grow their own army of cloned humans/soldiers with our current technology?
We don't have artificial wombs so there will always be a birth mother and biological father. They're still going to be little kids who have to grow up and get an education. Either way, you're going to have to sequester a bunch of women as brood mares and steal their infants; whetehr or not those infants are all genetically identical doesn't seem to make much of a difference to me. |
What exactly does it mean that a vaccine/booster etc is X% effective?
Typically they are referring to “Relative risk reduction”. Take 100 people and give them vaccine, 100 people who don’t get it (or better, get a placebo). After some period of time, see how many in each group got the virus. Let’s say 2 in the treatment group got the ‘vid and 20 in the placebo group. 2/100=0.02 20/100=0.2 So the relative risk reduction is= (0.2-0.02)/0.2 = 0.9 So the relative risk reduction is 90%. You can think of it as having a 90% less chance of catching the virus if you get the vaccine compared to if you don’t. |
What's the difference between two experiments using laser emitters and slits, one of which demonstrates "quantum physics" behavior and the one showing diffraction? Is diffraction involved in the quantum one or is it unrelated?
I have a big question and a minor question about experiment(s?) involving shining light through narrow slits. I've seen descriptions and videos of the double slit and how it's "quantum mechanics". I also did an experiments in high school physics to demonstrate diffraction using a laser emitter and a slit that was similar. It was a while ago so I don't remember all the details. But you achieve the "venetian blind" pattern because the waves cancel each other out in dark spots and amplify in the bright spots. My first question is: What are the differences between the diffraction experiment and the quantum physics one? From what I can remember they seem to be the same experiment, but I doubt that's the case. Second question: In the quantum experiment, I remember reading about "when the light was being observed it converges into two slits", and something about how when the data collected was set to automatically be erased (so it was still collected, but then discarded), it... did something, but I can't remember how it affected the results. Anyone know?
There aren't any, that's the point! One involves light and the other involves a stream electrons (or any other particles) aimed at the slits. The fact that they both result in a diffraction pattern demonstrates that it's not just light that has wave-like properties, but matter too! i.e. the wave-particle duality |
How do electromagnets work?
Electric currents produce magnetic fields. If you have an electric current traveling through a resistive medium, there are energy losses to heat. If the resistance of the medium is zero, like in a superconductor, there are no losses to heating. |
Why are surgical leg amputations done either above the knee or below the knee but not right through the joint itself?
Why not just separate the joint and take the tib-fib portion?
I think the question relates to why not leave the end of the bone as the termination point - that is to say why not simply remove the tibia, fibula and patella, leaving the smooth articulating surface of the femur as the termination point. |
Age of the Galaxy
So, the recent article about the discovery of the oldest spiral galaxy got me wondering how old our own Milky Way is. A quick google and I find that the internet gives an answer of around 13 billion years - almost as old as the universe itself and more than 2 billion years older than the newly discovered ancient spiral galaxy. Seeing as our own galaxy is spiral, what is going on here?
The discovery isn't of the oldest spiral galaxy thought to exist, it's the oldest spiral galaxy we've ever taken a picture of. Because light takes time to go through space, the light we're receiving from the galaxy is that old. All galaxy are effectively the same age. The discovery is of a galaxy that is both old and spiral, and is the oldest spiral that we have seen to date. |
Why aren't all mammals omnivores? Is there a benefit on being able to eat nothing but plant/meat compared to being able to eat both like humans are?
While herbivores have a more easier access to food than carnivores, they are more susceptible to droughts and changes in the environment than carnivores are. While carnivores do not need to eat as often, they have to use more energy to catch and kill their prey which can be a more riskier lifestyle than grazing in a field. Is there any evolutionary benefit on being a carnivore like a cat, a herbivore like a cow than an omnivore like a human or a goat? Are omnivores in general any better at surviving times of famine than carnivores or herbivores?
why aren't all mammals omnivores? So, let's start here. Mammalian diets are more flexible than the boxes "omnivore", "carnivore" would indicate. Here is a great example of an herbivore engaging in predation. Certainly not common, and animals will be a very small minority of their diet, but not that strange either. We also can see examples of obligate carnivores eating plants; you might be able to observe that behavior in your own home! So, don't think of strict categories like "carnivore" and "herbivore", think of a range. And think of another scale that ranges from "specialist" to "generalist". While carnivores do not need to eat as often This is a false generalization. I have done some small mammal field work, and often if you leave traps out overnight any shrews that wandered into your trap will be dead in the morning of starvation. These little opportunistic predators eat insects, worms, salamanders, baby mice, nuts, etc, constantly in order to stay alive. This overnight death of starvation doesn't happen to any other of the small mammals you'll commonly catch, which are generally more herbiverous in nature. So, our shrew would be a mostly carniverous generalist. Next I would recommend thinking about the different biological tools that organisms might need to eat various items. What teeth do you need? What digestive track? What behaviors and metabolism? And who else are you competing with? Let's imagine a source of food that is easy to get a hold of: A fruit. Fruits are made by plants largely in order to spread the seeds around; they are a payment to the organisms that eats them, in energy, that hopefully allows the spread of those seeds far away. Do we need special teeth to eat the fruit? No. Special digestion? No. However, we do need to outcompete all the other organisms that are trying to eat that same fruit! You can think of lots of ways we can compete. Compare to a resource that is very difficult to eat: let's say, grass. Very tough for you and I to get much nutrition our of it; you need specialized tools to do so. So, cows and other ungulates have special teeth and digestive tracks to help break the grass into something that is nutritious. There is a cost and a benefit: the cost is that they have to grow lots of extra disgestion, have specialized teeth that aren't good for other things, as well as constraints on their size. The payoff is access to a foodsource that a lot of other critters can't eat. An example of extreme end member specialists are pandas (eat bamboo) and koalas (eat eucaltyptus). Bamboo and eucalyltus are barely even food, but these extreme specialists make a fine living. On the other hand... if the food you specialize on dies, you are in big trouble. Hyper-predators are also specialists in a similar way. A lion has very specialized teeth, but their bodies and behavior are also very specialized towards active hunting. The payoff is that they are getting a food resource that other animals don't get access to: Healthy antelopes. But the lions have to pay for that, as well. They need to be able to deal with uneven times between meals; they need to be able to run incredibly fast and attack an often larger animal , they need to defend that dead meal from everything else out in the world (a dead antelope is much more like a fruit; you don't need to specialize to eat it, you just need to compete with everyone else that is trying ot eat it!) Every kind of animal is a solution to the problem of staying alive. Every single one has a evolved into a niche that is benefitical, and there are a million tradeoffs along the way that come into play. Making generalizing statements about "herbivores", "carnivores", or "omnivores" probably isn't very useful because those aren't biologically real categories, they are labels that we have put on organisms that generally describe what they happen to eat. If you want to start generalizing, also think about whether an organism is a specialist or a generalist. Think about the environment they live in: it is stable, or does it vary a lot? |
Do plants need "rest"?
