Quantum mechanics and everyday nature

Question: Is there a phenomenon visible to the naked eye that requires quantum mechanics to be satisfactorily explained? I am looking for a sort of quantic Newtonian apple.
Comment: Can't post an answer but the rainbow reflections that you see on a CD are a good example. They are caused by interference between the light reflected from a 'quantum grating' - a surface which is periodically reflective and less reflective. Different frequencies interfere differently.
Comment: @jwg: There is nothing quantum-mechanical about the fact that a CD acts as a reflection grating. That's pure classical E&M.
Comment: Fire and stars have different colors at different temperatures. This is explained by the quantum mechanical explanation of blackbody radiation.
Comment: @BenCrowell, you are right of course.
Comment: Note: Please see the description of big-list:
DO NOT USE THIS TAG! "big-list" is applied to list-like questions which have no single answer. A few of these were asked early in the site's history, but they are no longer allowed.
Comment: "*I am looking for a sort of quantic Newtonian apple*", gravity can be quantified and predicted by relativity, but is not yet explained, "graviton", or any other satisfactory explanation is still to be discovered. We know how to accurately calculate the parameters of the fall, but almost 4 centuries after the "gravity idea", we still ignore why the apple falls.
Answer: Use a prism (or a diffraction grating if you have one) to break up the light coming from a florescent bulb. You'll see a bunch of individual lines rather than a continuous band of colors. This comes from the discrete energy levels in atoms and molecules, which is a consequence of quantum mechanics.
If the audience you have in mind is more advanced, you can present the ultraviolet catastrophe of classical mechanics. Classically, something with finite temperature would tend to radiate an infinite amount of energy. Quantum mechanics explains the intensity vs. wavelength curves that we actually see.
Comment: Or see reflection on CD/DVD.
Comment: Is it acceptable to describe the rainbow as a quantic phenomenon?
Comment: @user1975053: A rainbow can be more or less completely explained by water having an index of refraction that varies with frequency. Once you have this, no QM is needed to get a rainbow. I don't know if the dispersion of water is a QM phenomenon.
Comment: A diffraction grating doesn't give you a continuous band of colors when you look at a florescent light. It's line-dark gap-line-dark gap etc. That's the QM phenomenon.
Comment: I'm not convinced that spectral lines have to be quantum. A mechanical tuning fork has a mechanical response frequency and a pair of mirrors has an optical resonance frequency. Neither of those examples is quantum.
Comment: "*A rainbow can be more or less completely explained by water having an index of refraction that varies with frequency. Once you have this, no QM is needed to get a rainbow*" but the index of refraction itself cannot be explained by classical mechanics, it's [quantum mechanics](https://www.feynmanlectures.caltech.edu/I_31.html).
Answer: Reflections on Everyday Quantum Events
In one sense, it's hard not to see quantum mechanics in everyday life. For example, the existence of complex chemistry and the volume occupied by ordinary matter are both direct consequences of something called Pauli exclusion. That's a quantum rule that requires that every electron in the universe maintains a unique address that consists of its location in space (three numbers), its momentum (more-or-less velocity for same-mass electrons, and also three numbers), and one more odd one called spin orientation (binary, either up or down). When negatively charged electrons are packed tightly together by, say, the positive attraction of an atomic nucleus, these unique-address rules cause the electrons to take up unique positions and orientations around atoms (chemistry), and to resist being squeezed together beyond a certain point (volume).
Atomic bonding in chemistry -- without which we would not be here to discuss this! -- would largely disappear without that last odd address rule about up-down spins. The ability of two electrons to share the same space by having opposite spins gives some atoms the ability to steal an electron from other atoms by providing a cozy shared-address home away from home, an effect that in chemistry is called ionic bonding. In other cases the up-down pairing rule enables a pair of electrons to be shared equally by two atoms, which is also called covalent bonding.
Seeing is Believing
However, I think your question was really focused more on finding "a phenomenon visible to the naked eye that requires quantum mechanics," and that what you wanted was something a bit more profound and large than simply summing up the large-scale impacts of many very small quantum events. I suspect you were hoping for something you can see without your unaided eye, without the need for a lab filled with exotic equipment.
Such things really do exist. In fact, you very likely looked straight into an example just this morning. They are called mirrors.
That is, it the ability of polished metals manage to reflect beautifully accurate images of the worlds around them while most (not all!) other substances are dark, dull, or transparent, is a type of large-scale quantum event that is every bit as odd as exotica such as laboratory Bose condensates. It's a classic example of familiarity generating indifference: Metallic reflection is so common and easy to observe that we forget just how profoundly odd and non-classical it is.
Spacing Out, For Real
So why is metallic reflection deeply quantum?
It's quantum in multiple ways, actually. The first step is that you have to send a huge number of electrons into a sort of curious alternative form of space, one in which the coordinate systems for finding the an electron no longer consists of three directions of space, but instead must be expressed in three directions of momentum.
How can an electron possibly get "lost" in ordinary space? The way they get there is surprisingly uncomplicated and ordinary sounding: In metals, certain electrons are given the freedom to run around freely throughout the entire volume of the metal. That is, metal atoms are firm believers in a sort of community-wide sharing of some of their electron children, caring not in the least if their own electron ends up very far away indeed, as long as other electrons stay close enough to cancel out their positive charges.
A roaming electron does not sound all that unusual until you realize that electrons are so very light that quantum mechanics cannot be ignored. What quantum mechanics does to very light objects is cause their quantum descriptions to start taking up space across the entire volume of the metal over which they roam. That is, instead of an electron moving back and for the across a crystal as an massive classical object would, an undisturbed and freely roaming electron is most accurately represented as being equally located at all locations in the metal at the same time.
Try to pull that trick with your car!
What is Your Address, Please?
However, since  in any given piece of metal the super-light shared electrons are all doing the same "I am everywhere!" trick at the same time, there arises a problem with that address issue I mentioned earlier: Every electron in the universe must have a completely unique address.
If these lost electrons are all sharing space it the same chunk of metal, it means that they also are sharing essentially identical (even if odd) locations in ordinary space... and that simply will not do. It means that each such electron in the metal must find some new way to maintain a unique "address" within the universe. The up-and-down option helps, but only allows two electrons to share the same address. So, the only option left is for the electrons to start climbing into the only remaining set of coordinates, which is the diverse range of speeds and directions (velocities) called momentum space.
Now I should point out that when observing this process from our perspective of ordinary space with XYZ coordinates, electrons climbing into momentum space just looks like they are all acquiring different speeds, which doesn't sound all that exotic. But to electrons moving into momentum space, the view is very different indeed. Here's the main reason why: The electrons can actually bump into each other once they enter into momentum space, just like water molecules filling up a container in ordinary space. All of that bumping and jostling for momentum space forces the electrons to spread out and take up more room there, again in a way that is strikingly similar to how water molecules pile up in ordinary XYZ space.
Quantum Splish-Splash
In fact, the process of electrons jostling around and spreading out in momentum space is so similar to the way water molecules fill a container that such collection of electrons in metals are called Fermi sea. (An aside: Enrico Fermi must have had a really good press agent working for him, given all the cool stuff that is named after him in physics.) This type of momentum-space liquid even has a well-defined surface, just like an ordinary liquid.
However, recall that from our perspective in ordinary XYZ space, the electrons piled up in momentum space just appear to be moving at different velocities. This equivalence means that electrons closer to the surface of the Fermi sea in momentum space must necessarily also be moving faster in ordinary XYZ space. In fact, for a good conductor such as silver the electrons at the surface of the Fermi sea end up moving very fast indeed. Since speed for a small object is the same thing we call heat, just how hot (how hot) do these electrons end up being?
We're Feeling Hot Hot Hot
Well, if the electrons at the top of the Fermi sea in a large piece of silver suddenly lost all of their energy, it would be emitted in the form of X-rays. The burst would be so energetic than anyone standing nearby would be killed. That's pretty hot! Fortunately for jewelry wearers, this flatly cannot happen because all of those electrons lower in the Fermi sea refuse to budge. They really like their much cooler locations in momentum space, and they are not about to give them up!
Mirror Mirror On the Wall
Now it's time to bring all of this back around to your question of whether you can "see" quantum effects on the scale of ordinary life.
The quantum magic begins whenever you look into an ordinary mirror. As soon as you do, you are already gazing into a sea of electrons that from a quantum mechanical perspective don't quite exist in ordinary space. They are "lost" in the XYZ space we know best, a space in which their accurate quantum representations are in some cases as large as the entire surface of the mirror.
And most of those lost electrons are also hidden! That's because light that we see bouncing off a mirror comes from only a very tiny percentage of the Fermi sea electrons, specifically only the extremely hot ones at the very top of the Fermi sea. This is because they are the only electrons that have any "wiggle room" left to accept a photon and play catch with it.
What happens is this: An electron at the Fermi sea surface can accept a particle of light, a photon, and by doing so speed itself up just a little more. But unlike the electrons further down in the sea, when an electron at the surface speeds up it creates an "empty spot" in the Fermi sea. The process is closely akin to the ways a splash of water can rise up into air, but then realizes it no longer has any water below it to keep it supported. Unlike the water in the see, the splash above the surface is not stable: It has to fall back to the surface.
Very much like such a splash of water, an electron at the Fermi surface that has been "splashed up" by an incoming particle of light (photon) has no support underneath it to keep it there. So, it must fall back to the surface of the Fermi sea. As it does so, it gives up the photon energy that it held ever so briefly by re-emitting a nearly identical version of the photon it just absorbed. This re-emission of a photon from an electron at the Fermi surface is the smallest and most fundamental unit of reflection, the event from which larger-scale reflections are composed.
Simplicity from Complexity
Now the really neat thing about such re-emissions is that if your metal is smooth and consistent and polished on the surface, each such re-emission effect ends up being directed by the high symmetry of both the flat metal surface and of its smooth Fermi sea of electrons, causing the emitted photon (or more precisely, many photons interacting over the entire surface) to emerge in a very precise fashion that we call the angle of reflection. It's a case where a lot of complicated physics guided by even more complicated math ends up having a gorgeously simple outcome, and event we simply call reflection.
And most amazingly, that simplicity is deeply dependent on quantum effects that cross the entire mirror. It requires electrons that has collectively lost their way in ordinary space, and taken up refuge in a space that is not like the space we usually see, yet still allows them to bump into each other. They form a liquid in this peculiar momentum space, a sea that flips upside down our very understanding of what an "object" or "liquid" is and how it should behave. The tiniest sliver of these hidden electrons then wave back to us as they surf the surface of their hidden, showing off the incredible speeds they have reached by tossing photons back at us in a coordinate juggling act that we see as in the vivid brightness of a mirror, or the beauty of a sparkling ornament, or in a bit of bright silver or gold.
Finale: Take a Moment to Reflect
So, metallic reflection is a deeply quantum event, one that takes place on a human scale, and one that is uniquely beautiful and useful. If you find your universe a bit dull some mornings, take a moment to say hello to this lovely bit of quantum weirdness when you look into a morning mirror! And reflect a bit on your reflection to remind you how a remarkable universe we live in.
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Addendum 2015-06-20: Vision as Quantum Physics
I must add an example of large-scale quantum phenomena that is much closer to home than a mirror. It is the fact that you can see at all.
Lenses, including the ones in your eyes, are profoundly quantum devices. If it were not for quantum mechanical conversion of the large-scale shape of a lens into guidance for how microscopic particles of light (photons) travel, the lenses in your eyes would be as opaque as steel, and you would not be reading this text.
The problem is this: Since light is emitted and received as tiny particle-like units of energy, or photons, classical physics requires that these photons remain particles during their travels between those two points.
And that is a problem. After all, how does an electromagnetic photon travel through a lens full of electron-rich atoms that should bat it this way and that like a maze of mind-boggling complexity, let alone form an image? It might bounce around for a few moments in the outermost atomic layers of the lens, but it would have no chance of penetrating deeper before being lost or absorbed.
It is quantum mechanics that rescues us from the paradox of classical-photon blindness.
Mathematically, quantum mechanics allows a single photon to "explore" the entire shape of a lens through a process called the integration of all possible histories. This process makes no sense at all classically, since it is as if the photon had explored literally every possible path between its starting and ending points. Those virtual explorations are then added up in a special way to produce the wave function of the photon, which tells which bundle of paths is the most likely to contain the actual photon.
It is this infinite array of virtual photon paths that allows a single photon to "sniff out" the overall shape and form of a lens such as the ones in your eyes. Given the incredibly tiny amount of energy contained in a single photon in comparison to a huge, human-scale lens, that is a rather remarkable feat. It is roughly like taking a small penlight into orbit and "seeing" the shape of the entire earth by shining it onto the night side. Remarkably, every photon must do this on its own, since the result of shining every photon in a beam of light one-by-one through a lens is the same as what you get by shining them all at once.
The bottom line is this: Every single form of reflection, refraction, or transparency that you see using ordinary light is pretty much a miracle of quantum mechanics. None of those effects can exist without the photons "sniffing out" the large-scale shape of a mirror, lens, or window (which is really just a flat lens) in a way that allows them to ignore the incredible complexity of those objects, and focus instead on their overall shape and optical properties.
So how far do you need to go to see profoundly quantum effects in everyday life? Not far at all, for the very act of using your eyes to look for such effects is itself deeply quantum.
Comment: Wow. I know very little about physics beyond a lot of very general "that's something like this", but this answer is absolutely fantastic. Thank you for this fascinating glimpse into the incredible things we know about the world.
Comment: This answer is too good.
Comment: @TerryBollinger Excellent answer indeed. It reminds me about the "how does a magnet work?" someone asked to Feynman once : https://www.youtube.com/watch?v=wMFPe-DwULM . Of course quantum mechanics is at work in reflection. Yet reflection should appear sufficient for dealing with ... reflection ;-) Still, the Pauli principle is THE effect to mention indeed.
Answer: It's funny that you mention "the naked eye", because all you have to do is to close your eyes. As it turns out, the reason why we don't see anything when we close our eyes is quantum mechanics.
Sean Carroll explains it nicely: There is a lot of black body radiation in the infrared range inside your eyes. Even though the total energy of this infrared light is much higher than visible light that enters through our lenses, it isn't absorbed by the receptors, because according to quantum mechanics it can only be absorbed in quantized packages (photons). And each individual photon doesn't have enough energy to be absorbed.
Comment: Whilst the electronic transition of the chromophore molecule does require a specific band of energy, this is not the only mechanism for isomerisation. The other major source is thermal (transition energies of infrared radiation). Our eyes are sensitive to it, but not very. Most thermal fluctuations do nothing, and there are many more due to internal processes than external IR. They eye itself absorbs a lot of incoming IR and to observe a difference one would need to have enough infrared radiation to significantly warm your retina (quickly). We don't see IR because of the signal/noise ratio.
Comment: Point is, there is plenty of isomerisations caused by the blackbody radiation, it's just background that we filter out.
Comment: Also, this is basically why colour vision doesn't work at night.
Answer: Magnetism is a nice example, you can explain the spin-spin alignment only with quantum mechanics (see exchange interaction), it is even possible to prove the Bohr-Van leeuwen theorem, which states that no classical theory can explain how a magnet works.
Reference: Feynman's Lectures on Physics
Comment: The problem is that the macroscopical effects of magnetism are perfectly explained by Maxwell's equations, in a classical framework. What needs quantum mechanics is its microscopical description, that is not a 'naked eye' thing. Of course your answer is OK, but in my opinion is not really convincing. We should find something that had no explanation before QM.
Comment: @Bzazz The microscopic origin of magnetism was not a problem in classical physics in the sense that it was unable to speak about _why_ materials are magnetic - they just "were" magnetic. I hardly think that counts as "explaining".
Comment: As a logical extension to this, you need QM to explain why a paper clip is attracted to a magnet.
Comment: I don't see how QM explains gravitation, maybe you should share your discoveries with the world, however, I prefer discussing about physics rather than philosophy, I've only pointed out an ubiquitous macroscopical phenomena that is completely quantum mechanical
Comment: By this logic, the naked eye phenomenon of gravity also requires quantum mechanics because the classical framework only gives us non-explanatory cruft like $\frac{gmM}{r^2}$, and doesn't explain *why* there is gravity associated with mass.
Comment: @Kaz You are right: the classical framework does not _explain_ gravity. Neither does current quantum theory, either! _That_ is the main drive in fundamental theoretical physics today. (Heard of quantum gravity? string theory? GUTs? TOEs?)
Comment: I agree with @Emilio.  This is probably the best example, not sure why it's not at top
Comment: @Bzazz Ferromagnetism, diamagnetism and paramagnetism had no explanation before QM, yet its effects are far from microscopic. Yes, you are right, we don't "see" QM with naked eyes, but you don't see gravity either.
Comment: To add to the original answer: magnetism is a _relativistic_ quantum mechanical effect - this is what you deal with when attaching a magnet on the fridge :)
Comment: This example is misleading - there is both macroscopic explanation of magnetism in terms of EM theory (Lorentz force, Maxwell's equations) and microscopic explanation of magnetism (with so named tools in the  microscopic EM theory of particles or with terms of the simplified model based on Hamiltonian for long-range interaction of magnetic moments. Spin-spin interaction in quantum theory is formally similar to the latter).
Answer: You and your environment still exists! If it was not for quantum mechanics everything would spontaneously disintegrate, as atoms are not stable in Classical mechanics, due to radiation emitted by an accelerating electron.
Comment: @JánLalinský In general all accelerating electrons would radiate as far as I understand. While I cannot do a detailed calculation to say that all matter is unstable I would presume it would be true classically.
Comment: Wonderfully pithy! So obvious and yet I never thought of it.
Comment: The collapse of classical atom Bohr talked about is valid only for isolated system with retarded fields. In reality, atoms are not isolated but subject to EM fields of other atoms and to external background EM radiation, so the premises of the collapse argument are not really plausible in classical EM theory.
Answer: The definitive, visual proof that quantum mechanics is required to describe our world is the observation of superfluidity in liquid helium that has been cooled to below the lambda point. Below this temperature (2.17K at STP) a macroscopic fraction of the atoms have condensed into the ground state. This leads to macroscopic correlations that cause the fluid to flow in highly non-classical, unusual ways. For example, the fluid can flow up (against gravity) the sides of it vessel to a nearby reservoir:

