# Faster than light information travel

by JohnLuck
Tags: information travel
P: 915

The photons need to be compared......every time/one.

The photon A (on earth) needs to be compared with photon B (on mars).
P: 20
 Quote by San K The photons need to be compared......every time/one. The photon A (on earth) needs to be compared with photon B (on mars).
On Mars they would look at angle A and know that polarized means 1 and non polarized means zero. So why would they need to be compared?

PF Gold
P: 5,322
Faster than light information travel

Your rules are more or less correct. There is no question you can delay measurement of one photon of the pair until after the other. The issue here is that you must properly label the photons and consider what you are doing to each. So we have Alice on earth trying to send a message to Bob on Mars, and you want the message to be communicated in let's say 1 second (which would exceed the speed of light c). Alice measures her photon on Earth at angle A 1 second before Bob's photon arrives on Mars.

What happens? Alice sees either an + or a - (1 or 0 or however you want to label it). There is nothing much for Alice to do here, as there is nothing different for Bob to see regardless of whether Alice does or does not measure her photon, and regardless of what angle A she choses to measure at.

Bob sees a random pattern every time. + - + + - + - - - + - + - + - etc. Not much of a way to send a message. So you can see that your idea that Bob can tell the difference between a "polarized" photon and an
"unpolarized" photon is incorrect. Because when Bob observes ANY photon, it will yield a polarization result.
 P: 133 You are wrong about the 'reset' affecting both photons. It will only affect the photon you are doing it on, and will break entanglement.
PF Gold
P: 5,322
 Quote by georgir You are wrong about the 'reset' affecting both photons. It will only affect the photon you are doing it on, and will break entanglement.
The reset mentioned by JohnLuck is just a colloquial way of describing things, and should not be taken too literally. The rule that actually applies is Malus.

I would like to point out that the mechanism of breaking of entanglement is not fully understood. Therefore it is difficult to back up the comment that measuring one particle in the pair "causes" the break in the strictest of manners. Actually, measuring either one could be said to accomplish the same thing. That is because there is no sense in which time ordering of the measurements on the pair matters.
 P: 133 It seems quite simple and obvious to me. "Entangled" means that the current polarizations along all possible angles are related (in this case, opposite). Measuring the polarization on one of the photons also changes it - at least along other angles. So measuring the polarization obviously breaks the relation, or breaks entanglement.
PF Gold
P: 5,322
 Quote by georgir It seems quite simple and obvious to me. "Entangled" means that the current polarizations along all possible angles are related (in this case, opposite). Measuring the polarization on one of the photons also changes it - at least along other angles. So measuring the polarization obviously breaks the relation, or breaks entanglement.
So I would say what is simple and obvious is something of a matter of perspective. And your comment is a good first cut.

Again, the devil is in the details here. For example, what is a measurement? If you use a beamsplitter to measure the polarization, but then recombine the H and V output properly, the original entangled state should be restored. If that is the case, then the original measurement did NOT cause collapse. See for example:

http://www.optics.rochester.edu/work...ybellsineq.pdf
PF Gold
P: 5,322
 Quote by georgir Why do they bother to have pairs of photons and loops in both directions, instead of looking at one direction and just blocking the opposite beam in the corresponding loop on the right instead of on the left? The results should be the same... or should they not? Granted, it is an interesting idea to test out, but it significantly complicates the setup. They need perfectly separated photons and perfect detection and counting, etc... with a setup in just one direction they can easily make do with a regular beam and just measuring its brightness at the end.
There is an explanation, although you may or may not agree with the reasoning.

Because of significant issues with detection and pairing of entangled photons, the "brightness" itself can be somewhat misleading. Instead, you could almost call it the "difference in brightness" that is being measured at particular angles. That should follow the QM prediction.

