# All Questions

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### how is Helmholtz free energy related to grand partition function

According to my source of notes $$A = kT \left ( N \ln z-\ln Z(z, V, T)\right)$$ where $z=e^{\beta \mu}$, $\mu$ being chemical potential, $Z(z,V,T)$ is grand partition function. The derivation ...
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### Energy conservation in Hubble Expansion

So space expands due to Hubble Flow. Some light is observed, which is seen to be red-shifted due to the space expansion. It is less energetic. Where did the energy go?
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### Writing Breit-Pauli spin-spin-coupling Hamiltonian as a sum of irreducible spin tensor operators

The spin-spin interaction part of the Breit-Pauli Hamiltonian for two electrons is essentially (omitting constants and the contact term) \frac{3(\mathbf S_1 \cdot \mathbf r)(\mathbf ...
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### Feynman diagram; $\pi^0+\pi^0\rightarrow \pi^++\pi^-$

For the reaction below draw three Feynman diagrams, one that proceeds through exchange of a gluon, one through a photon and one exchanging a weak W-Boson. $\pi^0+\pi^0\rightarrow \pi^++\pi^-$ Which ...
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### What would it look like if the earth were suddenly put on the anti-node of a gravitational standing wave with a huge amplitude?

Or could gravitational standing waves even exist?
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### Is it necessary to consider relativistic effects when looking at an electron-photon collision?

I am looking at the collision of a photon into a stationary electron with the aim of computing the velocity of the electron after the collision in terms of the initial photon wavelength. Of course ...
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### The existence of gravity waves has just been demonstrated by direct observation, 2016

How does the apparent confirmation of gravity waves effect String Theory today
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### What is the proof of gravity being curvature of spacetime?

According to general relativity gravity is an illusion caused by curvature of space-time, rather than real force. As I understand there is overwhelming experimental evidence to support general ...
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### How is it that gravitational waves can be detected by measuring distances?

If gravitational waves stretch space itself, how can they have an effect on the interference of two laser beams? According to my understanding, compression or dilation of space itself (as opposed to ...
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### Is all energy quantised?

I'm wondering if all energy is quantized.Can a particle with kinetic energy have any value of kinetic energy or is kinetic energy also quantized?My reason for asking this is that if a particle is ...
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### Gravitational waves now detectated

Based on the information released so far, have we learned anything new from the detection of gravitational waves at LIGO?
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### Electromagnetic Induction, specifically induced voltage

Why is it that if a conductor 'cuts through' (what does this mean?) a magnetic field that a potential difference is produced? I read this recently in a textbook...
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### Special Relativity and the Light Clock and which direction was the laser pointing?

If I were to make a light clock, I would have some sort of laser pen attached to the top of one of two mirrors. That laser pen would be perpendicular to the surface of either/both mirrors. I would ...
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### What is the converted force from the weight force to the rotational force?

What is the converted force from the weight force to the rotational force? How far the 2,000 lbs (gross weight) car can drive (or move) with the rotational force that uses gears to converted from a ...
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### How do large interferometers work?

In very large Michelson interferometer a such as LIGO, how can we keep the two light paths at the exact same distance in order to avoid any unwanted and noisy fringes shift? When I used to make ...
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### At which point are gravitational waves generated when two black holes merge?

I was reading today's announcement of the gravitational waves and was wondering about this situation where there are two orbiting black holes. Did the wave come from the final merging or was it from ...
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### The Ideal Gas Equation in higher dimensions

Basically I wanted to know whether or not the ideal gas equation, $PV=NkT$ would hold in higher dimensions? If so, how would you go about proving this? I can't see any reason as to why it shouldn't ...
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### How did LIGO detect the source location of the black holes mentioned to be the cause of today's announcement?

Today LIGO announced discovery of Gravitational waves. What method was used to determine the source location of the waves?
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### Can two different objects or system of molecules have different temperatures, but having same internal kinetic energy?

If I take an extreme case, where a body has only an internal potential energy with zero internal kinetic energy, does this body have a temperature? Another question related to it: if two objects A and ...
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### Force required for balancing a three-bodied frictionless and gravityless system

Hello everyone, I made calculations over calculations but I think my free body diagrams were wrong. There are three bodies here: on two of them (the 2 and 3) are applied two forces (F2 and F3) ...
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### Redshift of merging black holes

How did they found that the gravitational waves where emitted at redshift $z=0.09$? I understand the measurement of redshift for an electromagnetic wave where we have measured in a lab various ...
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### Gibbs isotherm and calculating interfacial tension change from first principles

Question: Is it possible for a solid particle to change the surface tension between two phases? (or: Does a solid particle have a chemical potential?) This question stems from the more ...
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### Einstein's original papers predicting gravitational waves?

I came across the original handwritten papers in which Einstein predicted gravitational waves: and since LIGO announced they've detected a signal confirming the predictions I was wondering if ...
In chapter 10 section 2 (on pomology) of Weinberg's QFT volume 1, he shows $G$ has a pole when the external line goes on shell. In the proof, he inserted a complete set of single-particle states ...