Why Do Some Theories Suggest Speeds Faster Than Light?

[kfx]

I was wondering whether it makes sense to talk about "speeds" faster than the speed of light. For example, Wikipedia mentions that the "speed" of quantum entanglement has a minimum lower bound of 10,000 times the speed of light. - http://en.wikipedia.org/wiki/Quantum_entanglement#Concept

But I've heard that the concept of speed faster than light is meaningless, because for any space-like interval there exists a frame of reference where the start and end points occur "simultaneously", i.e. the speed of communication is infinite.

That is, if one can reach c+1, then one can reach any speed by changing the frame of reference; including communications back in time, i.e. time travel. Why does it make sense to speak of speeds like 10,000 *c?

 

[ZapperZ]

I was wondering whether it makes sense to talk about "speeds" faster than the speed of light. For example, Wikipedia mentions that the "speed" of quantum entanglement has a minimum lower bound of 10,000 times the speed of light. - http://en.wikipedia.org/wiki/Quantum_entanglement#Concept

And yet, if you read all those papers that that Wikipedia has cited for such effect, none of those papers claim to have contradicted Special Relativity. Why do you think that is?

Is it because in the quantum entanglement phenomenon, there is nothing that is moving, or traversing, and therefore, nothing is actually moving faster than c? In QM's description, these "communication" is inherently non-local, but nowhere in there is there something that's moving.

But I've heard that the concept of speed faster than light is meaningless, because for any space-like interval there exists a frame of reference where the start and end points occur "simultaneously", i.e. the speed of communication is infinite.

That is, if one can reach c+1, then one can reach any speed by changing the frame of reference; including communications back in time, i.e. time travel. Why does it make sense to speak of speeds like 10,000 *c?

You need to learn about Lorentz transformation. You are thinking of Galilean transformation, which is what we are all familiar with. Velocity addition doesn't work the way you think at speed approaching c.

Zz.

 

[kfx]

I don't think you understood the question.

What does the number 10'000 c refers to?

 

[ZapperZ]

I don't think you understood the question.

What does the number 10'000 c refers to?

It refers to, if you look at the ACTUAL paper, the speed "something" would have to travel to be compatible with the entanglement measurement. This experiment was done by the Gisin group on entangled photons measured at about 18 km apart[1].

We don't have that "something" that moved from one location to another.

So where did I misunderstood the question?

Zz.

[1] D. Salart et al., Nature v.454, p.861 (2008).

 

[kfx]

If you didn't, then why are you answering to something else?

Well, thanks for nothing. I looked at the Nature news and abstract of the actual paper, and they do mention testing all possible frames of reference. The technical side of it is still not clear to me, but doing this actually gives a sense to the number 10000c.

I'm still not sure about the time travel thing. So quantum entanglement coupled with Special Relativity is supposed to allow to build something that could be called "quantum time machine". It could look like this - two entangled particles at points A and B, and an observer that travels from A to B at speed c-epsilon. If a measurement is done at point B, for the observer it will look like the effect at point B happens before (in his reference frame) the cause at point A. For him it will look like quantum information traveling backwards in time.

But it appears that to get reverse causation we have to accelerate close to speed of light. Is that right? Seems like a major problem in building the machine :) Could this be done in some less expensive way? E.g. by accelerating only the experimental setup, instead of accelerating the observer? Can someone comment on this?

 

[ZapperZ]

If you didn't, then why are you answering to something else?

Er.. if I didn't misunderstand the question, when why did I answered to something else? What "something else" did I answer to? There is nothing that travels at 10000c. Which part of that did you not understand? It means that your original premise is faulty!

Well, thanks for nothing. I looked at the Nature news and abstract of the actual paper, and they do mention testing all possible frames of reference. The technical side of it is still not clear to me, but doing this actually gives a sense to the number 10000c.

I'm still not sure about the time travel thing. So quantum entanglement coupled with Special Relativity is supposed to allow to build something that could be called "quantum time machine". It could look like this - two entangled particles at points A and B, and an observer that travels from A to B at speed c-epsilon. If a measurement is done at point B, for the observer it will look like the effect at point B happens before (in his reference frame) the cause at point A. For him it will look like quantum information traveling backwards in time.

But it appears that to get reverse causation we have to accelerate close to speed of light. Is that right? Seems like a major problem in building the machine :) Could this be done in some less expensive way? E.g. by accelerating only the experimental setup, instead of accelerating the observer? Can someone comment on this?

I have no idea to what you are responding here. Time travel? When did this come into this thread?

I was responding to the original post. You are trying to change this thread into a completely different topic. That was not what I was addressing. I was pointing out that you had a faulty understanding of claiming that we have seen faster than light speeds. We haven't. To have a "speed", it implicitly implies a ds/dt, i.e. a continuous displacement over a period of time of something. There is neither a continuous displacement, nor a something, in this phenomenon.

Zz.

 

[atyy]

Yes, you are right. In principle, wave function collapse is supposed to be instantaneous. It doesn't violate relativity, because it is covariant (like a plane of simultaneity), not invariant.

Their main result is that what they observed was consistent with instantaneous collapse. The 10000c they got was their attempt to estimate some sort of "error" for their measurement of instantaneous collapse. To do that, they assume that they might not be in a perfect inertial frame, due eg. to the Earth's rotation.

http://www.nature.com/nature/journal/v454/n7206/full/nature07121.html
"Taking advantage of the Earth's rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of the influence. For example, if such a privileged reference frame exists and is such that the Earth's speed in this frame is less than 10-3 times that of the speed of light, then the speed of the influence would have to exceed that of light by at least four orders of magnitude."

 

[kfx]

There is nothing that travels at 10000c.

But quantum information is exchanged at least in this, well, "speed"! I did put "speed" in quotes in the OP. If it's unacceptable to use this term in this context, then you could have pointed it out. What term should be used then?

Time travel? When did this come into this thread?

Dunno, in the OP?

 

[atyy]

Basically, they must state a frame. Even slower than light speeds are "meaningless" (in the sense of the OP) since we can always set them to zero.

 

[atyy]

But quantum information is exchanged at least in this, well, "speed"!

I'm not sure "quantum information" is the right term. Anyway, you are right, quantum something (wavefunction collapse) occurs instantaeously. But no classical information can be exchanged faster than light by this quantum means.

Here's a review: http://arxiv.org/abs/quant-ph/0212023.

 

[kfx]

Basically, they must state a frame. Even slower than light speeds are "meaningless" (in the sense of the OP) since we can always set them to zero.

Yes, a good point. Probably this was obvious to a physicist. :)