In a more elaborate set-up, we can see the fountain effect:

I find this to be the most convincing, visible phenomenon that requires QM.
Comment: "*For example, the fluid can flow up (against gravity) the sides of it vessel*", said like this, this could be explained by "*[Capillary action](https://en.wikipedia.org/wiki/Capillary_action) (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces like gravity.*"
Comment: Sure. The implication here (not obvious to some, I guess) is that "flow" means, unlike in capillarity, that it keeps going.
Answer: This video and this article both show you how to make a quantum eraser experiment at home using only a laser pointer and some polarising filters, which in the video are obtained from 3D glasses*.
One could argue that this doesn't really count, since if you really think about it, you should expect the same result if light were just a wave. However, if you accept that light is photons then it very nicely demonstrates that the interference pattern disappears if there's a way to know which path the photon took, which is a very distinctly quantum phenomenon.
(*) In my experience, most 3D glasses tend to have circular polarising filters rather than linear ones. This doesn't seem to be addressed in the video, but it probably changes what you'd need to do in order to see the result. However, I have used at least one pair with linear filters, which were from an IMAX cinema.
Comment: I think QM is not really needed to explain their experiment. Wave nature of light too explains it well. In order for two waves to form constructive or destructive interference their polarization should either be random or (anti/)parallel.
Comment: @user10001 I know, that's what I meant when I said "one could argue that it doesn't really count, since if you really think about it, you should expect the same result if light were just a wave." (Though I suppose I could add that QM *is* required to understand why the laser pointer works!)
Comment: Sorry, I read only first line :)
Answer: Among the naked-eye visible effects that require a quantum explanation are fluorescence, phosphorescence, and electroluminescence. Concepts like band gap energy and the connection between energy and wavelength are needed to form plausible, detailed hypotheses which address the readily observed aspects of these phenomena.
Answer: The transparancy of glass is a quantum phenomenon. It is owed to the fact that the electrons in the silicon crystals require an inordinate amount of energy in order to get exited into a higher orbital. This means that low energy photons like visible light can pass through unhindered. Meanwhile, UV light has enough energy to be absorbed.
Glass is transparent, but you don't get a sunburn.
Answer: As an illustration of Dan's answer I get myself a diffraction grating and start looking at florescent lamps. I was frankly really disappointed because you really don't get to see the lines clearly with a grating alone. What you would see is more or less like this

But I moved on and told myself that I can make myself a spectroscope and I did just that! I had a big poster and rolled it to a tube. From a brochure I cut myself a thin slit and taped them together.

