# All Questions

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### Is Information Considered Matter?

The new internet distribution technology called LiFi is supposed to employ light in the process of transferring internet packets (which are essentially ...
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### Commutator of Operators

So in a spin-orbit interaction there's a hamiltonian and a corresponding perturbation. The spin of the particle is $1/2$. The unperturbed hamilton, $H$, commutes with $L^2, J^2, S^2$. How do you find ...
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### Binary Black Hole Solution of General Relativity?

This is rather a technical question for experts in General Relativity. An accessible link would be an accepable answer, although any additional discussion is welcome. GR has well known solutions ...
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### The size of an electron

Considering that an electron is a quantized excitation of the Dirac field, why are there still discussions regarding the "size" of an electron? Isn't the "size" of an electron simply defined as the ...
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### Turbulent spacetime from Einstein equation?

It is well known that the fluid equations (Euler equation, Navier-Stokes, ...), being non-linear, may have highly turbulent solutions. Of course, these solutions are non-analytical. The laminar flow ...
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### Why do we need virtual particles?

I understand the $\Delta t \cdot \Delta E \geq \hbar / 2$ relationship and the idea behind them. However, I don't understand why do we need them at all. I'm a physics undergraduate. As far as I know, ...
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### Is it Possible to Burn a fuel (coal especially) without a flame or spark?

Actually there was a topic of quality determination of coal.. coal is heated upto 750-950 degrees in order to burn its mixed volatile substances. I was wondering how coal itself doesnt burns.. same ...
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### How to expand the eigenvectors of $J_x$ on the basis of $J_z$?

Consider a system whose state space is four-dimensional and for which the possible eigenvalues of the operator square of angular momentum $J^2$ are $j=0,1$. We can consider then the operator $J_z$ and ...
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### Is electric potential a field?

I'm just learning about electric potential in my second-semester physics class. My textbook is emphasizing the fact that electric potential is a property of the source charges and that I'll soon learn ...
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### What's the bubble's wall made up of in false vacuum decay? [on hold]

It is well known that for some kind of double well potentials, there are two minima with one is unstable called the false vacuum while the other stable one called the true vacuum. The tunneling is ...
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### Eccentric binary black holes

Comparable-mass binary black hole inspirals and mergers are expected to be an important source of gravitational wave signals for current and future ground-based detectors. It is generally expected ...
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### What would become of the earth's orbit if half of the sun's mass were suddenly to disappear?

I shall assume that the sun is at rest, so before the half mass disappear, the orbit of earth is an ellipse.
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### Motion of a bullet through a solid block [on hold]

I cannot understand why R=-ma, please can anyone explain me?
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### Is a creeping flow with $u_r \sim \ln r$ physically possible?

I was wondering if it is possible to have a 2D cylindrical flow where the radial velocity scales with $ln (r)$. I understand that a flow with $u_r \sim 1/r$ corresponds to a line source or sink. Also ...
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### Cooling of atomic clocks

I have read of cooling atoms to one-quarter of one-thousandth of a degree above absolutely zero. According to Physicist William Phillips, this improves the measurement of the ticking frequency. I have ...
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### Tachyons and Lorentz velocity transformation

In general, is it possible to apply the Lorentz velocity transformation to a tachyon? I have tried to do so but the results seemed very illogical. Here's my attempt: suppose an evil spaceship named ...
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### Are deforming force and stress always equal? If they are equal, then why does shape of the body get deformed?

If the deforming force is equal to the restoring force then there is no impact of the deforming force on the body. Then the shape of the body remains constant. How does the body deform, then?
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### At what temperature does the vibrational degree of freedom becomes significant for an ideal diatomic molecule?

For ideal diatomic molecules such as $\text{H}_2$, $\text{N}_2$ and $\text{O}_2$, at what temperature does the vibrational degree of freedom significantly contributes to the calculations such as that ...
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### Rolling a ball into a cone; what should the forces overall be?

This is my solution to finding angular displacement/velocity/acceleration on cone so far. Consider a cone, with an apex of half-angle $\psi$ pointing down, and a height of $h$. If I roll a ball into ...
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### What makes objects fly out of bike basket?

When riding a bike on a bumpy road, objects (let's say, a baseball weighing 150 grams) sometimes fly upwards and out of the basket. However I cannot seem to be able to explain this physically. ...
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### How small would the patch of sun be that delivers heat to the earth?

The sun, being a ball, projects its EM radiation and light in all directions. We earthlings only receive a light from a portion of the sun, not all of it. What size is this patch of the sun's surface ...
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### Stationary waves on a laminar liquid flow near collision with a surface [duplicate]

Look at the picture below I know about plateau rayleigh instability. And my opinion is that the reason of this phenomena is plateau rayleigh instability. But there's a question, in the picture ...
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### Why didn't helium continue to generate after a few minutes post-Big Bang?

Helium stopped generating after a few minutes after the Big Bang, but what made it impossible for its generation to continue?
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### Degrees of freedom in a diatomic gas in 2-dimensions

Question: What is the specific heat capacity at constant volume of a two-dimensional diatomic ideal gas of N particles at room temperature? My answer: A diatomic gas can move in both directions, can ...
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### Definition of pole of a spherical mirror

what can be a suitable definition to a pole of the spherical mirror. It cant be the geometrical center of a spherical mirror as if a mirror is cut then its pole does not change. moreover if a ...
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### How this Poisson bracket transform when we change the coordinates?

I'm studying the book Geometric Mechanics by Darryl D. Holm and there's one exercise in the book I'm not quite getting what has to be done. The same discussion the author makes in the book is made on ...
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### Explain the notion of light/electromagnetic waves/photons to a non-physicist

A non-physicist asked me about special relativity. My explanations naturally were based on gedankenexperiments involving light. This forced the question: "What is light? It is particles, isn't? Or is ...
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### Why doesnt flow of liquid from a orifice depend on atmospheric pressure?

While calculating the velocity of efflux, ie, the velocity of a liquid when it flows from a orifice which is made in a beaker filled with liquid. Since at the orifice, atmospheric pressure acts along ...
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### is Ù the same as omega as the unit of resistance?

I was making an exercise about DC motors and the resistance was given in Ù. Is this the same unit as Ω? Or is there a relation between these two units?
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### Are acoustic phonon frequencies always linear in k?

I'm confused by a discussion in Ashcroft and Mermin's textbook on pg. 512-513. They say that if we have a bunch of ions in a solid and neglect the effect of the conduction electrons, then waves will ...
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### Principle behind electrostatic shielding?

If we have a solid conducting sphere with charges around it, then the electric field inside the sphere is zero, otherwise the electrons of the sphere would not be in equilibrium as there would be a ...

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