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  • 0 posts edited
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  • 1,734 votes cast
Apr
25
comment Are black holes hollow?
There is no gravity inside a hollow sphere. Things don't fall outward to the shell. I don't know what happens inside hollow black holes, but it seems unlikely that things will fall upward towards the horizon.
Apr
24
comment Addition of $N$ spin halves
Is this the right formula?
Feb
16
comment Do black hole merger simulations include regions inside event horizons?
@dmckee: this is one of the things I remember from a talk about numerical simulation of black holes ... and I've forgotten who gave it.
Feb
16
comment Do black hole merger simulations include regions inside event horizons?
Things inside the event horizon are simulated, but not necessarily very far inside. Remember that you can't even decide where the event horizon is without looking at the state globally ... thus, cutting the simulation off exactly at the event horizon is pretty much impossible.
Feb
15
comment Difference between theoretical physics and mathematical physics?
@Yvan: I didn't think it was derogatory. I just wanted to emphasize that Witten did not receive his Fields Medal for string theory, which is a misapprehension that a lot of physicists have.
Feb
15
comment Difference between theoretical physics and mathematical physics?
@Yvan: as far as I can tell, Witten did not receive the Fields medal for string theory. The citation said merely "proof in 1981 of the positive energy theorem in general relativity". The honorarium lecture gives details on this, rigidity theorems inspired by string theory, and topological quantum field theory. For the first, Witten gave a complete proof. For the second and third, Witten's work gave the tools that let other mathematicians give a complete proof.
Feb
14
revised Is Bekenstein entropy limit inconsistent with universal continuity?
added 17 characters in body
Feb
14
answered Is Bekenstein entropy limit inconsistent with universal continuity?
Feb
12
comment How were the solar masses and distance of the GW150914 merger event calculated from the signal?
The farther away the event was, the smaller the amplitude.
Feb
10
comment What will be final velocity of three charges $q$, $q$, $2q$?
Are you looking for the shape of the triangle as time goes to infinity, or the exact trajectories of the particles? Because the shape of the triangle might be reasonably easily calculable, while the exact trajectories of the particles are going to be horrible.
Feb
10
comment Why is particle superposition still part of quantum mechanics?
Superposition and quantum entanglement can occur.
Feb
10
comment Can a magnet be tuned to attract only to one other magnet?
@user100712: (1) they haven't created real magnetic monopoles. They have created quasi-particle magnetic monopoles, which means they behave like magnetic monopoles, but are made out of elementary particles which are not magnetic monopoles. (2) magnetic monopoles with a single unit of magnetic charge, which is what was created, aren't going to result in macroscopic magnets.
Jan
16
revised Is it probable for particles to become entangled under natural conditions?
added 71 characters in body
Jan
16
answered Is it probable for particles to become entangled under natural conditions?
Jan
10
comment How do we imagine a Hadamard gate acting on the Bloch Sphere?
If I'm not getting the sign on $i$ wrong, $R_x(−π/2)$ leaves $|+\rangle$ unchanged. $R_z(-\pi/2)$ takes it to $\frac{1}{\sqrt{2}}(|0\rangle - i |1\rangle)$, and the last rotation takes it to $|0\rangle$. Look at the Bloch sphere and remember that $\pi/2$ is a quarter turn.
Jan
10
comment How do we imagine a Hadamard gate acting on the Bloch Sphere?
It's not a rotation of $\pi$ (which would take $|+\rangle$ to $|-\rangle$), but $\pi/2$.
Jan
8
comment Confusion about quantum probabilities depending on how coarsely grained the measurement apparatus is
You need to replace "eigenvectors" with "eigenspaces" in your comment above to get something that's close to correct. (It's a measurement operator, but it's not the most general form of measurement operator.)
Jan
8
comment Confusion about quantum probabilities depending on how coarsely grained the measurement apparatus is
No, there's not. They teach this simplified version of measurement in QM classes, and it's completely inadequate when you start asking questions like this. The book Quantum Theory: Concepts and Methods by Asher Peres is quite good for these more general types of measurements.
Jan
8
comment Confusion about quantum probabilities depending on how coarsely grained the measurement apparatus is
It's not ad hoc. You just have to realize that integrating amplitudes to get the coarse-grained amplitude is not the right way to do things. You seem to be trying to do this using the simplest form of quantum measurement (probably the only one you've been taught) where everything gets projected onto orthogonal one-dimensional vectors. This doesn't work ... you need to understand more general quantum measurements. I don't believe there is any such thing as the "eigenvalue spectrum of the coarse-grained position operator" in the sense that you want.
Jan
8
comment Confusion about quantum probabilities depending on how coarsely grained the measurement apparatus is
Maybe one way to see why your calculation is wrong is to imagine the amplitude around -1 made up of two components, one positive and one negative. In the measurement with coarse resolution, your recipe would say that the chance of measuring -1 is zero, because the integral is 0. But in the measurement with fine resolution, you have a reasonable chance of measuring both -.99 and -1.01. Do you think quantum measurements work like that?