# All Questions

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### What are the proper domains of the position and squared angular momentum operator?

I am looking at the position operator on a compact set $K \subset \mathbb{R}^n$ and the squared angular momentum operator ( so essentially the Laplace-Beltrami operator where I just look at the ...
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### How does it seem to be that space/time always equals c?

Bit of a noob question perhaps, but the way objects move they seem to be going all the same "velocity" so to speak, that velocity being the speed of light. Except, velocity is displacement/time, so if ...
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### Does gravity stack?

If I am standing on a planet on the opposite side from its sun will I feel the downward pull of both the planet's gravity and the star's gravity? In other words, does gravity "stack" (Any D&D ...
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### How should a math undergrad student prepare himself to study GR and QM?

I'm quite sure that similar questions like this have been asked for more than thousands of times on here but since each person's background and interests are unique I believe questions like this ...
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### P-N junction voltage under polarization

All the books that deal with the p-n junction under applied bias asume that the same equations used for the equilibrium case (no bias) can be used for the biased case provided that the juntcion ...
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### Counting Problems in Physics

What are some classic counting problems in physics? I'm trying to think of interesting examples to give in a math class on the matter, and I feel as if physics should have some ones to offer.
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### what is the composition of magnetic field?

whenever I read electromagnetism topic then i was confused by magnetic field that what is the composition of it and why magnetic field forms from outside the current carrying conductor ...
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### Reference on electrodynamics with tempered distributions

Back in my undergrad I had a course on classical electrodynamics where the fields had values in the space of tempered distributions. In this way one could correctly treat self-interaction and ...
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### The DLCZ Protocol

I have been reading the article review for Optical Quantum Memory (http://arxiv.org/abs/1002.4659) when I came across this section about DLCZ protocol. I don't understand about Beam splitter. For what ...
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### why do some of the dimensionful physical constants take on the values they do?

so this might have some duplicated inquiry that this question or this question had, and while i think i have some of my own opinion about it, i would like to ask the community here for more opinions. ...
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### Derivation of the first term of the expansion of the conformal block

In the book Conformal Field Theory by Francesco, Mathieu, Senechal, how do we get equation 6.190 $$\mathcal{F}_{1}=\dfrac{(h_{p}-h_{2}-h_{2})(h_{p}+h_{3}-h_{4})}{2h_{p}}$$ , the first term in the ...
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### In terms of physics, does the phrase “time slows down” mean the same thing as “things happen more slowly?”

The common definition of "time" is a type of measurement, like size. But the sentence "size gets bigger" doesn't make any sense. Is "time slows down" an odd phrasing of "events occur more slowly" or ...
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### Potential Energy and Kinetic Energy [on hold]

how does potential energy related to height? how does kinetic energy related to velocity? how does potential energy related to those kinetic energy? how does potential energy and kinetic energy ...
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### Why is Silver-108 unstable

Why is Silver-108 unstable if silver-107 and silver-109 are stable? I found it on crash course but no answer was given.
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### Is topology of universe observable?

There is an idea that the geometry of physical space is not observable(i.e. it can't be fixed by mere observation). It was introduced by H. Poincare. In brief it says that we can formulate our ...
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### Derivation of force law in special relativity

I've seen force defined in special relativity as the rate of change of 4-momentum $${\bf{F}} = \frac{d {\bf{p}}}{dt}$$ Can anyone comment on the following derivation of that relation? Take one ...
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Any object can emit and absorb radiation and the power of emission can be represented by the Stefan-Boltzmann law: $$P=A\epsilon\sigma T^4$$ In many texts the net power radiated is the difference ...
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### Why do bubbles make a sound?

I have an understanding of how bubbles work. They encapsulate air (or other fluids) in a membrane caused by surface tension. When they pop, there is often a sound. Sound is a type of energy, kinetic ...
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### How do I know which force to use in moments calculation?

I'm working on some Statics problem, with the method of joints, etc and for all of them, we use sum of moments = 0. However, I'm very confused about which forces I should use to find the sum of ...
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### How much could one slow down aging using existing spacecraft?

Imagine that I board a spacecraft and head away from the Earth at top speed until I've aged twenty years, then I turn around and come back. How much younger will I be than the unexciting folk who hung ...
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### Is there a textile like material that can act as a spring?

Imagine some sort of textile (as in very light, not metallic) that is tightly rolled up, can be easily unrolled by applying some force (i.e pulling it) but when released will eventually return to it's ...
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### compact and non-compact gauge theory [on hold]

From the answer to this question, I see that the gauge theory has to be compact if the charges need to be quantized. I am not sure in what sense these are necessary and sufficient. So here is a ...
I think about a rotating torus (simplified tire) filled with ideal gas. Mass of gas is $m$ and molar mass is $M$. Pressure in non rotating torus is $p_0$. Temperature is constant $T$. Inner radius of ...