# All Questions

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### Can a web community write papers? [closed]

the internet has changed science drastically. Not only in terms of distributing knowledge e.g. via online encyclopedias as wikipedia and freely available sources of publications as arXiv but also as a ...
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### "Natural units" of mass

Gravitational attraction is given by $\frac{GMm}{r^2}$ while attraction due to electric charge is given by $\frac{q_1 q_2}{r^2}$. Why does gravity need a constant while electric charge doesn't? ...
• 339
3k views

### What if physical constants were increased or decreased? [closed]

(Probably related to this one, and probably should be CW.) A very long time ago, I had the good fortune to read George Gamow's excellent series of Mr. Tompkins books. That introduced me to the idea ...
9k views

### Deriving the speed of the propagation of a change in the Electromagnetic Field from Maxwell's Equations

I've been told that, from Maxwell's equations, one can find that the propagation of change in the Electromagnetic Field travels at a speed $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (the values of which can ...
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### How to determine the (n,m) dimensions of a carbon nanotube?

I've been reading about nanotubes lately, and I keep seeing the $(n,m)$ notation. How does this describe a nanotube's structure? How do I determine which is $n$ and which is $m$ ? I'm familiar with ...
• 1,453
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### Calculating de Broglie wavelength [closed]

Hey, trying to finish an assignment but having some trouble with it. I will show all my work. The topic is on wave/particle dualty, uncertainty principle (second year modern physics course). So the ...
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10k views

### Which experiments prove atomic theory?

Which experiments prove atomic theory? Sub-atomic theories: atoms have: nuclei; electrons; protons; and neutrons. That the number of electrons atoms have determines their relationship with other ...
30k views

### Accelerating particles to speeds infinitesimally close to the speed of light?

I'm in a freshmen level physics class now, so I don't know much, but something I heard today intrigued me. My TA was talking about how at the research facility he worked at, they were able to ...
• 1,118
470 views

### Creation of the Electromagnetic Spectrum [closed]

After seeing this image: http://mynasadata.larc.nasa.gov/images/EM_Spectrum3-new.jpg And reading this: "The long wavelength limit is the size of the universe itself, while it is thought that the ...
700 views

### Magnetism-Related Terminology

A few questions about a magnet and a paperclip: What do you call a material that attracts another material via magnetism? (i.e. the magnet) What do you call the material that is attracted in #1? (i.e....
604 views

### CPT and heat equation

I haven't understood this thing: Physics is invariant for CPT trasform...But the Heat or diffusive equation $\nabla^2 T=\partial_t T$ is not invariant for time reversal...but it's P invariant..So CPT ...
• 1,344
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### Nonlinear optics as gauge theory

the widely used approach to nonlinear optics is a Taylor expansion of the dielectric displacement field $\mathbf{D} = \epsilon_0\cdot\mathbf{E} + \mathbf{P}$ in a Fourier representation of the ...
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4k views

### An iPhone falling on carpet is fine, is it true? [closed]

I heard that an iphone falling on carpet would not cause damage to it, both internal and external. Any physical explanation for this?
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1 vote
2k views

### Relation of angular speed of a rigid body to Euler's Angles

My Question was like this and i have realised few things and still have some doubts I have a book in which a paragraph goes like this Now, $\dot\phi$, $\dot \theta$, $\dot\psi$ are respectively ...
• 1,461
2k views

### Separation of variables, eigenfunctions of the Dirac operator

Disclaimer: I am not a physicist; I am a geometer (and a student!) trying to learn some physics. Please be gentle. Thanks! When solving the Schrödinger equation for a particle in a spherical ...
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### Why are there 3 quarks in a proton? [closed]

A few quark related questions (I don't know much about them other than that there are 2 flavors concerning protons and neutrons). Why are there 3 quarks in a proton or neutron? Why not 2 or 4? Is ...
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422 views

### Is it possible to determine timescales of electron dynamics from the natural linewidth of an electronic transition?

A lot of work has been done recently on electron dynamics using attosecond pump-probe techniques; for instance in this paper. In this particular paper, the authors photoionized the neutral ...
18k views

### Is it possible to obtain gold through nuclear decay?

Is there a series of transmutations through nuclear decay that will result in the stable gold isotope ${}^{197}\mathrm{Au}$ ? How long will the process take?
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### Why are quark types known as flavors?

