# All Questions

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### How is thermodynamic entropy defined? What is its relationship to information entropy?

I read that thermodynamic entropy is a measure of the number of microenergy states. What is the derivation for $S=k\log N$, where $k$ is Boltzmann constant, $N$ number of microenergy states. How is ...
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### Some questions about chapter I.1 (by Minahan) of the “Review of AdS/CFT Integrability”

These questions are in reference to this beautiful review article by Minahan - http://arxiv.org/pdf/1012.3983v2 I gained a lot by reading some of its sections but not everything is clear to me. I ...
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### Why can free lithium atoms not take part in an Auger process?

Shouldn't it be possible for an incoming photon to excite one of the 1s electrons to a 2p state (or one of even higher energy) and then for the excited electron to drop back to 1s and kick out the 2s ...
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### Water droplet evaporation due to humidity and temperature

I am conducting an experiment that requires comparison to models for a water droplet's change in size due to evaporation. I've found plenty of papers, but they all seem to require measurement of the ...
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### Hydrogen as a fuel in Sun

The source of Sun's incessant energy is hydrogen; which is continuously converting to helium through nuclear fusion reaction releasing energy. Why does not all hydrogen convert into helium in one big ...
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### How are the comoving coordinates NOT a prefered reference frame?

Physics me this: The equivalence principle has rigorous physical definitions that say, for one, that the laws of physics are the same in all inertial reference frames. This is to say that the ...
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### Do days and months on the Moon have names?

On Earth we have various calendars, for example, Days: Monday, Tuesday, Wednesday, etc., etc. Months: January, February, March Does the Moon have names for its "daily" rotations, etc.? It sounds ...
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### What corresponds to this Lagrangian density?

Is there a physical example of a field that would have the following Lagrangian density $$L= \sqrt{1+\phi_x^2 +\phi_y^2+\phi_z^2}$$ where the subscripts denote partial derivatives and $\phi$ is a ...
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### Mass of a galaxy via Luminosity [closed]

Is there a way of calculating the mass of a galaxy, or even a nebula from the luminosity? EDIT I'm deleting this, and moving the question to Astronomy Stack Exchange - thanks david
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### Fermionic interaction potentials

Are there any examples of fermionic particles or quasiparticles for which the interaction potential is a globally smooth function? i.e. no singularities or branch points. As an example, in Flügge's ...
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### Do the laws of physics work everywhere in the universe?

Do the laws of physics change anywhere in the universe?Or will they change from place to place in the universe?
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### What are the easiest ways to get data out of a FITS file without a library?

There are FITS libraries for most programming languages (see this list), but FITS is a simple enough format that it should be pretty easy to extract data without the need for such libraries or ...
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### What happens when a lightning rod is not earthed?

A Lightning rod is fixed to the building and it is not given earthing then what happens? How does the lightning reach the ground through the building?
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### Church–Turing Thesis

Can the Church–Turing Thesis be proved assuming classical mechanics, how is the proof or disproof? Edited: I was looking for a proof of "everything computable by a device obeying CM is computable by ...
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### When is the “minus sign problem” in quantum simulations an obstacle?

The "minus sign problem" in quantum simulation refers to the fact that the probability amplitudes are not positive-definite, and it is my understanding that this leads to numerical instability when ...
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### Phase of Elements

There are 11 gaseous elements and two liquid elements at standard temperature and pressure. The rest are solid. Can phase be predicted from quantum mechanical principles?
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### Why so many arguments for the transformation equations of generalized coordinates?

For a system of $N$ particles with $k$ holonomic constraints, their Cartesian coordinates are expressed in terms of generalized coordinates as \mathbf{r}_1 = \mathbf{r}_1(q_1, q_2,..., q_{3N-k}, ...
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### How to find the principal point in an image?

I need to find the principal point in an image. Its a point where the principal axis intersects the image sensor. Due to misalignment this point is not at the center of image always(or image sensor). ...
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### Partially polarized light in laboratory

Does anybody know a way to create (in laboratory, but with no super-sophisticated equipement) a beam of light with partial linear polarization whose degree of polarization can be adjusted (at least ...
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### Work of Marie Curie?

I've been reading about the work of Marie Curie recently after a friend filled me in on what she did (never having had much of an idea previously) and it's all very interesting. What I can't ...
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### Cases of any known fundamental physical constants changing within our locality?

Has there been any cases where the only explanation has been that at least one of the physical constants must have changed to explain an experiment/event/obervation? I am not intrested in large scale ...
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### How does capacitance work?

I have a circuit whit a AC source a capacitor and a resistance all in series. I find that the difference of potential between the capacitor leads begin to change after some instants as it should. But ...
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### How do we know that black holes evaporate?

This has been bugging me for some time. As I understand it, Hawking radiation is the result of the mismatch between the vacuum state of a quantum field as seen by a free falling observer (falling ...
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### Iron Man repulsor question [closed]

Ignoring the whole movie situation along with the power demands, what do you think Iron Man's repulsors use to "repulse" his targets? I don't think they are lasers or electric beams, or "reverse ...
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### Is Quantum Physics used in manufacturing CPUs?

