# Tag Info

76

Entanglement is being presented as an "active link" only because most people - including authors of popular (and sometimes even unpopular, using the very words of Sidney Coleman) books and articles - don't understand quantum mechanics. And they don't understand quantum mechanics because they don't want to believe that it is fundamentally correct: they always ...

46

I wish to complete @Luboš Motl's answer, to which I agree. My point is on why people continue to make this mistake of an active link. This mistake is connected with one of the most interesting properties of quantum mechanics, Bell's theorem. One can argue that any physical theory is an hidden variable theory, the hidden variable being the description of the ...

42

Since general relativity is a local theory just like any good classical field theory, the Earth will respond to the local curvature which can change only once the information about the disappearance of the Sun has been communicated to the Earth's position (through the propagation of gravitational waves). So yes, the Earth would continue to orbit what ...

35

The thing about the speed of light $c$ is that it's not just a number associated with a certain type of particle. While we could talk about the mass of the proton, and there would be no problem assuming non-protons had greater or lesser masses, the value $c$ is an entirely different beast. $c$ is an intrinsic property of spacetime itself, not of the ...

25

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 ...

25

Gravitational influences do propagate at the speed of light, not instantaneously. The question of what would happen if the Sun instantly disappeared is actually a funny one in general relativity. The equations of general relativity imply as a mathematical consequence that energy must be locally conserved. Therefore, there is no valid solution to the ...

22

It's not possible to communicate faster than light using entangled states. All you get out of entanglement is a correlation between the values of two measurements.; the entanglement doesn't allow you to influence the value measured at another location in a non-causal way. In other words, the correlation only becomes evident after combining the results from ...

21

(There's a couple of these questions kicking around, but I didn't see anyone give the "two boosted copies" answer. Generically, I'd say that's the right answer, since it gives an actual causality violation.) In your scenario, the two planets remain a hundred thousand light years apart. The fact is, you won't get any actual causality violations with FTL that ...

18

Shine a flashlight on a wall. Rotate the flashlight so the illuminated spot moves. Q: How fast does the spot move? A: It depends how far away the wall is. Q: How fast can the spot possibly move? A: There is no limit. Put the wall far enough away, and the spot can move with any speed. Q: What is moving across the wall? A: Nothing. The light that makes up ...

17

Nobody has explained to me how Shor's quantum factorization algorithm works under the transactional interpretation, and I expect this is because the transactional interpretation cannot actually explain this algorithm. If it can't, then chances are the transactional interpretation doesn't actually work. (I have looked at some of the papers that purport to ...

17

All observations are consistent with standard GR so far, but I don't think the speed of gravity, in particular, has ever been measured. Experimental measurements of the speed of gravity was quite a controversy a few years ago when a paper came out claiming that the speed of gravity was very close to $c$ as measured by the Shapiro delay. To see papers on the ...

15

In special relativity, you think of a 4-dimensional space-time. The key point here is that two events, 1, and 2 happening at $t_{1}, x_{1}, y_{1}, z_{1}$ and $t_{2},x_{2},y_{2},z_{2}$ have a distance given by ${}^{1}$ $$(\Delta s)^{2} = -c^{2}(\Delta t)^{2} + (\Delta x)^{2} + (\Delta y)^{2} + (\Delta z)^{2}$$ Now, we can therefore give any two events a ...

15

Edit regarding 3+1 spacetimes and causality I'll keep adding to the answer as I get more information, and hopefully everything will just evolve along. At the very least, I'll have a set of notes to work from in the future :) This is also the first, broadest, cut at an actual answer regarding causality. Alcubierre sets out to find his warp drive metric ...

14

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 ...

13

In reality there's no such thing as a perfect rigid body. There will always be a delay in the motion propagating along the body. Under "normal" conditions you don't notice this delay as it's infinitesimal when compared to the size of everyday objects you interact with. However if you had a rod several light years long (assuming that's at all possible) the ...

13

Suppose you and I have a conversation from a long distance away. We're at rest with respect to each other and communicate much faster than light. I say "How are you", and you wait a short time and say, "I'm fine thanks." From our point of view, you were responding to my question. However, from a reference frame moving from me to you at relativistic ...