Some of the issues with detection include the fact that often a photon arriving at one detector cannot be paired with a photon at the other anywhere close to the requisite coincidence time window. Now, is there some bias that would affect part of the universe and point us to a false result? The way around that is to also look for the output with the opposite polarization. This provides a convincing way to demonstrate that when one polarization stream decreases, the other is in fact increasing. Inference is not required because you can simply measure it.
 P: 20 Ok, so someone suggested that are entangled and have opposite angles A, breaks their entanglement when we measure angle B and does not regain it after we measure angle A again. But this still allows for a machine as I see it. This time we still have a continuous stream of photons traveling to mars, only this time we use millions of photons pairs per packet. The photons that come out the right side of the machine we filter so that those who doesn't point up at angle A are destroyed and thereafter we send the photons into our mirror system. Those that come out of the left side of the machine we also filter at angle A but this time only the ones that do not point down are removed. This ensures that all the pairs we do not filter away are entangled in the same way. Then just an instant before the beam is detected on mars, we decide if we want to change the default configuration of the photon packet, which we can do by measuring angle B of the photon packet on earth. On mars they measure the angles of the photon packets and if they are all pointing down, we know the machine on earth did not change the default, but if they are all pointing in a more or less random direction, we know that they did. Thus we have send a bit of information. There will of course be a very low chance of either all photons being filtered away in a packet or having all photons pointing down, even though they were reset, but this is extremely unlikely and again just amounts to normal network noise, which we already have protocols to account for. georgir I never said that changing the entangled property of one photon would change the other. The machine I describe above only needs to be able to "reset" an entangled photon at one angle and nothing else. Which paper did you read by the way, I got the explanation from here: http://www4.ncsu.edu/unity/lockers/u...pers/bell.html (a bit patronizing, but written in layman terms)
 P: 446 I'm not sure that I understand your idea. If you know what the state of the photon is, then it will not be entangled. You will just be sending a polarized beam of light to Mars and measuring a separate polarized beam on Earth.
P: 20
 Quote by DrewD I'm not sure that I understand your idea. If you know what the state of the photon is, then it will not be entangled. You will just be sending a polarized beam of light to Mars and measuring a separate polarized beam on Earth.
No, I know the state of the photon at angle A and it is entangled. I think you are confusing this with the uncertainty principle?

EDIT
Actually I don't know the angle of any of the actual photons, I only know that the photons with my desired angle will not have been intercepted and that it is very probable that more than 0 photons have gone through the filter.
 P: 446 No it has nothing to do with the uncertainty principle. It has to do with the fact that preparing entangled particles is not very easy. If you need to measure the polarization in order to select which photons pass, you may no longer have entangled photons. If you extract the full information that can be encoded in the polarization after the entanglement, the photons will no longer be entangled unless there is some way that the information can be erased (there is a lot of evidence supporting this, but there is still some question if this is generally true and I don't know enough about all of the experiments ). Since I was having trouble understanding the particulars of your post, this may not be the reason that it won't work. If it makes sense to someone else, perhaps they can explain why it won't work.
P: 20
 Quote by DrewD No it has nothing to do with the uncertainty principle. It has to do with the fact that preparing entangled particles is not very easy. If you need to measure the polarization in order to select which photons pass, you may no longer have entangled photons. If you extract the full information that can be encoded in the polarization after the entanglement, the photons will no longer be entangled unless there is some way that the information can be erased (there is a lot of evidence supporting this, but there is still some question if this is generally true and I don't know enough about all of the experiments ). Since I was having trouble understanding the particulars of your post, this may not be the reason that it won't work. If it makes sense to someone else, perhaps they can explain why it won't work.
I do not know whether measuring polarization would destroy entanglement. Further, I do not know if filtering the photons counts as measuring. I will have to look at other experiments to find this out.
 P: 133 I am repeating myself now, but you still seem to fail to grasp the obvious. A) After you pass a photon through a filter, you have broken entanglement, as the filter has changed the photon. (It may be possible to undo the change and restore entanglement if you merge the two possible paths of up/down photons just right, but that is irrelevant to your case.) B) Even if you did manage to filter and send a stream of photons with known A polarization to Mars that also have entangled counterparts on Earth, measuring the Earth photons on B does in no way affects the Mars photons. They will keep their known A polarization. How is it so hard?
P: 20
 Quote by georgir I am repeating myself now, but you still seem to fail to grasp the obvious. A) After you pass a photon through a filter, you have broken entanglement, as the filter has changed the photon. (It may be possible to undo the change and restore entanglement if you merge the two possible paths of up/down photons just right, but that is irrelevant to your case.) B) Even if you did manage to filter and send a stream of photons with known A polarization to Mars that also have entangled counterparts on Earth, measuring the Earth photons on B does in no way affects the Mars photons. They will keep their known A polarization. How is it so hard?
A) It is not obvious to me that filtering breaks entanglement. It depends on whether the filtering interacts at all with the photons that pass through it. Do you know the properties of all types of polarization filters?

B) You are wrong. Measuring the angle B would set the angle A at random for all the photons. This is what is so special about entanglement, the entangled pair affect each other instantaneously over a distance, (Einstein called it "spooky action at a distance"). I suggest you read up on it. Also what did you think entanglement was?
Mentor
P: 17,210
 Quote by JohnLuck A) It is not obvious to me that filtering breaks entanglement. It depends on whether the filtering interacts at all with the photons that pass through it. Do you know the properties of all types of polarization filters?
All polarization filters place the photons into an eigenstate, if they do not do that then they are not filters. Entanglement only happens when the state is a superposition of eigenstates, so it is self-contradictory to claim that you have placed a pair of photons in an entangled eigenstate.

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