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Why laplacian appear in so many places? [duplicate]

diffusion equation, Schrodinger equation, poison and laplace equation,sound wave equation these all equations are having one thing common, LAPLACIAN.why this quantity pops up everywhere? is there any ...
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1 answer
25 views

What will happen if the inner walls of the perfect blackbody (shown in the image) is made perfectly reflecting?

I am curious about making the inner walls of this blackbody perfectly reflecting instead of perfectly absorbing. If I keep on giving in incident radiation,the photon density inside will increase, ...
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How is energy conserved in interference of light waves? [duplicate]

How is energy conserved during interference of light waves? I already know a textbook explanation that in constructive interference the resulting intensity is four times the initial intensity and for ...
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1 answer
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Confused about the scattering Operator in LSZ reduction formula

In Greiner's Field quantization book, Chapter 9 on the LSZ reduction formalism, he states $$S_{fi}=\langle q_1,...,q_m;\text{out}| p_1,...,p_m;\text{in}\rangle\tag{9.10}$$ where $S$ is the scattering ...
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Experiment on gravitational redshift with receivers at different altitudes

Is there a repeat of the Pound-Rebka experiment on gravitational redshift, where the receiver was placed at two different height distances from the transmitter? What was the result of the experiment?
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2 answers
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Why are composite particles grouped with force carrying particles?

This Huge confusion with Fermions and Bosons and how they relate to total spin of atom says a hydrogen atom is a boson. I'm confused. I thought bosons were force carrying - e.g. a photon, gluon, etc?
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3 answers
47 views

Force of an accelerating ball?

By Newton's second law, we know that $F=ma$. If I were to drop a ball from some height, it would accelerate to the ground with gravitational acceleration. But if I were to drop the same ball from a ...
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·Can Physically Apply + Positive Newton's III Third Law of Motion as Cycle System in a Motor Engine [closed]

*Physically Can Apply Positive/in favor Newton's III Third Law of Motion as a Cycle System in a Motor Engine Device? 3rd Thermodynamic Law Applicate in a Power-Plant Device. The Reaction at From of ...
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1 vote
1 answer
44 views

Do the null geodesics of photons emitted by Hawking radiation arise from the event horizon?

It is a well-known explanation of Hawking radiation that it originates from the quantum fluctuations near the horizon. Does it mean that one can look at the photons (part of the radiation) and follow ...
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2 votes
1 answer
52 views

To what extent can phase transitions be made rigorous?

It seems a lot of physical intuition in statistical mechanics, for example phase transitions, critical temperature, scaling hypothesis, renormalization group methods etc. should have a purely ...
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1 answer
22 views

Can I treat fluid a rigid body calculating Moment of inertia? If not, how would I calculate it?

I wanted to set out calculating how much moment of inertia a bottle of water had so as to see how fast a bottle had to spin to create a non-zero constant angular velocity in the water. As I understand,...
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Confusion on FFT grid size used in ab initio calculations for periodic systems with planewave basis

Sorry it might be a long post, I'll try to write down all my thinking on this topic, and it is appreciated to point out whether and where I make mistakes. We are talking about algorithms used in ...
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1 answer
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Why does temperature remain constant in this case?

Consider two ideal gases of differing heat capacities. They occupy separate compartments of the same total volume. Assume their initial temperatures and that the amount of substance for both are the ...
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5 votes
2 answers
438 views

Do supermassive black holes at galactic centers and the galaxis containing them spin in the same axis?

If the galactic mass is rotating around a central supermassive black hole, should their spin axis not be the same, just as we would obtain for the rotation of a star and its planets ?
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Why does magnetic dipole's north pole pointing in the direction of magnetic field increases overall magnetic field?

I'm studying magnetic moment- current loop, electron/proton/charge revolving, magnetic dipole etc. This is about paramagnetism of materials. According to me magnetic dipole's south pole is attracted ...
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1 vote
1 answer
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What causes the flow of electrons from anode to cathode in a Daniell cell?

In a standard galvanic cell, I understand that electrons flow from the anode to the cathode. For example, in a Daniell cell, electrons flow from the Zn anode to the Cu cathode. Before the two half-...
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1 vote
1 answer
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Absence of velocity in energy conservation

Here we see a rod at rest hinged about a point. We want to know the angular speed of the rod when it becomes vertical as shown in the figure. The solution which is given in the books goes like this. ...
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Gravitational Aharonov-Bohm Effect and Conservation of Energy

Like for the electromagnetic Aharonov-Bohm effect of QED, the gravitational Aharonov-Bohm effect has recently been proven in an experiment done by Ch. Overstreet (2022), showing that the gravitational ...
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3 answers
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Can you see on Miller's planet?