It didn't take 10 minutes but the result was really satisfactory:

I was also able to see some of the Fraunhofer lines while I was looking at the sun at 1 or 2 hours before sunset but I couldn't take a photo of it because of my poor cellphone camera.
I hope this helps illustrating Quantum Mechanics from daily life with the help of daily objects!
Answer: If you are looking for something visible to the naked eye, but that not necessarily happens naturally around you, then the quintessential experiment displaying the quantum nature of light in a way that is visible to the naked eye is Young's double slit experiement.
The great thing about this experiment is that it can easily be performed in the comfort of your own home.  See this Physics.SE post: Is it possible to reproduce Double-slit experiment by myself at home?
Comment: Young's experiment actually just demonstrates wave nature of light. If you wanted to show its quantumness, you would need to show wave-particle duality using single photon sources and single photon detectors which I doubt people usually have lying around.
Comment: Sure, you can explain Young's experiment with macroscopic light intensities using quantum physics. But the question asks for phenomena that can be explained **only** using quantum mechanics. Therefore you need single photons interfering to have the need to turn to quantum mechanics. Otherwise, you do not even need Maxwell's equations and are perfectly happy with a wave-optical explanation.
Comment: @OndřejČernotík I respectfully disagree with your characterization. The mere fact that (as you point out) we *can* shoot individual photons at the screen shows that the electromagnetic wave model of light is deficient.  Once we note this and perform the experiment with a stream of particles, we are forced to invoke quantum mechanics to explain the resulting interference pattern.
Comment: @OndřejČernotík Here is my point: once you accept that a laser beam consists of a stream of photons, how do you explain the interference pattern without quantum mechanics?  I agree that if the only experiment we ever did with light were the non-single-particle-at-a-time experiment, then the non-quantum wave model would be sufficient.  If a student were to ask me whether the double slit experiment could be satisfactorily explained without quantum mechanics, I would feel like I were not being totally honest if I were to say yes.
Comment: but @joshphysics accepting the existence of photons in a beam of light is a circular argument, because a photon is the quantum of light.
Comment: @joshphysics I agree with you that once you accept that light beam is made of particles, you need quantum physics to explain Young's experiment. But the problem here is the acceptance bit. If you try and explain this to someone who knows nothing about quantum physics, they'll ask why you assume light is made of particles and how they can see it. So you get nowhere and still need a convincing everyday phenomenon that illustrates the importance of quantum physics.
Comment: (This probably depends on educational background. Some people 'know' from school that light is made of photons, but don't care about / have trouble understanding quantum phenomena. Anecdotally, I became really curious about QM after reading about the double-slit experiment, and consequently about delayed choice quantum eraser, on Wikipedia. – speaking from layman's perspective)
Comment: @joshphysics, note that laser beam does not consist of single photons even in quantum theory of light. It is often modelled by coherent states, which are far from photon states and rather mimicking classical EM field.
Answer: One very "simple" example is that of the reason why we are not going through the ground while atoms (and hence matter) are mostly empty.
Although it is still debated which dominates in this effect between electrostatic repulsion and the so called Pauli exclusion Principle (a quantum effect), it is pretty much admitted that electrostatics only is not sufficient.
Quantitative estimates of this quantum repulsion are done on a daily basis by people calculating ab-initio (solving equation of quantum mechanics for the electrons) intermolecular potential parameters to be put then in simulations at the molecular scale (basically these calculations explain why it is almost fair to represent atoms as hard beads and therefore they already explain how come two empty atoms cannot overlap).
Another simple case is that of a piece of metal whose rigidity (or at least some measure of it) owes a lot to the exclusion principle (see Eq. 494 of the link and the following sentence) satisfied by the conducting electrons in the system.
Answer: Superconductivity of course. Classically, you cannot explain perfect diamagnetism and perfect conduction from a disordered system at the same time.
The most striking experiment is the levitation of a superconductor on a magnetic field, aka Meissner effect. You just need a high-Tc superconductor and a few N-liquid. The stricking fact is the disapearance of the effect when the nitrogen entirelly vaporized.
A lot of videos on internet about that. See e.g. this one: http://www.ted.com/talks/boaz_almog_levitates_a_superconductor.html
By the way, superconductivity is THE demonstration of quantun weirness at the macroscopic level.
Quantum Hall effect is an other interesting effect, but it requires more materials (fridge, ...).
In general, one can safely say that almost (if not) all the true effects of quantum physics are matter-field interactions of some types... the Meissner effect and the quantum Hall effect are just two specific matter-field interaction (magnetic field applied on low temperature collective excitations of electrons).
It should in principle be possible to measure the spectrum of some atoms in a table-top experiment (after all, it's a late 19-th experiment), but it's less impressive than levitation I believe. All spectroscopy properties can only be perfectly understood using quantum mechanics, and can well be "seen" by naked eyes, like fluorescence (it sometimes requires IR glasses, but still I believe it's naked eyes since you can really see the fluorescence using this glasses).
Generically, all condensed matter problems require quantum mechanics to be perfectly understood: band theory (including band gap and crystal symmetry leading to the huge field of semi-conductor for instance), electronic  propagation in disordered system (including Mott insulator for instance, ... (see also Kaz's answer https://physics.stackexchange.com/a/65464/16689 on this point).
Tunnel effect can be thought of a striking effect of quantum mechanics, even if it is hard to see with naked eyes. See nevertheless jinawee's answer https://physics.stackexchange.com/a/65416/16689 on this page.
Comment: Although a bipoloar junction transistor may require the tunnel effect to work, it is accurately described by the Ebers-Moll equations that do not make a direct reference to quantum mechanics. At some level, everything requires quantum mechanics in order to *work*, even a 100 kilogram mass oscillating on a big steel spring.
Comment: @Kaz Thanks for your remark, I modified the answer accordingly.
Answer: You also need the tunnel effect in order to explain many aspects of electrical conductivity. For example, why oxidized copper wires are still well conductors instead of insulators.
Another interesting quantum mechanical effect is produced in photosynthesis. The process is called "hopping" and it happens when a chlorophyll absorbs a photon and then it emits an exciton that will propagate until it reaches a special type of chlorophyll molecule, which produces an electron transfer. There are some references like: http://www.chemphys.lu.se/old/Archive/annual_96/primarynew.htm .
There is also the hypothesis that quantum entanglement is produced in certain birds to allow navegation. See: http://prl.aps.org/abstract/PRL/v106/i4/e040503 .
Answer: In some sense all chemical reactions are fundamentally quantum mechanical, but in the case of chemiluminescence and related atomic light-emitting phenomena like the aurora, quantum physics enters another way: excited states of the molecule or ion can only decay and emit a photon because the electron in that state is being continually shaken by vacuum fluctuations, microscopic fluctuations in electric fields due to quantum uncertainty.
Answer: The sun is visible to the naked eye. The only reason the sun shines is quantum-mechanical tunneling. Without tunneling, fusion reactions would be impossible at the temperature of the sun's core.
Comment: But note the Sun would still shine without nuclear reactions - if it was hot for other reasons, like gravitational collapse.
Answer: Stuff it explains --

Normal force
Electric conduction
Why some atoms are stable (motivating issue)
Zeeman effect (motivating issue)
Why the universe isn't just a continuous cloud of matter
Why are masses discretised (the difference between the masses of two particles)
Much of chemistry
Comment: Friction is not particularly quantum, and "electricity" is too vague, if you mean "electrical conduction", yes, true. But the rest are good.

Quantum mechanics and everyday nature


Size of a square in a square

Question: Given a unit square (blue in the picture), pick a point on one edge and label it A. Label the distance from the nearest corner to A as x. Pick one of the corners opposite A and label it B. Call the remaining edge C. There is a unique square with one corner at A, one corner on edge C and with the remaining two corners forming an edge passing through B. What is the area of the new square in terms of x?

