2
votes
1answer
51 views

Gravitational redshift in a general stationary metric

Suppose you have a general metric $g_{\mu \nu}(t,r,\theta,\phi)$ which don't depend explicitly on $t$ coordinate, i.e a stationary metric. Light travels along a geodesic from A (at which the frequency ...
0
votes
0answers
23 views

Current density in covariant formulation [on hold]

I know that I have messed it up quite frankly, but don't know where- Why? $$qu_\alpha=J_\alpha$$ in covariant formulation.
0
votes
1answer
21 views

Two supersonic planes. Do they hear each others sonic boom?

Plane A is traveling at Mach 2 and is over taken by plane B traveling at Mach 3. Does plane A hear the sonic boom from plan B? If so when? Does plane B hear the sonic boom from the slower Plane A? ...
1
vote
2answers
74 views

Klein-Gordon Green's function: derivative of delta distribution?

In Peskin/Schroeder there is an explicit calculation showing that the retarded Green's function of the real Klein-Gordon field $$D_R(x-y) ~\equiv~ \theta(x^0 - y^0) \langle 0 | [\phi(x), \phi(y)] ...
2
votes
3answers
444 views

Breaking the sound barrier underwater

Theoretically, if I were to launch something faster than the speed of sound in water (around 5 times that of air), what would happen?
10
votes
3answers
1k views

Why does splitting hot tea from one glass in two glasses makes it cool faster?

For a while now I've noticed that if I take a cup of hot tea and pour it into two cups and leave it then both cups will cool faster than the single cup. How is it that nature cools two cups of tea in ...
0
votes
0answers
30 views

Does the effect of spaghettification mean that (speculative) traversable wormholes can only exist from supermassive black holes? [on hold]

Assumption that wormholes really exist. Even if humanity could produce wormholes from quantum black holes. These would be never traversable due to the effect of spaghettification? Or does the exotic ...
2
votes
1answer
48 views

What is the physical significance of Compton wavelength?

Wikipedia says, The Compton wavelength represents the quantum response of mass to local geometry. What does it actually mean?
1
vote
2answers
203 views

Electron as a standing wave and its stability

1. When it was an era of classical mechanics we used to believe in the Bohr's atomic model. It interpreted electrons as particles (although I couldn't understand how come Bohr who interpreted ...
0
votes
1answer
9 views

Nature of photoluminescence at a semiconductor heterojunction

Do I understand it correctly that photoluminescence at a semiconductor heterojunction occurs because of intralayer recombination? If so, why can't photoluminescence occur because of interlayer ...
0
votes
0answers
55 views

Why is it harder to stand still than to walk? [on hold]

I can walk for miles, but I'm exhausted after an hour of standing. I think it is not psychological. Is there any explanation according to physics?
0
votes
2answers
62 views

Why is this circuit component on without a potential difference?

From the following graph, determine whether (h) will be on or off: I think (h) should be off because there is no potential difference across it. Is this correct? Edit: Using the method provided ...
4
votes
3answers
76 views

Can a standing wave form on a string with both end open

I am fascinated with an idea of an standing wave forming on a string with both end open. If we assume two identical waves coming in of an infinitely long string then for a short period of time, they ...
0
votes
0answers
33 views

What is the supernovae? [on hold]

Supernovae are exploding stars. They represent the very final stages of evolution for some stars. Supernovae, as celestial events, are huge releases of tremendous energy, as the star ceases to exist, ...
-2
votes
0answers
49 views

Is this idea practically possible? [duplicate]

I have to make a working model for my science exhibition. I have planned to make a wind turbine with some innovations. My model of wind turbine will consist of magnet it at the tip of every fan blade. ...
0
votes
0answers
28 views

How to explain the homogeneity of the universe through a physic model

If in general, two objects homogenize themself by combining it's parts, why the horizon effect, based on a big bang model, excludes such an important physical interaction process? I wonder if this ...
0
votes
1answer
60 views

Why doesn't the two poles of a u-shaped magnet attract each other?

Why doesn't the two poles of a u-shaped magnet attract each other? Since opposite poles attract each other why cant the north and south pole of a u-shaped magnet collapse by attracting each other?
2
votes
1answer
38 views

What do field lines between two like-charged parallel plates look like?

