2
votes
3answers
969 views

Unpolarized light vs. randomly rotating polarized light?

I am confused with physical picture about unpolarized light. Is unpolarized light very fast rotating polarized light? or co-existing state of two orthogonal polarization? (or something else?) If ...
2
votes
3answers
502 views

Problem on apparent height of postage stamp underneath paperweight

A postage stamp is placed on a surface and a glass cube of refractive index $1.5$ is placed over it. When observed through the cube, the stamp appears at a height of $1.5cm$ from the bottom of the ...
2
votes
2answers
372 views

How is wavelength actually related to space (/distance)?

Is a photon of 400nm in wavelength "smaller" than a photon of 1km in wavelegth?
2
votes
1answer
180 views

representation of conformal group in d>2

In P. Di Francesco, P. Mathieu, D. Snchal they fix the generators of the conformal group acting on a scalar field by somewhat arbitrarily defining $$\Phi'(x)=\Phi(x)-i\omega_a G_a\Phi(x)$$ and by ...
2
votes
2answers
60 views

Where the extra power comes from?

Suppose we have two radio waveforms each has amplitude of 1, then the total power is 2. Suppose these two waveforms add up some where constructively, then the amplitude become 2, and the total power ...
2
votes
2answers
3k views

Is squared motor gearbox ratio proportional to inertia ratio?

I read an interesting article http://m.machinedesign.com/news/motor-sizing-made-easy It is very interesting, but I can not follow the 2nd last paragraph. I don't understand why it is true. ...
2
votes
1answer
746 views

Diagonalization of a hamiltonian for a quantum wire with proximity-induced superconductivity

I'm trying to diagonalize the Hamiltonian for a 1D wire with proximity-induced superconductivity. In the case without a superconductor it's all fine. However, with a superconductor I don't get the ...
2
votes
2answers
185 views

A question for the generalization of gauge transformation with two antisymmetric indices

I have a question about the generalization of gauge transformation with two antisymmetric indices. Starting from Eq. (3.7.6) in Polchinski's string theory book p. 108. $$S_{\sigma} = \frac{1}{4 \pi ...
2
votes
2answers
2k views

Calculating the expectation value for kinetic energy $\langle E_k \rangle$ for a known wave function

I have a wavefunction ($a=1nm$): $$\psi=Ax\exp\left[\tfrac{-x^2}{2a}\right]$$ for which I already calculated the normalisation factor (in my other topic): $$A = \sqrt{\frac{2}{a\sqrt{\pi a}}} = ...
2
votes
2answers
271 views

Photons moving relative to photons

I know we can't move at the speed of light, but if we were to travel on a photon how fast would we see other photons going? The speed of light is constant... So do photons see other photons moving at ...
2
votes
2answers
297 views

How do you travel in a circular orbit around a massive body?

I am trying to figure out how an object could achieve a perfectly circular orbit. Given a mass for the planet or other body the object is orbiting and a distance from the center of mass, how fast ...
2
votes
1answer
197 views

Why do new towels dry better after a few uses?

Most of you will be familiar with the phenomenon: you have bought a new towel and you first have to wash it or use it a couple of times before it starts to work properly, i.e. dry your body after ...
2
votes
2answers
330 views

Light in Different Reference Frames

I think I'm just confused, but for some reason I thought that light moving straight in one frame would have to move in the same direction in another frame. I know there are photons-but because I have ...
2
votes
1answer
773 views

Canonical transformation generated by hamiltonian?

Someone told me that, in a hamiltonian system, the hamilonian function is the generating function of the canonical transformation given by time translation. However, this statement doesn't make any ...
2
votes
1answer
112 views

Compton scatering problem - How to solve this large problem faster using the momentum system?

I have encountered a hard exercise which i cannot quite solve. Could anyone help me with it? This is the exercise: Lets say we have a photon whose energy $W_f$ is equal to the relativistic ...
2
votes
4answers
9k views

Transformer: primary side & secondary side current 180 degree out of phase

I am a novice in electrical engineering. I notice that in transformer the secondary side current & current referred to as primary are 180 degree out of phase from each other. But why it is so, I ...
2
votes
1answer
214 views

How to derive Eq. (1.2.17) in Polchinski?