(I haven't been able to work out a complete answer to this yet.)
Comment: It is more a mathematical question than a puzzle, unless there is a clever shortcut that solves it without much calculations.
Comment: pretty good question. thanks for it. I had a good time solving, putting into latex and so took more time to solve it :P
Comment: @FlorianF Yes, and the solutions so far are all tedious algebraic calculations.
Comment: @hexomino Not so much solving the quadratic as setting up the equation and doing the algebra.
Comment: Wow. Three very different and clever solutions. I'll wait one day to give others a chance, but it's going to be tough to pick an accepted answer.
(I did consider posting this on the math exchange, but it seemed more like a puzzle than a simple question.)
Comment: @xnor I'm not sure I would consider solving one quadratic equation as tedious.
Comment: Oh no, probably the first puzzle on Puzzle.SE I could have solved, because it's math and not some strange language stuff. But it already has answers.
Comment: Man, it's still super close. I'm going to have to give it to @Oray though as both the (barely) oldest and (tied for) highest voted.
Comment: Weirdly proud I found... something resembling a solution... using only early highschool math.
Answer:
As this tessellation of the tilted new square
neatly matches an overlapped tiling of the unit square,
the $w{\small\,\times\,}x$ overlap of unit squares equals
the $1 \! - r^2$ difference in the areas of the two types of squares.

 \begin{align}    w \!\;x  ~=~ & 1 - r^2 \\[.5ex]    \Longrightarrow~~                  x(1{-}w) ~=~ & r^2 \! - y   \kern3em (~ y = 1-x ~) \\[-1ex] \\                    r^2 ~=~ & y^2 + (1{-}w)^2 \\[.5ex]          x^2 r^2 ~=~ & x^2 y^2 + (r^2\!{-}y)^2 \\[.5ex]    \Longrightarrow~~                        0  ~=~ & (r^2)^2 - (x^2\!{+}2y)(r^2) + y^2 \!{+} x^2 y^2 \\[-1ex] \\ \textsf{area of new square}           ~=~ & r^2 \\[.8ex]  ~=~ & x^2\!{+}2y ~\pm\sqrt{ x^4 \!{+} 4 x^2 y {+} 4y^2                                     {-} 4y^2 \!{-} 4 x^2 y^2 } \over 2 \\[.8ex]  ~=~ & x^2\!{-}2x{+}2 ~\pm\sqrt{ x^4 \!{+} 4 x^3 (1 \!{-} x) } \over 2 \\[1.5ex] ~=~ & 1 - x + {x^2 \over 2} \left( 1 + \sqrt{{4\over x}-3} ~ \right) \end{align}

(The "$ \small\pm $" was deduced to be
"$ \small +  $"
by testing the formula on the case where
$    x   = \large{1 \over 3}       $,
$ \theta = 45^\circ                $,
$    r   = \large{2\sqrt2 \over 3} $  and
$   r^2  = \large{8 \over 9}       $.)
Here are some tiling experiments,
beginning with the easiest-by-hand 45° case,
that led to selecting the straightforward version presented.
The 45° case's symmetry naturally created some fun red herrings.
Comment: Now that's an answer!
Comment: Very cool answer!
Answer: Here is the answer:

 $\frac{(2 - 2 x + x^2 + \sqrt{4x^3 - 3 x^4})}{2}$

More is coming!
Here is our original diagram completed with

 the biggest square that I will show it will be square

for sure later;

the length $|DE|$ is our $x$ and let's put some specific angle that we are going to work with in our main square as below;

I call the length of side of the other square as $y$ and as you can see

from the $\alpha$ values, right triangles, and hypotenuse as $y$ all four right triangles in the biggest square is the same triangles. I do not want to get into much detail since it is kinda obvious. ($\Delta {GEI}$,$\Delta {EFC}$,$\Delta {FHK}$ and $\Delta {HGJ}$)

so we now know that;

 $ICJK$ is a square and I will call the side length of that biggest square as $z$ from now on.

And Let's zoom where we want to focus and put our known equations;

Method 1
We know that from the figure above;

 $tan(\alpha)=\frac{z-1}{x}=\frac{1-x}{z-1+x}$

from here we solve $z$ as;

 $z=\frac{\sqrt{4 x - 3 x^2}-x}{2}  + 1$

then using the equation below

 $(z-1+x)^2+(1-x)^2 =y^2$

changing z value in terms of x later;

 $(\frac{\sqrt{4 x - 3 x^2}+x}{2})^2+(1-x)^2 =y^2$

simply we find $y^2$ which is the area of the square we are looking for;
Method 2

 $\cos{\alpha }=\frac{z+x-1}{y}$

 $\sin{\alpha }=\frac{1-x}{y}$

 $\cos{\beta }=\frac{1}{\sqrt{1+x^2}}$

 $\sin{\beta }=\frac{x}{\sqrt{1+x^2}}$

and we  know something else from sinus;

 $\sin({\alpha+\beta})=\sin{\alpha}\cos{\beta}+\sin{\beta}\cos{\alpha}$

and we also know that;
5.

 $\sin({\alpha+\beta})=\frac{y}{\sqrt{1+x^2}}$

if we combine these without using $1$ and notated that as $\cos{\alpha }$ alone we get;

 $\sin({\alpha+\beta})=\frac{1-x}{y}\frac{1}{\sqrt{1+x^2}}+\cos{\alpha}\frac{x}{\sqrt{1+x^2}}=\frac{y}{\sqrt{1+x^2}}$

as a result;
6.

 $\cos{\alpha }=\frac{y^2-1+x}{xy}$

and using 6. and 1. we are going to figure out what is $z$ in terms of $y$ and $x$ as below;
7.

 $z=\frac{y^2-x^2-1}{x}+2$

so let's find our y value using these knowns;

 $(z-1+x)^2+(1-x)^2 =y^2$

then put our new z value in terms of x and y as in 7;

 $(\frac{y^2-x^2-1}{x}+x+1)^2+(1-x)^2 =y^2$

and solve for $y^2$ which is the area of the new square we are trying to find;

 $Area=\frac{(2 - 2 x + x^2 + \sqrt{4x^3 - 3 x^4})}{2}$

and z in terms of only x becomes;

 $z=\frac{\sqrt{4 x - 3 x^2}-x}{2}  + 1$
Comment: Yessss I simplified my answer and it comes to the same as yours. Hurray, we got it!
Comment: I think you lost an x in your final Area equation, comparing it to your answer from Method 1.
Comment: @JoshuaTaylor nope, I dont think so, both equations must be equal to each other when simplified. you may try wolfram alpha ;)
Comment: Oh, I see. The powers under the square root are different.
Answer: Firstly let's label everything:

We have five similar right-angled triangles, which must be enough to get a lot of information about the interrelationships between the quantities $x,y,d,e,f,p,q,r$.

 The $p,d,x$ triangle is similar to the $1-d,y-p,r$ triangle. Compare the hypotenuses and longer adjacent sides to get $$d-d^2=py-p^2.$$

 Use Pythagoras's theorem in the $p,d,x$ triangle to get $x^2=p^2-d^2$. Combining this with the relation from point 1, we find $$x^2=py-d.$$

 The trapezium with parallel sides $d,e$ and perpendicular side $x+(1-x)=1$ has areas $\frac{d+e}{2}$. But it's also three right-angled triangles together, so it has area $\frac{py}{2}+\frac{dx}{2}+\frac{(1-x)e}{2}$. So we have $d+e = py + xd + (1-x)e$. Using $py=x^2+d$ from point 2, we find $d+e=x^2 + d + xd + e - ex$, which gives $0 = x+d-e$ therefore $$d = e-x.$$

 The $p,d,x$ triangle is similar to the $y,1-x,e$ triangle. Compare the non-hypotenuse sides to get $x(1-x) = de$. Using the relation from point 3, this means $x(1-x)= (e-x)e$, which gives a quadratic equation for $e$ in terms of $x$. Solving this, and using the fact that $e>x$ to know which root to sue, gives $$e = \frac{x+\sqrt{x^2-4(1)(x^2-x)}}{2}.$$

 Finally, use Pythagoras's theorem in the $y,1-x,e$ triangle to get $y^2=e^2+(1-x)^2$. Since $y^2$ is exactly the area of the new square, and using the relation from point 4, we get that the area of the new square is $$\left(\frac{x+\sqrt{4x-3x^2}}{2}\right)^2+(1-x)^2.$$

Simplifying this expression, the final answer is

 $$\frac{x^2+(4x-3x^2)+2x\sqrt{4x-3x^2}}{4}+\frac{4-8x+4x^2}{4}$$ $$=\frac{x^2-2x+2+x\sqrt{4x-3x^2}}{2}$$
Answer: I think this matches up with the other two solutions but uses coordinate geometry which is quicker here.

 Let $B$ be the origin in the Cartesian plane $(0,0)$ then $A$ is located at the point $(x,1)$.
 Let $C$ be located at the point $(1,y)$. I will designate the Cartesian coordinates as $(X,Y)$ so as not to confuse with $(x,y)$ above.
 The line $AC$ is given by the equation $(y-1) X  + (x-1)Y + (1- xy) = 0$.
 This means that the perpendicular distance from $B$ to $AC$ is $\frac{1-xy}{\sqrt{(x-1)^2 + (y-1)^2}}$.
 Also the distance between $A$ and $C$ is $\sqrt{(x-1)^2 + (y-1)^2}$.
 In this instance these two quantities must be the same so we set them equal and solve for $y$. $$\frac{1-xy}{\sqrt{(x-1)^2 + (y-1)^2}} = \sqrt{(x-1)^2 + (y-1)^2} $$ $$\Rightarrow 1 -xy = (x-1)^2 + y^2 -2y + 1$$ $$\Rightarrow y^2 + (x-2)y +(x-1)^2= 0$$ and this means that $$ y = \frac{(2-x) \pm \sqrt{(x-2)^2 - 4(x-1)^2}}{2}$$ Note here that the plus sign puts $y>1$ so we take the minus sign. Then the area of the square is just $$(x-1)^2 + (y-1)^2 = (x-1)^2 + \left(\frac{x + \sqrt{(x-2)^2 - 4(x-1)^2}}{2}\right)^2 $$
Comment: Your solution matches up with mine and Oray's, but I still need to read this and understand the method :-)
Comment: @Randal'Thor, this is the brute force tool in the geometry toolkit :)
Answer:

 area $F(x) = (1 - x)^2 + \left({x \over 2} + \sqrt{x - {3 \over 4}  x^2} \right)^2$

 ${d \over dx} F(x) = 0 \implies x = {1 \over 3} \implies F( {1 \over 3} )$ is minimum.

 Be $\vartheta$ the angle between AC and the horizontal,
 then $\vartheta(x) = \arctan \left( { {x \over 2} + \sqrt{x - {3 \over 4}  x^2} } \over {1 - x} \right) \implies \vartheta({1 \over 3}) = 45°$
Answer: I tried this problem for fun, but got quite a different answer than the others. Posting it for commentary, and educational purposes.
Definitions:

Points A, B and C and length x as described in the problem
Point D, diagonally across from B on the original square
Point E, top left of the original square
Length y (also: AC), side length of the created square

First, I bisected the quadrangle ABCD along the line AC.
The area of the triangle ABC is simple, as both the base and height are y. Therefore, the area is y^2 / 2
The area of ACD is the same base (y), but the height is sqr(2)-y. The diagonal of the original square, minus the height of the other triangle. (y*sqrt(2) - y^2)/2
This means that the area of ABCD is y*sqrt(2)/2
The area of ABD is half the square, minus ADE. In other words, ABD = (1-x)/2, or AD/2
By the same logic, we can say that the area of BCD is CD/2, we just don't quite know what CD is, yet.
Knowing the area of ABCD and ABD, however, BCD = y*sqrt(2)/2 - (1-x)/2. This means that CD = y*sqrt(2) - (1-x)
The Pythagorean triangle ACD is of course y^2 = (1-x)^2+(y*sqrt(2) - (1-x))^2
That's a bit of a garbled mess, but if we work it out (it's just calculation by this point) we find that y = sqrt(2)(1-x)
Finally, we go from a length to a square, so the area is 2(1-x)^2

Mind you, this is clearly wrong, as it implies AD = CD... which in many cases can't possibly be true... I fear I may have made a mistake in calculating the area of ACD, that's the only place where the error would be like this.

