I understand that faster-than-light communication is impossible when making single measurements, because the outcome of each measurement is random. However, shouldn't measurement on one side collapse the wave function on the other side, such that interference effects would disappear? Making measurements on "bunches" of entangled particles would thus allow FTL communication, by making observed interference effects appear or disappear. How does such an experiment not:

1) Clearly imply that faster-than-light communication is possible?


2) (if #1 is rejected) Imply that measurement of one half of an entangled pair does not cause the collapse of the other half's wave function.

Why doesn't this thought experiment clearly show that if we maintain that FTL communication is ruled out, we must also rule out "universal collapse" in the Copenhagen interpretation?

EDIT: Here is an example of an explicit experiment (though I think experts could come up with something better):

You can entangle a photon with an electron such that the angle of the photon is correlated with the electron's position at each slit of a double slit experiment. If the photon is detected (it's outgoing angle measured), then which-path information is known, and there is no interference. If the photon is not detected, the interference remains.

The experiment is designed such that the photon and electron go in roughly opposite directions, apart from the tiny deflection which gives which-path information. You set up a series of photon detectors 100 ly away on one side, and your double slit experiment 100 ly away in the opposite direction. Now you produce the entangled pairs in bunches, say of 100 entangled pairs, each coming every millisecond, with a muon coming between each bunch to serve as a separator.

Then the idea is that someone at the photon detector side can send information to someone watching the double-slit experiment, by selectively detecting all of the photons in some bunches, but not in others. If all of the photons are detected for one bunch, then the corresponding electron bunch 200 ly away should show no interference effects. If all of the photons are not detected for one bunch, then the corresponding electron bunch 200 ly away would show the usual double-slit interference effects (say on a phosphorus screen). (Note that this does not require combining information from the photon-detector-side with the electron-double-slit side in order to get the interference effects. The interference effects would visibly show up as the electron blips populate the phosphorus screen, as is usual in a double-slit experiment when which-path information is not measured.)

In such a way the person at the photon detectors can send '1's and '0's depending on whether they measure the photons in a given bunch. Suppose they send 'SOS' in Morse code. This requires 9 bunches, and so this will take 900 milliseconds, which is less than 200 years. The point is that such an experiment would only work if you assume that the measurement of the photon really does collapse the wave function nonlocally.

  • $\begingroup$ Related: physics.stackexchange.com/q/3158/2451 and links therein. $\endgroup$ – Qmechanic Apr 15 '13 at 20:05
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    $\begingroup$ @Qmechanic, unfortunately, having read the whole thread, I don't think it is very closely related. I am still looking for an answer to this question. That thread does not at all touch upon the question of communication by "turning on and off" interference effects through measurement. $\endgroup$ – user1247 Apr 15 '13 at 22:45
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    $\begingroup$ Luboš Motl's answer is unreasonably obtuse. He should have simply said that the process of entangling the two particles is a measurement that destroys the interference pattern, without further conditioning on additional information. You'd only be able to "measure" (i.e. post-process) into two different outcomes by conditioning on information, as the unconditioned observation at either site does not (and cannot) change from actions at the other site that is causally disconnected. $\endgroup$ – Nimrod Sep 4 '18 at 7:10
  • $\begingroup$ Just wanted to add my 2 cents on the subjects. What you're asking is a legitimate question many have pondered over. Check the wikipedia article en.wikipedia.org/wiki/Delayed-choice_quantum_eraser In the article, there's an explanation, even if complicated, for why you can't violate causality with delayed choice experiments as they're currently implemented. However, some may argue that such a causality violation is possible if the experiment is modified in some way. $\endgroup$ – iliar Jul 5 '20 at 17:12

There is no experiment in which genuine information could be sent faster than light and there is no contradiction between this fact and quantum mechanics – as built by the Copenhagen school. Quite on the contrary, the proper, Copenhagen-like interpretation of quantum mechanics is needed for a description of known experiments that is compatible with special relativity and its most general consequences, locality and causality.

You would have to describe your experiment in detail if you wanted the interference and its disappearance to be discussed seriously.

However, quite generally, if there are entangled pairs produced, a single particle from this pair won't contribute to an interference pattern by itself. (A typical example is an entangled electron-photon pair where the electron participates in a double-slit experiment and the photon is used to "look" at the electron. The photon gets entangled with the electron but the electron's own interference pattern disappears.) The interference pattern may only be glimpsed if one compares some appropriate measured properties of both particles in the entangled pair. But that's only possible much later, when these results of measurements are communicated to a single place, and because the comparison occurs much later, it can't be used to transmit any information faster than light.

