Quantum Entanglement

Discussion in 'Techforge' started by Lanzman, May 14, 2019.

  1. Lanzman

    Lanzman Vast, Cool and Unsympathetic Formerly Important

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    Hey, want to make your brain hurt? For inexplicable reasons, the subject of quantum entanglement came up at work. Someone pointed me at a paper that explains certain aspects of the phenomenon, because I mentioned that I had thought entanglement was a binary situation - a pair of particles are either entangled or not. But it turns out that there's such a thing as partial entanglement, and multiple particles can be entangled. I'm not going to pretend to understand this fully, and the math completely defeats me, but I think I grasped enough of it to say "ow, my brain." And to appreciate that, while the rules underlying reality may ultimately be fairly simple, they can give rise to a near-infinite complexity.
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  2. Paladin

    Paladin Overjoyed Man of Liberty

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    Entanglement is really...spooky (to use Einstein's description).

    You can rig up an experiment where two entangled photons with opposite (but unmeasured) polarization are generated, and the determination of one photon's polarization INSTANTLY causes the other photon--no matter how far away--to assume the opposite polarization. This isn't a case of them simply having determined but unknown polarization beforehand. Experiments can be done that show the two photons cannot have "hidden variables" that only emerge when the measurement is performed. Measuring one really does change the other instantly.

    Unfortunately, there doesn't seem to be a way to use this phenomenon for communication. :(
  3. Bailey

    Bailey It's always Christmas Eve Super Moderator

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    Got any more info on those experiments? That's something I've always wondered about.
  4. spot261

    spot261 George "The Animal" Steele tribute act

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    What's the issue with communication? If you know the initial states of the two photons what's to prevent encoding of successive patterns of change?

    I've often wondered about this (and I'll gladly admit being the proverbial intelligent non expert here), where does this leave relativity given the instantaneous nature of information transfer?
  5. Paladin

    Paladin Overjoyed Man of Liberty

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    This video covers it pretty well, but it's a bit of a mind-bender.

    You can't chose the polarity of the first photon, you can only know, once you've made the measurement, that the other photon is the opposite. No matter what you do on one end, the other end looks random.
    Relativity survives because information can't be transmitted faster than light this way and so causality is preserved.

    What I'd wonder with respect to gravity is this: suppose we had two entangled electrons and sent one down into the gravity well of a black hole, and the other out into normal flat space far away from the black hole. What does "instantly" mean when the two things being measured are in regions where time is passing at very different rates from one another?
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  6. spot261

    spot261 George "The Animal" Steele tribute act

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    Help me out here, I'm probably being stupid, but couldn't that be calibrated by observers on each end? If the effect is consistent despite changes in distance what's to stop you observing the two simultaneously in close proximity to each other then sending one on a space ship along with a pre arranged cipher?

    Also if one is affecting the other instantaneously at any distance, surely that means information is passing between the two FTL, even if we can't make use of that for practical purposes for whatever reason I'm missing with the above?

    Your thought experiment seems to hold even without the black hole, time dilations due to travelling frames of references would produce a similar problem would they not?
  7. Paladin

    Paladin Overjoyed Man of Liberty

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    Once you measure one, both are determined. I'm not certain, but I believe the entanglement is then broken. (edit: Yes, that appears to be the case.) Even if not, you still can't use it for information transmission.
    Information in this sense is any kind of non-random message. The universe seems to forbid this. If this could happen, it might break causality; that is, we could send messages to the past in a way that creates a paradox ("I sent myself a message telling me not to send myself a message").

    (Aside: relativity does allow for an event in one observer's future to be in another observer's past, but there is a weird catch that prevents time travel: if your future event is in my past, then it must occur far enough away from me that it can't give rise to any break in causality. You and I can disagree about whether an event is in the future or the past, but we can never get cause and effect out of order. The universe seems to require this in order to keep paradoxes from emerging.)

    The fact that measuring one instantly changes the other does not mean information is passing between them. It just means that the two are linked in a way that transcends the space in between them.
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  8. Paladin

    Paladin Overjoyed Man of Liberty

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    Here are a couple of videos that elaborate on this points:



  9. Bailey

    Bailey It's always Christmas Eve Super Moderator

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    Don't see the differing rate of time passing as being a problem, after all that's the case everywhere, not just around black holes. Despite the stretching of time it's still occurring in the same direction.
  10. spot261

    spot261 George "The Animal" Steele tribute act

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    That would explain it, even if it does sound distinctly like a ST plot device, "We'll only get one chance at this Captain, once we've sent the message the link'll be broken...."

