4
votes
1answer
399 views

Is there a fully quantum-field theoretic treatise of Planck's Law for black-body radiation?

I recall from my undergraduate statistical mechanics and QM classes that Planck's Law may be derived fairly straight-forwardly by considering the density of states of EM radiation in thermal ...
5
votes
1answer
277 views

Why can't fermions be affected by effective gravity in non-linear quantum electrodynamics?

Quantum electrodynamics based upon Euler-Heisenberg or Born-Infeld Lagrangians predict photons to move according to an effective metric which is dependent on the background electromagnetic field. In ...
1
vote
1answer
159 views

Time evolution of wave spectrum

A useful way of thinking (not only) oceanic waves is to consider them as a superimposition of linear modes: the elevation η of the sea surface is given by: 1: $\eta({\bf x}, t) = ...
8
votes
4answers
1k views

Effects of space mining on Earth's orbit

I was reading a post about space mining, specially lunar mining. I was thinking about what would change in Earth's orbit if we start bringing tons of rocks to it? I mean, in a huge scale. So, would ...
6
votes
2answers
2k views

How to count photons

How are photons counted? What is the experimental setup used to count photons from a laser or even a lamp? Of course, in the case of the lamp, I would be able to count only the photons that pass ...
22
votes
5answers
4k views

What is the usefulness of the Wigner-Eckart theorem?

I am doing some self-study in between undergrad and grad school and I came across the beastly Wigner-Eckart theorem in Sakurai's Modern Quantum Mechanics. I was wondering if someone could tell me why ...
2
votes
2answers
7k views

What causes gravity?

What causes gravity? Why is there attraction between masses? Is it due to time or space distortion?
13
votes
6answers
3k views

Why do physicists believe that there exist magnetic monopoles?

One thing I've heard stated many times is that "most" or "many" physicists believe that, despite the fact that they have not been observed, there are such things as magnetic monopoles. However, I've ...
5
votes
2answers
696 views

Trying to model pinball physics for game AI

I'm working on an AI for a pinball-related video game. The ultimate goal for the system is that the AI will be able to fire a flipper at the appropriate time to aim a pinball at a particular point on ...
3
votes
1answer
1k views

Why must the deuteron wavefunction be antisymmetric?

Wikipedia article on deuterium says this: The deuteron wavefunction must be antisymmetric if the isospin representation is used (since a proton and a neutron are not identical particles, ...
5
votes
1answer
2k views

time difference with airplane travel direction

a fight from east to west, (e.g. Australia melbourne to perth) takes longer than travel backwards. (west to east). What's the reason?
0
votes
2answers
455 views

Matter in superconductive state [closed]

We distinguish between the states of matter: gas, liquid and solid. Possibly we could add the plasma state and/or the superconductive state as new states of matter. Phase transistions at certain ...
4
votes
3answers
2k views

Why do white dwarfs shine white?

The hotter something is glowing the more white/blue it appears. A dying medium sized star expands, cools and becomes a red giant for a while, but eventually it is going to gravitationally collapse ...
9
votes
5answers
555 views

How important is the cosmic censorship conjecture?

I would like to know how important the cosmic censorship conjecture is? Should a quantum theory of gravity must obey this? It was never rigorously proved in classical GR too. What would be the ...
3
votes
2answers
902 views

What happens in string theory beyond the Hagedorn temperature?

What happens in string theory when the temperature exceeds the Hagedorn temperature? Is that even possible? If yes, what is the nature of the phase transition and the phase beyond that? What happens ...
3
votes
4answers
720 views

Is there any physics that cannot be expressed in terms of Lagrange equations?

A lot of physics, such as classical mechanics, General Relativity, Quantum Mechanics etc can be expressed in terms of Lagrangian Mechanics and Hamiltonian Principles. But sometimes I just can't help ...
18
votes
1answer
1k views

Why is there a deep mysterious relation between string theory and number theory, elliptic curves, $E_8$ and the Monster group?

