# All Questions

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### Finding maximum deceleration [on hold]

Given $$\frac{d^2s}{dt^2}=-Ae^{s/B}\biggl(\frac{ds}{dt}\biggr)^2$$ Show that maximum deceleration (where $A,B$ are constants, $v=v_0$, and $s=-\infty$) is $\frac{v_0^2}{2eB}$. I'm so lost on the ...
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### BCS Theory of Superconductivity

I'm currently taking an introduction to Solid State Physics class, and is now on the subject of superconductivity (SC). Currently I'm reading about the BCS theory, and how this works on a microscopic ...
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### the story about pre-big bang scenario [on hold]

I came across this subject today while reading about BKL singularity. Doesn't seem to be explained concisely elsewhere, would someone kindly provide a quick summary of the story here? Thanks!
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### are there any “known unknowns” that could affect the possibility of a false vacuum?

(Although Donald Rumsfeld was mocked for talking about "known unknowns" and "unknown unknowns", I think it's an truly important distinction.) Periodically, I hear about how the universe might be in a ...
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### Equivalence of the real and complex canonical equations

This might be too simple to be posted as a question, but I'll take my chances: How is it shown that for: $u_j=\frac{q_j+ip_j}{\sqrt{2}},u_j^*=\frac{q_j-ip_j}{\sqrt{2}}:$ ...
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### How do gravitons and curved space time work together?

I've heard two different descriptions of gravity, and I'm wondering how they work together. The first is Gravitons: "The three other known forces of nature are mediated by elementary particles: ...
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### $2\pi$ and Feynman Rules

I notice a $2\pi$ term in the $\delta$-function when trying to construct an amplitude using the Feynman Rules. The $2\pi$ also appears as an integration measure to enforce normalisation in the phase ...
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### What sets AdS radius of the Vasiliev dual to the O(N) vector model?

In $\mathrm{AdS}_5$/$\mathrm{CFT}_4$ the AdS radius $R$ is determined in terms of the string length by the gauge theory t'Hooft parameter as follows \frac{R}{l_{\rm s}} \sim ...
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### Neutrinos arrived before the photons (supernova)

A while back I read about the super Kamiokande detector detected a large neutrino flux and then several hours later a supernova was seen. Anyone know of this with sources? I don't recall the source at ...
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### Explanation of the classical coupling of the Higgs Field to Electromagnetism

I'm interested in learning about the classical coupling of the Higgs Field to Electromagnetism. There are numerous sources explaining the Higgs mechanism quantum mechanically, i.e. How does the Higgs ...
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### How do charge carriers move thermal energy? (Peltier effect)

I am having hard time understanding how the charge carriers (electrons and holes) are able to move thermal energy. I am on a high school physics level, so I will probably have a hard time ...
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### Symmetries of relativistic Lagrangian and Hamiltonian systems

In non-relativistic mechanics, the conserved quantities found using Noethers theorem in Lagrangian mechanics are the same as those quantities which are conserved under canonical commutation with the ...
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### Can capillary forces be used to make hydro electricity?

Could a device with very thin columns of glass or something that attracts water more be used to pull water up and then release it to drive generator and perhaps add vacuum. I have been wondering ...
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### Components of forces?

Let's say we have an object lying on a slope, the weight of the object can be divided into two components one horizontal and one vertical. My question is does the weight act only in these two ...
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### is gravity a force…explain it [on hold]

if gravity is a force than why doesn't it follow the rules of inertia. If you push massive object it accelerates slowly compared to object which is of same size but less density/mass. However, as we ...
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### Units of the PDF in the Lattice Boltzmann Method

In the Lattice Boltzmann method we require (based on mass conservation) that the sum of the distribution functions for a node is equal to the density, i.e. $$\sum_i f_i = \rho$$ But what units do ...
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### Coverage of squares by randomly putting circles with width following a Gaussian distribution

For some reason, I need to know the coverage of squares, if I put circles randomly on them. The radius of my circles follow a Gaussian distribution. For a better understanding see the attached ...
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### Reference for the calculation of $[E_i(x),B_j(y)]_{x_0=y_0}$ in Coloumb gauge [on hold]

Can you suggest me a reference textbook where this calculation is done?
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### Bubbling Aluminum vapor through liquid Helium

What sort of a microstructure would you get on bubbling Aluminium vapors through a large tank of Helium?Would it be a glassy microstructure and/or would you get nano-powders and/or a new crystal ...
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### Riemann curvature tensor symmetries confusion

In the context of spacetime, reading Schutz, I'm confused about the symmetries of the Riemann curvature tensor, which I understand are: ...
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### Forces in a solenoid

I understand that the windings in a solenoid experience a Lorentz force $\mathbf{f} = \mathbf{J} \times \mathbf{B}$, which tend to cause an outward pressure where $\mathbf{B}$ is directed along the ...
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### Some small questions about quantum spin and rotations

I'm studying about quantum-spin (in a syllabus about non-relativistic quantum-mechanics though), but I have some trouble understanding everything. So I would like to ask some small questions, which ...
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### How to smooth the spectrum of a light source?

