# All Questions

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### How does one make a particle with particular wavefunction?

I was always curious how scientists achieve a particle with particular wavefunction (of location and spins etc.) So how do they achieve it? Or is this impossible?
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### Counterpart of the Klein Gordon Equation on the “Coordinate Shell”

The relation $$\psi=Ce^{i/\hbar(Et-\mathbf{p}\cdot\mathbf{x})}\tag{1}$$ satisfies the Klein Gordon equation on the mass shell, i.e. for $E^2=p^2+m^2$. Now let's think in the reverse direction. ...
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### Is there a deep reason why springs combine like capacitors?

I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ...
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### Relating theta_QCD to neutron EDM

How do I relate the topological $\theta_\text{QCD}$ parameter to the electric dipole moment (EDM) of the neutron? I am very familiar with chiral perturbation theory. I just need to know how to take ...
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### Was Jupiter's mass “guessed at” by Kepler or Galileo?

Following Kepler's publication of his 3rd law of planetary motion1, $$p^2 / r^3 = 1$$ in 1619, it would have been possible to use telescopic observations to arrive at an estimate of the orbital ...
310 views

### Conservation Laws in Photon Parametric Down-Conversion

As Wikipedia explains, one photon passing through a crystal sometimes down-converts to two photons. Wikipedia says total energy and momentum are conserved by just considering the three photon states; ...
1k views

### Why does it take until the middle of summer before lakes have warm water, but desert sand heats up in hours?

My sister asked me this question and I keep thinking that water would conduct heat much faster than sand. Hence the energy transfer of heat across the lake does not allow it to heat up soon. Sand on ...
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### What should a physics undergrad aspiring to be a string theorist learn before grad school?

The question I guess is pretty clear. I am a physics undergrad wishing to pursue research in quantum gravity(string theory?). What are the subjects I should learn other than the usual compulsory ...
258 views

### Fields of Steady Currents Using Electrostatics

Suppose you have a uniform ring charge rotating at constant angular velocity so that you also have a uniform ring of steady current, and thus you can use the Biot-Savart Law to compute the magnetic ...
321 views

### Improved energy-momentum tensor

While still dealing with this issue, I've stumbled upon this answer to a question asking about the conserved quantity corresponding to a scaling transformation. It mentions that in accordance with ...
1k views

### Why does inverting a song have no influence?

I inverted the waveform of a given song and was wondering what will happen. The result is that it sounds the exact same way as before. I used Audacity and doublechecked if the wave-form really is ...
229 views

### classical dynamics on group manifold SU(2)

I am trying to understand how to formulate classical dynamics on group manifold SU(2). This will be an exercise for me to the more advanced subject of path integral on group manifold. Does someone ...
779 views

### Force applied perpendicular to direction of motion, static or kinetic friction?

Let's assume we have a 1 kg cube on a flat uniform surface. The coefficients of friction between them are $\mu_k = 0.25$ and $\mu_s = 0.50$. This cube is moving at 1 m/s in the y direction, with ...
2k views

### Direction of friction on particle placed on a rotating turntable

If a particle is placed on a rotating turntable then the particle has a tendency to slip tangentially with respect to the underneath surface... So the friction should act tangentially to the ...
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### Is time continuous or discrete?

While working on physics simulation software, I noticed that I had implemented discrete time (the only type possible on computers). By that I mean that I had an update mechanism that advanced the ...
803 views

### representation of SU(2)

The question is regarding SU(2) group and SU(2) algebra. The SU(2) group can be generated by exponentiating the generators of SU(2) algebra $X_a$ as $exp(i t_a X_a )$ with $t_a$ being three ...
184 views

### How is an ideal mirrored box of photons distinguishable from massive particles?

Suppose you have an ideal mirrored box that contains enough photons as to have a relativistic mass equivalent to the [rest mass + kinetic energy] of an electron. In other words, the two systems have ...
511 views

### How does Doppler effect differ between EM -waves in Electrodynamics and Sound -waves?

I have an electrodynamics -course that contains doupler -effect but unfortunately with little explanations. Is it the same thing as the classical doppler effect for example with sound, more here, or ...
177 views

### why nontrivially space-like connected event horizons do not respect unitarity?

I want to understand the assertion that the gluing between distant event horizons is forbidden by unitarity. What is exactly the argument that unitarity will necessarily forbid topological nontrivial ...
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### Is there a difference between a postulate and a principle in physics?

Is there a difference between a postulate and a principle in physics? Both seem unproved statements taken as true. If thats correct, why the different names?
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### How can we define BF theory on a general 4-manifold?

(I have rewritten the question some, with new understanding) 4d BF theory is classically presented as the TFT arising from the Lagrangian $B\wedge F$, where $B$ is an abelian 2-connection (locally ...
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### Inductance of loop to a square wire? What is the induced current and the induced $\bar E$?

I am practising for the elementary-physics -course Tfy-0.1064, this puzzle is a simplified version of the problem 3A here (sorry not in English). My basic idea here is to understand a scenario with ...