Are there particle's or masses that travel faster than light?

  • #26
is something traveling faster than light equivelent to something "going back in time"?
 
  • #27
is something traveling faster than light equivelent to something "going back in time"?
I don't think so. I may be incorrect.
 
  • #28
2,193
2
I can't help but find this fact to be quite unfair :)

I fail to see why the double slit experiment with entangled photons can't be used as a communication device.

The setup is like this: One half (photon A) of a photon pair with entagled momentum is sent through a double slit and then detected (by detector A), while the other half (photon B) is captured by another detector (B).

Since detector B is able to determine his photon's momentum, and hence the momentum of the entangled photon A, detector A doesn't see an interference pattern.

This changes when you put a lens in the right place front of detector B that "maps" all B photons to the same spot, and makes their momentum indistinguishable.

With the lens in place, the pair's momentum is undecided, and detector A sees an interference pattern.

This setup seems to be able to transmit FTL messages. You have 2 easily distinguishable signals at detector A (absence/presence of interference) and can send these signals by placing/removing the lens at point B.

Can you give me some pointers to why this can't work?

I wonder if a "delayed choice quantum eraser" might shed some light on this. For example consider this text:

A delayed choice quantum eraser is a cross between a quantum eraser experiment and Wheeler's delayed choice experiment. This experiment has actually been performed and published by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlan O. Scully[1] This experiment was designed to investigate some very peculiar consequences of the well known double slit experiment in quantum mechanics, as well as the consequences of quantum entanglement. As this experiment is a more complex version of the quantum eraser experiment, study of that article may make this article easier to understand.

Source: http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser
 
  • #29
Cthugha
Science Advisor
1,941
298
The setup is like this: One half (photon A) of a photon pair with entagled momentum is sent through a double slit and then detected (by detector A), while the other half (photon B) is captured by another detector (B).

Since detector B is able to determine his photon's momentum, and hence the momentum of the entangled photon A, detector A doesn't see an interference pattern.

This changes when you put a lens in the right place front of detector B that "maps" all B photons to the same spot, and makes their momentum indistinguishable.

With the lens in place, the pair's momentum is undecided, and detector A sees an interference pattern.

This setup seems to be able to transmit FTL messages. You have 2 easily distinguishable signals at detector A (absence/presence of interference) and can send these signals by placing/removing the lens at point B.

Can you give me some pointers to why this can't work?
This is a common misunderstanding about these kind of double slit experiments.

In the double slit scenario you have to distinguish between two kinds of interference. There is the usual interference for single photons and there is two-photon interference, which is more accurately described as the interference of two probability amplitudes leading to one certain event.

Basically each of the two entangled photons behaves like thermal light, which means it has a rather short coherence time and volume. To see an interference pattern, you need the two slits to be positioned rather far from the photon source as this means, that you choose a smaller part of the photon source, which increases coherence.

For two-photon interference, you need to position the source nearer to the slit as this means that you will have a larger collection of pairs of k-values. You will see a larger part of the interference pattern if you include a larger range of k-values. The two-photon interference however, is only visible in coincidence counting, so you need to compare the results from both detectors, which makes ftl communication impossible, because you are not able to get the information from detector A to detector B faster than the speed of light.

Zeilinger has shown (see for example the Dopfer thesis - there is some other paper as well, but I do not know where it was published so I need to check) that these two kinds of interference forbid each other. The closest distance from the source to the double slit, which allows 100% visibility of the two photon interference pattern is already shorter than the minimal distance, which allows single photon interference. So if you see an interference pattern without looking at the entangled partner, the entangled partner does not matter anyway.
 
  • #30
22
0
Since detector B is able to determine his photon's momentum, and hence the momentum of the entangled photon A, detector A doesn't see an interference pattern.
The problem here is that by measuring the photons momentum, you then in fact change it.
 
  • #31
5,601
40
No information, intelligence, can be sent faster than light. Entanglement appears to be instantaneous, yet no information is communicated. "c" appears to be one of the fundamental constants in this universe....and one of it's most limiting ones.
 
  • #32
ok this thread is changing into something else.
 
  • #33
if a particle travels faster than light,with just it's presence in the past,it will violate causality?
 
Last edited:
  • #34
what is a light cone?
 
  • #35
10
0
No information, intelligence, can be sent faster than light. Entanglement appears to be instantaneous, yet no information is communicated. "c" appears to be one of the fundamental constants in this universe....and one of it's most limiting ones.
Yep. I just looked up the experiment I quoted, and it involves a coincidence counter.

Transmitting information probably requires transmitting energy, and hence mass. This would rule out any FTL communictaion...
 

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