Can plants go through photosynthesis nonstop without a break? Or do they need some time to slow down? EDIT: Let me rephrase, if a plant is given light 24/7, watered when needed, and new/changed soil when required, would it work itself to death?
In general, most plants have increased photosynthesis rates during the day and decreased or no photosynthesis during the nights. This is simply because there is light during the day which is necessary for the light dependent reactions of photosynthesis (the light independent reactions of photosynthesis do not require light but occur alongside the light dependent reactions). Stoma pores also open during the day and close during the night for gas exchange (photosynthesis needs carbon dioxide and releases oxygen). One interesting exception to this is CAM plants. CAM is a specialized type of photosynthesis that exists in plants from extremely hot, dry, or windy climates. CAM photosynthesis has adapted to these particular conditions. Adaptations include the stomata opening during the night instead of the day for gas exchange. This is because hot, dry, or windy conditions cause evaporation of water from leaves or other plant material to occur faster. So for example, cacti and other CAM plants store carbon dioxide in the form of an organic acid during the night, and release the carbon dioxide for use in photosynthesis during the day when there is light. |
Help me figure out a weird sky phenomenon I saw this morning.
I'm an early riser, so even on my days off I'm up at 6. I was outside just before 7 am - the sun wasn't up yet. I hopped in my car to drive to the store for some coffee, donuts, and (you guessed it) bacon. I pulled out of my driveway and drove west. I live in a valley and so I was looking at mountains; beyond those mountains is the Pacific Ocean. The Sun was not quite up yet, still tucked behind the mountains to the east. The sky was quite clear, one or two high fluffies here and there and a little mistiness along the horizon to the north and west. Here is what I saw: in the western sky extending from the tops of the mountains to about a third of the way through the sky was a column of shadow. That's the best way to explain it - the sky was bright (the sun was almost up) except for this one column which was considerably darker. It was like a reverse searchlight was mounted behind the mountains to the west. The "sides" of the column, then, were not parallel to each other, but instead spread out slightly. The sides, though, did seem to be true - they weren't wavy and did not bend (except to follow the arc of the sky). I drove to the store, looked at it from the parking lot, went inside to shop. When I came back out, the column was still there. It was not as dark and not as focused. I'm not sure about this, but it also seemed to have changed its angle in the sky - where before it was coming roughly straight up from behind the mountains, it now seemed to be tilted slightly to the south. This may have been an illusion based on the fact that it's really hard to judge angles across a spotlessly clear sky in the daytime. It was gone by the time I got home - maybe 15 minutes total from the time I noticed it. Here's what it wasn't - the shadow of a jet contrail. That was my first guess, but jets do not travel that direction in my neck of the woods. There is nothing to the west except for the ocean, and it's kinda big. Jets travel north/south or variations thereof (northwest/southeast, etc.) I also should mention that one of the clouds in the sky looked a bit like the FSM. Perhaps the shadow column was one of His Noodly Appendages? I forgot to buy bacon.
I couldn't exactly paint a mental picture of what you are describing here (maybe I need an ms paint pic to help sort it out), but one possibility may be Anticrepuscular rays , shadows of clouds beyond the horizon being cast upwards instead of the normal case, downwards. A good site with an overview of various atmospheric phenomena is http://www.atoptics.co.uk/ . The site can be a bit obnoxious to navigate. |
What would happen if Antarctica melted instantaneously?
This question came up because I was experimenting with and put a nuke on Antarctica because I was bored. Would the water displace equally in all directions? Or would currents,etc have anything affect on this displacement. Would the entire world flood water world style? How quickly would it happen(if it happened)?
