# All Questions

667 views

### Moving black holes

What happens to the fabric of space in the wake of a moving black hole? Is space permanently deformed by a moving black hole or does it rebound as the black hole passes?
127 views

### Where can you get a photon detector?

Say I'm doing the double-slit experiment with photons as the particle and want to add a detector just behind each slit to eliminate the interference pattern. Where would I get a detector like that?
461 views

### Mechanisms of mass generation for Dirac neutrinos

If neutrinos are Majorana particles, one way of explaining their small masses is the seesaw mechanism. Now say I'd like my neutrinos to be Dirac, for symmetry to the quark sector. What mechanisms ...
340 views

### Classical (or semi-classical) interpretation of photoelectric effect?

This site says that "it has recently been proven that the photoelectric effect can be interpreted classically (or at least semi-classically) in non-particle, wavelike terms". Is anyone familiar with ...
893 views

### Origin of lepton/quark generations?

What theoretical explanations exist for the fact that there are three generations of leptons and quarks? I'm not so much asking why there are exactly 3 generations, but rather what makes electron, ...
58 views

### Do unoriented strings possess asymptotic states?

In QFT based particle theory, SU(N)-colored particles are not really present as asymptotic states, then raising some problems to build a S-matrix or other more axiomatic approaches to the theory. ...
851 views

### Why are Navier-Stokes equations needed?

Can't we picture air or water molecules individually? Then, why are Navier-Stokes equations needed, after all? Can't we just aggregate individual ones? Or is it computationally difficult, or ...
22k views

### How to deduce E=(3/2)kT?

It says in my course notes that a particle has so-called "kinetic energy" $E=\frac{3}{2}kT=\frac{1}{2}mv^²$ Where does this formula come from? What is k?
32 views

### Impurity scattering [duplicate]

Possible Duplicate: Impurity scattering temperature dependence Is there any temperature dependence of relaxation time in impurity scattering of conducting electrons? It seems to me that ...
3k views

### How is the topological $Z_2$ invariant related to the Chern number? (e.g. for a topological insulator)

This question relates to the $Z_2$ invariant defined e.g. for topological insulators: Is it correct to relate $Z_2$ = 1 to an odd Chern number and $Z_2$ = 0 to an even Chern number? If yes, is it ...
423 views

### Effect of gas or liquid within a compound lens system

Hi my question is if a compound lens system if filled with gas or a liquid how does it affect the system when compared to the lens system being separated by air alone. Does this affect the focal power ...
3k views

### How exactly does static discharge work?

Assume I have built up a pretty high charge by rubbing the floor or something. I want to understand these situations: I almost always get shocked when I touch a metal doorknob with my bare hand. I ...
644 views

### What is the energy of interaction between a point charge and an infinite cylinder?

I don't remember enough from my electromagnetism course and I can't find any simple, full example on this subject. I know, that I can consider the cylinder as a wire with the same charge density ...
139 views

### Kerr solution for finite collapse time

The Kerr black hole solutions gives an analytic continuation that is asymptotically flat. Some people have argued that this is another universe, but others state that the analytic continuation ...
110 views

### NGCC 1277— a recoil ejection?

Recent calculations agree that a merging pair of supermassive black holes can emit enough gravitational waves to eject themselves from a galaxy. Could NGCC 1277, a small galaxy with a 17 billion ...
169 views

### 4-momentum and an $y$ component of momentum

I have 2 coordinate systems which move along $x,x'$ axis. I have derived a Lorentz transformation for an $x$ component of momentum, which is one part of an 4-momentum vector $p_\mu$. This is my ...
147 views

### Will Randall-Sundrum extra dimension scenario become defunct if not supported by LHC?

The Randall-Sundrum extra dimension scenario had been one of the most extensively studied class of theories. This offered a solution to the hierarchy problem. However, if this picture is not supported ...
701 views

### Why the Hamiltonian and the Lagrangian are used interchangeably in QFT perturbation calculations

Whenever one needs to calculate correlation functions in QFT using perturbations one encounters the following expression: $\langle 0| some\ operators \times \exp(iS_{(t)}) |0\rangle$ where, ...
546 views

### Time evolution of a reduced density matrix

For a bipartite quantum system evolving under some master equation, is the time derivative of the reduced density matrix equal to the partial trace of the time derivative of the matrix? In other ...
628 views

### Where to place a fan in order to cool a room the fastest [closed]

Imagine I have room that is very hot and it is colder outside. I want to cool it down using a fan. Scenario 1 There is only one door. I have one fan. Where should I put it and which way should it ...
143 views

### Stiffness tensor

Let's have a stiffness tensor: $$a^{ijkl}: a^{ijkl} = a^{jikl} = a^{klij} = a^{ijlk}.$$ It has a 21 independent components for an anisotropic body. How does body symmetry (cubic, hexagonal ...
179 views

### Are there any good reading materials for variational approach in many-body theory? [closed]

I need something like a summary of existing results, including the treatment of BCS Hamiltonian and Hubbard model. Auerbach's book is a good one but I still hope to get more comprehensive review. My ...
630 views

### Why is the “canonical momentum” for the Dirac equation not defined in terms of the “gauge covariant derivative”?

The canonical momentum is always used to add an EM field to the Schrödinger/Pauli/Dirac equations. Why does one not use the gauge covariant derivative? As far as I can see, the difference is a factor ...
320 views

### Direction of friction on rollers with plank resting on top

In a question, its given that a plank is resting on 2 rollers like: Then the explanation, as part of computing the forces, for the direction of friction on the rollers look like: Why are the ...
178 views