I couldn't find any image to answer my question on Google. But I guess it looks somewhat like this, correct?
0
votes
1answer
23 views

How does the size of conducting sphere affect the distribution of charge?

I am doing my homework. One question gives individual charges on two spheres. And asks what the resulting charge is on each after touching. But one thing interest me is that in the answer key, it's ...
0
votes
3answers
112 views

If time is relative, how could time pass? [duplicate]

EDIT: I appreciate people who answered below. But it does not answer the question, so I will clarify my questions: -It seems like everyone is saying that time passing is actualized by physical ...
-7
votes
0answers
20 views

Senses in three-dimensional universe [on hold]

Our senses perceive only a single threedimensional universe and the only non-physical entities we know are mental, are our thoughts in other dimensions?
0
votes
1answer
38 views

Can we extract positrons from gamma rays?

If gamma rays undergo pair production is there a way to say, deflect and collect the positrons using magnetic fields?
0
votes
3answers
64 views

Which one of the two objects does the gravitation force vector point to?

My book tells me that the arrowhead should point to whichever is responsible for the field. Am correct in a assuming that it's whichever has the larger mass or bigger electric charge?
1
vote
1answer
39 views

Transfer function of an RC circuit

I'm trying to reach the transfer function of an RC circuit which I am told is $$\frac {V_R} {V_\text{in}} = \frac{R}{\sqrt{(R^2 + \frac {1}{\omega^2 C^2})}} \cos(\omega t - \theta) \, .$$ Is this ...
0
votes
1answer
38 views

Expanding physical quantities in dimensionless parameters

I have a system with two dimensionful parameters, say, chemical potential ($ \mu $) and temperature ($ T $). Now I want to write down an ansatz for any physical quantity (e.g, Greens function) at ...
3
votes
3answers
122 views

What is the binding energy of a neutron star?

Neutrons which constitute a neutron star have a rest mass that is greater when separated from the star because they are bound with a certain potential energy. This potential energy causes the system ...
1
vote
2answers
52 views

Tension in a massless string as it bends around a concave shape

I understand that tension in a massless string must be the same throughout when the string is straight, to prevent any section of it from accelerating at an infinite rate. However, I fail to see how ...
0
votes
1answer
39 views

When will a ball running between two downwards slopes stop going back and forth?

Two angles $a$ and $b$ meet at a point, making an angle $c$ between them. $a$ and $b$ are all $0 < \theta < π/2$. $c$ is $π/2 < \theta < π$ $a$ is to the left and opens to the left, $b$ ...
0
votes
0answers
12 views

Where to get the AME2012 isotope mass table?

what's the official site for getting the latest isotope mass table, i.e., AME2012? All links I have found (NIST, IAEA) point me to a site at the Chinese Academy of Sciences, but the link ...
0
votes
0answers
66 views

Finding the tension/acceleration of a pulley

I redid the entire thread with pictures. Please click the link to see a picture of pulley with it weights. You see the equation at the bottom of the picture. ...
3
votes
1answer
42 views

Can stable non-circular orbits exist in 2 or 3 dimensional hyperbolic space

I know that stable non-circular orbits in euclidean space exist only in 3 spatial dimensions but what about if the spatial dimensions are hyperbolic instead? Are there any stable non-circular orbits ...
0
votes
2answers
47 views

Concerning the dark bands in the light “wave” interference pattern

I'll begin with a with a brief and familiar example to frame the question: |_>EXAMPLE ...
0
votes
1answer
52 views

Magnetic field tangential surface integral

When deriving: $\vec{E}=-\frac{\partial\vec{A}}{\partial t}-\nabla\phi$ from Helmholtz decomposition of $\vec{E}$, It seems necessary to show: $$\iiint \nabla' \times \frac{\vec{B}(\vec{r')}}{|\vec{r} ...
0
votes
1answer
16 views

Why can't objects made of insulating material “store excess charges”?