I have a super stupid question about deriving Eq. (1.2.17) in Polchinski's string theory, vol 1. The book seems to derive from $$\tag{1.2.16} h_{ab}=\frac{1}{2} \gamma_{ab} \gamma^{cd} h_{cd} $$ ...
2
votes
2answers
583 views

Does trade affect Earth's rotation? [duplicate]

Every country is trading with other countries around the world, some more than others. I was wondering if there would be any change to the Earth's rotation because of the imbalance of trade between ...
2
votes
1answer
1k views

Drag force at high speeds [duplicate]

The drag force on a spherical body according to Stokes' law is given by $$F = 6π\mu rv$$ Where $\mu$ is the dynamic viscosity of the fluid, $r$ is the radius of the spherical object, and $v$ is its ...
2
votes
2answers
177 views

Is an ice globe the worst possible way to cool a drink (with ice)?

Given an alcohol solution and a mass of ice in whatever shape you like, is shaping it into a sphere the worst possible way to cool your drink without diluting it? If the ice starts off at a sub-zero ...
2
votes
4answers
2k views

Total angular momentum of deuteron

I'm studying for my nuclear physics exam and the book we use is Introductory Nuclear Physics by K.S. Krane. In the chapter on Basic Nuclear Structure, we research the deuteron. However, when ...
2
votes
2answers
383 views

Bertrand's theorem

I found in Goldstein's Classical Mechanics that the condition for closed orbits is given by $\frac{d^2 V_{eff}}{dr^2}>0$.(bertrand's theorem). Can somebody explain to me, how this inequality is ...
2
votes
2answers
231 views

Is a permutation of coordinates or labels really equivalent?

To construct a N-body anti-symmetric wave function some derivations start with the requirement that the N-body wave function should be anti-symmetric under a permutation of coordinates, other ...
2
votes
2answers
12k views

How to estimate the physical size of a molecule?

I'm reading some chemistry-related papers that employ concepts of droplet evaporation. Since I am no chemist, I am wondering: How can I estimate the actual size of a molecule, say succinic acid? An ...
2
votes
2answers
204 views

How is everything a field?

I've heard before that everything in physics can be thought of as either a field, or its excitation. Is there some intuitive explanation of how I can look at gravity, light, electromagnetism, etc as a ...
2
votes
3answers
417 views

On Einstein notation with multiple indices

On Einstein notation with multiple indices: For example, consider the expression: $$a^{ij} b_{ij}.$$ Does the notation signify, $$a^{00} b_{00} + a^{01} b_{01} + a^{02} b_{02} + ... $$ i.e. you ...
2
votes
1answer
156 views

Supersymmetry in Quantum Mechanics (Does it apply?)

Suppose we try to apply supersymmetry in quantum mechanics to a particular potential. If you come up with two partner potentials, and two partner Hamiltonians, and then look at the energy of the ...
2
votes
1answer
1k views

What is the difference between Feshbach resonance and Fano resonance?

What is the interpretation of "resonance" in Feshbach resonance and Fano resonance? What is the difference of Feshbach resonance and Fano resonance?
2
votes
2answers
96 views

Gauss's Law understanding

In the case of a point charge $q$ at the origin, the flux of $\vec{E}$ through a sphere of radius r is, \begin{equation} \oint \vec{E}\cdot d\vec{a} = \int \frac{1}{4 \pi \epsilon_0 ...
2
votes
2answers
2k views

Find E and B from vector potential

I have a vector potential given by: $\mathbf{A}(x,t) = \mathbf{e}_{y}\frac{1}{2} e^{-(x-ct)^{2}/{4a^{2}}}$ Now, the question is "Determine the E and B under the condition that the scalar potential ...
2
votes
3answers
336 views

Why is the Coulomb potential in pseudo-2D experiments proportional to the logarithm of distance?

Inspired by this question, I ask another. Theoretically, Coulomb potential in 2D is proportional to the logarithm of distance; In experiments, though electrons are constrained in a pseudo-2D ...
2
votes
2answers
1k views

Do black holes have charges?

Do black holes have charges? If so, how would they be measured? Also, does electricity behave the same way? Black holes affect photons, which are carriers of EM radiation, so do black holes have any ...
2
votes
1answer
941 views

Why vary the action with respect to the inverse metric?