### fitting van der waals law on real data

For an examples class on thermodynamics, I would like to fit Van der Waals law on data of real gasses, say $CO_2, H_2O_2, CH_4$. I want to set out measurements of pressure and volume at constant ...
1k views

### Why isn't temperature measured in units of energy?

Temperature is the average of the kinetic energies of all molecules of a body. Then, why do we consider it a different fundamental physical quantity altogether [K], and not an alternate form of ...
246 views

### observable quantities are gauge invariant?

I have a simply question, that is whether spatial velocity is gauge invariant. It is seems that under a infinitesimal coordinate transformation the velocity is just transform as other vectors, and it ...
163 views

### Why doesn't the firewall argument also apply to far away ingoing modes?

Gidom Mera's answer at http://physics.stackexchange.com/a/45511 is illuminating, but on closer analysis, it brings up further puzzles. Backscattering works in both directions. Let's see what we get ...
301 views

### How to express $ds$?(when we know expression for $ds^{2}$) [closed]

We know that $$ds^2 = g_{\mu\nu}dx^{\mu}dx^{\nu},$$ Can you say how to calculate $ds$?
314 views

### What makes an equation an 'equation of motion'?

Every now and then, I find myself reading papers/text talking about how this equation is a constraint but that equation is an equation of motion which satisfies this constraint. For example, in the ...
179 views

### Other Gross-Neveu like theories?

By "Gross-Neveu like" I mean non-supersymmetric QFTs whose partition function/beta-function (or any n-point function) is somehow exactly solvable in the large $N_c$ or $N_f$ or 't Hooft limit. ...
682 views

### Can thick-film reflection holograms be used to create true mirrors?

Experimental alert: Someone may be able to answer this question experimentally simply by going to a shopping mall and finding the right piece of holographic jewelry. My question is whether the type ...
488 views

### Building a LED lamp, which is the brightest? 3 LEDs of 2500 mcd or 1 LED of 6000 mcd?

I want to build the brightest lamp, for it to be seen at daylight, should I use 3 2500 mcd LEDs or 1 6000 mcd? And also Why?
4k views

### What is the farthest-away star visible to the naked eye?

My girlfriend and I were watching Cosmos, and something Carl Sagan said got us wondering what the farthest-away visible star is. Obviously "visible to the naked eye" is a fuzzy concept that might have ...
455 views

### using a unitary matrix to transpose

A unitary matrix U is a matrix such that the conjugate transpose of U, when multiplied on the right with U, yields identity. My question is, is it possible to obtain the transpose of any density ...
79 views

### How to express answers where inequalities are involved?

Part of the floor of a workshop is inclined at 10 degrees to the horizontal. This is to allow the safe storage of steel cylinders. A cylinder of mass 7000 kg is stored as shown in the diagram (c the ...
136 views

### Quantum cryptography: encryptions

I am studying quantum cryptography and I have a very basic question. Suppose A and B share a secret key k, where k=0 or 1. A wants to send one qubit to B. What A does is, if k=1, she 'flips' the qubit ...
384 views

### How accurate are Mayan astronomical “ephemerides”?

Because of the hype surrounding the "end" of the Mayan calendar (along with the usual cultural relativism and Western guilt) it is nearly impossible to find an objective quantitative assessment of the ...
5k views

### Confused on how to properly use right hand rule

I am having trouble using the right hand rule properly and often find myself putting my hand in awkward orientations. I know you point your hand in the direction of $r$ and then point your fingers in ...
849 views

### Ice skater increase of energy

This may be a very basic question but I am not seeing how it works. Consider the standard example of an ice skate rotating about his/her center of mass and pulling in his/her arms. The torque is zero ...
1k views

### Pump pressure required to fill water tower

I have a water pump at a distance of x meters from a water tower located h meters above the pump through a pipe with diameter d. I am not concerned with the exact value just want to make sure my pump ...
466 views

### Is man-made Carbon Fusion possible?

In a type Ia Supernova, the carbon accumulated in the earlier stages of a stars death fissues to create even heavier elements. Could this be used by humans aswell? Is it theoretically possible to ...
212 views

### How do physicists and astronomers handle leap seconds?

I'm confused by the many contradictory descriptions I see about how UTC leap seconds are accounted for. I understand that there are various ways to handle them in common practice, and I've seen a ...
251 views

### Directionality of angular momentum

I was told that the sum of linear and angular momentum is conserved. Given that angular momentum's direction as a vector is completely arbitrary (I believe there is no physical reason for choosing ...
188 views

### Accelerating expansion of universe - entire universe a non-inertial frame of reference?

If the expansion of the universe is accelerating, doesn't that mean that the entire universe is a non-inertial frame of reference? And if so, what implications does this have (if any)?
1k views

### A problem from Pathria, canonical ensemble, how to calculate $\left\langle \left(\Delta E\right)^{3}\right\rangle$

This is problem 3.18, from the book Statistical Mechanics by Pathria. Show that for a system in the canonical ensemble \left\langle \left(\Delta E\right)^{3}\right\rangle =k^{2}\left\{ ...
1k views

### Kinetic Energy And Rotational Motion

The problem is, "A metal can containing condensed mushroom soup has mass 220 g, height 11.0 cm and diameter 6.38 cm. It is placed at rest on its side at the top of a 3.00-m-long incline that is at ...
74 views

### transition between extremal and nonextremal black hole states

Extremal black holes are at zero temperature, hence they do not radiate. my question is twofold: 1) is extremality of micro black holes a stable property? electric charge is quickly emitted from ...