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Relative Entanglement Nightmare

  1. Sep 29, 2013 #1
    I just woke up from a nightmare that terrified me to my core. I had recently seen this video from Sixty Symbols regarding the apparent paradox of time dilatation http://goo.gl/dwOvg1 on a train moving near the speed of light heading towards a tunnel with guillotines at each end.

    The nightmare was that i was watching this same experiment unfold, but in a way that challenged causality.

    Maybe I just need someone who really knows to tell me it'll be ok.

    In my dream I was watching the experiment unfold, except this time the observer at rest (relative to the mountain tunnel) used entangled particles. Depending on the state of the particle the guillotine it arrived at would operate or not. On one end, however, the particle was received into a new entangled set that was kind of like a quantum computer where each qbit had its probability adjusted and more-or-less controlled into a pre-determined state regardless of the original input value. The other entangled particle of the original pair was just held at the other end of the tunnel guillotine for the pre-determined amount of time before a measurement was made.

    The experiment played out pretty much as it does in the video, with both sides agreeing that regardless of the fact that one event happened before the other, that both events happened. Only because of the "delayed quantum choice" the probability was shifted from an even 50/50 to something more like a 25/75. But both observers still agreed.

    Then it started up again and I found myself riding on a new train, also going near the speed of light from the OPPOSITE direction. In that moment I realized that the events that had played out in one direction were now reversed so that the cause and effect looked to be opposite from that perspective. Except, because of the entangled particle being "modified" at the destination, I realized that the results were now different since the particle that was being observed without the extra "tweaking" was happening first, supposedly changing the entangled particle at the other end of the tunnel before it could be "adjusted" to be something different.

    So I awoke in a cold sweat and I tried to find an answer online, but couldn't.

    If Train A sees the probability as 25/75, and train B sees it as more like 50/50, what does that say about causality? Doesn't everything all fall apart?

    Need help on this one so I can sleep at night. And really, I did dream this. I suppose I've been struggling with the twists of science a bit much. I take it as a sign of my science muscle growing.
  2. jcsd
  3. Sep 30, 2013 #2


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    Staff: Mentor

    The probability of what?

    There is no difference between measuring at point A, "collapsing" the state at point B, then measuring at point B, and measuring at point B, "collapsing" the state at point A, then measuring at point A. From the point of view of QM, the order does not matter.
  4. Sep 30, 2013 #3


    Staff: Mentor

    People have a big misunderstanding of how entanglement works. You can do all sorts of interesting things with entanglement, but the interesting effects all require information from the other entangled particles. Without that information there is nothing unusual about an entangled ensemble.

    For example, suppose you have two entangled ensembles, A and B. If you measure the entangled property of A you get some random distribution. If you measure the entangled property of B you also get a random distribution. When you compare A and B you find some (potentially perfect) correlation between the measurements at A and B. There is nothing you can do at A which can help you determine whether or not something has been done at B. Once you compare the measurements at A and B it doesn't matter which measurement happened first, the correlation is the same.

    Delayed choice experiments do nothing to change this picture. In a traditional delayed choice experiment, for example, the interference pattern is not visible in the general population of the entangled photons. It is only when you take the subset of photons which are paired with the interfered photons that you get the entangled interference. You have to get information about one ensemble in order to see anything other than noise in the other ensemble.
  5. Sep 30, 2013 #4


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    That video is a nightmare, no wonder you have been entangled by it. See this thread where it was discussed at length:

    Relativity Paradox – RoS: Trains, Tunnels & Guillotines

    Mike's statement concerning the train spotter, "at the moment the train disappears inside the tunnel, he pulls a lever which briefly makes these guillotines come down and then go back up again", is impossible. I suspect that this has caused you to consider other mechanisms that determine the timing of the guillotines based on a single detector/observer. But there is no such mechanism. This is an unfortunate misrepresentation of the so-called guillotine paradox.

    It would be possible to have two detectors (or observers) near each end of the tunnel that would control the actions of the guillotines. The one just before the far end of the tunnel would activate his guillotine when he sees the front of the train at his location and the one just inside the front of the tunnel would active his guillotine when he sees the end of the train at his location. Then after the scenario plays out, we could determine that in the rest frame of the tunnel, both guillotines were activated at the same time, but this spoils all the fun, doesn't it?
    Last edited: Sep 30, 2013
  6. Sep 30, 2013 #5


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    You could also have a single detector/observer in the center of the tunnel that pulls a lever when the front of the train arrives and which then transmits signals at the speed of light to activate both guillotines but this is not what Mike was saying in the video.
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