I have read this question: Miller's world would be fried by a strong flux of extreme ultraviolet (EUV) radiation. The cosmic microwave background (CMB) would be blueshifted by gravitational time ...
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1 answer
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Confusion on number of component of Cauchy stress tensor

The Cauchy stress tensor is often presented as a tensor having $(2,0)$ tensor having nine components in any given basis. However, I think it should actually be $6 \times 3 =18$ because a cube has six ...
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Finding the stress tensor in the Stokes equation from free energy

I have found free energy for a system of particles. The free energy is a functional of a scalar field which is the area of the particles. So I have the following free energy $F(A,p)= \int dx dy \: f[A(...
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Would Quantum foam wormholes move at relativistic speeds?

I have heard theories that fluctuations in the quantum foam create subatomic wormholes. If this is correct, would subatomic wormholes have so little mass that they would move at relativistic speeds, ...
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7 votes
2 answers
533 views

Contradiction in my understanding of wavefunction in finite potential well

Most things like to occupy regions of lower potential. So the probability amplitude should be higher in a region of lower potential. I denote the potential by V. However, we also know that the kinetic ...
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21 views

Notation for the metric of $\rm dS_4$ and/or $\rm AdS_4$

4D de Sitter and anti-de Sitter spaces may have their metrics inferred from the induced metric on a hyperboloid embedded in 5D Minkowski space: $$ -( x^0)^2+( x^1)^2+( x^2)^2+( x^3)^2+( x^4)^2=\pm \...
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1 vote
3 answers
58 views

Using quasistatic processes to calculate quantities

This question is inspired by Reif Problem 5.5. Note that it is not a homework problem and, even if it were, my question only loosely relates to it. A vertical cylinder contains $N$ molecules of a ...
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Are there any theories of the beginning or the end of the universes where all fundamental symmetries would be violated?

There are theories trying to explain the origin of the universe (such as inflationary models) and also theories that try to predict how will the universe end (like the "big freeze", the &...
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  • 517
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1 answer
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Moment of inertia regarding square lamina

The above is a square lamina of length $a$ and mass $m$. We are interested in finding the moment of inertia of it when it is rotating with a diagonal as the axis. Now my approach is using ...
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  • 579
3 votes
0 answers
26 views

Nonlinear symmetry realization: what is it for and caveats

I have several doubts regarding the nonlinear realization of a spontaneously broken symmetry and hope they are approppriate to be grouped, and I appreciate any insights. Consider the group breaking ...
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0 votes
2 answers
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Why are Eigenvectors of a 1D quantum ising hamiltonian real

I was modelling the 1D transverse quantum Ising model and made a Kronecker product loop to find the Hamiltonian of the system, for a given magnetic field configuration. Now, my question is that when I ...
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0 votes
1 answer
17 views

Effect of coil radius on magnetic induction

We've all seen the how dropping a magnet through a coil will produce a "blip" of voltage across the free ends of the coil, e.g.: https://www.youtube.com/watch?v=jaZfTXUv_lI How does the ...
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0 answers
19 views

Snell shortest path : would the path depend on the observer classically?

Fermat principle for deriving Snell's law reads formally as minimizing : $$T=\frac{x}{v_1\sin i}+\frac{L-x}{v_2\sin r}$$ Where $\sin i=\frac{x}{\sqrt{h^2+x^2}}$ is the incidence angle sine and $\sin r=...
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2 votes
0 answers
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Measurement operator in a Bell experiment

I'm trying to figure out why a Bell experiment gives rise to the payoff (measurement) operator used in this paper on quantum game theory. Two players are each in control of one half of an entangled ...
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-6 votes
1 answer
53 views

Why perpetual motion never works? [closed]

Thermodynamic laws prevent such a thing.
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1 answer
35 views

Why for cylindrical problems the order of the Bessel function is an integer value?