Size of a square in a square


Why comments and not answers?

Question: Why is it that so many people leave answers in the "comments" section, as opposed to the "answer" section? (You may leave your answer in either section )
Comment: if I don't have time to formulate a quality answer (sources, quotes, etc) I will leave it as a comment
Answer: I leave comments that are undeveloped answers so others can build on them to make an answer if they want.
Answer: If I don't have an answer at all but have some information that can lead to an answer, I will often post it as a comment.
Answer: Mi Yodeya has very strict quality standards. When I have an answer which is not of high-enough quality - for example, it lacks sources - I often post it as a comment. When I do post it as an answer, it gets downvoted quite quickly, or attacked. I wish the site was friendlier... But as it is, I rather post it as a comment then get negative feedback for posting it as an answer.
Comment: If something's worth saying, it's worth saying as well as possible and then trying to improve.
Comment: Negative feedback helps you improve ideas by showing problems. You wouldn't want a Chavrusa with someone who always just said "You're right!" How dreadfully boring. (And inaccurate as none of us is always right. One shouldn't be afraid to find out one is wrong, especially on a site like this where all one has to lose are magical unicorn points.)
Comment: @Fred The other answer to this question discusses contributing underdeveloped ideas for consideration. That answer has 9 upvotes currently. Knowing that one's ideas are half-baked and acting accordingly is healthy and productive. It's thinking one's answer is good and then being upset that others differ that belies an issue with either you or everyone else.
Comment: LN6595, I don't understand what you mean by "if we could." Are you contending that @DoubleAA or others would delete your post? I claim that that's false. Do you mean "if we could without receiving any requests for improvement"? I agree with DoubleAA that requests for improvement are a good thing and should be welcomed by anyone who cares to write in public in the first place. || What debate are you talking about? Can you point to a Meta answer that says that sources are (or should be) required? Can you point to one that says that they aren't (or shouldn't be) preferred?
Comment: Are you, in fact, claiming that a post that makes assertions implicitly based solely on personal knowledge would not be more useful, and therefore should not be considered more valuable, if sources are added? Because otherwise, I don't see any disagreement here about policy or even standards at all.
Comment: @LN6 I dont downvote posts just for having no sources. You are making that up. If a post doesn't have sources, I might downvote it if I, a random internet user, think the answer provided by another random internet user is wrong. My word vs theirs. (There are unfortunately lots of users out there who think they are smarter than they are, including many who claim "rich store[s] of Judaic knowledge accumalated...over...decades of study") Are you even reading the meta posts Isaac linked for you? I state this explicitly there yet you continued to misrepresent me and the community in general
Comment: @IsaacMoses If someone has no time constraints, that's true. I agree that posting a bad answer as a comment solely to evade downvotes is inadvisable, but posting something useful as a resource so that others can improve the question or write good answers is a good idea if you don't have the time, inclination, or wherewithal to write a proper answer yourself. Incidentally, it's possible that a useful, answer-like comment would be downvoted if posted as an answer because it's either missing a reference, is only a reference, or is so bare-bones that it would be wanting as an answer.
Comment: For information about the strictness of Mi Yodeya's answer-quality standards, particularly with respect to presence and quality of sources, please see [these](http://meta.judaism.stackexchange.com/questions/712/better-to-post-an-answer-with-no-source-or-not-to-post-at-all) two [Meta posts](http://meta.judaism.stackexchange.com/questions/1444/) .
Comment: @DonielFilreis See my user profile... (But yes that was the reference.)
Comment: @DoubleAA yes, the Resh Lakish, Rabbi El'azar's Rabbi Yochanan problem. But here he want to say that the answer is not ready and he want to help despite that
Comment: Note plenty of answers on this site are highly upvoted and lack any sources. But if you don't bring sources, you better be sure you are 100% right and no one argues.
Comment: @DoubleAA Per the reasoning in my above comment, I voted for the Yishai's answer (post undeveloped answers as comments to help other people answer). I'm a bit less clear on this answer, which seems like it could be interpreted in a couple of ways: 1) Post potentially unpopular, unsourced answers as comments to get your opinion out there while dodging downvotes. 2) Post answers without sources because, while they aren't good enough as answers and will be downvoted, they are still useful as comments that could improve the question or lead to good answers (similar to Yishai's answer)...
Comment: @DoubleAA I think #2 is appropriate behavior, regardless of whether the specter of downvotes is motivating that behavior. However, if someone is posting unsupported comments that they know the community overwhelmingly thinks are wrong (per #1) (e.g., `The real answer to your question is that Judaism is bad, and Jews are too stubborn to care!` - obviously, LN6595 wouldn't write anything like that), then that's not appropriate commenting behavior.
Comment: @IsaacMoses That gets to the heart of the problem. If you look at this answer http://meta.judaism.stackexchange.com/a/824/6599 you will see that posting answers without sources is hotly debated, with nearly equal upvotes and downvotes. Some of the big users, specifically Double AA who has made his opinion known here, are sticklers for quality and will comment and dovnvote a post that does not meet their quality standards...
Comment: ...But users like me and those who voted for my answer here, agree with Yirmiyohu in http://meta.judaism.stackexchange.com/a/824/6599 and would be happy to post away, drawing on our rich store of Judaic knowledge accumalated sometime over our decades of study - if we could.
Comment: @DoubleAA R' Yochanan would agree with you (BM 83a). He died as a result of someone always telling him he's right.
Answer: I will usually leave a comment when I have no formal source for it, as a proper answer needs a source.
Usually more of "I heard from my Rabbi" or "a Rabbi" which I guess is a source for me but not one suitable for Mi Yodeyah's answers. Of course, perhaps I should go to my Rabbi and ask him for the source that I am going to quote. But then maybe he heard it from his own Rebbe...
True Torah She'baal Peh still exists..
Comment: The middle paragraph of this answer is incorrect. According to previous Meta posts, "my rabbi" is explicitly considered a [valid source](http://meta.judaism.stackexchange.com/a/2022/2),  preferably with [as much detail as possible](http://meta.judaism.stackexchange.com/a/716/2), and anyway, sources in answers are generally [not required, but strongly recommended to make the answer as useful as possible](http://meta.judaism.stackexchange.com/q/1444/2).

Why comments and not answers?


How to get the emails sent on anther computer?

Question: I use Foxmail to handle email in my company, now I'm back in school for a few days and I need to get a few emails that I sent in company. But I didn't see these options in Foxmail or Thunderbird, any suggestions?
Answer: The ideal solution is to have an IMAP server for your company, and an IMAP server for your school and for both to be accessible through a remote Internet connection, either directly or through a VPN. You cannot create such a situation if youy only have control of the POP/IMAP client such as FoxMail or Thunderbird.
What many email clients do allow is that you can save a copy of sent emails locally (for example to your notebook/laptop hard disk) and save a copy of received emails there too ("offline working")
In the worst case you can use your email client to forward all received emails to a private account accessible from all locations. Use BCC to also send it a copy of all outgoing messages. This is quite a lot of effort though. Explore your email client's "offline" capabilities.


Execute content of buffer as vimscript

Question: I have a nofile buffer that contains vimscript, and I want to execute it.  However, :so %  will not work because it requires a saved file, not a buffer.  Is there a vim command to do this by default?  If not, is it possible to create a function which does the same things?
Answer: Yes, you can yank the whole buffer with ggyG or :%y and execute it with :@".
In short:
:%y|@"<CR>

Used in a mapping:
nnoremap <key> :%y|@"<CR>

Used in a flexible command that works on the whole buffer by default or an optional range:
command! -bar -range=% Foo execute <line1> . ',' . <line2> . 'y|@"'

Comment: you have a missing colon in first command.
Comment: Also, this does clobber the `"` register.  I imagine you could use another register if you wanted to, or you could just save the `"` register and restore it afterwards.

Execute content of buffer as vimscript


70728/2 Ninjago piece too short?

Question: My son is putting together the 70728 set and the grey tall pieces in book two at step 8 seem to be too short, they leave a gap. By about a mm. Has anyone else had this experience?
Comment: My first thought is: Is everything seated completely? It's amazing how fast tolerances can add up. One way to check those tall pieces is to get 5 regular bricks (anything of brick height) and stack them up and compare height to the seemingly problematic 1x2x5 light grey pieces, which should be equal.
Answer: As this set is built on a plate-built base, it is quite possible that they are not pushed together tightly. I have ran into this numerous times when building the LEGO Modular Buildings as the second and third floors have a plate-built floor. Look through everything that has been built so far and make sure that the plates are pushed together all the way. LEGO does have some tolerance and viggle-room, but it is much less than a millimeter.

70728/2 Ninjago piece too short?


Can I present google analytics data to users within my application?

