# All Questions

769 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 ...
691 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 ...
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 ...
452 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 ...
552 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 ...
2k views

### Why can't massless particle exceed speed of light?

Why massless particle can't exceed speed of light?
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### 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 ...
648 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 ...
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 ...
270 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 ...
3k 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 ...
807 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 ...
217 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 ...
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 ...
5k 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?
278 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 ...
228 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 ...
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 ...
241 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?
535 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 ...
869 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 ...
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 ...
1k 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 ...
145 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 ...
533 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 ...
401 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 ...
658 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 ...
4k 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, ...
8k 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 ...
587 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 ...
668 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 ...
281 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 ...
3k 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?
2k 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 ...
3k 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?
2k views

### Is kinetic energy a scalar? [closed]

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

### A basic question on the derivation of the wave equation

Today I saw the derivation of the wave equation in class, and I did not understand the following step. We are modeling a uniform-density string as being made up of tiny masses spaced a small amount ...
2k views

### Green functions in Quantum Mechanics

How the Green's functions and the Quantum Mechanics are related? Do they can be used to solve the Schrödinger equation of an particle subjet to some potential that is not a Dirac's delta? And the ...
808 views

### How should 500ml water bottles be stored in the fridge so they will be coldest?

Should they be taken out of the wrapping and stored as single units, or should I leave them in the package? EDIT to add I mean, the package is 24 bottles bundled together. Should I just leave them ...
1k views

### $N=4$ supersymmetric yang-mills theory and S-duality

[Question suggested by @Lawrence B. Crowell's answer to the question on T-duality] There are three parts to my question: A. What is the action for $N=4$ SUSY Yang-Mills and what is the physics of ...
6k views

### Do light and sound waves have mass

I have been reading Hawking's 'A Brief History of Time' and it has gotten me thinking about Einstein's theory of relativity, in that it assumes that an object must have infinite mass if it is to be ...
3k views

### How does electron excitation relate to atomic orbitals?

Taking off from the Rutherford-Bohr model, Figure 1. Rutherford-Bohr model. when an electron absorbs energy (from light, heat, or otherwise) it jumps to a higher energy level, or shell. Then when ...
667 views

### Energy transfer to a driven oscillator

Why is the amount of energy transferred to a driven mechanical oscillator largest when the drive frequency is the same as the natural frequency of the oscillator? Why are they exactly out of phase by ...
361 views

### neutrinos by formation of “neutron pairs”

Here : http://www.newscientist.com/article/dn20084-neutron-star-seen-forming-exotic-new-state-of-matter.html are news on superfluidity in a neutron star. The necessary bosons they say are pairs of ...
590 views

### Lattice QCD and string theory

I've heard the claim that some aspects of string theory are used to improve Monte-Carlo simulations of lattice QCD, for example by people working at the LHC. I know a bit about Monte-Carlo methods in ...
232 views

### Why are the lighter halogens gases?

In the periodic table of the elements, only a few of the elements are gases at standard temperature and pressure. Those elements include the noble gases some of the halogens, and a few of the elements ...
751 views

### Treatment of boundary terms when applying the variational principle

One of the main sources of subtlety in the AdS/CFT correspondence is the role played by boundary terms in the action. For example, for a scalar field in AdS there is range of masses just above the ...
813 views

### Please clarify how entropy increases when matter gravitationally coalesces

On John Baez's website, http://math.ucr.edu/home/baez/entropy.html, he discusses the problem of how entropy increases when a cloud of ideal gas collapses gravitationally (no black holes - keeping it ...