Whenever I have read texts which employ actions that contain metric tensors, such as the Nambu-Goto, Polyakov or Einstein-Hilbert action, the equations of motion are derived by varying with respect to ...
2
votes
2answers
1k views

Density Operator, Expectation Value, Coherent States

How would I go about evaluating expectation values like $\langle X \rangle$ and $\langle P \rangle$? Work I've done: I've done the integration over $\phi$ and rewrote $\rho$ as: $\rho = ...
2
votes
4answers
6k views

Capacitor Charging and Discharging when connected to the ground

When we charge a capacitor using a battery and then remove the battery, the plates of capacitor becomes charged. One holds positive charge and the other one gets equal negative charge. o. k. ? Now ...
2
votes
2answers
705 views

Asymmetric uncertainties

Inspired by these two question on tex.SX Asymmetrical tolerancing Asymmetric uncertainties with siunitx package I'd like to ask for a nice explanation for these kind of uncertainties, like ...
2
votes
2answers
246 views

Can acceleration feel like constant gravity for indefinitely long?

So here's the setup: I'm in a spaceship, without windows as always, and the ship is accelerating upwards at a constant rate of $1\,\text{g}$. So inside the spaceship it feels like I'm being pulled ...
2
votes
1answer
126 views

Interacting particles

We are familiar with the grand partition function for the grand canonical ensemble. This makes me wonder: what kinds of modifications would be required if the particles interacted? Thanks.
2
votes
1answer
1k views

Does universal wave function exist?

Does universal wave function exist? What the science tells us?
2
votes
2answers
147 views

When does $T^{ij} = T_{ij}$?

Suppose we have some tensor with components $T^{ij}$. Then suppose that we also have $T_{ij}$. When would $T^{ij}T_{ij} = (T^{ij})^2 = (T_{ij})^2$?
2
votes
2answers
386 views

Most efficient type of heat pump

Can I make a heat pump beat the Carnot efficiency? Why is the Carnot process the most efficient one? If I have a heatpump that is sphere shaped, and cascaded in layers like a onion can I beat Carnot ...
2
votes
2answers
327 views

Empirical bound on sum of electron and proton charge

Followup to "Why do electron and proton have the same but opposite electric charge?". It is argued that even a tiny residual charge would result in huge amounts of electricity in bulk matter, ...
2
votes
3answers
278 views

Flushing water-Is it related to Coriolis force? [duplicate]

There are videos and articles on the internet which demonstrate that water flows down a flush clockwise in Northern Hemisphere and anti-clockwise in Southern Hemisphere. Here are a couple of links ...
2
votes
1answer
124 views

Locally every force admits a potential?

I have a little doubt about a force being or not conservative. Well, as I understood, some forces cannot be expressed as exterior derivative of some scalar potential because the work done by the force ...
2
votes
2answers
309 views

Dark matter and QFT

My understanding is that the particle is a somewhat artificial notion in QFT (see: Quantum Mechanics: Myths and Facts), and that in general it is possible for a quantum field to have unstable ...
2
votes
1answer
1k views

Particles entangled after the big bang

Is that true that the big bang caused the quantum entanglement of all the particles of the universe so every particle is entangled to each other particle of the universe?
2
votes
1answer
740 views

Why is the free energy minimized by the Boltzmann distribution?

Can someone show me, without glossing over anything, why $F = E - TS$ is minimized when $p_i = e^{-U_i/k_bT}/\sum_ie^{-U_i/k_bT}$? I understand it conceptually, but am having difficulty showing it ...
2
votes
1answer
586 views

Physical Significance of Operator Norm/Spectral Norm of a Quantum Operator

Is there any physical significance of operator norm/spectral norm of a hermitian operator?
2
votes
3answers
25k views

Small car colliding with large truck

A small car collides with a large truck. Why do both vehicles experience the same magnitude of force? Wouldn't the large vehicle experience less force than the small one?
2
votes
1answer
563 views

How to get the new direction of 2 disks colliding?

I'm developing a 2D game including collisions between many disks. I would like to know how I can get the angle corresponding to the new direction of each disk. For every disk I have this information ...

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