# All Questions

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### A commuting decomposition with respect to a fixed operator

Let us assume we have a have a fixed hermitian operator on an Hilbert space $\mathcal{H}$, that written in its eigenbasis is $$H = \sum_k e_k \vert e_k\rangle \langle e_k \vert$$ and we have a ...
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### Homopolar Motor and Rotational Symmetry

Take a battery, stick a magnet under it and then place a perfectly circular wire on top of the battery, in such a way to form a circuit. We now have an homopolar motor with a core that has perfect ...
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### BCS energy gap in superconductivity: Microwave apsorption as proof

A quick question. The BCS theory predicts the existence of the energy gap below the Fermi level. The experimental proof is the absorption of the EM radiation in the microwave spectrum. Why/How? Could ...
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### Resistivity of non-ohmic devices

Is resistivity of non-ohmic devices constant with constant temperature, or it depends on some other factors as well? I'm aware that for non-ohmic devices, the ratio V/I is not a constant which means ...
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### Dark matter/dark energy in Einstein's equation as manifestations of entropy production

It is well known that pressure adds a contribution to the gravity sources in Einstein's equation. That contribution is unknown in Newton's theory. What about entropy itself, or more precisely the ...
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### Determinism in quantum mechanics - a thought experiment [duplicate]

How does modern quantum mechanics treat determinism? Is the world predictable? According to the uncertainty principle, when we measure a micro-object with a measuring device, we cannot predict what ...
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### Hamiltonian of a system of an hydrogenic atom in EM field

In an interaction between an EM wave and an hydrogenic atom (atom with single electron) we have in the semi-classical approximation (I think that's what is called, basically treating classically EM ...
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### Why is this the definition of the scattering matrix and reflection/transmission coefficients?

I've been having trouble with the choice commonly used to name the elements of the scattering matrix in quantum mechanics. Let's say we are dealing with a simple potential barrier in 1D, such that if ...
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### Classical Green function

What is the physical reason why the classical Green's function is not defined as a principle value integral? In a recent discussion (Classical Green's function) it was said that the classical ...
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### Why can't a mass be accelerated to the speed of light if its mass does not increase with speed?

The standard (though possibly/allegedly technically wrong/ out dated) explanation for why you can't accelerate a body to the speed of light by applying a constant force for a long time is that the ...
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### Laser diffraction by synthetic Opal: how to determine "$d$"?

I'm doing some measurements with laser diffraction on a cut cube of synthetic opal. Here's a stock photo of it: The shimmering colours are due to diffraction of visible light on intermolecular planes ...
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### In special relativity, is it true that mass is conserved and energy is conserved?

If so, it would seem to follow that a photon has mass, because an atom that emits a photon loses mass. If the photon has no mass, then the total mass will be less and mass will not have been conserved....
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### Location of a particular spatial frequency in a Fourier transform plane

I am trying to assess structure of a Fourier transform generated by a thin lens. I am using scalar diffraction theory and Fresnel approximation such that input $U(x,y)$ transparency located at the ...
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### Effect of homogenous strong magentic field on a spherical ball: why $H$ is not zero after stopping it?

Consider a spherical ball of radius $R$. Firstly, the ball has been magnetized by putting it into a very strong homogeneous magnetic field B while keeping temp = $T_1$. Then, the external magnetic ...
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### If a force is applied on block such that it is stationary, then which force counteracts it ? Tension or friction?

I imagine the title wasn't very clear, so here is the question in a bit more detail. Consider a block attached to a nonextensible and massless string as shown in the finger below. The static friction ...
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### How exactly does a fixed pitch wind turbine start from standstill/stall?

I might have a crucial gap in my understanding of the aerodynamics of a wind turbine but I am struggling to get my head around how a fixed pitch wind turbine can start spinning from a standstill? I ...
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### Gravity in quantum mechanics [closed]

Many popularizers of physics throw around the term "quantum gravity," before listing a whole series of reasons why we haven't been able to come up with such a theory yet. My question is, are ...
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### The law of impulse saving assignment [closed]

I need help with solving this physics problem. I only determined that $\upsilon_2\cos{\beta}=65\ m/s$ but i can’t find that angle to determine the velocity separately. Tried to determine $\beta$ ...
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### Matter is Apparently just Field Excitations, But How the Heck does that Work? [duplicate]

Ok so, around four-ish months ago, I was told by a Cosmologist that what we perceive as matter doesn't actually exist and that the alleged particles that we think it's comprised of are actually "...
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### Can the apparent weight in a elevator accelerating downwards be compared to that of body submerged in a fluid

