# All Questions

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### 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 ...
171 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 ...
150 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 ...
726 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, ...
580 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 ...
696 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 ...
150 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 ...
185 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 ...
679 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 ...
338 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 ...
182 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 ...
252 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 ...
165 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 ...
303 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 $ds$?
316 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 ...
187 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. ...
756 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 ...
521 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?
6k 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 ...
477 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 ...
83 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 ...
138 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 ...
392 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 ...
894 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 ...
524 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 ...
215 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 ...
283 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 ...
195 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 ...
77 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 ...
160 views

### Ejected Electrons with 0 KE?

So I was taught that: Kinetic Energy (of electron) = Energy (of photon) - Ionization Energy If E(photon) = IE, then KE=0 of the electron. What does this physically/theoretically mean? My current ...
238 views

### Theoretically speaking what are the limits for super-capacitors in terms of their storage capability?

According to wikipedia, commercially available super-capacitors in 2010 can store 30Wh/kg, with 85Wh/kg achieved in a lab in 2011. Have any theoretical calculations been done to show what can be ...
153 views

### Boundary conditions for fields in Kerr/CFT

I am reading a paper by Guica et al. on Kerr/CFT correspondence (arXiv:0809.4266) and I'm not sure if I got this. They choose the boundary conditions, like a deviation of the full metric from the ...
2k views

### Why is temperature constant during melting?

This is an elementary question but I do not know the answer to it. During a phase transition such as melting a solid to a liquid the temperature remains constant. At any lower temperature the heat ...
658 views

### What happens to the vapor pressure when I expel the liquid phase from the vessel?

Suppose I put water in a closed vessel and heat it, so the vessel becomes pressurized above ambient pressure. Now, there should be liquid and gaseous water inside the vessel. Since it's under ...
364 views

### Euler-Lagrange Equation

A particle moving towards the origin has initial conditions $x(t=0) = 1$ and $\dot{x}(t=0)=0$ If the Lagrangian is L:=$\frac{m}{2}\dot{x}^2 -\frac{m}{2}ln|x|$ This should satisfy Euler Lagrange ...
271 views

### micro black hole forces

A black hole would radiate mass optimally for interstellar-travel applications in the range between $10^7$ and $10^8$ kilograms. Assuming a light-only radiation emission spectrum, with a parabolic ...
670 views

### Why do we study the scalar field in QFT when there is no such thing in nature?

The Klein-Gordan equation describing a spinless scalar field is one of the first things one studies in a QFT course, but there are no elementary spin-0 fields in nature. Is the scalar field to QFT ...
168 views

### can we investigate physics through investigation of pure number? [closed]

If the consistency between the two is so absolute, why can we not investigate the physical nature of the universe through analysis of pure number? Particularly at the quantum scale?
482 views

### What are the current possibilities for large-scale storage of electrical energy?

I'm curious as to how large-scale electrical storage is managed now, how much they can store, how well they scale and for how long (and what is their loss rate). (I'm not asking about technological ...
583 views

### What is the pressure in a vertical pipe that has moving fluid through it?

I have a system like this When the valve is closed, pressure along any point along the thin tube may be found thru pgh. 1. What about when the valve is opened? 2. Does the top of the jar need to be ...
740 views

### Faraday law, third Maxwell's equation in Mathematica

Three question about this equation: $\displaystyle\nabla\times\mathbf{E}=-\frac{\partial \mathbf{B}}{\partial t}$ 1 If I solve this equation with Mathematica, I find the magnetic field ...
133 views

### What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process for ideal gas?

What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process (in simple form) for ideal gas? I think that you need work to preserve this process and you bring thermal ...
750 views

### Why laundry dry up also in cold/frost?

Why laundry dry up also in cold/frost? When you have frost, water in the clothes should freeze, but if clothes are dry, then it should be possible that steam in the clothes does not have time to ...
Since the charged pions decay into two particles, a muon and a muon neutrino, seems quarks disappeared!, The decay proceeds by the weak interaction $W^{+}$ and can be visualized in terms of Feynman ...