Why is there a deep mysterious relation between string theory and number theory (Langlands program), elliptic curves, modular functions, the exceptional group $E_8$, and the Monster group as in ...
4
votes
2answers
474 views

Glass melting at near absolute zero?

I read this report and summarise here but my question is - if quantum mechanics will make glass melt at temperatures near absolute zero and it is near absolute zero then wouldn't this be a huge issue ...
4
votes
2answers
586 views

Superconformal algebra

I had earlier also asked a question about super conformal theories and I am continuing with that, now with more specific examples. I am quite puzzled with it given that I see no book explaining even ...
7
votes
2answers
2k views

Why can't massless particle exceed speed of light?

Why massless particle can't exceed speed of light?
7
votes
3answers
662 views

Bosonic M-theory and moonshine gravity

Last month I asked about a 27-dimensional origin of the heterotic string. Now I'm looking at Witten's "Three-dimensional gravity revisited", where he proposes that pure gravity on AdS3 is dual to the ...
1
vote
1answer
730 views

How to determine velocity and mass of asteroid 2011 CQ1

As you may have seen, a small asteroid had a near miss with the earth a few days ago. As a physics teacher teaching momentum, I think this could be an excellent problem for my students, however, I ...
14
votes
3answers
2k views

Significance of the second focus in elliptical orbits

1.In classical mechanics, using Newton's laws, the ellipticity of orbits is derived. It is also said that the center of mass is at one of the foci. 2.Each body will orbit the center of the mass of ...
1
vote
1answer
274 views

How to vizualize a heat pump microscopically?

Hey guys, while learning thermodynamics i wondered how a the principle of a heat pump would look at a microscopic level, not on a quantum mechanical level. I learned that when a hot and a less hot ...
1
vote
1answer
4k views

Finding the steady state solution of a damped oscillator

A damped harmonic oscillator with $m = 10$kg, $k = 250$N/m, and $c = 60$kg/s is subject to a driving force given by $F_0\cos(\omega t)$, where $F_0 = 48$N. (a) What value of ...
2
votes
2answers
929 views

Electric potential in a capacitor

Im a writing a school report regarding the electric energy and potential of a capacitor. In short: An experiment was carried out where we charged a capacitor, and then decharged it with an electric ...
2
votes
3answers
222 views

Will watering tea down make it clearer?

If I poured water into my tea, would I see more or less of the bottom of the tea-cup? Intuitively, there would be as many particles blocking as many photons, and so I'd see the bottom just as clearly ...
5
votes
2answers
1k views

Black hole analog experiment?

This question is directed mostly at people giving lectures on black holes, but input by other physicists or students is very much appreciated. Do you know a good (home)-experiment with a black hole ...
8
votes
6answers
6k views

Why can't photons have a mass?

Why can't photons have a mass? Could you explain this to me in a short and mathematical way?
1
vote
2answers
283 views

Under what conditions do the Galilean transformatons become more accurate?

For S and S' in standard configuration, the Galilean transformations are: x' = x - vt, y' = y, z' = z, t' = t From the Lorentz transformations for v << c: x' = x - vt, y' = y, z' = z, t' = t ...
5
votes
1answer
235 views

Why aren't string compactifications by generic conformal field theories considered more often?

Most string theory compactifications analyzed so far have as backgrounds a conformal field theory corresponding to a nonlinear sigma model with a Calabi-Yau target space, or some relatively classical ...
8
votes
1answer
2k views

Where does CMB come/emit from?

Where exactly does CMB come from. I've seen it in documentaries as a huge sphere with Earth in the middle. But if all this radiation was ejected from the start of the universe some time after the big ...
2
votes
2answers
254 views

What about the neutron stars with CMB temperature as dark matter?

Inspired by How do we know that dark matter is dark? and What is the temperature of the surface and core of a neutron star formed 12 billion years ago now equal to?
6
votes
2answers
593 views

Does perturbation theory break down for quantum gravity?

