# All Questions

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### Is tetrad postualte independent of gauge field?

Here is what I know, $g_{\mu \nu} = e^{a}_{\mu} e^{b}_{\nu} \eta_{ab}$ and the tetrad postulate is D_{\rho} e^{a}_{\nu} = \partial_{\rho} e^{a}_{\mu} - \Gamma^{\lambda}_{\rho \nu}e^{a}...
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### Calculating the speed of an ejected electron [closed]

I'm working on some physics exercises and came across this one. Sadly, there is no solution provided, so I was wondering if at least my line of reasoning is correct. ...
64 views

### How is the change in internal energy zero for the following scenario:

in the problem, they are removing partitions one by one. when they remove the first partition they've mentioned in the solution that internal energy will remain constant in the process (and after ...
1 vote
45 views

### DMRG for anyons

I want to do some DMRG calculations for anyons. For example, consider the golden chain model for fibonacci anyons. https://arxiv.org/pdf/cond-mat/0612341 I have two anyon types: $1, \tau$. However, ...
• 163
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### As per Friedmann Equations, is big-bang singularity necessary?

The Friedmann Equations do not directly require that the scale factor $a(t)$ was zero in the beginning. Since Einstein's static universe is still a valid solution, is it possible that before the Plank ...
• 1,161
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### How steep is too steep to tow a caravan up? [closed]

My family plans to buy a static caravan but we have been told it may not be possible to move it from the road onto our property. The entrance to the property is a bit steep – but is it too steep to ...
96 views

### Circuit quantization procedure

In circuit QED, we construct the hamiltonian of a circuit following Devoret's procedure. We start from a classical treatment, in which we impose Kirchhoff's voltage laws on fundamental loops and ...
• 113
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### Kinetic energy of a disc rotating about an inclined axis [closed]

What is the kinetic energy of a circular disc of mass M and radius R rotating with angular velocity w about an axis that is inclined to its surface by an angle of 45 degrees? (Please answer using the ...
84 views

### What does the word "Observable" mean in Quantum Gravity?

I have seen the statement in several places that the only "observables" in general relativity or Quantum Gravity are measured at temporal or spatial infinity. This is often used as a ...
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### Battery in non-closed circuit with charged body and Earth

Looking at the clips on how the battery works, I understand that eventually one of the ends releases the electrons which then move through the conductor to the other end, to continue the reaction. Now ...
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### What is the charge distribution on a parallel plate capacitor when one plate is grounded? [closed]

Let's say a large parallel plate capacitor is connected to a DC power supply. Then, the charge on the two plates will be +CV and -CV. If I ground the negative terminal (both the capacitor's and the ...
48 views

### Potential of circle and $n$-gon

Consider power central fields $f \sim r^k, - \infty < k < \infty$, and unit circle, "charged" in the sense of the field. We will be interested in the potential within the circle, i.e. ...
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### Will expanding space rupture an empty box floating in outer space

Under the theory that space itself is expanding, but the space inside of atoms and molecules doesn't expand because nuclear and electromagnetic bonding forces exceed the forces that expand space, ...
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1 vote
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### What is the correct way to think of position?

How accurate would it be to think of position (along some axis) as the component of radius vector. Example: $$\textbf{r} = x \hat{\textbf{i}} + y \hat{\textbf{j}}$$ And if that is correct, we could ...
1k views

### Quantum: why linear combination of vectors (superposition) is described as "both at the same time"?

I want to get a better understanding of quantum phenomena and out world in general. Before long I've thought of Schrödinger cat as being both alive and dead (or spin both up and down). Now after some ...
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### Could you make a fire torch with electricity? [closed]

Similar to a blowtorch, is there a way to generate a flame without using gas or liquid fuel(like butane, propane, etc)? (source) For example I'm thinking about those portable air heaters, could one ...
• 193
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### Why does moment of inertia stop at 1/2 as solidness of a cylinder increases?

So I have two things about moment of inertia: The I for a hoop is $I=MR^2$ and the I for a solid disk is $I=\frac{1}{2}MR^2$ I've noticed that as "solidness" goes up from 0% (completely ...
1 vote
43 views

### Viable values for the $K$ parameter in the FLRW metric

The FLWR metric is sometimes given as $$c^2 d\tau^2 = c^2 dt^2 - \frac{a(t)^2}{(1-KX^2)} dX^2.$$ I am not interested in the tangential motion so I set $d \Omega = 0$ although it is of interest in ...
• 5,677
40 views

### Frozen coffee that will not defrost at room temperature [closed]

A few weeks ago I froze some Starbucks iced mocha in an aluminum water bottle that is not insulated. Three days later the coffee which has been kept on the kitchen counter has still not defrosted in ...
16 views

### Why Does Every Member of the Ensemble Perform Work Equal to the Change in Free Energy During a Quasiequilibrium Process?