There are six types of quarks, known as flavors. Why where these types called flavors? Why do the flavors have such odd names (up, down, charm, strange, top, and bottom)?
• 407
6k views

### Spherical wave as sum of plane waves

How can we do this computation? $$\iiint_{R^3} \frac{e^{ik'r}}{r} e^{ik_1x+k_2y+k_3z}dx dy dz$$ where $r=\sqrt{x^2+y^2+z^2}$? I think we must use distributions. Physically, it's equivalent to find ...
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7k views

### Ising model for dummies

I am looking for some literature on the Ising model, but I'm having a hard time doing so. All the documentation I seem to find is way over my knowledge. Can you direct me to some documentation on it ...
1k views

### Light emission spectrum units

Do someone knows the units of the spectra provided here ? It seems obvious enough that it's said nowhere, but even Wikipedia and other sites are quite blurry on this point. So, is it power ($W$), ...
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5k views

### Why does my watch act like a mirror under water?

I have a digital watch, rated to go underwater to $100 \rm m$. When it is underwater it can be read normally, up until you reach a certain angle, then suddenly, it becomes almost like a mirror, ...
• 3,097
1 vote
598 views

### What is an analog to QM's Hilbert space in GR?

I've read that QM operates in a Hilbert space (where the state functions live). I don't know if its meaningful to ask such a question, what are the answers to an analogous questions on GR and ...
• 2,102
2k views

### How many Onsager's solutions are there?

Update: I provided an answer of my own (reflecting the things I discovered since I asked the question). But there is still lot to be added. I'd love to hear about other people's opinions on the ...
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### Does the energy of a magnetic field decrease when it moves a conductor carrying a current?

When a charged particle moves in an electric field, the field performs work on the particle. Thus, the energy of the field decreases, turning into kinetic energy of the particle. Does the magnetic ...
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62k views

### How efficient is an electric heater?

How efficient is an electric heater? My guess: greater than 95%. Possibly even 99%. I say this because most energy is converted into heat; some is converted into light and kinetic energy, and ...
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311 views

### Can I parameterize the state of a quantum system given reduced density matrices describing its subparts?

As the simplest example, consider a set of two qubits where the reduced density matrix of each qubit is known. If the two qubits are not entangled, the overall state would be given by the tensor ...
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### Is there a conserved quantity that enforces planar orbits in central force motion?

From what I remember, one of the first steps in finding the equations of motion for an orbiting body is to argue that the body's motion has to be restricted to a plane, because the central force has ...
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### Why is the mapped universe shaped like an hourglass?

I've watched a video from the American National History Museum entitled The Known Universe. The video shows a continuous animation zooming out from earth to the entire known universe. It claims to ...
6k views

### How far does a trampoline vertically deform based on the mass of the object?

If a baseball is dropped on a trampoline, the point under the object will move a certain distance downward before starting to travel upward again. If a bowling ball is dropped, it will deform further ...
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### Relation between density and pressure for a perfect fluid

what is the relation between mass density $\rho$ and pressure $P$ for a perfect fluid ?
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1 vote
172 views

### Behaviour of mass and momentum distributions under Newtonian Gravity

In the context of this question should mass distribution $\rho(r,t)$ and momentum distribution $p(r,t)$ be well behaved ? By 'well behaved' it is meant that derivatives of all orders exist everywhere. ...
• 2,102
829 views

### If time standard clocks and any memories about the time standard are destroyed, can we recover the time standard again?

Assume the time standard clocks and any memories about the time standard are destroyed. Can we recover the time standard again exactly? Recovering the time standard again means we can determine the ...
3k views

### Why some nuclei with "magic" numbers of neutrons have a half-life less than their neighbor isotopes?

It's easy to find the "magic" numbers of neutrons on the diagrams of alpha-decay energy: 82, 126, 152, 162. Such "magic" nuclei should be more stable than their neighbors. But why some nuclei with "...
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9k views

### Reciprocal Lattices

Is there an easy way to understand and/or visualize the reciprocal lattice of a two or three dimensional solid-state lattice? What is the significance of the reciprocal lattice, and why do solid ...
2k views

### Appearance of atoms

I was watching a documentary entitled "The Atom" and one of the statements made was that Atoms behave differently when we look at them. I wasn't too sure about the reasoning behind this and i'm hoping ...
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221 views

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1 vote
482 views

### Teleportation by quantum isolation

This is a thought experiment, so please don't treat it too harsh :-) Short: If we could isolate two places A and B in the universe from all and any interaction with the surroundings, is there a ...
1 vote
306 views

### Is there a theory about kinetic energy "particles"? [closed]

We have a model of electricity which says electrons flow from one place to another. We have a model of optics which says that photons go from one place to another. As I understand, there is currently ...