# All Questions

614 views

### Hydrostatic equilibrium of a star - simplification math error

I am trying to do this question and getting stuck at interpreting what the question is asking for. $$dP/dR = -g(R)\rho(R) = -[GM(<R)/R^2]\rho(R).$$ Thus when we descend a distance $\Delta R$ ...
8k views

### If both radio waves and gamma rays can travel through walls

and they are on opposite ends of the electromagnetic spectrum, then why can't light travel through walls which is right in the middle of the spectrum? This question has already been asked here. ...
1k views

### Entropy of an empty universe

After watching the first episode of wonders of the solar system, one question came up which is not explained. Bryan Cox says that ultimately the universe will be devoid of matter, so not even a ...
20k views

### Why do tsunami waves begin with the water flowing away from shore?

A sign of a tsunami is that the water rushes away from the shore, then comes back to higher levels. It seems that waves should be both + and - polarized and that some tsunamis should go in the ...
927 views

### Angular momentum and EM wave

Is there any sense in saying that circularly polarized EM waves have angular momentum?
1k views

### How does mass relate to resistance (Ohm's Law)

How can I relate $mg$ to $R$? Just to give you a scenario: If a free hanging weight was tied to a motor... I know the voltage ($V$) and current ($I$) so I can use $V = IR$ to find $R$. What if I ...
313 views

### What do we know of superconductivity in thin layers?

motivated by another question, i wonder if there are special properties of superconductivity when restricted on 2D or very thin layers related to the effective permittivity in function of the ...
328 views

### Effect of expansion of space on CMB

Is it true that the expansion of space time cause the CMB to become microwaves from a shorter wavelength. If it is has the amplitude been increased? Seeing as the amplitude has decreased; why hasn't ...
758 views

### Scaled energy output of a hydrogen atom

I'm trying to get a grasp of the power within a single hydrogen atom (e.g. the power released when whatever process happens in a hydrogen bomb)... If we could enlarge a hydrogen atom up to the size of ...
3k views

### Chemical potential of particles with zero mass

Why massless particles have zero chemical potential?
424 views

### Are we crystals?

Can we say that we are crystals because just like crystals we are made up of very small unit (cell) making up almost the same shape (our body) everywhere.
7k views

### Software for geometrical optics

Is there any good software for construction optical path's in geometrical optics. More specifically I want features like: draw $k \in \mathbb{N}$ objects $K_1,\dots,K_n$ with indices of refraction ...
352 views

### Mixed conductive and convective heat transfer

I want to make a finite element analysis of a cold airflow through warmer pipes. In particular I want to see how the pipes cool down and the air heats up, as it travels through the pipes. Wich are the ...
1k views

### How to write units?

In a scientific article, should I write "3m", "3 m", "3 meters", or "3 [meters]" ?
1k views

### How long would it take for electricity to flow from one terminal to other, via a 1 LY long wire?

Basically, how long does it take for electricity to determine there is a closed circuit and how does it know that the circuit exists? I'm curious to know how it knows there is a closed circuit at any ...
394 views

### vector cross products

Lets say you have a free particle in a rotating frame of reference with constant angular velocity $\mathbf{\omega}$. By free, I mean there are no real forces on it. Lets call the moving system ...
470 views

### How much energy can be extracted from hydrogen?

http://en.wikipedia.org/wiki/B-L says that the difference between baryon number and lepton number is conserved. Ordinary hydrogen has one of each, but turning it into helium releases only the binding ...
592 views

### What is the closest general-relativistic equivalent of a “time slice”?

In a newtonian universe, one can talk of a "time slice", that is, the state of the universe at a given point in (global) time. In a "typical" classical universe, a time slice would contain enough ...
920 views

### dynamic casimir effect

A few years ago, when i studied the casimir effect interpretation as the filtering out of vacuum modes with appropiate boundary conditions, i had the following dilemma; supposedly the derivation of ...
678 views

### Is it possible for one side of the universe to “meet” the other?

I've variously heard the shape of the universe being described as multi-dimensional, like a helix or mobius strip, and super string theorem seems to say there are lots of universes all piled up next ...
882 views

### What prevents the accumulation of charge in a black hole?

What prevents a static black hole from accumulating more charge than its maximum? Is it just simple Coulomb repulsion? Is the answer the same for rotating black holes? Edit What I understand from ...
543 views

### Can the Kramers–Kronig relation be used to correct transfer function measurements?

In experimental physics, we often make measurements of linear transfer functions; these are complex-valued functions of frequency. If the underlying system is causal, then the transfer function must ...
1k views

### Physics of simple collisions

I'm building a physics simulator for a graphics course, and so far I have it implementing gravitational and Coulomb forces. I want to add collisions next, but I'm not exactly sure how to go about ...
350 views

### Will physic object in a perfect environment last/exist forever?

We know,i.e. wood/steel tables in earth will completely broken or disappear in a very future day. If we put the table in a perfect/ideal environment (maybe in vacuum), will the disappear/broken still ...
637 views

### Is This The Answer To Artificial Gravity?

Einstein teaches that as an object gets faster, its relativistic mass increases... Newton teaches that as an object's mass increases, so does it's gravitational pull... So... if you a tethered some ...
374 views

