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  • 0 posts edited
  • 17 helpful flags
  • 1,692 votes cast
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?
Jan
7
comment Confusion about quantum probabilities depending on how coarsely grained the measurement apparatus is
For the continuous case, you integrate the absolute value of the square of the amplitude to get the probability, not the amplitude. The integral of the area under the square of the curve is the probability, not the integral of the area under the curve, which is what your transformations are assuming.
Jan
7
answered Confusion about quantum probabilities depending on how coarsely grained the measurement apparatus is
Jan
7
comment Does matter shrink uniformly without restriction in freefall to central region of black hole?
@John: maybe I'll give my own answer (somebody should mention that most black holes kill you by stretching you and not by squishing you -- if somebody else wants to do that, go ahead; I don't care about reputation).
Jan
7
comment Does matter shrink uniformly without restriction in freefall to central region of black hole?
Winterberg's paper has this theory that looks totally crazy to me: that at some point after a black hole starts to form, it explodes and turns its entire mass into gamma rays. I don't need to know much about physics to know that this is very much not the conventional wisdom.
Jan
7
comment Does matter shrink uniformly without restriction in freefall to central region of black hole?
And "Also take a look at Friedwardt Winterberg's Gamma-Ray Bursters and Lorentzian Relativity. He says an infalling body gets destroyed on the way in." A totally unexplained reference to a highly dubious paper, and I don't see how it's relevant to your answer.
Jan
7
comment Does matter shrink uniformly without restriction in freefall to central region of black hole?
Where is it confusing? "And meanwhile the "coordinate" speed of light is decreasing. That's the speed of light at some location as measured from this location." Do you expect the OP to figure out what the "coordinate speed of light" means from this? I don't think I could do it if I didn't know what you meant already.
Jan
6
comment Does matter shrink uniformly without restriction in freefall to central region of black hole?
I'm not the downvoter, but this answer is quite incoherent in places. Maybe not as incoherent as the question, but we give more leeway to questions.
Jan
6
answered Physics after a Theory of Everything
Jan
5
comment Can we speed up the evaporation of black holes manually by accelerating it?
While black holes follow geodesics, if you accelerate them by slingshotting them using a large mass object, shouldn't they behave exactly the same way as twins? Not all geodesic paths undergo the same proper time; geodesics just maximize proper time locally.
Jan
5
answered Can we speed up the evaporation of black holes manually by accelerating it?
Jan
3
revised A question about the universal quantum cloning machine (UQCM)
added 298 characters in body
Jan
3
comment A question about the universal quantum cloning machine (UQCM)
There must be something cloning-like that you can do with entangled states, and possibly somebody has figured it out and published it. But you can't use the UQCM without modification and expect it to work with the same success probability.