If I remember correctly, I heard some people saying that the transistors on CPUs today are so small, that they have to use quantum physics to make CPUs. Is that correct?
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### Spinning bucket of water in zero gravity

Everyone knows how the surface of a spinning bucket of water would look like on earth - parabolic. But what if we turned off gravity (for instance by doing the experiment in a freely falling lift)? ...
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### Is nature symmetric between particles and antiparticles?

Is nature symmetric with respect to presence of particles? Do we have an antiparticle for every particle thought of? Are there any proven examples where we don't have an antiparticle? And what about ...
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### Given that matter cannot escape a black hole, how did the big bang produce the universe we see today?

Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. If the matter contained within our ...
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### Asymptotic Invariants in General Relativity

I was trying to understand Witten's proof of the Positive Energy Theorem in General Relativity by reading the original argument given by Witten. I am comfortable with the overall argument, but I would ...
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### Is it possible for the universe to be infinite considering Zeno's and Hilbert's paradox? [closed]

I'm only in high school (junior) so goes easy on me. But, how do physicist refute these paradoxes? Considering there are a number of theories regarding the origin of the universe and some postulate ...
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### Where does this equation originate from? (found in the Big Bang Theory)

Recently, I've been watching "The Big Bang Theory" again and as some of you might know, it's a series with a lot of scientific jokes in it - mostly about Physics or Mathematics. I understand most of ...
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### Is there a consensus on the fate of our universe?

We all know that our universe is inflating from what is known as the Big Bang. However, will our universe continue to inflate at the current rate? Or after reaching a maximum size, will it collapse in ...
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### What is the official difference between a planet and a dwarf planet?

I'm trying to understand how objects are classified as planets, moons, or dwarf planets. Can someone please explain the differences between them? I'm really curious about why Pluto is a dwarf planet, ...
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### generalizing spin rotations

I have a question about the relation: $\exp(-i \vec{\sigma} \cdot \hat{n}\phi/2) = \cos(\phi/2) - i \vec{\sigma} \cdot \hat{n} \sin(\phi/2)$. In my texts, I see $\phi\hat{n}$ always as c-numbers. My ...
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### vander waals and casimir forces

Does one need to invoke quantum mechanics to explain casimir force or vander waals force. I see that textbooks show derivation of vander waal force with no QM but casimir force is typically described ...
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### What happens when two smooth surfaces touch?

I am wondering what will be the physics to explain how two neutral, chemically nonreactive objects stick. I know that using van der Waals formalism, we can treat neutral body electrodynamic forces and ...
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### Why is gravitation force always attractive?

Why is the gravitational force always attractive? Is there another way to explain this without the curvature of space time? PS: If the simple answer to this question is that mass makes space-time ...
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### How can one predict the length of the Synodic month? Why is it irregular?

I'd like to write a program that uses the exact (down to the second) amount of time from one new moon (or full moon) to the next. Yet, I am told that this period is irregular. Yet, it seems to be ...
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### Terminal speed of a body in a gravitational free fall? [closed]

What speed would a small body moving towards a large body reach under gravity alone? More importantly, is there a 'terminal speed' for the small body due to the increasing energy required to ...
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### How to place a mirror parallel to a wall?

For one of my experimental setup I need to place a mirror perfectly parallel to a wall. It can be placed at any distance from the wall. I would like to use any method other than direct measurement. I ...
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### Hail stones in summer?

This has always puzzled me. Yesterday (in London) it started hailing despite it being about $20^oC$. A couple of years ago I experienced hail in Sicily when it was about $35^oC$ in the shade!. How is ...
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### The Higgs field a new Luminiferous aether?

As of this writing it has been made clear to me that classical physics' Luminiferous aether was a terriblly poor discriptor of space. With the advent of Special Relativity and General Relativity, that ...
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### Why is the surface of Venus so hot?

Whenever I have seen Venus described, its high surface temperature is attributed to an intense greenhouse effect. This seems to make sense, as its atmosphere is roughly 96% CO2. But on Earth, the ...
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### What is the simplest way to prove that the Earth orbits the Sun?

Assume you're talking to someone ignorant of the basic facts of astronomy. How would you prove to them that the Earth orbits the Sun? Similarly, how would you prove to them that the Moon orbits ...
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### What is the simplest way to prove the Earth is round?

Assume you've come in contact with a tribe of people cut off from the rest of the world, or you've gone back in time several thousand years, or (more likely) you've got a numbskull cousin. How would ...
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### What is the meaning of the Fourier transform of Feynman propagator?

I know $K(a,b,t)$ is the probability amplitude that a particle that starts at point $a$ is found at point $b$ at a time $t$ later. There is also an expression that sometimes is called green function: ...
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### How viable is reuse of nuclear waste at this point, and what are future prospects? [closed]

I keep seeing documentaries describing the pile-up of nuclear waste from nuclear power stations in the US and in other countries. Countries such as Finland are working to deposit waste in old mines in ...
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### How to make a small tokamak?

$\require{mhchem}$I made a fusor once, like the easy science project: deuterium-deuterium ones, but they're really inefficient. I was wondering if it would be possible to make a small tokamak; not one ...