### Could a bubble of photons make a spaceship massless? [closed]

I'm not sure how theoretically possible this is but my question is... If we could somehow make a perfect bubble of photons (a massless bubble) and put a spaceship inside it, could it therefore ...
822 views

### Are the solutions in radicals of cubic and quartic of any use in physics?

We all know that there are analytic formulae to solve quadratic, cubic and quartic polynomial equations. But it seems to me that the only solution that widely used is physics is the solution of ...
491 views

### Why is a suspended spinning body of liquid prolate?

Instead of a flat disc one would expect the centrifugal forces to push it in.. The body is not in a container but suspended in air spinning horizontally (i.e. left to right or vice-versa) - how to do ...
3k views

### Gravitational force between two masses

I get it that there will be a gravitational force between objects attracted towards gravity but can there be a gravitational force between two objects resting on horizontal plane? In other words, does ...
113 views

### Is there a chance for Earth to have an additional satellite someday?

I am just an space enthusiast and not a physics professional so pardon me if this is not the right place to ask. As title says, is there a chance for our Earth to have additional satellite(s) like our ...
13k views

### Why is the decibel scale logarithmic?

Could someone explain in simple terms (let's say, limited to a high school calculus vocabulary) why decibels are measured on a logarithmic scale? (This isn't homework, just good old fashioned ...
3k views

### Time as a Hermitian operator in QM?

In non-relativistic QM, on one hand we have the following relations: $$\langle x | P | \psi \rangle ~=~ -i \hbar \frac{\partial}{\partial x} \psi(x),$$ \langle p | X | \psi \rangle ~=~ i \hbar ...
217 views

### Can Gravity be described in terms of velocities and rotations of local Minkowski space?

In the paper The River Model of Black Holes: Am.J.Phys.76:519-532,2008, Andrew J. S. Hamilton, Jason P. Lisle http://arxiv.org/abs/gr-qc/0411060 The authors give a way of describing the action of a ...
1k views

### What sort of jobs do physics graduates with B.Sc/M.Sc get? [closed]

I've been told that in countries like Israel the truth of the matter is that the only available work for B.Sc./M.Sc. in physics/math is only to be high school teacher. To do serious stuff you need to ...
6k views

### What is the electric field generated by a spinning magnet?

Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical ...
90 views

### Why are unorientable strings with reversed orientations different?

If a string is unorientable, why is the a string with reversed orientation different from the initial string? Why do we have Kalb-Ramond 2-forms?
1k views

### Active gravitational mass of the electron

In PSE here electrons are added to a sphere and gravitational modifications are expected. My question is: Is there any experiment that show that a negatively charged object is source of a stronger ...
330 views

### Singing: Resonance body open-closed or closed-closed?

Googling yields contradictory results, so here my question: When I sing, my vocal chords vibrate, but my whole body is the resonance body, right? So I would say that when I think about standing waves ...
1k views

### Are modified theories of gravity credible?

I'm a statistician with a little training in physics and would just like to know the general consensus on a few things. I'm reading a book by John Moffat which basically tries to state how GR makes ...
143 views

### Star trace in the sky

If the space of universe is expanding at fast rates..(even faster than light) Why we do not see the trace of the stars? A possible answer could be that the stars are only moving away but mantaining ...
1k views

### How does fluoroscopy (x-ray) imaging work in practice?

I know the basics, that by measuring how much x-ray signal reaches each 'pixel' on the receiver we can measure how much has been absorbed. But this gives only a single channel of information, e.g a ...
197 views

### What is the stress-energy distribution of a string in target space?

If $| \psi \rangle$ is a string mode, how do you compute $\langle \psi | \hat{T}^{\mu\nu}(\vec{x}) | \psi \rangle$ where $\vec{x}$ is a point in target space? This information will tell us the energy ...
5k views

### Why are Saturn's rings so thin?

Take a look at this picture (from APOD http://apod.nasa.gov/apod/ap110308.html): I presume that rocks within rings smash each other. Below the picture there is a note which says that Saturn's rings ...
670 views

### Gravity theories with the equivalence principle but different from GR

Einstein's general relativity assumes the equivalence of acceleration and gravitation. Is there a general class of gravity theories that have this property but disagree with general relativity? Will ...
404 views

### Maxwell's Demon - laser cooling

There’s an interesting article on Scientific American that tells how to cool individual atoms to within a very tiny fraction of Absolute Zero. It uses two laser beams acting on a very cold rarified ...
160 views

### Knotted token-ring network

Suppose we have a rigid token-ring network. An observer at any node can seemingly determine the angular momentum of the network by measuring the time it takes for a packet to travel around the ring ...
700 views

### What allows the modified Urca process to work at lower density than direct Urca in neutron star cooling?

The dominant method of neutron star cooling is neutrino emission. There are two regimes usually presented, the "direct Urca" and "modified Urca" processes, each of which are sequences of neutron decay ...
5k views

### Rigor in quantum field theory

Quantum field theory is a broad subject and has the reputation of using methods which are mathematically desiring. For example working with and subtracting infinities or the use of path integrals, ...