6
votes
1answer
60 views

Is the subadditivity of quantum entropy valid in the infinite-dimensional case?

Does the subadditivity (and strong subadditivity) of quantum entropy hold for infinite dimensional quantum systems as well? Unfortunately the books in my hand give proof for finite dimensional cases ...
5
votes
1answer
1k views

How could we travel to the nearest supermassive Black hole?

I have just watched a trailer for the upcoming movie Interstellar and started to wonder about some physics involved. In the end of the trailer, they are obviously plunging into a Black hole hoping to ...
1
vote
0answers
116 views

Double dot product in Cylindrical polar coordinates - Strain Energy

I'm working with a problem in linear elasticity, and I have to calculate the strain energy function as follows: $$2W=σ_{ij}ε_{ij}$$ Where σ and ε are symmetric rank 2 tensors. For cartesian ...
0
votes
0answers
31 views

Transmission of porous membrane with vertical cylindrical pores

There is some porous dielectric membrane with strictly vertical cylindrical pores. I would like to find theoreticaly optical transmission spectra of this membrane, depending on the bulk material ...
3
votes
1answer
339 views

Instantons in Witten's supersymmetry and Morse theory

I'm reading Witten's paper on supersymmetry and Morse theory and am confused about the details of the instanton calculation which he uses to define a Morse complex (beginning at page 11 of the pdf) . ...
0
votes
0answers
34 views

How to restore the potential energy? [duplicate]

How to restore the potential energy, of a particle with variable mass and variable coordinate, by сartesian coordinates and mass of the particle? ? Dimension - one dimensional. Data that is to ...
6
votes
2answers
6k views

What is the escape velocity of a Black Hole?

The escape velocity of Earth is $v=\sqrt{\frac {2GM}{R}}$, where $M$ is the mass of the Earth and $R$ it's radius (approximating it as a sphere), and is much less than light speed $c$. But I want to ...
0
votes
1answer
79 views

Showing a fourth rank tensor in $\epsilon$'s reduces to one in the metric $g$

Consider the fourth rank tensor $$S_{\mu \nu \rho \sigma} = a(\epsilon_{\mu \sigma}\epsilon_{\nu \rho} + \epsilon_{\mu \rho}\epsilon_{\nu \sigma})f(x^2),$$ in 2D where $a$ is a constant and $f(x^2)$ ...
2
votes
2answers
234 views

Bloch theorem, Energy, Free electron

I'm trying to learn on my own a bit of solid physics to tackle semiconductors afterwards. I'm struggling with the Energy versus $k$ diagrams for a free electron which shows that for a single value of ...
1
vote
2answers
159 views

Airplane on a treadmill [duplicate]

I've heard conflicting answers, and would like to see the record set straight: An jet/propeller airplane is traveling on a giant treadmill at takeoff speed. Will the plane takeoff, or will it remain ...
1
vote
1answer
86 views

What is the speed of light from a star moving towards a observer.? [duplicate]

We can say that in vacuum speed of light is constant. But if the star is moving with a certain velocity does it add that velocity to the velocity of photons emitted out of that star?
1
vote
1answer
87 views

What is the significance of the difference in the eigenvalue equations of Bloch functions for electrons vs photons?

any text on photonic crystals will highlight the almost perfect analogy between electrons in a periodic potential and photons in a periodic dielectric. The analogies are: $$V(\vec r + \vec R) = ...
3
votes
0answers
91 views

Partition function of classical quadrupole in an electric field

The partition function of a dipole in an electric field is a well-known problem, analytical solvable (nice integral, can be calculated with pen and paper), for example in the Langevin treatment of ...
2
votes
2answers
3k views

How can sound waves propagate through air?

We know that the sound waves propagate through air, and it can't travel through vacuum. so the thing that help it doing that is the air's molecules pressure. So my question how can that happens? I ...
2
votes
0answers
60 views

Is the second law of thermodynamics a real law? [duplicate]

Everytime I ask people why the second law of thermodynamics is true, they say because the probability of entropy increasing is very large. This for some reason doesn't seem right for me. Just because ...
1
vote
1answer
93 views

A well-defined quantum probability in the beginning of the universe?