Question: I'm creating an auction site and would like to provide my sellers with analytics related to their product pages - visitors, search terms, etc. I could roll my own analytics but want to know if it is possible (or forbidden) to use google analytics within my application and present that data to my users. Is this possible, has anyone done this and, if so, how did you go about it. Thanks in advance
Answer: Sure, you can do this.
The easiest method is to give your sellers direct access to GA.  I have no idea how you have your GA interface setup but you can setup profiles in lots of different ways (filters, segments, etc..) to have the profile show data only for the specific seller, and give their GA account access only to their profile.
Alternatively, GA provides an API for retrieving data from them and displaying on your own site.
Answer: Also you can provides Real time data reporting via Google's Real Time API - https://developers.google.com/analytics/devguides/reporting/realtime/v3/devguide
Which will provide several data in the manner of dimensions and metrics


Age of the universe

Question: A Galactic year is estimated to be around 250 million "terrestrial" years. That means there will be about 4 rotations per billion years. And age of the universe is estimated to be near 13.8 billion "terrestrial" years. It looks improbable to think that our galaxy has rotated 50 times since the beginning of time. Could someone explain this please?
Comment: "It looks improbable..." sounds subjective at least. Why do you think it's improbable?
Comment: It would probably be helpful, if you could specify as precisely as possible, what property of the Milky Way appears strange to you, and edit the question accordingly.
Comment: Looking at the distribution of stars in the Milkyway, I thought it would require more stirring than 50 times.
Answer: Your calculation sounds correct. It is however based on assumptions that are non-trivial. An analogy with the Earth would give that the Earth rotated around the Sun 13.8 billion times since the Big Bang. Which is meaningless since the Earth was created only a few billion years ago. Our galaxy, the Milky-Way, may have had a long and quiet history since 10 billion years ago (http://arxiv.org/abs/astro-ph/0702585), which is not too far from the age of the Universe. Our Galaxy, however, evolved, and its rotation speed too. As for the solar system, the duration of a "year" (ie one rotation over the Sun) on Earth is very different from the duration of a "year" on other planets. It is the same in the Milky Way where the revolution period is different depending on the distance from the center of the galaxy. As an order of magnitude, and considering in first approximation that our Galaxy is old, and that its properties did not change too much, an order a magnitude of a few tenths revolutions since our Galaxy is a stable disk sounds plausible. As a first approximation. I hope this will help you to find that such a value is not improbable.
Comment: This is a sound explanation. Let's see what others think about this. Thanks.
Answer: Currently Astronomy relates to Time in a manner similar to the Geocentric / Heliocentric syndrome that plagued early Astronomers that did not have enough information available to accurately describe how the Solar System maneuvered.
Earthlings use Time as it relates to Earth to measure the Age of Celestial objects because we don't currently understand how Time works. More specifically we use the J2000 epoch to tell time on Earth because it was designed hundreds of years ago, appears to work for the time being and is commonly agreed upon.
The problem is that we believe our planet is approximately 4.5 billion years old and a year is an approximate amount of time that is consistent over long periods of time. So we extrapolate out and use that to measure objects outside our system since that is the only reference point we know and understand and we do not have access to a Time system that explains other Systems and is capable of measuring them.
The Universe if it is the largest encapsulating structure needs its own time system that starts from its origin and is consistent to measure against other systems. For example using a second instead of a year (presuming a second takes the same duration in all systems).
A Galaxy would need its own Time reference from its origin with in the Universe using a similar consistent standard like a second so you could accurately compare their ages.
If you just look at the Earth year we don't know how long a year was a billion years ago, it may have only been 100 days and we don't know how long one will be in a billion years, it may be 1000 days, just to make an example. So an Earth year is a poor way to measure the age of other Celestial objects but it is the best we have at the present time so we continue to use it. Technically we shouldn't be able to measure anything older than approximate 4.5 billions years because theoretically that time doesn't exist because Earth didn't exist. You can see how this resembles the Geocentric / Heliocentric problem.
In the future we will realize the J2000 epoch is based on the birth of Jesus Christ and that we are not accurately measuring time on our own planet based on its beginning but rather basing it on an individuals life. At some point we will agree on a common starting point for the planet and develop technology to determine that moment similar to how we developed carbon dating.
You can see how this will even effect the term light year as we realize the length of a Earth year changes over long periods of time and is not accurate to measure distances that light travels relating them to orbital period of our planet. Note also light appears to travel a linear fashion whereas our planetary orbit is circular. The key would be to have a time standard that is linear in fashion as well.
Your point about how many times the galaxy has rotated is similar to the age of an earth year problem, no one from Earth really knows how fast or how long a galaxy evolves so they are just estimating based on the best information available and that is commonly agreed upon.
Comment: J2000 has nothing to do with keeping time. It is simply how we assign coordinates to stars (see this wiki page on astronomy http://en.wikipedia.org/wiki/Epoch_(astronomy). Astronomers use epochs to correct for things like stellar proper motions and the precession of the earth, which change the RA and Dec of stars over time. Chaonomy is either a crank or very poorly informed.
Comment: Your statement that "...we don't currently understand how Time works." is demonstrably false. Time is but one of four relevant coordinates which are addressed in both the Special and General theory of Relativity. I think you're confusing yourself on the subject.
Comment: @chaonomy, Your explanation about galaxy rotation is interesting. Let's see what others think about this. Thanks!
Comment: @Chaonomy, how in the world is any of this related to the Heliocentricity/Geocentricity problem? That was about the structure of the solar system, and had nothing to do with time! Not a great answer.
Comment: It is an analogy. People use to think the Earth was the center of the Solar System until they later learned the Sun was the center. Currently people think the that the Time of the Universe and other objects in the Universe is based on the Time on Earth and the concept of Earth years because they don't understand how it really works. It is a good answer but most people on Earth aren't aware that Humanoid Extra Terrestrials live among us. We will realize that as a Planet before we will realize any of the true mysteries of how Outer Space works.
Comment: It's a sociological answer to a scientific question.

Age of the universe


Does :reload work globally in clojure?

Question: Say I have two source files in a clojure program and I reload a changed external module using :reload in one of the modukes. Does the reload only occur in scope of the module which calls reload or is the reload in effect for all modules?
Answer: If a clojure file is reloaded, then it is reloaded. This is not subject to a scope - it just loads and evaluates whatever is in the file.
What exactly happens then is dependent on what's in the file. For instance, reloading a (def) form overwrites the root binding for that var. Another interesting consequence is that deleting a var in a file and then reloading it will not remove the var.
Require and use also have side-effects beside loading, and a :reload does not mean all other require and use statements that refer to the same file are re-evaluated.
Answer: i'm guessing that, by using :reload, you update a namespace on your repl (on your current instance of the java runtime). therefor, my answer would be: it is global (because now your instance of the runtime has the new code)

Does :reload work globally in clojure?


DSLR vs CamCorders for video tutorials?

Question: I'm planning to record video tutorials for a project of mine without compromising on the quality and I would like to know the best option to shoot them
Comment: This question is a bit broad. You might want to give us more details. What kind of tutorials do you want to make, what specifically do you need help with deciding in producing your tutorials? There are many factors in shooting videos. Asking for the best option is similar to asking "What is the best option to make music".
Answer: DSLRs tend to have much larger image sensors than camcorders, which tends to mean much less depth of field (DOF) for a given field of view.  Shallow DOF is great for isolating subjects from their background, but it's a nightmare when you actually want the subject and its context to be in focus together.
Most DLSR lenses have a limited zoom ratio and/or limited maximum aperture compared with camcorder zooms.  A typical DSLR kit (EOS 80D with 18-135mm f3.5-f5.6 zoom), can be equated to a 35mm equivalent of 29-216mm f3.5-f5.6 zoom lens.  A Canon Vixia G40 camcorder has a 35mm equivalent of a 26.8 - 576 f1.8-f2.8 zoom lens.  That's more than 2x longer at the long end with 2 f-stops more light-gathering ability.
Combine longer range, wider aperture, and more depth of field, and you can see why news organizations tend to send camcorders into the field.
On the other hand, depending on your subject and your aesthetic, you may prefer the shallow DOF look, such as this short video demonstrates: https://vimeo.com/100437145
Answer: Another factor to consider in deciding between DSLR and Camcorder is the ergonomics. While the pictures from a DSLR have a nice cinematic look, they are harder to use.
As mentioned by Michael, the larger sensor gives you shorter depth of field, so anything other than static subjects can be challenging to keep in focus - and this is coupled with the fact that the displays are generally too low resolution to judge critical focus on-the-fly. So most people end up bolting on an external monitor.
The lenses are something else to consider: stills lenses often have the focus distances marked like "1ft—2ft—infinity". So a follow focus rig is a useful addition, meaning that you'll want rails to mount it all on. Most stills lenses also change speed as you zoom, so a close-up will be correctly exposed, but when you pull out for a wide shot you'll be a stop over.
And the shape of the camera is different. A DSLR is harder to hold steady while filming and looking at the screen, compared to a camcorder with a viewfinder that you jam onto your eye. So people often add shoulder rigs to them. So in addition to the camera you end up with a frankenrig with monitor, follow-focus, batteries external mics and recorder, and shoulder mount all hanging off it. If the look is worth it to you, then this might be acceptable, but if ease-of-use makes a difference this is a big minus for the DSLR.

DSLR vs CamCorders for video tutorials?


What are the rules for releasing ArcObjects from memory in .NET?

