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12 views

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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0answers
7 views

How do core electrons factor into the $f$-sum rule?

Kubo gives in Statistical-Mechanical Theory of Irreversible Processes section 8 the sum rules for conductivity, $$ \frac{2}{\pi} \int_{0}^{\infty} d \omega \operatorname{Re}\left(\sigma_{\mu \nu}(\...
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0answers
15 views

How do you determine these coefficients experimentally?

So in our lab today my friend and I had to examine the time it takes for cylinders to roll down an incline. We managed to find an ansatz on the following form after some work: $$ t = c(c_1 + x^{c_2})^{...
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0answers
10 views

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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1answer
18 views

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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1answer
9 views

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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0answers
18 views

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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0answers
24 views

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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0answers
6 views

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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1answer
33 views

In 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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0answers
12 views

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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5 views

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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1answer
17 views

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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0answers
13 views

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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0answers
23 views

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 ...
-1
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0answers
14 views

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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0answers
32 views

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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2answers
28 views

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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0answers
17 views

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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0answers
18 views

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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0answers
15 views

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 ...
0
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1answer
24 views

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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1answer
31 views

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 \begin{equation} V(x) = V_0\theta(x) - w\delta(x) \end{equation} in which $\theta(x)$ is the unit step function, $\delta(x)$ is ...
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1answer
18 views

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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0answers
32 views

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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1answer
33 views

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 ...
1
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1answer
27 views

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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0answers
20 views

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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0answers
22 views

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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2answers
23 views

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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1answer
19 views

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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2answers
24 views

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 ...
4
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0answers
27 views

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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0answers
10 views

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 ($\partial M$) ...
1
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1answer
24 views

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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2answers
46 views

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 ...
-1
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1answer
21 views

A boat pulling a water skier [closed]

Is the force on the boat greater than the force on the water or is it less, or is it the same as the water skier? The mass of the water skier is 68kg. My answer was that the boat has a higher force ...
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0answers
18 views

Is it really necessary for a Hamiltonian density to vary with a total derivative under symmetries?

I am studying the effect of infinitesimal transformations on a hamiltonian field theory, $\mathscr{H}(\mathscr{Q}_n,\mathscr{P}_n,\partial_i\mathscr{Q}_n,x)$, with scalar fields and momenta $\mathscr{...
1
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1answer
19 views

How to calculate the maximum Elongation of Mercury seeing from the Earth?

I want to calculate the maximum Elongation of mercury seeing from the Earth but not by this assumption that its orbit is circle but it is an ellipse with semimajor axis a = 0.387 AU and e = 0.2 and ...
1
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1answer
39 views

Angular Frequency of Wave Function

I encountered this while finding separable solutions to Schroedinger's equation: $$i\hbar\frac{\partial\Psi}{\partial t}=-\frac{\hbar^2}{2m}\nabla^2\Psi+V\Psi.$$ By writing $\Psi$ as $\psi(x)T(t)$, we ...
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1answer
22 views

Derivative with respect a partial derivative in lagrangian density [duplicate]

I am reading about field theory and lagrangian densities, and I found the following lagrangian density in my book: $$ \mathcal{L} = \dfrac{1}{2} (\partial_\mu \phi)^2 - \dfrac{1}{2} m^2 \phi^2\ , $$ ...
1
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4answers
62 views

How does the speed of light being measured by the observer who is in motion?

I am trying to understand the idea that the speed of light is constant for each observer regardless of their motion. But I don't understand how come? I also don't understand how this was measured. Let'...
3
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1answer
33 views

00th component of the electromagnetic energy-momentum tensor

I'm working through relativistic electrodynamics, but I'm stuck on determining the $00$-th component of the energy-momentun tensor. The (contravariant) 2-tensor (in Heaviside-Lorentz units) reads: $$T^...
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0answers
14 views

Can we solve this using conservation of angular momentum? [closed]

This can be solved using impulse, but if conservation of angular momentum is used for the system of them, then the answer comes out to be different. Two cylinders, one of radius α and other of radius ...
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0answers
32 views

Klein paradox in the massless case

I have a question about the Klein paradox in the massless case, for a potential step of height $V_0$ (this is exactly the situation described by Wikipedia). I don't have a problem to understand the &...
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0answers
19 views

What does it mean to take the absolute value of a fermion correlation function?

I’m trying to understand the details of Weingarten inequalities (Tong has a good summary of them in his Gauge Theory notes). I’m confused about the following: Say we have a fermionic operator $B_\...
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0answers
18 views

Derivation of the force between two current loops [duplicate]

Could you explain me how to derive the force between two current loops carrying currents $I_1$ and $I_2$, written as a double loop integral: $$ \vec{F}_2 = \frac{\mu_0}{4 \pi} \oint \oint \frac{I_2 ...
1
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1answer
26 views

Dirac equation with an external Yang-Mills field

We know that the Dirac equation of a fermion coupled with a Yang-Mills field contains a term $i\gamma^{\mu}A_{\mu}\Psi$ describing this interaction (i.e. the interaction does not depend on the value ...
1
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0answers
13 views

Casimir conformal generator of $SO(d+1,1)$

The purpose of this post is to ask the help of derivation of equation 2.8 of https://arxiv.org/abs/2106.10822 Let $P_i$ be a point on the conformal boundary of $AdS_{d+1}$ and $Z_i$ be a polarization ...
2
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0answers
62 views

How was this warp bubble generated?

The Debrief just reported that DARPA just "accidentally" created the world's first warp bubble. The actual paper is available here. My layman understanding is that warp drives were so far ...

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