Global average sea level rise would be 60-70 meters. I don't remember the exact number, but that's okay because the exact number isn't known anyway. Large parts of Antarctica remain unexplored, without ice thickness measurements. However, the sea level rise would not be uniform worldwide. The biggest effect would come not from ocean currents but from the loss of the gravitational pull of the ice itself (or, strictly speaking, from the loss of concentrated gravitational pull near the pole). Near Antarctica the sea level would actually fall, as the ocean would no longer be attracted by the ice sheet. To compensate for this, sea level would rise more than the average in the far field. To complicate matters further, this pattern of sea level rise would be temporary. The Antarctic continent is presently depressed by hundreds of meters by the weight of the overlaying ice, and when that weight is removed it will slowly rise up to a new equilibrium position over the course of thousands of years. This rise will lower local relative sea level as the land will rise relative to the ocean. However, because the mechanism of the rebound is slow creep of mantle rock to fill in the space underneath the continent, the mass deficit that caused the change in the Earth's gravitational field will eventually be erased. Eventually absolute sea level will be raised by a constant amount worldwide, but relative sea level on the Antarctic continent will still be lower because the whole continent will move upwards. This is all posted from my phone while I'm waiting for my clothes to finish at the laundromat. I can provide a source when I get home. Mitrovica et al., 2001 |
Does charging your phone slower, by connecting it to a pc by usb-a, makes the battery last longer than connecting directly into a outlet?
This is no longer correct with the new charging methods, like "Quick Charge" that boost the current and the voltage substantially. Normally USB 2 would be at 5V at 2.5 Watts, Quick Charge is up to 18 Watts. This will boost the charging Voltage to up to 20V. This allows you to very rapidly charge your phone, but will cause the battery to heat up much more. This extra heat will cause the battery to degrade more quickly than if charging by 5V at 500 mA (usb2) or 2000mA |
Are there chemical elements that no longer exist? Could we continue to create man-made elements past what we've already made?
The big bang created almost nothing besides Hydrogen and Helium. The reasons for this are complicated, but basically boil down to all the fusion pathways from Helium to heavier elements take a long time, and Big Bang Nucleosynthesis was only occurring for a few . Every element found in nature that is heavier than iron is made in supernovae, via several different processes. The theoretical upper limit for the elements that can be created by supernovae is for the number of (total neutrons and protons), and the upper limit is thought to be around 270 . Thus the heaviest element produced in nature is probably just past the actinides, possibly Meitnerium or Darmstadtium (elements 109 and 110), and of course they will only exist for a few seconds before decaying. Heavier elements created in labs are created by bombarding heavy elements with other heavy elements, a process which does not occur in nature, therefore these higher elements (like the recently created Ununseptium) can not occur in nature, even the most extreme types of nature like supernovae. |
Why would our sense of smell become selected to be worse than what we had before? (Evolution)
It's not that it was a disadvantage to have a strong sense of smell, but that this was no longer an important trait. Individuals were just as likely to survive long enough to reproduce regardless of how well they smelled. |
If the cyclin/cyclin-dependent kinase inhibitor p21/WAF1/CIP1 were to be silenced in humans, could its absence help reduce the chance of breast cancer?
So basically, we have what's called the BAF180 ( a subunit of the PBAF chromatin complex) which, when mutated, binds to the p21 gene and arrests the G1 phase of the cell cycle. The p21 gene apoptosis and does not under any circumstances induce cell death. In breast cancer, the PB1, which encodes BAF180, is found to be mutated. While I know that cancer is caused by multiple mutations, there is still a chance to significantly reduce the risk if a method to silence the p21 gene were to be implemented. What does Reddit think?
This Google Scholar search may be a good place to look for relevant journal articles. A few look promising, such as this one "Linkage of Curcumin-Induced Cell Cycle Arrest and Apoptosis by Cyclin-Dependent Kinase Inhibitor p21/WAF1/CIP1" And this one "Cyclin-Dependent Kinase Pathways As Targets for Cancer Treatment" |
While earthquakes are considered a natural disaster now, were they a big deal back when we were just hunters and gatherers?
Thinking about it, the only reason earthquakes are destructive to us now is because of the sheer amount of buildings we have built everywhere. There's also the tsunami afterwards but ancient people away from the coastline shouldn't have problems even with the strongest of earthquakes. Am I missing something here, or are they just not as big of a deal before than it is now?
Our built environment along with population density in earthquake prone places certainly does increase the hazard posed by earthquakes, and in detail the majority of damage and death in earthquakes are at least in part related to buildings (e.g., Doocy et al, 2013 ). In the absence of buildings, the earthquake shaking itself is unlikely to be deadly. There are however associated hazards that come with either the shaking or the displacement from the earthquake that can be deadly. You mentioned tsunamis, and these can indeed be devastating to coastal communities regardless of the level of infrastructure (and critically could happen without warning in the time before a global seismic network or telecommunication because they can travel very far from their original source, i.e., much further away than where you would feel the earthquake shaking, and still be incredibly damaging). Another earthquake hazard that wouldn't require a building to kill you are seismically induced landslides, which can be destructive to anything living in steep terrain or at the bottom of the slope where a landslide happens. These can also produced delayed hazards that can effect lowland areas, e.g., a seismically induced landslide blocks a river in the mountains effectively producing a dam that can start to impound water, but eventually this dam will fail which can lead to extremely large and rapid flooding downstream (e.g., Huang & Fan, 2013 as an example of a discussion of the broad risk from earthquake induced landslides along with floods from landslide dam breaches). Even in lowland areas, far away from coastlines or mountains terrain, surface deformation from earthquakes can have potentially deadly results, though very localized. Take for example the New Madrid earthquakes , which produced abundant small scale landslides and local flooding of the Mississippi river and tributaries (e.g., Johnston & Schweig, 1996 or Guccione et al, 2002 ). As with many of the other hazards, these are relatively localized and to some extent, you as a hunter gather would need to be a bit unlucky (i.e., wrong place - wrong time), but these are still potentially deadly. You might also be interested in the response to this somewhat related question from a few months back . a lot of death and destruction associated with earthquakes are in part a product of our modern (or past) built environment, i.e., buildings collapsing on us, fires from ruptured gas lines, etc. However, there are certainly hazards associated with earthquakes (tsunamis, landslides, landslide dam breach induced floods, etc) that could still kill you even if you were not living in a building, but the effects would be more localized than what we see today for the most part. Another way to look at this would be the prevalence of discussions of earthquakes (or other natural hazards) in folklore and oral tradition. These events, or descriptions thereof, are pretty pervasive (e.g. Ludwin et al, 2007 ), giving us a sense that while these may have not been as deadly in our early history, they were still potentially damaging (and terrifying) enough for them to persist in these stories/histories. |
Why do digital cameras have real shutters? Can't they turn on and off the sensor electronically?