I'm studying Jarzynski's 1997 paper on the Jarzynski Equality. In the second part of the proof (from equation (9) to equation (10) in the paper), Jarzynski considers the following process: considering ...
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### Terminal velocity of a bar magnet in a long conducting tube (quantitative)

If a bar magnet is dropped into a copper (conducting) tube, it reaches a terminal velocity. I'm trying to derive this terminal velocity. We know from Lenz's law that $e=\frac{-d\phi}{dt}$. I'm facing ...
1 vote
26 views

### What actually happens when light goes into a waveguide?

I have only dealt with a rudimentary theoretical treatment of how light propagates inside a waveguide, such as those available in Griffiths. One thing bugging me is that I still do not know how ...
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### How to turn a sum into an integral?

In, An Introduction to Thermal Physics, page 235, Schroder wants to evaluate the partition function $$Z_{tot}=\sum_0^\infty (2j+1)e^{-j(j+1)\epsilon/kT}$$ in the limit that $kT\gg\epsilon$, thus he ...
31 views

### Bloch oscillations and umklapp scattering induced by applied electric field

In Kittel's book chapter 8 there is a statement that says "The electron accelerates from k = 0 toward the zone boundary; when it reaches k = pi/a it reappears (as by an Umklapp process) at the ...
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### Flux change through a loop

I'm having difficulties understanding this thing. It would be lovable if someone could show me its derivations and prove it: In a conducting loop, there is a relationship between the total displaced ...
31 views

### I am trying to derive an expression for the magnetic field of charged particles travelling at significant fractions of speed of light [closed]

I tried using lorentz transformation , which gives me correct answer for speed of light c but when i try to input any other values nearby c it becomes a constant B = 10-7 . q/r² ( (v - u)/ 1- uv/c²)
146 views
+50

### Do lasers with optical isolator or synchrotron radiation sources allow for Rabi cycles?

In Rabi cycle two-level system cyclically transitions between ground and excited state - bringing question where their energy difference goes during these transitions? For transition from ground to ...
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### How do I calculate the absorption of single layers of a multilayer stack using the electric field in each layer?

I'm trying to calculate the absorbance of individual layers using the transfer matrix method but I'm stuck on how to do it. The code I am using can give the total absorbance of the system, and the ...
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59 views

### Under what circumstances can a 4D singularity occur in General Relativity?

I've tried to find on the literature about 4D (single point) singularities, but most of the theorems about singularities pertain to either space-like or time-like singularities, which always have some ...
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### Can one make a jet of air bounce without spreading?

Similar to this reaction on this image (left most) how it keeps its mass without spreading: (Video Link) Is there a way to make a jet of air bounce without spreading? (source)
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1 vote
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### Cone vs. small circle parallel transport

I'm having trouble reconcile the following two seemingly contradicting conclusions (in 2d space): A cone is flat, because you can unfold it and it's a flat 2d surface. A cone as shown in the picture ...
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1 vote
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### Why do objects in circular motion move outwards instead of inwards? [duplicate]

I know that whenever an object undergoes circular motion then it has a centripetal acceleration towards the centre and a tangential acceleration. Due to these accelerations I had thought that ab ...
52 views

### Spinning a rope conundrum

Sometimes I will take a rope or string and spin it in the air to see what will happen. Now usually, if I am spinning it vertically, the end of the rope will extend outwards due to Newton's first law, ...
69 views

### What happens if we differentiate spacetime with respect to time? [closed]

Essentially, what would differentiating space-time with respect to time provide us with? What are the constraints associated with such operations? Is it possible to obtain a useful physical quantity ...
70 views

### Regarding the absorption property of a blackbody

Consider a blackbody of surface area $S_b$ and at temperature $T_b$. It is placed inside an evacuated chamber (to neglect all the effects of convection), with walls of chamber at temperature $T_c$ and ...
35 views

### Is there a way to achieve a long jet of air?

As can be seen in the example image bellow from a fog/smoke generator machine, the smoke spreads as soon as it leaves the nozzle. (source) Is there a way to achieve a longer jet of air with less ...
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### Does gravity accelerate you towards the geodesic of light between you and the mass?

If there's a planet far away, you will accelerate straight towards it due to gravity. If you place a Schwarzschild black hole right in the middle between you and the planet (the distance between the ...
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### What would the temperature of the tropics be if the atmosphere did not try to balance out heat the latitudinal surplus and deficit

The atmosphere acts to even out the heat imbalnce between the surplus at the tropics and the deficit at the poles. Assuming that the composition and density of the atmosphere stay the same, what ...
37 views

### Argument for relationship between density of states and degrees of freedom and energy

From Reif's Fundamentals of Statistical and Thermal Physics chapter 2.5 Behaviour of the density of states: Can anyone give a clear explanation for why the density of varies rapidly as Energy ($E$) ...
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1 vote
I am trying to solve excercise 8.5 d) from Straumanns book on general relativity (here V is an n-dimensional Minkowski vector space): Prove that the orthogonal complement of a null vector is an $(n-1)$...