Could somebody please tell me if there's a reasonably cheap substance or device can I use to smooth the spectrum of a light source? For example, if the spectrum has spikes as in the blue graph below, ...
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### What are the arguments for gravity not being a force? (in quantum gravity)

In quantum gravity the standard assumtion is that gravity is a force, although there is a small but persistent group of theorethical physicists who think otherwise. What gives us the motivation to ...
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### Finding minimum time [on hold]

A particle is moving on the circumference of the circle $x^2 + y^2 = r^2$ such that the magnitude of $x$ and $y$ components of its velocity, can never be greater than $v$. Find out the minimum time in ...
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### Best way to check for anisotropy given a metric tensor

Carroll gives the definition of isotropy at a point as given vector $V$ and $W$ in $T_{p}M$, there is some isometry that can push $V$ forward such that it ends up parallel to $W$. I understand what ...
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### Does a grain of salt have exactly equal amount of sodium and chlorine? [migrated]

Does a grain of salt have exactly equal amount of sodium and chlorine? I know that the formula unit of salt is 1:1, but for example if there is 10000000 atoms of sodium is it possible that that there ...
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### Most interesting topic in classical mechanics [on hold]

Which is the most interesting topic in classical mechanics? I just want a general view of everyone
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### Milgrom's MOND: 2 questions [on hold]

QUESTION 1: "Can a satisfactory theory encompassing both General Relativity and MOND be found?" - Stacy McGaugh. See also the MOND pages QUESTION 2: If in the standard form of Einstein's field ...
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### What is the relation between electromagnetic wave and photon?

At the end of this nice video, she says that electromagnetic wave is a chain reaction of electric and magnetic fields creating each other so the chain of wave moves forward. I wonder where the photon ...
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### Derivation of Noether's theorem - A problem with physical significance

My question is about the field theoretic version of Noether's theorem. I am deeply troubled by one of the hypotheses of the theorem. As it is the standard textbook for Lagrange mechanics, I'll follow ...
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### Physical description of momentum flux tensor

In the field of fluid mechanics, what is the momentum flux tensor? Is there an easy explanation for how it "works"?
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### How to show that probability and statistics are very important in Quantum Mechanics?

I'm doing a research for my stats class in high school and I chose quantum mechanics as my subject. I narrowed down to electron localization in an atom and radial probability distribution. However, I ...
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### Relative linear velocity of a particle to a rotating object

I am trying to calculate the "relative linear velocity" of a particle moving over a rotating object. According to this paper (section 2.2) I am reading the relative linear velocity is calculated by: ...
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### Freezing water in a closed container [duplicate]

We know that density of ice reduces by about 8% during freezing, this means it expands to have little higher volume. But if I fill water in a container (entire volume) which has very low coefficient ...
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### causal sketches [on hold]

I don't have much of an idea of how to draw causal sketches. I know that you need to work out the gradient of the light cones, which can be done using a given metric and using null vectors. But how do ...
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### Peskin's book page 334 proof of $Z_1=Z_2$ to all orders in QED perturbation theory

Peskin in his QFT page 334 argued that $Z_1=Z_2$ to all orders in QED perturbation theory, but I couldn't understand his argument: ... With a generalization of the argument given there (section ...
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### Pressure difference between 2 side of wings of aircraft

I understand why the pressure is reduced above the wings of aircraft, but when I read books and look for information, it says that the air is compressed below the wings, I do not understand why it is? ...
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### Does salt $NaCl$ as a solid float in molten salt?

I know ice floats in water because it's crystalline structure causes $H_20$ solid to be less dense than $H_20$ liquid. Is the same true for salt because it is crystalline? If not why?
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### What do we see - continuation

So far, the only answer to my question is that I must track every light beam in order to establish what do we see. I need help for this, and I give some arguments to volunteers. If we need to ...