In mathematics or statistics, a well defined probability requires a large sample space. However, in the beginning of the universe, when the first quantum collapse happened, the sample space contains ...
2
votes
2answers
289 views

Who proposed the bulk-edge correspondence principle?

Who proposed the bulk-edge correspondence principle? The principle is often quoted in counting the number of zero energy states localized on the interface between two insulators with distinct band ...
0
votes
2answers
199 views

What does temperature look like at the subatomic level?

I am trying to get a better understanding of the definition of temperature at the subatomic level. I have a background in molecular biology with some college physics, but no deep quantum mechanics ...
0
votes
0answers
22 views

How to find the potential energy? [duplicate]

how to find the potential energy of a particle with variable mass and variable coordinate ? Data that is to compute, only the mass and Cartesian coordinate of the particle, like this: x;m ...
2
votes
2answers
92 views

Explanation for $M{\ddot{r}}=-\nabla \phi$

Could someone please explain this equation $$M\bf {\ddot{r}}=-\nabla \phi$$ Where $\bf r$ is a position vector and $\phi$ is the potential function. Could someone brief explain the potential function ...
4
votes
3answers
228 views

How can a detector distinguish between a photon and a gluon

Considering that both gluons and photons have no mass, no charge and spin 1, I was wondering how one can tell the difference, if they hit a detector after a collision at the LHC. I know that gluons ...
0
votes
1answer
115 views

Macroscopic Forces from QED

In QED the carrier for electromagnetic interaction is a photon, while macroscopic forces are due to electromagnetic interaction (by macroscopic forces I mean: normal force, object collision, friction ...
13
votes
4answers
2k views

If I shine a bright light on a neutron star, what would it look like?

If there are no orbitting electrons in a neutron star's makeup to interact with EM, what happens to light that strikes it?
7
votes
1answer
256 views

Recommendation on mathematical physics book of Symplectic geometry

I want to learn the applications of symplectic geometry in physics. Which mathematical physics textbook will have a detailed and heuristic explanation of this aspect?
1
vote
1answer
118 views

Does spin-0 or spin-2 describe massive or massless particles?

spin-0 is massive or massless? How does we separate the massive and massless degrees of freedom for spin-2? What is the partially massive?
2
votes
2answers
121 views

Can the magnetic fields of EM radiation be harnessed or measured?

We use the electric component of EM radiation to create the EM radiation and to detect it (antennas and Etc.), but does anyone know of a situation where the magnetic component of EM radiation is used? ...
2
votes
2answers
57 views

Reference on electrodynamics with tempered distributions

Back in my undergrad I had a course on classical electrodynamics where the fields had values in the space of tempered distributions. In this way one could correctly treat self-interaction and ...
3
votes
5answers
1k views

Why aren't units with powers, like cm³, surrounded by parentheses?

Since $\renewcommand{\unit}[1]{\,\mathrm{#1}} 1\unit{dm} = 10^{-1}\unit{m}$, it follows that $1\unit{dm^3} = 10^{-1} \times 10^{-1} \times 10^{-1} \unit{m^3} = 10^{-3} \unit{m^3}$. However, in ...
3
votes
2answers
142 views

Why doesn't water in water barometer boil?

I have read that the pressure in a water barometer at the top of the water column is around 0.5 psi and at such low pressures water should boil at around ~26°C (Room temperature). [1] [2] How ...
3
votes
1answer
191 views

Negative energy/mass bounds on de-Sitter spacetime

There exists a Positive Energy theorem for General Relativity in Anti-de Sitter and asymptotically flat spacetimes, but there is no equivalent theorem for de Sitter spacetimes Question: Is there ...
0
votes
1answer
117 views

Magnetic field of a stationary electron

As far as I know, a magnetic field can only be produced by a moving electric charge, or from a particle's spin (this is how a permanent magnet works, all the spins are in the same direction) What is ...
-1
votes
1answer
79 views

When/Where will the planets colide?

Given N bodies of known position, mass, speed and direction, if they were to collide, where it will happen and when? PS: this is not a homework... im working on a physics simulation engine... ...
4
votes
2answers
103 views

Can quarks of different colors annihilate?