I'd imagine that it'd be quicker to do things electronically, avoiding the hassles of a physical object opening and closing.
A lot of point and shoot cameras have an electronic shutter. There are pros and cons to both devices. A mechanical shutter lets the manufacturer use a simpler and more efficient sensor (lacking the electronics needed to turn it of and on) with a higher fill factor. From http://www.steves-digicams.com/knowledge-center/why-digital-cameras-have-mechanical-shutters.html |
We've had an magnum opus of a thread about the speed of light, but I feel like I could use one about evolution and abiogenesis. Can anyone help explain to us how it all happened?
I know that the two are separate things, but they get confused a lot, and I think it would be nifty If we had a thread like one to help clear everything up. Also, how did life come into existence and what were the mechanics behind primitive nucleotides turning into humans and flowers?
Abiogenesis is incredibly difficult to study for several reasons. First, all of the evidence was sub-microscopic, on the scale of chemicals less complex than ordinary proteins. Even if any remnants from pre-cellular life still existed from then (keeping in mind that such stuff would serve as food for many living organisms) it would be nearly impossible to find and study today. Second, the earliest pre-cellular life or proto-life would have been far less capable at living compared to later life that there is no hope that any ancient examples could still be around exploiting some niche. Third, the time-scales involved (tens to hundreds of millions of years) make it impractical to study in the laboratory. Nevertheless, there has been a fair amount of research on the subject which provides interesting insights into the likely origin of life on Earth from non-living environments. Various experiments have established that certain fairly complex chemicals (including sugars, primitive amino-acids such as glycine, and nucleotide bases such as adenine) can be produced from simple chemical precursors in environmental conditions similar to the conditions on an early Earth (e.g. lightning, high UV radiation, etc.) The best example of this is the Miller-Urey experiment . The hypothesis of abiogensis is that some example of self-replicating molecule arose naturally from these primitive chemical ingredients, likely some variant of RNA. Such a chemical would catalyze the duplication of itself within an environment enriched in such pre-biotic chemical ingredients. However, duplication of that sort could take a very long time at such a stage (perhaps even years), waiting for the right chemicals to be formed and to interact in the right way. Over time the self-replicating chemical would change. Multiple copies of it would exist, but some would have errors (in the case of a short RNA snippet, additions, deletions, differences of single bases, etc.) These mutations would influence the properties of the chemical, perhaps facilitating self-replication in one way or another. In the case of RNA snippets they could form "ribozymes" which are similar to enzymes. One likely early property would be facilitating the formation of nucleotide bases. Over geological time (millions of years) such self-replicating proto-life-forms would gradually acquire various traits until they started to look like proper organisms, and then from there would evolve the various characteristics we associate with all organisms (the DNA->RNA->protein process, use of ATP, etc.) The existence of the ribosome (which is composed mostly of RNA) at the core of the genetic machinery of all life is perhaps evidence of the earliest days of life on Earth. As far as evolution, that is a remarkably simple process to describe. If you have a population of self-replicating creatures such that the characteristics of the creatures are encoded in heritable material (such as genes) that is not always copied 100% perfectly then evolution is a natural consequence. Even if the population begins as entirely identical clones over time slight variations (mutations) will lead to some individuals in the population having slightly different characteristics. If this leads to an improved chance of survival in the given environment then those genes will tend to predominate relative to the genes of the rest of the population, and the species will have changed very slightly. Note that the changes do not have to be extreme, because the advantages will express themselves over generations, much like compound interest. How much would 1% compound interest add up to over a hundred, a thousand, or a million generations? In this way small changes transform the gene pool, and then further small changes stack on those, etc, etc, until eventually you end up with big changes. This can cause populations to change completely or it can cause sub-populations to become significantly different from main populations (creating a new species). Over time populations die off, new populations form, and life becomes ever more diverse and complex. These processes are extremely simple, but over geological time (millions and then billions of years) they can lead to enormous changes. |
Conceptually, how does the speed of light relate to the permeability/permittivity of free space?
I know the equation that relates the three constants: c=(ε0*µ0) However, I never understood why this worked or if there was a conceptual reasoning behind it, so any explanation would be greatly appreciated.
Light can be described as an electromagnetic wave. Electromagnetism is described by Maxwell equations. When you work out those, you end up with the equation of wave motion for light, with speed value equal to c as you described. This by the way prompted a crisis: that speed value is the same regardless of the reference frame. So the speed of light is the same even if you chase it at 99% its speed. Einstein eventually explained this with special relativity. |
If two small moons or planets collide together in space, over time they can essentially create one big moon or planet. My question is, by that logic, shouldn't our own moon be getting closer and closer to us, instead of further away each year?
I'm not sure if I worded my question good enough in the title for others to understand so I'll try to go more in depth here. Just tonight, I saw a simulation on what it would be like if two planets collided together. The result was chaotic. Over time the debris from the two masses, through gravity, were able to come together and create a larger mass. I've also seen this being talked about by the likes of Neil deGrasse Tyson and other big name astronomers on the science channel. But going by that logic, shouldn't our moon be getting closer to us instead of further away? As in one day our moon should collide into the earth effectively making the two masses into one mass that is slightly larger than Earth? I appreciate any feedback. Thank you.
