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picture taken from this video for those who do not like to click on youtube links

youtube video - retrocausality through Double Slit & Bose

The answer will probably be no, but i am interested to know WHY this would/would not work.

He is using a nonlinear crystal to create two entangled photons, one of which he is sending through the Einstein-Bose condensate, hence it will arrive much later at the double slit. (other similar experiments use lasers or specific fiber optics cables to delay the photon(1))

He then decides in the future on if to break the delayed photon's wave function or not, by observing some quantum state/states.

According to the video, this will lead to the earlier arriving photon(2) to not create an interference pattern in case it has been collapsed in the future. Now i am not sure how he would be able to tell from just one photon if it does interfere with itself still when going through a double slit or not, but one can easily imagine to do this on a hundred or more photons. If just one of the hundred photons would appear on the screen where it should not appear, hence where there would be gaps normally because of the interference, we would know that one of the 100 delayed photons at least was detected(it's which path was/will be known)

By timing this properly, one could then send messages in the form of yes/no or 1/0 or heads/tail if he was to make his decision on either to observe the which path of the 100 delayed photons or not.

You could of course build a second such device, and send the information you receive another time back, going even further back in time, then a third and so on. OR you could simply use the same machine and do it again, looping the output into the input. You would be only restricted to the point where you created the device and wouldn't be able to send information back any further, should this work, which i doubt.

I cannot imagine any restro-causality to be possible. Imagine if you got the information you wanted from the future, but then decided to destroy the machine in the past, therefore you could not possibly ever have received the message in the first place.

It would mean that you cannot possibly act on the information you receive, like if you were a Zombie. Or if you tried, then the universe would make sure that something really really bad happened to you, and the sender in the future is actually someone else or other far fetched, impossible sounding explanations.

I wouldn't consider this a duplicate as i created this thread to discuss this specific experiment using the Einstein-Bose-Condensate, plus my question tries to answer other considerations(indirectly) about what it would mean if it WAS actually possible. Furthermore, i find this specific case more accessible to some, including myself. So yes, maybe the other thread does answer some of the questions but i think answering the question from the perspective intended in this thread might be useful to many.

David Z
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pZombie
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    To me it seems that the way you observe the photon in the present should be independent of the measurement in the future. The fact that they're entangled shouldn't mean that they are causally linked in any way that would allow you to send a message with them, it just means that when you get the two measurements you can apply your answers to get information about the other one but not influence its behavior. – Misc.nerdiness Apr 23 '15 at 11:42
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    @Misc.nerdiness yes. the confusion is between causality and correlation. Correlation ( entanglement) is not necessay and sufficient to imply causality. – anna v Apr 23 '15 at 13:09
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    I believe the beauty of this experiment is that it is simple to understand and simple to give an answer on what YOU believe would not work. For example, you could say that the collapsing of the wave function of the 100 photons in the future, would not result in being able to tell if they were collapsed by observing the past when doing the experiment as outline above, because of A, B, C reasons and then describe what would happen instead according to your theory. Because if what happens is as described by me, then how could this possibly NOT work? – pZombie Apr 23 '15 at 13:44
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    Furthermore, we do not have to be concerned about if this only works with events being within some lightcone or if this is not really ftl or any of the like. The simple question is, if you could for example, send a the information of a series of heads/tail coin throws back to the "past". And before defining what the past really is, or if this really is the past, i believe you all understand the idea. – pZombie Apr 23 '15 at 13:53

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