I read somewhere that for cylindrical problems the order of the Bessel function is an integer value while for spherical problems the order is of half integer value. I know that the Bessel functions ...
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0 answers
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Tilt angle of falling stick

In the ground frame, a stick of length $L$ oriented horizontally, is falling straight down vertically with a constant velocity $u$. In the frame of a person moving horizontally with respect to the ...
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Second born amplitude for the electron-hydrogen scattering

can someone explain how could they got the result of the second born formula written in equation 2.36 from 2.32 I failed to demonstrate it. The image from Byron 1973 here the link enter link ...
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1 answer
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Understanding the Cahn-Hilliard equation in terms of units

The Cahn-Hilliard equation may be formulated as $$ \frac{\partial c}{\partial t} = M \nabla^2 \left(\frac{\partial \hat f}{\partial c}\right), $$ where $c : \Omega \to [0,1]$ describes the ...
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0 votes
1 answer
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Magnetic susceptibility for ferromagnetic materials

Magnetic susceptibility $\chi_m$ is defined by following relation: $$\vec{M}=\chi_m \vec{H}$$ where $\vec{M}$ is the magnetization density and $\vec{H}$ is the magnetizing field, defined by $\vec{H}=\...
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Asymptotic States, Propagator and Commutation Relations

Following Fradkin's discussion in the book QFT Integrated Approach, the commutation relation for asymptotic states satisfies $$\left\langle 0\left|\left[\phi(x), \phi\left(x^{\prime}\right)\right]\...
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-1 votes
2 answers
52 views

Can we derive Ampere's Circuital Law from Gauss's Law or vice versa?

I was curious if it is possible to derive Ampere's Circuital Law from Gauss's Law as they are very similar and both can be applied for highly symmetrical problems $(Infinite\space wires,Rings..etc)$ ...
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0 votes
1 answer
31 views

Is the definition of work related to the nature of the fundamental interactions?

I am having troubles trying to understand why is work defined as it is. So, I know how work is defined: $W = \vec{F}\cdot{}\vec{d}$ (F is the force, d the displacement) and I am okay with it. This, ...
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1 vote
1 answer
32 views

UV & IR radiation are mostly absorbed by water. Our eyes are predominantly water. If our eyes were made of another liquid, could we see slight UV/IR?

Firstly, I'm not a physics guy. I just think it's neat, that's all. I was reading a post on here about why water is transparent (I was more interested in why milk isn't) and saw a graph that showed ...
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0 answers
18 views

Is there a theoretical equation for the $d$-spacing for an amorphous solid or a metastable crystal?

So I am trying to study and compare two different samples (thin films) of the same material but with different cooling rates. I made one that was cooled slowly, allowing it to nucleate and crystallize ...
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-2 votes
0 answers
20 views

A substance has the volume expansivity and isothermal compressibility: $β = 1/t $; $k = 1/p$ find the equation of state [closed]

a substance has the volume expansivity and isothermal compressibility: $β = 1/t $; $k = 1/p$ find the equation of state
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0 answers
20 views

Do Higgs fields commute with $W^1$, $W^2$, $W^3$, $B$, and why?

In Standard Mode, we have quantities such as : $\left(D_{\mu}\Phi\right)= \left( \begin{array}{cc} \partial_{\mu}+i\frac{g}{2}W_{\mu}^3+i\frac{g'}{2}B_{\mu} &i\frac{g}{2}W_{\mu}^1+\frac{g}{2}W_{\...
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1 vote
2 answers
78 views

How does string theory relate superposition and general relativity?

So I know that in general relativity, superposition cannot be true. However, since string theory supposedly connects quantum mechanics and gravity, how does it relate superposition and gravity?
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1 answer
46 views

Finding Equation of motion from Lagrangian

I am given the following Lagrangian: $$L = \frac{ml^2}{2}\left((\dot\theta)^2 + \sin^2{(\theta)}\dot\phi^2\right) + \frac{I\dot\phi^2}{2} + mgl\cos{\theta}$$ which is meant to represent a simple ...
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-1 votes
1 answer
35 views

Dispersion of $E$ and $p$

Is the dispersion of $E = c$. dispersion of $p$? Following that: $\begin{aligned}E^{2}=c^{2}p^{2}+m_{0}^{2}c^{4}\\ \Delta E^{2}=\Delta c^{2}p^{2}\\ 2\Delta E=2\Delta c^{2}p\\ \Delta E=c^{2}\Delta p.\...
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  • 659
3 votes
0 answers
28 views

What is the meaning of mutual quantum information?

I'm having some difficulty understanding the physical meaning of the mutual information that two subsystems share with each other. For example, if $\rho_{AB}$ defines the matrix of a bipartite state, ...
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4 votes
1 answer
88 views

The reasoning of the definition of $S$-matrix

The definition of the $S$-matrix is given by $$S=\lim_{t_{f}\rightarrow\infty}\lim_{t_{i}\rightarrow-\infty}U(t_{f},t_{i}).$$ Where $U(t_{f},t_{i})$ is the evolution operator, given by the $$U(t_{f},...
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