# All Questions

131,157 questions
4 views

### Stellar aberration: distal or local? Experimental proof?

Stokes put forward the theory (I am simplifying here.) that stellar aberration takes place out in space and that light from stars is already fixed in direction with respect to the Earth's surface by ...
4 views

### Could a material be created to effectively negate directed energy weapons?

You always see lasers and plasma guns being used in SF space battles, but couldn't a sufficiently reflective or absorbent material be utilized to essentially make these sorts of weapons useless? On ...
3 views

### Does bending a piece of steel change the speed of sound in it?

Specifically, I have a student investigating sound produced in a helical spring (slinky) when struck. When it is struck it produces a sound that changes in pitch - from very high to quite low. Also, a ...
33 views

### Is quantum mechanics misleading [on hold]

Mechanics implies dynamics. Yet all of the qm I have seen is empirical curve fitting of a static system. Maybe they should change the name? For example it seems the point of qm is to do an experiment,...
14 views

### Motion of the center of mass of rigid bodies in space

For the classic two body problem, I know that the motion of the center of mass is a straight line (with respect to an inertial frame), provided that the bodies are considered as point particles. Now ...
16 views

6 views

### Can Matrix Product States be used to describe ergodic states?

The eigenstates in the ergodic phase of the disordered interacting quantum many-body system follow a volume law of entanglement. Since MPS follows area law, can we use any MPS based algorithm (ITEBD ...
14 views

### Eddy Currents and magnet's orientation with respect to motion

I am doing a report on an experiment on harmonic oscillators. We added magnets to the cart (they move with it) to provide magnetic dampening, placing the magnets once horizontally and once vertically (...
18 views

### HELP in particle physics [on hold]

How to calculate the differential cross section for bhabha scattering in the center of mass system? and as a function of cos(theta)?
22 views

### Hyperfine structure of hydrogen

I have an expression $H=a \vec{I} \vec{S}$ which is a leading term in Hamiltonian for the hyperfine structure of p-states in hydrogen, but I don't really understand it. By Wigner-Eckart, hyperfine ...
37 views

21 views

### Why is the different p.d. same across different resistors in series connection? [on hold]

The total potential difference in a circuit is said to be equal to the total potential drops, why? The energy is lost as heat and the potential drops are always lower than the potential difference so ...
8 views

### Timescales of plasma recombination and fluorescence?

I am currently working on a very simple model for the radiation from electric arcs. As both fluorescence (internal electronic transition) and plasma recombination occur, I would like to compare the ...
63 views

### Physics mystery : why does it rotate?

In the beginning this thing was floating still and the candle was out. Then someone lit the candle and the flame somehow Made the object spin on the water. See the picture. I know a little bit about ...
14 views

### Electric field lines in a two point charge conducting surface [on hold]

The magnitude of E at the surface of an irregularly shaped solid conductor must be greatest in regions where the surface curves most sharply, such as point A and must be least in flat regions such as ...
22 views

### $y$-component and $z$-component not equal to zero

If we were to place a point on the perpendicular bisector of an evenly charged rod of positive charge, lying along the y-axis, By symmetry are there any other ways that the electric field's y and z ...
26 views

### Bose Einstein Condensate

In Bose Einstein condensate photons stop moving, but with reference to what frame of reference? Photons move at constant speed in all reference frames so what happens to Maxwell equations at zero ...
30 views

### C and T Symmetry of Free Dirac Lagrangian

I want to show the $C$ and $T$ symmetry of the free Dirac Lagrangian $$\mathcal{L}=\overline{\psi}\left(i\gamma^\mu\partial_\mu-m\right)\psi.$$ Following the notation of Peskin, Schroeder, we have ...
25 views

### What the difference is between Størmer Verlet and regular Verlet method?

I was wondering what the difference is between the Størmer Verlet method and the regular Verlet method, if there is any.
70 views

### What's wrong with this mechanics question? [on hold]

Just working through a question and I don't think it's possible to solve without more information. Am I right or am I missing something? A 66 kg ice skater comes to rest uniformly on rough ice from ...
45 views

### Getting the density of states for photons

I know that the density of states $g(\epsilon)d\epsilon$ is the number of states in the energy range $[\epsilon, \epsilon + d\epsilon]$. I considered a system of non-interacting free photons in 3 ...
92 views

### Having trouble making sense of Einstein's thought experiment

So I was reading about Einstein's thought experiment where he tries to show that simultaneous events in one frame may not be simultaneous in another frame. So, in the given pic, light from B' ...
36 views

### Necessary condition for Entropy to be a state function

In a thermodynamic process, entropy change from a state $a$ to $b$ is defined as $$\int_{a}^{b}\frac{\delta Q_{rev}}{T}$$ But, there can be infinitely many reversible paths to reach $b$ from $a$. ...
26 views

### What exactly is the time step in weather predictions?

The question basically says it all. I am confused as to what the 'time step' in weather prediction signifies-- is it the time interval between each measurement of the initial condition? Or is it the ...
38 views

### Why is force defined the way it is? [duplicate]

In classical mechanics an inertial frame is a frame of reference where: $\vec{F}=m\frac{\mathrm{d^2}\vec{x}}{\mathrm{d}t^2}$ holds. Couldn't we have expanded this set of inertial frames of reference ...