Perturbation theory presumes we have a valid family of models over some continuous (infinitely differentiable, in fact) range for some parameters, i.e. coupling constants. We have some special values ...
10
votes
2answers
1k views

Do traversable wormholes exist as solutions to string theory?

There has been some heated debate as to whether the laws of physics allow for traversable wormholes. Some physicists claim we require exotic matter to construct wormholes, but then others counter the ...
6
votes
9answers
3k views

What are the most important discoveries/breakthroughs in physics recently?

Can you provide a list of the most important discoveries/breakthroughs in physics recently? By recent, I mean the past decade or so. All branches of physics are welcome. Basically, I am interested in ...
1
vote
2answers
2k views

How long does it take a block to slide up and down a plane with friction? [closed]

So I am studying for my exam on monday for analytical mechanics. I am kind of stuck on a question, so wondering if someone can give me a pointer. I am actually looking for some pointers and some ...
2
votes
3answers
147 views

Ability to detect pulsars

Am I correct it remembering that unless a pulsars beams plane faces earth we can not detect them. And that similarly inbetween the pulses we can't see them either? If so how does this differ from ...
6
votes
4answers
579 views

How do we know that dark matter is dark?

How do we know that dark matter is dark, in the sense that it doesn't give out any light or absorb any? It is impossible for humans to be watching every single wavelength. For example, what about ...
4
votes
1answer
465 views

What is the effect of ice on an antenna?

A local FM radio station transmitting at 89.3 MHz recently announced that it would be running at 50% power due to freezing weather and a forecast of ice accumulation, as "when ice is forecast ... it ...
6
votes
2answers
735 views

How does one formulate boundary conditions in a variational approach?

Many equations of motion can be derived from a variational principle. To take a simple example, the wave equation $h^{ij} \partial_i \partial_j u = 0$ (where $h^{ij}$ is the Minkowski metric ...
5
votes
4answers
7k views

Light emitted by an object according to its temperature

According to this picture the light emitted by an object depends on its temperature. That makes perfect sense when we heat a metal. As its temperature raises we see it red at first, then orange, ...
10
votes
7answers
10k views

Electromagnetic fields vs electromagnetic radiation

As I understand, light is what is more generally called "electromagnetic radiation", right? The energy radiated by a star, by an antenna, by a light bulb, by your cell phone, etc.. are all the same ...
5
votes
1answer
630 views

Einstein's Field equations and impulse-energy tensor

I premise that I haven't yet studied General Relativity, but in Relativistic Electrodymaics I have knowed impulse-energy tensor of Electromagnetic Field. I know in Einstein's equations there is ...
0
votes
2answers
784 views

Understanding weight on an inclined plane

I'm trying to solve a problem where I have an object resting on an inclined plane, with the angle of the plan being alpha, and the weight being w. I'm having trouble figuring out how I can calculate ...
3
votes
1answer
284 views

A question about a vibrating membrane

The usual way to model a vibrating membrane is by using the wave equation. Is it possible to do that from "within"? Probably the answer is yes, but where can I see it done explicitly. What I mean is ...
-1
votes
3answers
4k views

Vector product in 2 dimensions [closed]

If I have a vector A=4i+3j and B=5i-2j, how can I find the vector product AxB? I know that given the angle, its C=AB sin theta, but how can I solve this without the angle?
8
votes
4answers
3k views

Water pressure in free fall

The increasing water pressure as you go deeper is generally explained in terms of the weight of the water column above the observation point pressing down. The question, then, is what would happen if ...
13
votes
2answers
4k views

Why two objects get charged by rubbing?

It is always told as a fact without explaining the reason. Why do two objects get charged by rubbing? Why one object get negative charge and other get positive charge?
-1
votes
3answers
2k views

Is kinetic energy a scalar? [closed]

Is it correct to say that kinetic energy is a scalar?

15 30 50 per page