Question: Do I need to release each object?  Is some memory management handled for me?
Answer: Most notably, always explicitly release cursors when you are done with them. I also release some enumeration objects which imply database access, for example IEnumRelationship you get from IRelationshipClass.GetRelationshipsForObject.
Also, when you create a lot of COM instances which are short-lived (especially in tight loops), it is also a good idea to release them explicitly.
There are also scenarios when is it advisable to release individual feature (row) references. For example, if you create a new geodatabase version, edit data, reconcile&post, attempts to delete the version afterwards might fail since there might be unreleased rows, which in turn keep reference to the version (workspace) you are trying to delete. Mostly, though, such scenarios are rare and you do not need to account for them in your day-to-day ArcObjects development. It would only make the code cluttered with extraneous cleanup, making it less maintainable.
It is also important to say when not to release .NET wrappers - never explicitly release RCW of ArcObjects which might be in use by any other managed code. One example of this - do not release IMap when in ArcMap. In general, do not try to release ArcObjects which you did not create.
Answer: For the most part .NET garbage collection works well.  There are some cases in ArcObjects that do important work on desctructors and the non-deterministic nature of the .NET wrappers can cause issues.  This help topic covers the primary cases to be concerned about and how to manage releases.
Comment: The same link in 10: http://help.arcgis.com/en/sdk/10.0/arcobjects_net/conceptualhelp/index.html#/Releasing_COM_references/0001000004tm000000/
Answer: Always destroy:

ICursors
IEnums

Be careful to not destroy something that's being used somewhere else.
Today I read a interesting discussion on ESRIs website in which Kirk participated. There were other very interesting opinions, such as the use of ReleaseComObject method and the FinalReleaseComObject (or something the like). Sorry I don't have the link on me right now.
Some even suggested to release IRows, but many agreed that it's just easier to let GC handle them directly.
I never release any IGeometry's. Anyone tried that?
Answer: I will use the ESRI.ArcGIS.ADF.ComReleaser.  That being said I'm not exactly sure which arc objects use a deterministic release pattern but I mostly attach it to the IServerContext object since that is the most crucial.
<code>using (ComReleaser comReleaser = new ComReleaser())
{

}
</code>
here's some information I was able to obtain at the 2011 esri Developer summit.
The big list I was remembering was for the singleton objects (which is two topics down in the help).
This is the link from the Best practices for using ArcObjects in .NET "Releasing COM references" topic:
http://help.arcgis.com/en/sdk/10.0/arcobjects_net/conceptualhelp/index.html#/Releasing_COM_references/0001000004tm000000/
And here is a post on the Geodatabase Blog to a fourm discussion that does contain a list of objects:
http://blogs.esri.com/dev/blogs/geodatabase/archive/2010/05/18/what_2700_s‑up‑with‑comreleaser_3f00_.aspx
(lastly a blog post with a link to help in case the url didnt work)
http://blogs.esri.com/dev/blogs/geodatabase/archive/2008/12/18/using‑the‑comreleaser‑to‑manage‑the‑lifetime‑of‑cursors‑in‑.net.aspx
Comment: I believe that ComReleaser is part of a 32bit assembly meaning that you won't be able to run ComReleaser in a 64bit process (e.g. ArcGIS Server).  I was writing an SOE and got burned on this.
Comment: When working with IServerContext, more crucial (than to release its .NET RCW as ComReleaser does) is to call ReleaseContext on it. Best way is to wrap the context in a .NET class implementing IDisposable (and following the standard dispose pattern including implementing the finalizer)
Comment: @Petr yes releaseContext is huge and something I haven't overlooked but thanks for putting it in writing for everyone else.
Comment: are there other options for Server releasing?
Comment: @VBAHole not that I've noticed. Even at 10.5.1 arcobjects.
Answer: Don't forget IWorkspace objects.  At the ESRI dev Summit a couple of years ago, I asked the question, and the answer from ESRI was ICursor and IWorkspace objects.
Comment: There is no point to releasing workspaces because they are cached by the workspace factory singletons.
Answer: are the rules different when working with Server Objects like a Cursor in an SOI?
I'm trying to use ComReleaser but it fails every time it gets anywhere near the method in my SOI code
Comment: Don't post a question as an answer to an existing question. Post it as a new question, with a link to the existing one.


Why did the Italians want Rocco killed by the Russians?

Question: Why did Papa Joe and his Italians want Rocco killed by sending him to assassinate the Russians? Considering that Rocco was known to be part of their gang, did they want to start a war?
Answer: They sent in Rocco as a way of killing two birds with one stone as they most likely see him (who they have given the mocking title of the funny man) as a liability and an embarrassment. They also sent him as a form of messenger with a message of hostility as it is explained that the Russians came to Boston as it is a fertile ground for crime. And in the spirit of glasnost they tell the Italians that they may preform criminal acts in Russia with no negative repercussions but the Italians refuse this deal and the Russians continue to push into Boston which is why Rocco was sent to kill the Russians. Two birds with one stone.
Answer: I don't think this would have started a war. Rocco was known as the Funny man he was basically looked at as a joke by almost everyone. Sending the least threatening guy to shoot the Russians was basically a joke on him. On the one hand he may kill the guy he was sent for if he gets killed they can say "sorry god that idiot thought he could get Made by doing this we are so sorry he was such an idiot." They don't lose anything since they don't value or care about him. Also it lets them know how well guarded the Russians are.


Image and Video Files not showing up in Gallery

Question: I am using Android 4.1.2 Jelly Bean OS Smart Phone, All the images and video files present in My Files(default file browser app for android) are not showing up in Gallery(default app), and Other Apps(e.g., MX Player) are also not able to detect the media present in devices internal storage. Please help me to fix this issue, and Does this behavior, drain my phones battery ?
Answer: I found that when I charged via computer USB all my photos were in the folder \Phone\DCIM\Camera.
All I did was move the photo files to a folder in the computer then copy them back to the camera folder and all of them could be seen again. Took about 2 minutes for 400 photos.
Answer: I am having this issue as well.
I have no idea why this works, but if i copy the folders to my computer, delete the ones on my phone, then copy them back, they all appear
Answer: Make sure there is no .nomedia file in the folder or in the parent folder (Sdcard). This prevents media files to be shown in the Gallery. Be aware that .nomedia is an hidden file, so you won't see it until you activate shown hidden files in your file browser.
Answer: I spent one hour with T-Mobile tech level 2 (supervisor) on this same issue. I followed the advice above, enabled Show Hidden Files and, sure enough, all the way at the bottom was a .nomedia file. As soon as I deleted that, the photos and videos started showing up in the Gallery.


How to maintain sentence variety when writing a self description?

Question: When I write to a "describe yourself" type prompt, I find myself wildly overusing "I am ..." type sentences. What are good ways to vary prose in this situation? (Preferably with examples)
Comment: Could you give us an example of a 'describe yourself' prompt? Does it have to be written in first person? Can other things be happening in what you're writing?
Comment: Related: https://writing.stackexchange.com/q/37492/14704 and https://writing.stackexchange.com/q/40680/14704
Answer: The problem with repeated "I am... " sentences isn't that you're not varying your sentence structure, it's that you're not communicating the underlying meaning with your reader.
Consider the standard Essay format:

Introduction
Body
Conclusion

By itself, each "I am..." statement is an introduction to a fact.  If you don't elaborate on that statement, the attribute you're trying to describe is meaningless.
For instance: "I am directionally challenged."
Okay, so what? How has this changed me as a person? Why should the reader care?  Is it merely a descriptor, like my eye color, or is it more than that?
"When I get off an elevator, I always pause, taking in my surroundings and trying to determine if I need to turn left or right.  Even when I get off the elevator at my workplace, the same one I've ridden 10+ times a week for five years. It doesn't bother me, and I'm not embarrassed by the fact.  However, if I'm travelling with coworkers, I will choose to walk behind them.  This is not because I feel lesser, or subordinate- it's simply more efficient.  I will hold my own during any technical discussion, argue with my boss and his boss with no fear of retribution.  But if I'm navigating to a new destination, even with GPS directions barked from my phone, I take it as a given that I will have to pull at least one U-Turn."
It's fine to start with the "I am" statement. After you start elaborating on what it means to you and the people around you, giving the readers the body and conclusion to support this introduction, you can revisit this statement and change it into something more.
"People sometimes underestimate me, when they see my hesitation upon entering a room.  What they fail to realize is that I am not weak, I am not scared; I am just directionally challenged."


How to prove that the exterior angle sum of an n-sided polygon is 360 degrees?

Question: Just wondering about the smartest and best way to prove such a question. I know of many ways, and also I don't want to use anything related to $180(n - 2)$.
Comment: By dividing the n-gon into n-2 triangles.
Answer: Intuitively it isn't too hard to see since as you trace round the shape you make one complete full turn so must go through a total of 360 degrees.
See this picture, link. Imagine moving the angles to a point along the lines, they make a full turn.
For a formal proof the only way I can think of at the moment is to use the fact that the interior angle sum is $180(n-2)$. Given the exterior angles $\alpha_1, \alpha_2, ..., \alpha_n$, the interior angles are $180-\alpha_1, 180-\alpha_2, ..., 180-\alpha_n$.
Summing these gives the interior angle sum to be $180n - \sum_{i=1}^n \alpha_n$. But this has to equal $180(n-2)$. Rearranging gives $\sum_{i=1}^n \alpha_i = 360$.
Answer: I don't know exactly what are you looking for, but perhaps try this:
You can divide the n-sided polygon into n triangles. Every triangle has the angle sum of 180°. We are not interested in the 360° from the circle around the middle point, so we have:
$n \cdot 180°-360°$
Comment: you should read my title - How to prove that the exterior angle sum of an n-sided polygon is 360 degrees?
Answer: This may be completely wrong, it seems far too easy. But I can't see any error, so here goes. This proof tries to bring the theorem closer to the "walking around the polygon" intuition.
Let $(u_1...u_n)$ be an n-tuple of vectors in $\mathbb{R}^2$, with $u_1 = u_n$. The angle between two successive vectors in this sequence is $\angle u_{i+1} - \angle u_i$, where $\angle v$ is the angle that $v$ makes with the positive x-axis. The sum of these angles is:
$$\sum_{1}^{n-1}{(\angle u_{i+1} - \angle u_i)}$$
$$=\sum_{1}^{n-1}{\angle u_{i+1} - \sum_{1}^{n-1}\angle u_i}$$
$$=\sum_{2}^{n}{\angle u_{i} - \sum_{1}^{n-1}\angle u_i}$$
$$=\sum_{2}^{n-1}{\angle u_{i} + \angle u_n - \sum_{2}^{n-1}\angle u_i} - \angle u_1$$
$$=0$$
The sides of a polygon and the exterior angles between them are simply a special case of this proposition.


