# Tag Info

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Quantum mechanics has not been shown to be non-local. Rather, hidden variable theories that make the same predictions as quantum mechanics are non-local. Quantum systems can be described in terms of observables that evolve locally. Those observables are represented by Hermitian operators, not by hidden variables, i.e. - single numbers. In entanglement of the ...

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Yes, QM violates locality. The fact that QM is non-local was fully proved by the experiments carried by the group of Aspect in the years after 1980, in base of the Clauser, Horne, Shimony and Holt inequality. Until today we don't know how does the nature work in order to achieve this non-locality. What yes we know, is that we cannot set hand on that ...

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The Schroedinger equation is non-relativistic and it propagates effects at an infinite velocity to begin with. It is thereof nonsensical to even talk about "locality". Schroedinger's equation doesn't describe local physics any more than a first order diffusion equation describes the speed of sound. There is no technical issue here, at all, you are simply ...

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The no-communication theorem is a no-go theorem from quantum information theory which states that, during measurement of an entangled quantum state, it is not possible for one observer, by making a measurement of a subsystem of the total state, to communicate information to another observer. The theorem is important because, in quantum mechanics, quantum ...

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Of course you know from special relativity that no information can propagate faster than $c$, which includes your directing the Mars end of the stick to move through your Earth end push. But you don't need to think about SR at all. Simply think carefully about what would happen if you did give the end of this rod a sudden shove. It has a great deal of mass ...

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I agree with @nmoy , it may be the speed of sound but nothing can go faster than the speed of light (as far as we know of). If it can happen, I would be very suprised to know that it would be a regular material found on Earth. For the stick to go faster than the speed of light, YOU would have to push it faster than the speed of light due to Newton's laws of ...

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The speed of the push would be roughly the speed of the sound in whatever medium the stick was made of. One thing is certain - it would not exceed the speed of light.

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In this paper by Gott et al., on p. 466 they define the "future visibility limit", saying in the published version that "No matter how long we wait, we will not be able to see farther than this", and on p. 7 of the arxiv preprint they similarly say "If we wait until the infinite future we will eventually be able to see the Big Bang at the co-moving future ...

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If our ideas about cosmic evolution are correct, galaxies that are visible today will in principle remain visible in the future. As time goes on, light from more and more distant galaxies will be able to reach us, and the number of observable galaxies will increase. However, there exists a cosmic event horizon$^1$, so this is an asymptotic growth: There's a ...

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Something is wrong with your scheme - it just can't work at all (we don't even need to invoke no-cloning): The positron travels to Alice, the electron to Bob. If Alice measures the positron in the spin down direction, Bob makes a lot of copies of the electron using a cloning device, and then measures them. If he gets all spin up, he knows Alice made the ...

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Because these particles would be photons- photons have a momentum of h/wavelength, so what would happen is that due to momentum conservation, one would have 2 times the momentum, and have 1/2 the wavelength (possibly absorbing the other). It would have twice the frequency, so twice the energy. Therefore, the photons speed remains the same.

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Yes, you are right. The four momentum of a virtual photon needn't to lie on the mass shell. Thus the zeroth component of the four momentum of a virtual photon is independent of its spatial components. The reason for this is that the zeroth component of the four momentum of a virtual photon arises from the Fourier transform of the step function. See S. ...

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The term virtual is used in other places in physics. For example in virtual images in a a mirror : we see an object in great verisimilitude, even ourselves. Why is the image called virtual and not real? Because it has the optical properties of the imaged object but not a large number of other attributes, mass being the simplest. In addition, its existence ...

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A few points: "the ant always gets to the end of the rope no longer how quickly it's expanding" is incorrect. The problem is that the rope cannot extend indefinitely, it will eventually slow down. Same with the universe, if it slows down, light that at the current expansion rate would not be able to reach us, will be able to do so at the slower rate. ...

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Superluminal worldlines are space-like: 'Time' on an FTL-spaceship would have to be measured with meter-sticks instead of clocks. This is just one of the issues with tachyons (hypothetical superluminal 'particles'). Others are a failure to be properly localizable, processes with endpoints of the same intrinsic type on both sides (emission-emission, ...

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What follows is certainly not a comprehensive answer addressing all of your concerns. It is an answer to the question is there a way to see something clearly pathological like superluminal signals in the heat equation? I would argue that yes, there is. The general solution to the initial value problem $T(x,0) = T_0(x)$ for the heat equation on the ...

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In the case of relativity, "information" refers to a signal that enforces causality. That is, if event A causes event B, then some signal must travel from A to B. Otherwise, how would B "know" that A had occurred. Some examples: Light (signal) from a candle (A) hits your eye (B), causing you to see it. Electricity (signal) flows from a connected switch ...

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In the context of relativity and nonproagation of information at greater than lightspeed between two separated points $A$ and $B$, "information" simply means any particle, feature of in a field (EM, quantum field, curvature in spacetime ...), message or so forth that could allow a causal link between $A$ and $B$, i.e. could make $B$'s physics depend on $A$'s ...

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