Wikipedia: "Antiparticles have exactly opposite additive quantum numbers from particles, so the sums of all quantum numbers of the original pair are zero." Is it possible to annihilate a ...
1
vote
2answers
125 views

Could the universe have evolved WITHOUT the non-determinism of quantum mechanics? [closed]

(I'm going to make a few conjectures here - please answer the question in light of them as if they were true, even though of course they may be overly simplistic or wrong) Assuming that: the ...
0
votes
2answers
127 views

Does Free Will Theorem imply that quantum mechanics plays crucial role in our brain’s functioning (consciousness)?

Is opposite statement of Free Will Theorem right: If elementary particles have a certain amount of free will, then so must we? Because to me elementary particles does have a bit of free will – quantum ...
0
votes
0answers
40 views

Conditions for hyperregularity

(Definitions from Marsden & Ratiu, ``Introduction to Mechanics and Symmetry''): A Lagrangian is regular if the Hessian $\partial^2 L/(\partial \dot{q}^i \partial \dot{q}^j)$ is weakly non ...
3
votes
0answers
123 views

Why is there a 'loophole' in Mermin Wagner for rotations?

I'm just starting out in my mathematics career by looking at some simple stuff on broken symmetries in statistical mechanics. Since 3D is 'hard' it would be very nice to look at 2D toy models of ...
8
votes
3answers
6k views

Why does the sky look black in pictures taken from the summit of everest?

In pictures taken from the summit of Mount Everest (such as this one), the colour of the sky is a very dark blue or even black in some pictures. I remember from my own experiences of hiking in the ...
2
votes
2answers
151 views

What is the difference between 'flow' and 'move'?

I just met with a very basic question.(Might even sound silly!) My textbook kinda says(not exactly), 'Whatever flows is a fluid'. That got me wondering because we are creating a whole category of ...
2
votes
4answers
722 views

Would the rate of ascent of an indestructible balloon increase as function of it's altitude?

Assume a balloon filled with Hydrogen, fitted with a perfect valve, and capable of enduring vacuum (that is to say, it would retain it's shape and so well insulated that the extremes of temperature at ...
5
votes
2answers
296 views

Impossible microwave interference?

I was doing a microwave experiment with the following set up: there is a Gunn diode which emmits microwave radiation and a receiver (both work with polarised light). The strange thing is that when ...
8
votes
2answers
134 views

Does the geomagnetic field rotate?

The Earth rotates about it's own axis. Do the geomagnetic field lines rotate due to this rotation or not?
2
votes
2answers
119 views

Can we correctly define momentum operator only by means of position operator and their commutation relation?

In "J.M. Ziman. Electrons and Phonons: The Theory of Transport Phenomena in Solids" the author formally introduces the position (displacement) operator and then defines the momentum operator with the ...
2
votes
0answers
142 views

From Berry's phase to artificial Gauge potential

How a nonzero geometric phase in a loop is used to generate artificial gauge potentials? If possible, can you also tell how to generate the non-abelian artificial gauge potentials.
3
votes
0answers
44 views

Active-sterile mixing for KeVins

People sometimes talk about KeV mass sterile neutrinos as a warm dark matter candidate. I think they call them KeVins (horrible name btw). Now, In order for it to be a good dark matter candidate it ...
4
votes
4answers
2k views

Why do we take small steps while walking on ice?

When we walk on ice we should take small steps. Small steps ensure: a.)larger friction. b.)small friction. c.)larger normal force. d.)smaller normal force. The correct ...
25
votes
4answers
15k views

Why do electrons occupy the space around nuclei, and not collide with them?

We all learn in grade school that electrons are negatively-charged particles that inhabit the space around the nucleus of an atom, that protons are positively-charged and are embedded within the ...
9
votes
2answers
1k views

How do I calculate the Reynolds number in multiphase flows?

I am modeling a gas flowing through a liquid. How do I calculate the Reynolds number in multiphase flows? And, at what Reynolds number should I consider the flow to be turbulent? The problem is of a ...
0
votes
2answers
95 views

Deriving spring oscilation period [duplicate]

We have a spring attached to a wall and at the other part an object on a frictionless surface. I tried to calculate the spring oscilation period. I used the conservation of energy and kinematics ...
0
votes
0answers
30 views

Question on envelope-carrier description of traveling wave

I'm doing a research internship in attosecond physics right now, and one of the really important things in the field is the description of a propagating laser pulse as the combination of a slowly (or ...

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