# All Questions

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### the definition of the vacuum state of quantum field by path-integral

In the review Entanglement entropy of black holes by Sergey Solodukhin(arxiv:hep-th /1104/3712, equation 13), I see a definition of vacuum state of quantum field by path integral over half of the ...
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### Can the Berry Connection be derived from a metric?

The Berry Connection is $$A_\mu(R)=-i \langle \Psi(R) |\partial_\mu \Psi(R) \rangle$$ which allows us to parallel transport a state indexed by $R$. We can integrate the Berry Connection to get the ...
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### Why physics hasn't broke through the “glassern ceiling” of homogenity in time?

Physics assumes in EVERY theory that a physical system is time-translation invariant and also space-translation invariant, i.e. physical laws are regarded to be the same on the whole spacetime. Why ...
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### How to accurately mark with arrows an electric circuit

As I understood, we choose to mark the current with arrows going from the plus pole to the minus pole (even though we know that in reality it is the contrary). As I'm looking at electric circuits, I ...
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### Point of no return when falling into evaporating black hole

Captain Foolhardy in his trusty spaceship The Test Particle is in radial free fall towards an evaporating uncharged black hole with no spin in an otherwise empty universe which asymptotes to ...
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### Are the electrons at the centre of the Sun degenerate or not?

Trying to find an answer to this question, I came across two different methods of determining whether electrons at the center of the sun are degenerate or not. The first method, used here, calculates ...
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### What is background independence and how important is it?

What is background independence and how important is it? In order to be a theory of everything, will the final string-theory/m-theory have to be background independent? Does the current lack of ...
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### resultant of 3 vectors along sides of equilateral triangle

It is a homework problem, but I really don't quite understand the question. It reads- "3 forces of magnitudes 10N, 20N, and 30N acting on a point are parallel to the sides of an equilateral triangle, ...
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### Does really m^3/m yields m^2?

Apart the evidence that such assumption works in physics formulae, how can someone make a statement like that? Maybe the formuale works just because are worked around that assumption? Why phyicist do ...
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### Is gravity's acceleration rate - squares of all natural numbers?

I've seen some science\history documentary in which they made a replica of Galileo's inclined plane experiment. They rolled a ball down the plane; and it's progressed in length units each unit of ...
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### How fast is a sample damaged when exposed to a femtosecond laser?

A femtosecond laser can cause damage to a sample, either due to thermal damage, ionisation effects or some other effects. A pulse picker can be used to reject some of the femtosecond pulses. This ...
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### Question of mechanics? [on hold]

There are two parallel train . One of the train overtake another one in 20 second and that train crosses another train in 10 sec. The length of train is 200m How to calculate the velocity ? Here is ...
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### Conduction bands and Valence Bands, so do electrons really exist in Conduction bands?

My understanding is that conduction bands are energy bands created from the electrons in the valence shells/bands absorbing energy to jump to the next level (is this true)? Now, do the conduction ...
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### momentum uncertainty of free particle

I've read several Q&A's regarding free particles and the associated wave packet in this website, but found the answer to my question nowhere. It's OK to attribute a Gaussian wave packet to the ...
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### How does phonon scattering change the distribution function?

For a one-dimensional structure, we know that the modified distribution function has the following energy dependency in equilibrium: Z(\varepsilon)\,f(\varepsilon) = ...
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### Why does a certain covariant derivative of the stress-energy tensor vanish due to diffeomorphism invariance?

In Polchinski's string theory volume 1, at chap 1.2, after equation 1.2.22, he says $\nabla_{a}T^{ab}=0$ as a consequence of diffeomorphism invariance. But I cannot derive it. My derivation is as ...
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### The correct integral for Newton's shell theorem

I am trying to prove Newton's Shell Theorem, and the natural integral I think of to use does not seem to give the right answer, so I am trying to find my mistake. For simplicity, I will work in one ...
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### Energy conservation in electrodynamic system?

Consider two charged particles initially at rest in the configuration below. Let us assume the following: Starting at time $t=0$, we apply a constant force $f$ to the the bottom particle so that ...
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### How to formulate pin joints in finite element modeling of solid frames

In frame analysis with finite element every node can be assumed with 6 Degree of Freedom (3 translation DoF and 3 rotational DoF). There are formulations over the web for creating stiffness matrix ...
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### Electric field of two non conducting spherical conductors [on hold]

The force between two point charges +Q and -Q placed r distance apart is f1 and the force between two spherical conductors, each of radius R with charge +Q and -Q is f2. IF THE SEPARATION " r " IS NOT ...
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### How can I understand a Vortex Tube and its efficiency?

A Vortex Tube takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. Apparently, it has been described as a Maxwell's Demon. Both ...
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### If all motion is relative, how does light have a finite speed?

