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Simple question about quantum entanglement from a layperson

  1. Nov 19, 2015 #1
    I've tried to find a relevant thread to post this in, but all seem to be way above my knowledge level and / or are very old. Perhaps this is not even the correct forum because my question assumes there is entanglement and we've also found a way to utilize it..

    Anyway, I just read this article from nist.gov, found it very intriguing, and I was wondering about the following scenario..

    Let's assume we've discovered a way to utilize QE to the point we have real time (faster than light speed) communication over any distance. Now let's assume we launch a powerful telescope and position it about 5 times the distance of the moon away, or roughly 5 light seconds from earth, and aim it back at ourselves.

    I position the telescope (with its super high 1TB sensor camera) aimed back at me standing in a field, about 5 meters away from me there is a table with my special quantum enabled remote trigger to the camera on the telescope. I walk over to the table, pick up the remote and fire the shutter on the camera.

    When I get the image, is it a picture of me standing 5 meters away from the table? (assuming it took me exactly 5 seconds to walk over and trigger the remote)
    Last edited: Nov 19, 2015
  2. jcsd
  3. Nov 19, 2015 #2


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    Staff: Mentor

    Sorry, can't be done. There's no way of using entanglement to communicate.

    You can see the problem in the simplest case: two spin-entangled particles, such that if I measure mine and get an "up" your result when and if you make a measurement along the same axis will necessarily be "down". So I measure my particle and get "up" - I have no way of knowing whether I've made the first measurement on the pair or whether you've already measured yours and gotten "down".
  4. Nov 23, 2015 #3
    Thanks for the reply - Yes I see why communication would be not possible via entanglement. So, in essence is the knowledge of how entanglement works just going to be a "nice to know" thing? In that yes we know it does this, but we don't know why it does this and we'll never be able to exploit this knowledge.

    On my original question, assuming we ever did find a way for FTL communication (via a wormhole, tear in spacetime etc.) would it be possible to photograph "the past".. or is this a question better suited for the general relativity board?
  5. Nov 23, 2015 #4
    Wormholes, if they exist at all, are thought would be very unstable structures that quickly collapse, and microscopically small too.
    I have read somewhere I forget, that in principle the collapse could be prevented, and that the wormhole once it exists, could be scaled up to be an accessible size, but that this would require a lot of hypothetical exotic matter. (IIRC this stuff would need to have a negative mass).
    Last edited: Nov 23, 2015
  6. Nov 23, 2015 #5
    We can exploit this knowledge for other uses, just not for faster-than-light communication. As for the "why", if quantum theory itself satisfies you as an explanation then we know why it happens, it's just a consequence of quantum behavior. So the question becomes more general, "why quantum physics", not just "why entanglement", which can be somewhat controversially be considered too philosophical a question.
  7. Nov 23, 2015 #6


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    Staff: Mentor

    I wouldn't say "never", but quantum mechanics is a theory about measurement results: Prepare a system in a given way, and QM will give the probabilities of getting various possible results out of the various measurements that you might make. It is (infuriatingly, to many people) silent about what mechanisms might account for these results. Thus, the "don't know why" isn't limited to entanglement, it's basically the entire theory.

    Photographing the past isn't hard, in fact it's the only kind of photography we ever do. The image that is formed in your camera is always of something that happened before you pressed the button on the camera, although the for nearby subjects the lag may only be a few nanoseconds. Here the past (light reflects from subject of photograph back then) is influencing the present (light reaches camera now after travelling through space).

    To understand the possible implications of FTL communication (which is generally accepted to be impossible) you might want to google for "tachyonic antitelephone".
  8. Nov 23, 2015 #7
    One of the things I read in one article that was referencing the latest closed loophole Bell test was the possibility of real time encryption, bypassing a middle layer. As an IT guy I find this interesting, but also I can't see how this would be possible either. If there is no data to communicate, there's nothing to encrypt..
  9. Nov 23, 2015 #8
    You can communicate, just not faster than light.
  10. Nov 23, 2015 #9
    Yes, photographing the past was a poor choice of words, every time I take an astral image I'm photographing anywhere from 1 second to billions of years in the past.

    Taking a picture of yourself taking the picture and getting a picture of yourself before you actually took the picture seemed a bit long winded though.. :- /
  11. Nov 23, 2015 #10


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    Entanglement makes some coordination tasks easier. Generating a shared secret key is one such task. Communication is not. Weird, right? But that's what the math says, and experiments confirm the math.

    Other examples of tasks that entanglement helps with are packing transmitted information more densely, or consistently winning the mermin-peres magic square game, or performing guaranteed-constant-time symmetry breaking.
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