# All Questions

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### Change of basis in quantum mechanics-spin vectors

Suppose ${x,y,z}$ are my spin vectors, such as spin 3/2. If I define an operator to be $W = e^y$, I can write it as $\sum_i e^{y_i} \lvert y_i\rangle\langle y_i\rvert$ where $y_i$ are the eigenvalues ...
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### Working on reflection 1 [on hold]

If a number of images are seen in a thick mirror then which image will be brightest? first second last
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### Would the detection of advanced waves be possible in curved space-time?

In the transactional interpretation of quantum mechanics reference 1, inspired by the Wheeler–Feynman absorber theory, a transaction is formed between the emitter and absorber by a superposition of ...
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### How can I redraw this circuit where I can apply Kirchoff's Voltage and Current Law to get correct Voltage at V?

I thought I can draw the -5V as a source but it did not turn out to be right, unfortunately.
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### Boundary conditions in Electrostatics

If I have a grounded conducting material, then I know that $\phi=0$ inside this material, no matter what the electric configuration in the surrounding will be. Now I have a conducting material that ...
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### Why can't we see virtual photons?

If photons are the carriers for the electromagnetic force, then why can we not see electromagnetic fields, given that photons are involved in this interaction?
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### What is a point transformation?

This problem comes from Goldstein. What does $s=e^{\gamma t}q$ mean? Do I just put $q=e^{-\gamma t}s$ into the Lagrangian? But I don't know what that means. I think the point transformation may ...
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### Theoretically if you passed the speed of light in a medium, would there be a sonic boom equivalent?

I know that it is technically impossible to reach the speed of light in vacuum since the mass of the object travelling would reach infinity. However in a medium, would there be some sort of ...
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### What is the smallest possible packing of earth air inside a box?

It is often stated that all the air in a room could suddenly rush into one corner but that there is simply a very low probability that it would do so. One possible way this could in fact happen is via ...
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### Problem in understanding the concept of 'superposition' as explained by Dirac

The concept of 'superposition' has really made me insane, actually. What I thought it was just simple superposition of matter waves. For instance, let's take the Double-Slit experiment: take the ...
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### What are the gradient lines of the magnetic field of a cylindrical magnet?

The problem is identical to that for a finite-length solenoid. I was interested in finding the family of curves for the equipotential lines, rather than the B field itself, which is more readily ...
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### Density Functional Theory (DFT) tutorial guide

I am going to start learning about DFT calculations. Could anyone advise me the best starting point for that? Simple example guiding tutorial with explanations would be great. Any input would be ...
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### Spinor field normalisation from poles in the propagator

In the theory of free scalar bosons (KG field) it is a basic result that the propagator $\Delta(p)$ has poles at $p^2=m^2$, with residue $1$ (or any other constant, depending on conventions). Thinking ...
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### How do radio signals contribute to gravity?

First, the inspiration for this question: I just read that it takes one hour to send a picture from the New Horizons space probe, to Earth. It also takes around 5 hours for that picture to reach ...
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### Can the mass of an object be changed by adding opposing magnetic fields

Apparently it does. Or is this vodoo physics. If not what is really happening in this case? https://www.youtube.com/watch?v=8N2TS3VReTA Boyd Bushman Changed the mass of a black box object, rock. He ...
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### Is work energy theorem $W=\Delta K$ or $W=\Delta E$?

I know that work is the change in kinetic energy. I saw on an equation sheet that work is equal to the change in total energy. Is this true? If so, how?
171 views

### Is it possible to make an electromagnet with two like ends?

Is it possible to create an electromagnet with one continuous wire with 2 like poles (i.e. both ends either north or south)?. Visualising it with the right hand screw rule for current carrying ...
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### What has the potential energy: the spring or the body on the spring?

Particles have gravitational potential energy due to its position in the gravitational field. We say the particle has potential energy and not the Earth (the body doing the work). Why is it not the ...
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### Exertion of force due to spin of Earth

If Earth were to spin at a different speed would I be experiencing a different weight?
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### Clustered-Entanglement thermodynamics

I want to understand the tools required to do calculations in entanglement thermodynamics in general, but I'm considering first how to model entanglement mathematically in terms of many-body systems ...
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### Struggling with a conversion. Showing number of atoms per cubic meter using pressure in Torr

I have been trying to show that the following is true: N = 9.65 x 10^24 x (P/T) Where N is the number of atoms per cubic meter of an ideal gas at pressure P (in Torr) and temperature T (in Kelvin). ...
29 views

### Gas Mass Calcuation in Galaxy Cluster

Rob Jefferies found this document for me that describes how the X-Ray emissions from galaxy clusters are used to calculate the mass of the cluster. I'm unable to follow the steps to calculating the ...
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### How does a change in birefringence correspond to a change in intensity?

In the optical Kerr effect, a pump beam causes birefringence in a sample, while a probe beam measures the return of the sample to equilibrium. My question is, how does the change in birefringence ...
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### Why do neutrino oscillations imply nonzero neutrino masses?

Neutrinos can pass from one family to another (that is, change in flavor) in a process known as neutrino oscillation. The oscillation between the different families occurs randomly, and the likelihood ...
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### Prove that Laughlin's 3-electron states are a complete set of states

In R. B. Laughlin's 1983 Physical Review B article, Quantized motion of three two-dimensional electrons in a strong magnetic field, Laughlin separates out the center of mass motion of the electrons, ...
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### How to find a metric of a general observer?

Yes, that's it. How to find a particular metric of an observer in general relativity? Let's say we have a static metric: ...
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### Why does all the current flow through a short circuit if its voltage drop is considered zero?

Path of least resistance vs. short circuit I know the path of least reisistance has been clarified already However, to derive the equations you need to assume that the voltage of each parallel ...
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### Does an object's redshift actually decrease with time?

I am trying to determine how an object's redshift (specifically, redshift due just to the expansion of the universe) changes in time. Starting with a definition of the Hubble parameter, H \equiv ...
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### Evolution of a system - Thermodynamic potential

There is something I don't understand about thermodynamic potentials. We know that the necessary condition for a system to evolve is an increase in entropy. For a system kept at constant volume and ...