How to recover securely deleted data

Question: Since we all know files are recoverable with programs after being deleted from the recycling bin, everyone is told to do secure wipes by putting random data over the files you're deleting on the disk itself. From what I understand with Hard Drives (and CDs, and Floppies, etc), the data is written to either be reflective/raised up (1) or non-reflective/sunken (0). My question is then how can a bad guy recover data that's been overwritten just once, since it would change the reflective or height of the gap?
Comment: "the data is written to either be reflective/raised up (1) or non-reflective/sunken (0)" - the bits are a useful abstraction, yes; alas, it hasn't been matching the physical reality of hard disks for years. The "bits" are more similar to "weeell, this bit is most likely 1; that bit is probably a 0; the checksum matches, so let's pretend they are". Quite clever and allows for cheap, consumer-level terabyte disks, too.
Comment: Your comment regarding reflective/raised etc may be relevant to CDs and DVDs but not for hard drives or SSDs. Hard drive use regions of magnetic polarity (various different kinds depending on technology used) and SSDs store the bits using circuitry.
Comment: [This is a good example of how the 'bits' are actually stored on a hard-drive](https://farm3.static.flickr.com/2646/4180253800_1a806d378d.jpg) (This screenshot is from popular hard drive "refreshing" utility SpinRite)
Answer:
See SSD (Flash Memory) security when data is encrypted in place and watch the discussion links on hard drives.
On older drives, it was likely possible to recover overwritten data. The density of modern drives for the past 5+ (and the plus may be 10, 15, or more) years has been so high that a single pass of random data means whatever was there before is just not going to come back unless the NSA knows something about this that the rest of the general public doesn't.
Modern file systems are such that the data may have been cached in several places, so unless the entire disk was wiped, the answer is to look for cached, duplicated, versioned, etc. copies of the file that were overlooked during the erasure process.
Comment: @Luke - As Jeff pointed out.  You should be worried about the other shadow copies that your operating system makes.  Those can be recovered.  Of course once deleted they are eventually going to be replaced with another file.  If you want to secure a mechanical HDD then you should still format it using a trusted method.
Comment: So it's basically old news? For modern drives it's not an issue as long as you do one full wipe?
Comment: Greetings! Do you know a relatively modern enough reference to a paper/study about recovering overwritten data on magnetic disks like HDD?
Answer:
how can a bad guy recover data that's been overwritten just once, since it would change the reflective or height of the gap?

There are three basic methods: multiple copies, error correction, and enhanced detection.
A hard disk, or CD-ROM for that matter contains a filesystem. A filesystem is an organization of files in a way that makes sense to the computer and allows you to arbitrarily create, modify, and delete files at random.
Multiple Copies
Some filesystems have performance enhancing features like quick writes or protecting an original file while modifying it. Some features may have copies of pieces of a file or even the entire file in a separate location on the disk.
Error Correction:
CD-ROM and DVD-ROM filesystems have error correcting codes so that if your disk is chipped or scratched and some bits of a file are unreadable, it can figure out that bits are missing and recreate them.
Enhanced detection:
Magnetic bits on a hard disk platter are writen and read by sensors called heads on the end of a arm which moves across the disk platter. The bits are written in concentric circles called tracks. The tracks are spaced far enough apart from each other to prevent a magnetic bit on a track from causing the head to read an adjacent track incorrectly. However magnetic fields are not clearly discrete and recording on a track will bleed slightly into the margin between the tracks. About ten years ago the size of a disk track that the heads could write were an order of magnitude bigger than what scientific instruments could detect. In that case it was possible to use scanning tunnling electron microscopes to look for the write patterns in the margin between tracks. However, doing so would have been extremely costly and difficult, and likely never happened in the wild.
Comment: Very nice write up
Answer: The general theory is that some residue of the previous state exists due to the
limited accuracy of the rewrite.  There may have been a proof of concept demonstration,
but I'm not aware of any claims that it's actually been done in earnest.  It would be
incredibly time consuming and expensive to even attempt.
Comment: Any links to how it would or what type of tools, even in proof of concept?
Comment: The same kind of sensors as are normally used to read the media, only more sensitive and under manual control.  Disk manufacturers must have them.

How to recover securely deleted data


Can v7 and v8 coexist on the same server

Question: Can I (and if so how) setup SC 7 and SC 8 on the same server? I do all my dev work in a VM and I need access to both at various times of the day. It would really make my life easier on my laptop if I could have them as different IIS apps on the same VM. Is it just a matter of changing the host/app/etc settings in SIM when I do the install? Any ideas?
Answer: You can have multiple versions of Sitecore on a single server.
Firstly, just install them into different folders and have them pointed to different Databases.
And secondly either have them both sit on their own domains so they can both run simultaenously, or if they need to sit on the same domain, then switch in IIS between the two. I'd recommend having them sit on different domains, this way you don't have to change any configurations when you want to switch between the two.
Answer: I love Sitecore because of its architecture. Everything related to Sitecore is in your instance's directory, so there are no other dependencies/dlls gets copied in windows/system etc.
so you can run multiple versions side by side. as mentioned by Jarmo, keep you host entry/domain different, Instance name so the database names.


Is this a design or a utility patent?

Question: In reference to the patent: US7878930
Answer: The cited patent is a utility patent. For US patents, the patent number for design patents is prefixed with a "D". Utility patents just have a number (currently in the 9 millions).
Design patents only cover the ornamental design of a functional object, not function. Thus most design patents consist mainly of drawings and very little text. The claims usually just refer to the drawing. Something like "1. The ornamental design for a container, as shown and described.". The original Coke bottle design patent D48,160 is a classic example. This Wikipedia article may be helpful.
Utility patents cover something inventive with utility (duh). There is a need to be novel, useful and non-obvious. Utility patents are usually longer, have an abstract, a background section and detailed description about how to implement the invention. In particular the claims section of utility patents are usually quite detailed and specific in their description of what is patented.
Answer: It's a utility patent. Design patents start with prefix "USD". Also design patents does not contains textual claims , and only contains images as claims.
Comment: Not to be critical, but generally its best if your answer adds something new if there is already an answer. Also, design patents do have textual claims, they just are really minimal references to the images (as was explained in the other answer).

Is this a design or a utility patent?


Is " independent and identically distributed" an assumption or a fact ?

Question: This is in the context of two random variables. A frequent assumption (e.g. of the error term in ANOVA) is of independent and identically distributed random variables. There is a question on this site asking how the assumption can be checked in a given dataset. Is it an assumption or a fact?
Answer: Just to add to the discussion, this is mostly an assumption that simplifies the mathematics of inference.
To take a concrete example, I am in the field of image processing and usually most algorithms will assume that the noise in the image is IID. This is hardly ever the case because most of the time we do some pre processing on the imaging (for ex: smoothing or averaging) and this will introduce correlation among neighbourhood imaging pixels. Also, pixels belonging to similar structures will have similar properties, the point spread function of the measurement device etc. will all make the IID assumption strictly not true.
In any real world case, it usually turns out to be an assumption but it depends on what you are trying to achieve to be able to tell whether the assumption is valid or not.
Answer: In practice being independent and identically distributed is an assumption; it may sometimes be a good approximation, but it's next to impossible to demonstrate that it actually holds.
Generally, the best you can do is show that it doesn't fail too badly.
This is what diagnostics, and to some extent hypothesis tests attempt to do. For example, if someone looks at an ACF of residuals (for data observed in sequence) to see if there's any obvious serial correlation (which would mean that independence didn't hold) ... but having small sample correlations doesn't imply independence.
[If you're trying to assess assumptions for some statistical procedure -- or especially if you're trying to choose between possible procedures -- it's generally best to avoid hypothesis tests for that purpose. Hypothesis tests don't answer the question you really need an answer to for such a purpose, and using the data to choose in that manner will impact the properties of whichever later procedure you choose. If you must test something like that, avoid testing the data you're running the main test on.]
Comment: On the contrary, having a small amount of serial correlation is extremely common. You can have instruments that are *very* slightly affected by the previous measurement, for example. Or measurements that are supposedly simultaneous are actually observed over time (measure this one, measure that one, measure the next one) and often environmental and other conditions are not quite perfectly controlled resulting in slight serial dependence. In many cases, variables can't be controlled or are necessarily taken over time or along some spatial dimension. Equipment undergoes wear. ...(ctd)
Comment: (ctd)... Perfect independence is in many cases an illusion, a convenient approximation to the (often quite small) dependence that is the rule, not the exception.
Comment: On what is this certainty based? Note that it can fail to hold in trivial ways, far below any ability to detect at any available sample size.
Comment: answer is good. But, I am sure that there must be some way to ascertain that it holds.
Comment: I accept your answer. However, I believe that if measurements are valid, there is virtually no scope for the serial correlation to exist.
Answer: De Finetti would say that conditional independence is a logical consequence of your assumption that the sequence of random variables whose values you can observe in your experiment is exchangeable.
Comment: ...and exchangeability is empirically testable via runs tests, etc.
Comment: failure to reject in a runs test doesn't imply that you actually have independence (nor exchangeability) though - just that it was too small to detect; independence is the assumption, but that dependence was no worse than *small* is the best you could hope to establish
Answer: It depends on the problem but iid is usually an assumption based on two random variables being approximately independent and identically distributed (or at least we have good reason to believe they are).  In most cases where we assume iid, we can't make the claim of perfect independence or that the distributions of the two random variables are perfectly identical, but we make the assumption anyway and then check the assumption based on the data.
However, there are some cases when iid could be considered a "fact."  For example, consider an experiment where you put a single die in a cup, shake the cup, and roll the die.  If you do this twice, I do not think anyone would have trouble accepting as fact that the two rolls of the die are iid.
Comment: I'd certainly be happy to treat the two rolls as iid, since it will be so close to true as to be pretty silly to do anything else - though it won't actually be *exactly* true. Dice, for example, [undergo wear and tear](http://img.photobucket.com/albums/v521/hentzau/photo13.jpg) so every time one is used, it causes a very small amount of wear. If I had to do a million rolls, I couldn't treat the first and last roll as identically distributed. I assume that's the sort of thing you're getting at with the use of quotation marks in the last paragraph.
Comment: The case I described is in fact a sequence (of length two) random variables that are iid.  Observed outcomes are the realization of random variables, whether it be ANOVA or a die rolling experiment.  In Anova, the errors that are typically assumed iid normal(0,$\sigma^2$) are also a sequence of random variables.  As stated on wikipedia, "repeated throws of loaded dice will produce a sequence that is IID": http://en.wikipedia.org/wiki/Independent_and_identically_distributed_random_variables