Can the apparent weight of a person ( here I mean what a weighing scale would read if that person were to stand on it ) in a elevator going downwards( NOTE: the acceleration is less than the ...
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### Method of Images problem

I was working on the problem of finding force on charge $q$ in the middle of two infinite conducting plates (where we have an infinite set of images) and I came up with an interesting idea to replace ...
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### Wheelbarrow stuff [closed]

We're working on a worksheet in physics and everyone in the class has different answers and the teacher is gone today. He allows us to use literally anything we need to try to figure out a problem, ...
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### Equations that establish gravitational effects in quantum mechanics [closed]

Many popularizers of physics throw around the term "quantum gravity," before listing a whole series of reasons why we haven't been able to come up with such a theory yet. My question is, are ...
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### Can the classically described singularity be a smooth fractal?

I plan to study general relativity in the next few months and for now I keep gathering information about it. I read here and there that Penrose and Hawking proved that general relativity as we know it ...
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### How to determine wave function for a free particle in a complex potential function?

The potential for a free particle in a potential field is given by $$V(x) = V_0\theta(x) - w\delta(x)$$ in which $\theta(x)$ is the unit step function, $\delta(x)$ is ...
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### Understanding how the Electromotive force is created

I was reading about the Faraday Law, and there is one thing that is not clear to me. If we have a wire loop, and we change the magnetic flux that passes through the surface that this wire encloses, ...
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### Stress tensor of complex scalar field

The canonical stress tensor is defined by $$T^{\mu\nu}=\frac{\partial\mathcal{L}}{\partial(\partial_{\mu}\phi_a)}\partial^{\nu}\phi_a-\eta^{\mu\nu}\mathcal{L}.$$ where $\phi_a$ are the different ...
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### Could Planck scale wormholes be a solution for non-locality?

When two entangled particles are separated, an observation made on one particle seems to be able to act at a distance to determine the state of the other particle. Is it possible, or has anyone in ...
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### Help with variation of the 3-dimensional $\sigma$-model action

Consider the following action $$S=\int\mathrm{d}^3x\sqrt{h}\left[R^{(3)}-\frac{1}{4}\mathrm{Tr}\left(\chi^{-1}\chi_{,i}\chi^{-1}\chi^{,i}\right)\right]$$ where $h$ is the determinant of the 3-...
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### Entaglement and causality violation. Devices that detect pure states

I have not seen a similar question to mine on the site. Please forgive me if there is any. When I studied quantum mechanics and the entanglement phenomenon, I was told that, although when one wave ...
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### What is the physical meaning of the anticommutator of two observables? [duplicate]

It is quite clear to me that when two operators commute it implies that two different observables associated with the respective operators can be measured simultaneously with the exact accuracy. But ...
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### Why the entropy of non-zero nuclear spin is zero at $T = 0$?

When reading Concepts in Thermal Physics (second edition) by Stephen and Katherine about the concepts of the third law, I met with such a problem. The text reads as follows: Consider a perfect ...
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### Net force if given tension angle and friction force

an inextennsible cable of negligible mass is used to drag a log on a rough horizontal surface .The tension in the cable is 850N and acts at an angle of 35 horizontally. the log dragged over a distance ...
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### Flux due to charge placed ON a sphere

Question If we place a charge, say Q, ON the surface of a sphere, will there be a flux through the sphere? My Understanding of the Concept I'm studying high school physics, so I have learnt Gauss's ...
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### Holography vs edge modes in Chern-Simons theories

There are two facts I have heard about Chern-Simons theories They are dual to a WZW theory on the boundary (e.g. Relation between CS/WZW and AdS/CFT). They require a chiral theory on the boundary due ...
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### Approximately Conformal Boundary Conditions for a 2d CFT

Is there any work on approximately conformal boundary conditions for a two-dimensional CFT? Consider a CFT on the upper-half-space $M$, and let the boundary conditions at the real line ($\partial M$) ...
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### Why aren't triplet states in Hydrogen molecule bound by Pauli's principle?

I am learning about the wavefunction of two electrons in a Hydrogen molecule and I came across following spin states: |↑↑⟩ and |↓↓⟩, which form two of the triplet states in the molecule. This confuses ...
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### Why does the speed of the escape velocity approch to 0?

I am a little confused about escape velocity. Does the escape velocity always approach 0 as we go infinitely far distance even if there is no friction? If so why does it approach 0? Shouldn't it be ...