This is quoted from my textbook. I saw plastic objects before being able to store excess charges. For example, if you rub paper against a plastic pole. Both will gain charges. But in lab, I never ...
1
vote
1answer
67 views

Question about Planck's constant

How did Dirac or whoever came up with it know that the momentum operator in quantum mechanics is $-i\hbar\frac{d}{dx}$? How did he know the $\hbar$ was in there? How did all these physicists know ...
0
votes
1answer
15 views

White light polarization by ground\sky

Why does light from the sun gets a slight (horizontal) polarization after bouncing from the ground? And same question for light which is scattered from the sky. Thanks.
0
votes
1answer
34 views

Why do semiconductors remain neutral outside the depletion region?

Why there is a sharp cut off of the charged region outside the depletion region, like on this image? For example why don't electrons on the conduction band in the n-type side rush towards the ...
2
votes
1answer
67 views

Maxwell's equations in integral form using differential geometry

So I've been trying to convert from Maxwell's equations in terms of differential forms to the integral versions of Maxwell's equations that we know from vector calculus. We have, in vector calculus ...
3
votes
1answer
44 views

What is the mix of ordinary mass vs. ordinary radiating energy in the universe?

I have seen data showing that the estimated mix of dark energy in the universe is 68.3%, the mix of dark matter is 26.8% and the mix of ordinary matter is 4.9%1. However, within "ordinary matter," ...
0
votes
2answers
29 views

Inductor - cause and effect [on hold]

The voltage on an inductor due to alternating current, also called back emf, is given by a beautiful, simple equation. It's just L times time derivative of current. According to Kirchoff voltage law, ...
21
votes
4answers
2k views

Why do physicists trust black hole physics?

Based on popular accounts of modern physics and black holes (articles, video lectures), I have come to understand the following: Black holes are predicted by General Relativity, a classical theory ...
1
vote
0answers
27 views

A functional average calculation confusion within Gaussian planar model's RG

I am trying to follow some detailed calculation in a famous paper [John, B. Kogut, Rev. Mod. Phys. 51, 659 (1979), An introduction to lattice gauge theory and spin systems]. More precisely, please ...
0
votes
1answer
32 views

Earnshaw's theorm and Effective potential

Earnshaw's theorm says "no stable equilibrium for any $\frac{1}{r}$ potential field in charge-free space". Now I am confused in some aspects, and I would like some helping hands. 1.)General physics ...
1
vote
1answer
79 views

How many days it takes to travel to Cygnus? [duplicate]

BACKSTORY I read today newspaper and made discovery of planet Cygnus (in news paper of course :v) that human can live there! The distance is 1400 Lightyear. Assuming I travel with the world fastest ...
1
vote
0answers
33 views

Why do particles in a medium have different speeds?

If thermal conduction occurs because of particles colliding with each other and imparting kinetic energy to each other, won’t the particles of an isolated medium collide with each other until the ...
1
vote
1answer
164 views

Why are mediums always represented as straight and parallel lines in Ray optics?

I have been reading Ray Optics lately and I always see that the interface between mediums are always represented as Straight lines in ray diagrams. There is no mention that they are considering an ...
2
votes
1answer
59 views

Basis states for many-particle system

I'm reading these notes about second quantization. In section 1.4 the author introduces many-particle wavefunctions. But I can't understand how basis are defined here. I know that if $\{\chi_i | i=1, ...
0
votes
0answers
38 views

Constancy of speed of light $c$ with perpendicular motion [duplicate]

Suppose light is travelling in a straight line parallel to $y$-axis takes time $t$ to reach from $y_1$ to $y_2$ in a reference frame $S$. Let there be another frame, $S'$, which is travelling parallel ...
1
vote
1answer
34 views

Coupling an electric charge to a gauge field. How is it done in this setup?

In page 9 of Tachikawa's N=2 susy dynamics for pedestrians it says that an electric particle with charge $n$ in the first quantised setup (in what sense first quantised?), Wick rotated to Euclidean ...
1
vote
1answer
49 views

Why isn't the electric field zero in the empty space?

A spherical portion has been removed from solid sphere having a charge distributed uniformly in its volume the electric field inside the emptied space is? Isn't the electric field suppose to be zero ...

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