Firstly, I'd like to apologize. This is probably a dumb question for whoever can answer it. My question is a conceptual one. I've often heard that Einstein shattered the notion of absolute motion ...
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### Eigenstates of sum of creation and anihillation operators

Does the operator $a+a^\dagger$ have eigenstates? If yes, what are they?
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### A misconception in an application of Newton's laws

When a body is kept in ground, and is at rest, the downward gravitational pull is balanced exactly by the Normal Reaction if we assume the earth to be an inertial frame. But this would mean that any ...
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### Why do we consider Lagrangian densities in QFT?

My question is: Why do we consider Lagrangian densities in QFT (as opposed to Lagrangians as in classical mechanics)? Is it simply because of the following? We wish the theories to be Lorentz ...
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### Can the stress energy tensor have nonzero value in a vacuum region?

In general relativity, when solving for the schwartzchild solution, we set $T=0$. 1) Is it possible for the stress energy tensor to have nonzero value in a vacuum region? 2) Is the stress $T=0$ in ...
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### Which theory explains the path of a photon in Young's double-slit experiment?

In Young's double-slit experiment, we know that a photon goes through either one of the slits but we don't know which one, and it ends up on a screen. I want to know which theory can predict to the ...
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### Electromagnetic braking hypothetical situation?

Let's say there is a conducting rod in space - with a constant external magnetic field. If you spin the rod - will electromagnetic braking make the rod stop eventually? I feel like there is a change ...
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### Ohm's law holds at constant temperature - what about Joule heating?

Ohm's law states that the resistance of a conductor is constant provided its physical conditions, such as temperature, remain constant. But what I'm thinking is that as you increase the voltage across ...
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### What role does “spontaneously symmetry breaking” played in the “Higgs Mechanism”?

In talking about Higgs mechanism, the first part is always some introduction to the concept of spontaneously symmetry breaking (SSB), some people saying that Higgs mechanism is the results of SSB of ...
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### Is helping others grasp difficult ideas, concepts or theories not considered giving an answer? [migrated]

I know you could just leave a comment, but not in this case I couldn't. I just found this awesome app while researching on my passion, physics. I stumbled across a question about time dilation, not ...
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### Exact meaning of locality and its implications on the formulation of a QFT

As far as I understand it, locality in physics is the statement that interactions can only occur between physical objects if the spacetime interval separating them is null or time-like. Thus, if the ...
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### $\sqrt{\frac{\omega ^2}{c^2}-k_z^2}$ in cylindrical harmonics

The radial component of the solution of the wave equation in cylindrical coordinates is $$J_\nu \bigg(\rho\sqrt{\frac{\omega ^2}{c^2}-k_z^2}\,\,\bigg)$$ But I always thought that $\frac \omega c$ was ...
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### “Skidding” to Brake on a Skateboard

When people are skateboarding, sometimes they kind of push out their back foot when they turn while carving, causing the board to skid a bit. I was wondering the physics behind this, any details on ...
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### Quantum Entanglement vs Universal Law Of Gravitation

I'm an 18 year old from India. I'm neither a science student, nor do I intend on pursuing it now. But I have a slight interest in the concept of Quantum Entanglement and I am heavily into movies. Now, ...
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### Resource for Symmetry Breaking in Terms of Roots and Weights?

I'm currently searching, for quite a while now, for a paper/book that discusses Higgs symmetry breaking in terms of roots and weights. Concretely I have in mind a discussion of what happens when we ...
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### Recommended books that describe emission and absorption of light and light/matter interactions

Hello I am a Chemical Engineer that has ventured into the field of optics and I was wondering if there is any recommended book/books to understand the maths and also some answered exercises or ...
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### Isn't the fact that gravity acts exactly like velocity seem profound? [on hold]

I was just reading something that I guess I knew all along, which is that time slows down as gravity increases. I guess it was the way it was phrased, because I knew that was true with black holes ...
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### Explain the Föppl–von Kármán equations

I am a newbe to elasticity. Could someone please explain to me briefly how the Föppl–von Kármán equations work? What are we trying to solve for? Is there some kind of intuition to the way they look? ...
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### Conservation of Angular Momentum, as related to a flywheel

Trying to work out some pesky flywheel dynamics for a project I'm working on, would love some for your assistance to better understand the underlying concepts. For a given flywheel (thin-walled ...
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### Magnetic field due to current carrying sheet

I have 2 infinite plates (x direction) with the width of A (y direction) and the distance between them is d (z direction). The current density is J on the first plate and -J on the second (the ...
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### Gravity on the International Space Station - General Relativity perspective

My question is an extension to this one: Gravity on the International Space Station. If all the outside views of the ISS was sealed, then the crew inside would not be able to tell whether they were ...
### Calculate pressure with the ideal gas law or $\rho gh$?
In a container the pressure on the walls due to a gas can be calculated using the ideal gas law $\bigl(pV=NRT\bigr)$. However, for a column of water, or the pressure at earth's surface due to a ...