I don't see the logic that because some things that crash melt together, nothing can move away from eachother..... Why would our moon have to be moving inwards? |
Is it possible for a person to be colorblind in only one eye?
Something like the heterochromia mutation?
Heterochromia is the result of more or less pigment in the iris of one eye. While there is usually a genetic component, there are a whole collection of genes that interact with each other and the environment to regulate melanin production. So it's a lot easier for a mistake to happen. Color blindness is more of an on/off thing though. It's caused by a single faulty gene and either a cell has that gene or it doesn't. So to be color blind in only one eye, a person would need two types of cells. Some of them with a normal working copy of the gene for color vision, and some cells with the defective version that causes color blindness. This is actually possible. It's called chimerism and is the result of two embryos fusing in the womb to develop into one organism. There's even documented cases of it happening in humans. https://en.wikipedia.org/wiki/Chimera_(genetics)#Humans The only problem is that which particular cell becomes which particular tissues in a chimera seems to be largely random. And the odds of getting a pair of color blind and not color blind twins fusing in exactly the right way is extremely tiny. |
Is the Mars curiosity rover waterproof?
The sealant to keep out environmental dust is as good as the sealant to keep out water. Electronic components are also in a sealed environment to facilitate temperature control. The rover probably isn't balanced with respect to buoyancy, so wouldn't function as intended underwater (let alone the instrumentation), but if you suddenly plopped it in water, barring physical damage like flipping over or whatever, it wouldn't suffer any electrical or motor issues. |
If a salmon's birth stream/river was filled in, what would they do?
Salmon go back to their birth stream to reproduce, right? What would they do if a dam or natural formation blocked off their access to the stream? Would they go somewhere else, reproduce at the closest point, do nothing and die, or what?
They would most likely hand around the base of said obstruction until either they die from starvation, spawn at the point they were stopped or leave. I am guessing a mixture of the first two. However being semelparous (die after one large spawning event), even if they were to spawn the adult population would die and then depending where in the river they spawned the juvenile population could die also from a lack of food etc. |
The soil triangle: Is there a name for charts of this type?
I am not going to release this question, but I will tell you that it is called a "ternary diagram" or "ternary plot" It is commonly used in many fields, such as geology, materials science, chemistry, and more https://en.wikipedia.org/wiki/Ternary_plot |
Massive black holes can spew out huge amounts of energetic material. Would this process convert heavy elements back into hydrogen and helium?
For example, if a nickle-iron planetary core is ripped apart, is it converted into protons and other light nuclei? For example, if a nickle-iron planetary core is ripped apart, is it converted into protons and other light nuclei? |
How can a car battery be rechargeable if the voltage is produced by a chemical process?
As far as I understand, lead and lead oxide are suspended and diluted in sulfuric acid. If this is the case, how can a battery 'die' if left charging something overnight, and how can a jump bring it back to life? I'd think that if the battery died, it'd be dead forever as there is no more chemical reaction taking place?
Charging a battery undoes the chemical reaction. You put in energy to put it back to the high-energy state. The full chemical equations are spelled out on the wikipedia page . This is also true in lithium-ion batteries. When you discharge the battery, the lithium ions move from negative electrode to positive. When you charge it, they go back from positive to negative. |
Is Magma the Same?
Is all magma the same? for example if you have a piece of shale and it melts into molten rock (magma), would the magma be the same magma as molten marble? Or would the magma be made of a different combination of materials.
No. The chemical compositions of your two hypothetical melts would be different and thus the physical properties would be different. This is more easily discussed in terms of more realistic magma/lava types that we see regularly. In considering types of magma, we usually think of them in terms of what types of rocks they will form when they solidify and crystallize. For lava (and rock types) we commonly see at the surface, we can think of a continuum between basalt (mafic) to rhyolite (felsic). The principle difference, or at least what we use to classify them, is the percentage of SiO2 (silica) but this accompanied with various differences in composition and thus resulting mineralogy. These differences in composition importantly cause differences in other properties like temperature of solidification, density, and viscosity. The ultimate cause for the differences in composition can be tied back to formation mechanism and location. This page from a course at Penn State goes through the basics of some of the physical differences in different types of magma/lava. |
Can other sources of light be concentrated through a magnifying glass to start a fire? Like the light from a flashlight, or from a full moon?
You can only focus light to reach the temperature of the light source. That is only true for thermal emission, not for light sources like LEDs (e.g. in a flashlight). |
What effects did the weight and size of a dinosaur have on the type of environment it needed to support it?
I am not 100% sure I used the right flair, if another would be more appropriate, please change it! I used to drive semi trucks, and one of our major rules was never leave a loaded trailer on anything but concrete, because it's supports would sink into the ground on anything else. In addition to that, I play and watch a number of scifi mecha type games, movies, and anime, and the discussion always comes up how mecha are unfeasible especially when they get big, because the ground wouldn't support their weight focused on where their feet were. That had me wondering if the huge multi-ton dinosaurs had similar problems. I am interested in hearing anything about how their mass/size/weight effected them.
