# All Questions

323 views

### Groups acting on physics - a clarification on electrons and spin

My first question is fairly basic, but I would like to clarify my understanding. The second question is to turn this into something worth answering. Consider a relativistic electron, described by a ...
134 views

### Do perturbative renormalization groups help one understand when perturbation theory can be used in general?

If, as I asked in this question, a relevant operator in a renormalization group transformation can't be used in a perturbative expansion since it becomes large as the transformations are applied, does ...
1k views

### How much electric charge do electromagnetic waves carry?

Since electromagnetic waves have both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. How much is ...
186 views

### Question about the perturbative renormalization group

I'm currently learning about the renormalization group (RG) in condensed matter physics and just want to clarify a couple of things: When doing the RG transformation, there's a flow to a fixed point. ...
225 views

### Do amorphous metals undergo conchoidal fracture?

Amorphous metals are often referred to as metallic glasses due to their quenched atomic disorder. Do they fracture in the same fashion as silicate glasses? If not, what failure mode(s) do they have?
273 views

### Given two boiling temperatures and pressures, how can I find the latent heat?

I am given the fact that at a certain pressure a liquid boils at a corresponding temperature, at a different pressure it boils at a different temperature, and then I am asked to find the latent heat ...
127 views

### How much energy does lowering an object into a black hole generate?

An object of mass m is slowly lowered into a black hole of mass 1000 m. Is the amount of braking energy larger than $0.6 mc^2$? Now what if, after lowering the mass close to the event horizon, we ...
137 views

### gauge invariance and Bohm-Aharanov effect

I am confused with the Bohm-Aharanov effect: though quantum mechanics is said to be gauge invariant, the presence of a solenoid imposes a gauge ... I used to think that a phase shift did not change ...
164 views

### Electron Fermi gas

My question is about 2-dimensional Fermi gas of electrons. What is magnetic susceptibility when $T<<T_F$ (where $T_F$ is Fermi Temperature) and, What is the ratio between Pauli and Curie ...
88 views

### Do Galactic Black Holes and Extra-Solar Planets change the mass balance of the universe?

With all the recent discoveries of supermassive black holes being at the center of nearly every galaxy, and the proposed "at least one planet" around (probably) every star. Does this affect the ...
143 views

### Is there a known optical design for a beam compressor?

With my little knowledge of optics I have come across some 'known' designs such as the Double Gauss for example, is there a 'beam compressor'. My requirements are to reduce an incoming parallel ...
321 views

### Hole in the disc shaped magnet

Let's suppose we have a magnetic disc. One half is magnetic north pole and the other half is magnetic south pole. Now suppose we drill a hole in the disc. Does anybody has an idea how the magnetic ...
2k views

### How to prove that a motion is Simple Harmonic Motion (SHM)?

I would like to know how one could show and prove that a given motion is simple harmonic motion. Once given an answer, I'll apply that technique to an example I am trying to figure out. Thank you ...
126 views

### Do asymptotically similar potentials yield similar energy levels asymptotically?

Let there be given two Hamiltonians $$H_1~=~ p^{2}+f(x) \qquad \mathrm{and} \qquad H_2~=~ p^{2}+g(x).$$ Let's suppose that for big big $x$, the potentials are asymptotically similar in the sense ...
187 views

### Why is the diffraction pattern for an annular aperture the difference of the two Bessel functions?

Inspired by this question, the pattern for the annular intensity is given by the square of the difference of the two Bessel functions. I would have thought the sum was more appropriate, could someone ...
207 views

### Hydrostatic equilibrium of a star derivation

I've been told to revise the derivation that proves $\frac{\mathrm{d}P}{\mathrm{d}r} =\frac{GM(r)p(r)}{r^2}$ where brackets indicate a function of, P is pressure and p is density. Rather helpfully he ...
303 views

### Physical interpretation of Wheeler - Dewitt equation

What is the mainstream ( if there is one ) interpretation of the Wheeler - Dewitt equation $\hat{H}\Psi =0$ ?
2k views

### To calculate pressure at point in a vertical water pipe of given diameter

Is it possible to calculate pressure at a point in a pipe, where a solenoid valve shall be placed, given that the pipe carries water (utility supplied, regular water) from an overhead tank (at a ...
283 views

### What are independent parameters in Hellmann–Feynman theorem?

A typical example in textbooks about the application of Hellmann–Feynman theorem is calculating $\left\langle\frac{1}{r^2}\right\rangle$ in hydrogen-like atoms. Wikipedia has a nice demonstration of ...
506 views

### What do physicists mean when they say “speed of light”?

Does it make sense to say, "The speed of light varies?" Some may say right off the bat yes, it changes as a wave passes through a different medium. However, I'd like to say no, because when I hear ...
127 views

### Black hole related questions

Well I have a couple of questions regarding Black holes so here they are, 1.Shouldn't they be called black spheres?(holes are in 2D right?) 2.What will happen if black holes collide?(If they can) ...
126 views

### Does electron go through a forbidden state when annihilate with positron?

Let's consider an electron-positron pair with total spin equal to zero. When it annihilates it can not emit only one photon because it would have zero momentum and nonzero energy. The pair emits two ...
387 views

### NASCAR drafting at Daytona

This year, the racetrack at Daytona has been repaved. The track was always faster than other tracks NASCAR raced at and several cars in a "train" were faster than single cars or smaller trains. This ...
399 views

336 views

### Which subjects in physics should I choose if I want to help tackling today's energy and environment related problems?

I was wondering what subjects a freshman in mathematics ought to choose in the future if s/he wanted to help working on energy and environment-related issues we are currently facing, and will very ...
351 views

### Can a disk like object (like UFO's) really fly?

UFOs as shown in movies are shown as disk like objects with raised centers that emit some sort of light from bottom. Can such a thing fly? My very limited knowledge in physics tell me that a disk ...
255 views

### Kinetic energy absorbing in order to avoid damages?

is this possible? To absorb kinetic energy and disable inertia force? http://www.youtube.com/watch?v=KnKPbAbJI0w http://www.youtube.com/watch?v=4_5oseSVUc4
334 views

### Restrictions on defining microstates (Entropy)

If we have an isolated system $Sb$ with thermodynamic entropy $Eb=X$ (and growing by the 2nd law of thermodynamics), we could define an abstract system $Sa$ (containing the system $Sb$) but define ...