13

This is really the same as Adam's answer but phrased differently. Suppose you have a single wire and you connect it to a battery. Electrons start to flow, but as they do so the resistance to their flow (i.e. the resistance of the wire) generates a potential difference. The electron flow rate, i.e. the current, builds up until the potential difference is ...

12

Excellent question. You are correct about wavepacket spreading, and in fact you do get superluminal propagation in non-relativistic QM - which is rubbish. You need a relativistic theory. You should read the first part of Sidney Coleman's lecture notes on quantum field theory where he discusses this exact problem: http://arxiv.org/abs/1110.5013 The short ...

11

Recall that commuting observables in quantum mechanics are simultaneously observable. If I have observables A and B, and they commute, I can measure A and then B and the results will be the same as if I measured B and then A (if you insist on being precise, then by the same I mean in a statistical sense where I take averages over many identical experiments). ...

11

In your example, the relevant speed isn't the speed of propagation of disturbances in the magnetic field, but rather the speed of the alignment of iron atoms. You are really asking "Does magnetization of a wire/metal propagate at the speed of light?" The answer is no; it propagates at the speed at which each individual iron atom can align its polarity. If ...

11

The problem is that if you had two such wormholes, you could still end up in your own past light cone. For example: suppose I took a wormhole trip to Alpha Centauri, leaving in the year 2050 and arriving in 2049 (according to some fixed inertial reference frame). Since Alpha Centauri is 4 light years away, this doesn't take me into my past light cone. But ...

11

What is different here? In some reference frames, your friend guesses the information and acts before you send it and in others, he guesses and acts after you send it. But there is no causality problem since his action is caused by his guess rather than the received information. In all reference frames, the guess precedes the action. Now consider the ...

10

This is an area rife with potential misunderstanding, so we need to be absolutely clear what we mean. Suppose I take a ruler and a clock and I use rulers to mark out $x, y, z$ axes in space and the clock to note the positions of events in time. Assuming spacetime is flat, I now have a universal coordinate frame that everyone who is stationary relative to me ...

10

In fact your view is quite close to the 'official' one; entanglement occurs just because both particles are described with one wave-function; the magic is in our classical habit of thinking that separate objects are described with separate "coordinates".

10

Your question was first asked by Laplace. The following is from the Wikipedia article on "The speed of gravity" Laplace The first attempt to combine a finite gravitational speed with Newton's theory was made by Laplace in 1805. Based on Newton's force law he considered a model in which the gravitational field is defined as a radiation field or ...

9

Just a nice analogue Prof. Jürgen Audretsch told me once: Imagine at home you put one glove in your coat without looking (and noticing it's only one of the two). After exiting the train you notice it's cold and you pull out that single glove. At this very instant you know it's either the left or the right glove, and you therefore know which one is left ...

8

Well, the problem in that paradox is that yes, one of the parties will measure the entangled particle to get the wave function collapsed and yes it will collapse for the other party. However, the other party will still have to measure the thing to learn what it is or has to wait for the initial party to send them a message telling what the wave function has ...

8

It's a combination of all these things and more. Most importantly, the TIQM interpretation is nonsense and all the positive words you hear about it are just unjustifiable hype promoted purely by John Cramer himself. Ontology - or "realism", as it is technically called in quantum mechanics - has been falsified in physics in the mid 1920s and it can never be ...

8

A very simple example is the following: Two observers A and B are spatially separated (one in London, the other in Cambridge). They are static with respect to each other and can therefore measure the same time. So, we can fix a particular time T. The two events $(T,\mathbf{x_{London}})$ and $(T,\mathbf{x_{Cambridge}})$ are spatially separated - one cannot ...

8

There can be at least two different flavors of paradoxes. In one, a result such as 2+2=5 is proved, and the problem must be either incorrect reasoning or a set of assumptions that was invalid. In the other type, exemplified by the EPR paradox, the correct result of an argument is so surprising that it seems like it must be a mistake. Based on the ...

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