Wow - this is a very cool question. I'm a geotechnical engineer, so I'm going to come at this from that perspective. A quick apology to most of the world, I'm going to do this in US Engineering units. I'll provide a couple conversions as I go. From a quick look at Wikipedia , it appears one of the heaviest known dinosaurs was Argentinosaurus huinculensis, clocking in at an estimated 212,500 lbs (96.4 metric tons). Spinosaurus seems to take the cake for bipedal dinos at 46,100 lbs (20.9 metric tons). So we have our upper-bound static loads. 212,500lbs/(4 legs) we get about 53.1 kips/leg (1 kip is 1000 lb), and 46,100lbs/(2 legs) we get 23 kips per leg. With these quick calculations, I can give a general answer to your questions. YES! Especially when you account for the dynamic effect of Dino movement, these are big loads. I can say I don't even need to consider foot-size and load distribution here. Your standard light-weight, single-story, pre-engineered steel building has maximum column loads of around 30 kips. Often I've had to recommend expensive ground improvement to support a building like that in delta deposits. A Titanosaur walking around with 50-something kip legs, applying dynamic loads to soil would settle a bit and would need to avoid some environmental settings entirely due to how much they would sink. That is my only conclusion from this quick look; that some environments (think river deltas, marshland, peat bogs, etc) would be incabable of supporting the weight of some of the very largest land dinosaurs. To try and determine just how limited they would be in terms of environment (what about loess plains, etc?) would require a lot more time, thought, and information. Like, do we have good information on what the gait of Titanosaurs looked like? Edit: Adding an edit to emphasize that the anecdote about a small building is not intended to compare the effect of prolonged static loading to a dinosaur walking around, but to give a sense of the magnitude of weight these guys were throwing around. |
Similar to the Chladni Figures, are there 1-dimentional nodes on the surface of the Earth during the earthquake, i.e. the lines (curves) of points that act like wave's nodes, with locally smallest amplitude?
Interesting question. My suspicion is probably not. Chladni patterns are standing waves because they are resonances on a plate, like a 2D analog of a vibrating string. Seismic waves, in contrast, radiate outward from the source and dissipate with distance, more like the ripples on a pond. |
When a doctor says to a patient that they have 6 months to live, how do they know?
Ultimately, what you are inquiring about is the prognosis. Once you have been diagnosed with a particular disease and it's severity, it is not too difficult to predict the outcome for you based on the statistical experiences of previous patients in your condition. Sure sometimes a patient that should die survives and vice versa, however these people are statistical exceptions and not the rule. |
At what point in time do we stop seeing wolf fossils near humans and start seeing "dog" fossils associated with humans?
Did dogs also go through intermediaries to transition between wolf and dog? Ie, we have H. erectus before H. sapien. Or are modern dog and modern wolves too similar ro be classified differently?
There's almost no chance wolves weren't a food source, especially from hunting. Realistically speaking they're less dangerous than a lot of large herbivores and if you've hunters capable of safely taking down deer, boar etc, you can handle wolves. And if you're following a herd of deer, you'll find wolves. |
What does it mean to be double jointed?
I've heard people being double jointed in wrists, thumbs and shoulders. Some people claim to be double jointed and are extra flexible in said region. I can twist my shoulders all the way around like in this video (that's not me). Is that double jointed-ness or something else? What is a double joint? Is it actually just a misnomer for something else?
'Double jointedness' refers to joint hypermobility. It can affect one, or multiple joints in the body and is characterised by an ability to extend or flex a joint beyond the typical range. Most commonly, hypermobility is caused by misaligned joints, abnormally shaped bones or some sort of connective tissue defect which results in weakened ligaments, muscles and/or tendons. It can also result from certain musculoskeletal injuries where healing does not restore perfect function (dislocations are a good example). Edit: Spelling |
On average, how far back do we need to trace an individual's family tree in order to find their most recent common ancestor with any other random person from the population?
I'm not sure why you think that statistics won't work in this case. Statistics never gives "the answer." It gives together with a range of confidence or probability. In this case a simple demographic model could be devised that provided an answer quite easily, but with a very broad confidence interval. On the other hand, demographic factors like overlapping adult generations, fluctuating rates of migration, and assortative mating can be included in much more complex models that will give better estimates. This paper develops population models that include several of these factors and finds the most recent common ancestor of all living humans was probably alive sometime between 3000 and 4000 years ago. This is not the question OP asked and I don't know of anyone who's attempted to answer that question, but it would involve essentially the same type of model. Taking the estimate from that paper (or even an estimate two or three times as old) and a basic understanding of the relatedness distribution of humans would yield an "average time to pairwise common ancestor" most likely less 1000 years. |
After a bone marrow transplant do you produce blood with a different set of DNA?
Yes definitely - and with this comes cells that may also react to "self" antigens in the new host so careful matching is required. Otherwise graft vs host disease may occur. On the other hand, in cases of cancer a tiny bit of this is OK as there may also be a graft vs cancer effect. http://www.ncbi.nlm.nih.gov/m/pubmed/19735262/ |
Why does air moved through a fan feel cooler?
Thank you for your submission! Unfortunately, your submission has been removed for the following reason(s): /r/AskScience /r/askscience For more information regarding this and similar issues, please see our guidelines. You can find the answer by searching for something like "fan cool air". Unfortunately, because of the high volume, a lot of great questions and answers can get buried quite quickly. The searchbar can be a great way of exploring /r/askscience and we encourage everyone to use it before posting. If you disagree with this decision, please send a message to the moderators. |
NPR's Science Friday wants you to "Ask a Quantum Mechanic"
Hey Reddit! Today at 3:10 Eastern Time, is talking quantum mechanics with MIT physicist and "Quantum Mechanic" Dr. Seth Lloyd. We'll open up the phone lines to hear from listeners, but we also want to hear from you. Ever wondered about a quantum internet teleporting data? Counterfeit-proof quantum cash? Or the welfare of Schrodinger's Cat (and kittens)? Now's your chance to ask those quantum questions. We'll pick a few of the best questions and ask them on the air. And please later today to hear the answers! I'll post a link to the live audio once the show is live.