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    $\begingroup$ @dmckee, I am going with the example provided my Lubos himself (an entangled electron-photon pair). Am I held to a different standard? I thought we had a shared vocabulary here provided by Lubos. $\endgroup$ – user1247 Feb 25 '13 at 16:55
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    $\begingroup$ @dmckee, I think this is a bit unfair, as either you understand my question or you don't. If you understand my question, such a stringent requirement is unnecessary: I'm not some crackpot trying to disprove SR here or come up with some novel experiment (I very much doubt my thinking here is novel), I am just asking a fairly basic question about EPR-type experiments. $\endgroup$ – user1247 Feb 25 '13 at 17:00
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    $\begingroup$ @user1247 - you wrote: "I think this is a bit unfair, as either you understand my question or you don't. If you understand my question, such a stringent requirement is unnecessary:" - No, this is invalid logic. We understood your question perfectly and we could also perfectly see that it wasn't complete so it couldn't have had an accurate enough answer. There is no contradiction between these two things. There are lots of comprehensibly formulated questions that still don't provide anyone with enough data to be given a meaningful unambiguous answer. $\endgroup$ – Luboš Motl Feb 26 '13 at 10:09
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    $\begingroup$ Hi Lubos (sorry I didn't see your response until now), your first paragraph (in response to my last) I completely agree with. This is indeed what I am saying. If the photon is detected to have had sufficient energy to give which-way information, then the electron should not show interference. If not, then the electron should show interference. This is all usual stuff. $\endgroup$ – user1247 Mar 5 '13 at 9:34
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    $\begingroup$ You say that I "implicitly suggest that it matters for the electrons' interference pattern whether the photons were processed differently after they interacted and got entangled with the electrons." No, I am giving an explicit example. If the photons are detected, then there is which-way information. If they are not, then there isn't. Right? $\endgroup$ – user1247 Mar 5 '13 at 9:36

I'm going to go ahead and answer my own question. I think the issue is that in my proposed experiment there would never be any possibility of observing an interference pattern without first destroying the entanglement that would allow (by measurement of the photon bunches) the switching on or off of the interference on the electron side. The entanglement between the electron and photon implies that there would be no interference pattern, regardless of whether or not the photons are observed. The only way to re-introduce interference would be to, for example, have the electrons go through a small slit prior to the double slit in order to spread their wave function. But doing this entangles the electron with the screen with the first slit in it and effectively erases its entanglement with the photon, unless the momentum of the screen with the first slit can be measured to sufficient accuracy after the electron passes through it. But interference will only be seen if the momentum of the screen cannot be measured to sufficient accuracy without compromising the corresponding uncertainty in the screen's position. Assuming that the level of this uncertainty cannot be controlled at will, the appearance of interference cannot be turned on and off by a distant photon/screen measurer.

  • $\begingroup$ It may be instructive to read up at Why does the Dopfer EPR experiment require coincidence counting? incl. answer, comments, references, ... $\endgroup$ – user12262 Oct 17 '13 at 21:35
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    $\begingroup$ @user12262, but there the top voted answerer claims in his linked article (vixra.org/abs/1103.0095) that FTL communication is possible in exactly the way asked about in this question. Given that the consensus appears to be against such a possibility, this only adds to my confusion... $\endgroup$ – user1247 Feb 24 '15 at 15:39
  • $\begingroup$ user1247: "but there ["Dopfer EPR", PSE/q/79427] the top voted answerer claims [...]" -- And there I've given my answer, too, which may well be understood as counter-claim. So, as mostly, you have to try and figure that out for yourself, too. Do you have any specific follow-up question(s) on my answer (or ensuing comments) there? Meanwhile I'll take another look at your answer here I try to come up with a specific follow-up question here (it may take a few days) ... $\endgroup$ – user12262 Feb 24 '15 at 23:18
  • $\begingroup$ @user12262, no I think I understand your particular answer, the difficulty is just in general that the literature seems a bit sparse on such experiments other than by Cramer et al who seem to represent a minority viewpoint. $\endgroup$ – user1247 Feb 25 '15 at 15:27

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