First of all, let me welcome Science Friday and Dr. Seth Lloyd to reddit! I'd also like to clarify to the redditors here, the questions will be answered on the air. This is not an "AMA-style" session, so don't be upset if you don't get a response in this thread. Science Friday is just collecting questions to answer on the show :) To get things rolling, I have a couple of questions about quantum entanglement. It's often said that quantum entanglement is "spooky action at a distance", and "instantaneous". However, it is still impossible to transmit information faster than the speed of light. So: What is meant by "instantaneous" in this sense? What is being "transmitted" between entangled particles? |
Everything in space orbits something, right?
No, not everything orbits something else. The largest orbits in the universe are those of galaxies within galaxy clusters, but those take such a long time (billions of years) that they aren't consistent or stable. At the supercluster scale, things stop being gravitationally bound to each other. We're not bound to the Virgo Supercluster, and we aren't orbiting it. On very large scales, the universe is rotating, and the peculiar motion of galaxies is insignificant compared to the spreading out of the universe, which precludes large-scale orbits. Most astronomers think it's quite likely that there is other life in the universe. One of the fundamental ideas underlying modern astronomy is the Copernican Principle, which could be summarized as "We are not special". The last half millennium of astronomy has been a continuous fight against the idea that we occupy a privileged place in the universe. The discovery over the past several years that planets are extremely commonplace only further supports the idea that life probably exists other places besides Earth. |
why does number of protons affect chemical and physical properties of an element?
How can an atom change from Nitrogen to Carbon each with completely different physical and chemical properties, by changing one particle in the atomic structure?
The electron structure of an atom determines most of its chemical and physical properties. The number of protons determines how many electrons you'll find in a neutral atom. |
If a drop of some liquid was extremely hot (really really hot) could dropping it in say.. a large pool of cold water - theoretically,heat up the entire pool?
It depends how hot you want the pool to get (and how big the pool). Theoretically if there was a magic substance that could be extremely hot and yet remain in the liquid phase, then you could bring any size pool from 0 degrees C to 100 degrees C from a single drop of this substance. But in practice this would be impossible because there just aren't any substances (that I am aware of) that remain in liquid (or even solid) phase above a few thousand degrees. And, depending on the size of your pool, you would want your drop of liquid well above a billion degrees. : relevant youtube video of a red hot nickel ball in water. |
If I am malnourished and typically consume less than 800 calories on average a day, and then suddenly eat 7,000+ calories worth of food in one sitting, what negative effects could come from it?
You should rephrase the question to not make it about any individual. Note that the thread the poster linked to states what is and isn't medical advice. Not only are you asking "what would happen to if...", you are asking "Why haven't I pooped it out". So you are asking for a diagnosis, asking for the prognosis. Medical advice isn't limited to "what should I do." Asking "what is happening to " is as well. The kicker is that in your edit, you tried to say you're not soliciting medical advice, ! If you have a concern about your body, please consult your primary care physician. |
How exactly does the immune system ward off external parasites?
My curiosity was piqued by reading about mange in dogs, specifically quotes from this page about how a healthy immune system keeps mites under control: Demodectic mange most often occurs when a dog has an immature immune system, allowing the number of skin mites to increase rapidly. Source: Since mites are outside of the body and presumably too large for white blood cells or the typical immune system defense against e.g. bacteria anyway, by what mechanism exactly would it be affecting their reproduction?
I will write this in the context of Demodex mites infecting the skin since that is your interest, but these defense mechanisms broadly apply to other lung or intestinal parasites as well. I will go over a typical barrier response in the skin and then describe how defective or immature immune system could cause mange. First, a "fun" fact. Everyone has Demodex mites living on/in them right now. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3884930/ Here is a review on the immune response against canine Demodex: https://www.ncbi.nlm.nih.gov/pubmed/24910252 To begin, Demodex colonizes the hair follicles and glands in the skin. It is thought that this colonization of mammals has occurred for millions of years. Normally, the host immune system tolerates the mites since they really don't harm mammals in any way. However, even tolerated organisms are detected by the immune system. In the case of Demodex, it is specifically keratinocytes in the skin that sense Demodex. The keratinocytes bear special receptors called Toll-like receptors (TLRs). There are many TLRs that detect conserved molecules present on foreign organisms like bacteria, fungi, and parasites. For Demodex, the TLRs detect chitin which is not found on mammalian cells. Though they are detected, the immune system doesn't usually respond against the Demodex. Immune responses themselves can lead to tissue damage and since the Demodex is normally on the "outside" of the body it is seen as harmless and ignored. Cue tissue damage. At some point of the colonization a mite might damage the epithelial barrier of the skin while living in the hair follicle or gland. Tissue damage is a great stimulator of the immune system. The damaged epithelial cells in the skin begin relaying to the immune system that they are damaged. This is done via signaling molecules called cytokines chemokines. Innate immune cells, which are some of the white blood cells, are recruited first. These include neutrophils, eosinophils, and basophils. Other cells, like mast cells, were already present in the tissues and act as surveyors against infection. Some of these cells (mast cells, eosinophils and basophils) are specialized for fighting against large external parasites. The cytokine signals produced by the epithelial cells direct the immune system to what type of organisms is causing the infection. The TLRs mentioned above are what drive the distinction. Different receptors trigger different signals. For external parasites a Type 2 inflammatory response is produced. This is the same response that causes allergies and asthma. Once recruited the immune cells perform their function as directed by the cytokines and chemokines. For mast cells, eosinophils, and basophils they essentially come in and dump packages of highly toxic compounds onto the mites. These compounds are meant to kill the mites. Macrophages come by and clean up the dead stuff. At the same time the innate cells are doing their job, special cells called dendritic cells are picking up bits of the mites (mostly protein) and hauling them back to the lymph nodes. In the lymph nodes, special cells called lymphocytes are activated (these are the other component of white blood cells). Specifically T cells and B cells. These cells were previously generated by the immune system and are waiting to be activated by recognizing the small bit of material the dedritic cell brought back. This is very generalized explanation on this process. Essentially what happens next is you get expansion of the T cells and B cells that recognize the mite bits. The end result that is important in this case is production of antibodies by the B cells that are specific against some part of the mite. These antibodies are dumped into the blood, diffuse into the tissue where the mites are, and bind to them causing them to be further recognized by the immune system. The antibodies can get into the tissue due to leaky blood vessels and fluid build up caused by the cytokines mentioned previously. Specifically, the antibodies in this case will largely be IgE and IgG antibodies. The IgE antibodies happen to also bind to the mast cells I mentioned previously and act as a trigger mechanism. So if a mast cell encounters a mite that has damaged the skin it will cause the mast cell to activate via the bound antibody. In this way, cellular and antibody mechanisms will keep the colonization of Demodex mites in check by periodically eliminating some of the mites infesting the hair follicles and glands. So now on to the defective immune system. The immune response is a complex process that involves a lot of cell types, a lot of receptors, and a lot of signaling molecules. You could imagine how loss of a specific part I described above could lead to an absent immune response against the mites. This is essentially what happens. Dogs with defective immune systems are lacking a specific part of the immune system normally used to keep the mite colonization in check. Without a strong immune response the mites are able to reproduce to much greater numbers. The large increase in mites causes more tissue damage and the disease. Say for instance the dog has a mutant TLR receptor that can't detect chitin. The keratinocytes will be less effective at sensing the mites. Or if the dog has lymphocyte deficiencies. They cannot make T and/or B cells which ultimately lead to antibody production. The dogs no longer produce antibodies against the mites which normally bind to the mites or arm mast cells to be triggered by mites. |
How do lungs clean themselves?
Most of the larger particles are caught by nasal hairs, then the smaller particles that evade the hair are caught in the mucus produced in the respiratory passages (up till bronchi). Further ahead in the respiratory bronchioles and alveoli (the part where actual gaseous exchange takes place) there are no mucus producing cells, and the even more smaller particles such as bacteria that do manage to reach there are eaten up by alveolar macrophages, which are basically your germ eating cells. |
For an extinction event-sized meteorite (e.g. the one that caused the Chicxulub crater, ~10km), how long would the object be visible in the sky before impact?
It depends a bit on the composition of the asteroid (it's only a "meteorite" after it's survived landing on Earth). Some materials are more reflective than others. We call the proportion of light reflected away the . Earth's albedo is about 0.3, for example. Icy objects like comets have higher albedos, while rocky ones have lower albedos. Asteroids typically have albedos ~0.1, though it varies. It also depends on where the asteroid is coming from relative to the Earth-Sun system. If they are crossing Earth's orbit from within Earth's orbit going outside, there may not be much of its lit face towards us (like a crescent, or even new, moon). Lastly, it's going to depend on its shape and how much surface area is visible, and how it's rotating. If it's cigar shaped and we're looking down the long axis, we'll see relatively little reflected light. If it's oriented or rotating so that we can see the long face, we'll see more. If it's closer to spherical, of course, the orientation doesn't matter so much. There's a chart here of absolute magnitudes of different sizes of asteroids assuming a range of albedos from 0.25-0.05. Magnitudes are a logarithmic system, where 5 magnitudes is a factor of 100 in brightness. Higher magnitudes are fainter. The usually accepted limit of human vision in a dark sky is 6 magnitudes (in a city, it's much worse - the brightest stars are around magnitude 0, and in the middle of a city like NYC, I don't know if even they are visible). Using that chart, a 10km object at phase angle 0 and at 1 AU from both the Sun and the Earth is at magnitude 13, well below the threshhold of human vision (not hard for even a small telescope, though). To be detectable at even a dark site by eye, it needs to be >7 magnitudes brighter, which is about a factor of 1000. Since brightness goes as 1/d if we keep the distance between it and the sun the same, that means it needs to be ~30 times closer to Earth than 1 AU, which is about 5 million km. To start being really noticeable to people in suburbs or people who weren't really looking for it, it would take until it was about 1 million km away (which is when, , you might start to notice a new star in a constellation). Assuming a relative Earth/object velocity of 25 km/s (a typical meteor speed, can't be too much more than a factor of 2-3 different), that would give you a time of about 11 hours from when it was getting noticeable to a lot of people, or a bit over 2 days from when it was at the threshhold of human vision. Again, I want to stress how much that could vary with viewing angle of the asteroid, the angle at which the asteroid approaches Earth relative to the Sun, the particular asteroid's speed relative to the Earth, and how reflective that particular asteroid is. It could easily not become very visible until a couple hours before impact, or it could be noticeable in the sky for a couple of days. This exercise demonstrates the importance of early detection systems with telescopes, so we can detect Near Earth Objects and map their orbits decades before a potential collision with Earth. About 95% of NEOs have been found, going by the rate of new discoveries. Orbits can change, though, as they interact with the Earth-Moon system or other planets, and new objects can be launched towards Earth from Jupiter and it's moons or from the Kuiper belt. Comets, in particular, can change their orbit with every pass as ice evaporates as it passes the sun and makes little jet engines of outgassing. If you want to do anything about a collision, like have a tow truck or engineering its orbit by changing the albedo , it's vital to have telescopes dedicated to early detection and orbit mapping. |
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