    That's a big "just" and sounds very much like the universe playing semantics. If one event is inextricably linked to another like this it's hard not to see that as causation.

    I'm just meandering through the (il)logic of this, so bear with me or feel free to disregard. Just musing.

    We have two events:

    Event A - I carry out an observation
    Event B - a particle several million miles away changes polarity as a consequence of my actions

    Crucially event B occurs before light could travel from the location of event A.

    Even if we allow for time being malleable as you say, that's still causality, albeit causality which in some manner bypasses relativity surely?

    If two objects can be linked in a way which transcends that space in between them and we take the word "universe" to mean "all that exists" then we have to see that as diminishing the universality of relativity. On the other hand if we take the word "universe" to have a more limited meaning that means the link must be in some manner mediated by another universe where different rules apply.

    In either case that's monumental stuff.
  11. Marso

    Marso High speed, low drag.

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    It just means we don't understand everything yet.
  12. spot261

    spot261 George "The Animal" Steele tribute act

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    I'm sure we never will, but this does seem to be pretty paradigm shifting stuff and damned exciting.

    I'd heard of the process, but didn't know it had (seemingly) infinite range.
  13. Marso

    Marso High speed, low drag.

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    Well, on a certain level it's as if both particles are in fact the same particle, and our own perception of them being separate in the first place is the kicker- goes back to a 'holographic universe' concept, in which everything exists on something like a 2-D membrane and is projected into a 3-D space, or something like that. There's books on this stuff, but it's been years since I read the last one on this topic.
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  14. spot261

    spot261 George "The Animal" Steele tribute act

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    Pretty damn fascinating stuff.

    I do often wonder if there's really any a priori to assume the universe must make sense in terms we can understand. Our brains are designed by evolution to deal with a certain set of problems and working within a fairly limited perceptual framework, a framework which is based on that which our sensory organs are designed to process. We can't really picture subatomic particles, we rely on the maths and then mentally force our models to fit with concepts we can understand. We accept the evidence of wave/particle duality but I don't think many (any?) of us can really grasp it on a perceptual level.

    We're already long past the point where we have to rely on mathematical modelling to illustrate concepts we can't conceptualise in the conventional sense using the software we have. What guarantees do we really have that the universe could not at some point turn out to be beyond even those models, working according to a logic that human brains, no matter how intelligent, simply cannot process? We are relying on an assumption that where our cognitive abilities struggle that the universe will at source obey the laws of mathematics, that it is the "music of the spheres", but the universe also has a habit of defying expectations we place on it.
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  15. Paladin

    Paladin Overjoyed Man of Liberty

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    Yes, because it's indistinguishable from random chance.

    You can't know what the state of the particle at Event B is until you "measure" it (that is, it interacts with the universe in a way where the quantum attribute under consideration assumes some definite value). Regardless of when you measure this particle--before or after the particle at A is measured--you still get a random result. You can't even determine whether the particle at A has been measured; you get the same random outcome either way.

    The chain of causality involves this measurement (interaction) which forces the particle to assume a definite value, but the value is always completely random from B's perspective, and the observer at A can't do anything that takes away this randomness.
    So far as is known, you can't use this phenomenon to "break" relativity. Whatever is the underlying truth of reality--and I'm not sure it's even possible for us to do anything other than describe it--this instantaneous phenomenon fits within the relativity framework.
    There are several interpretations of this phenomenon--some of them waaay out there like superdeterminism--but whatever reality is, I think it's more than our senses and brains are capable of experiencing. We can understand it to the extent that we can describe it, but no mental model that corresponds to reality as we normally sense it can be envisioned. Physicists can say something behaves like both a wave and a particle, but we can't really envision something that has both attributes, since nothing in our direct experience does.
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  16. Paladin

    Paladin Overjoyed Man of Liberty

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    That's right.
    The universe doesn't appear to be totally random, that there is some ordering mechanism to it. Yes, there's no guarantee this order is something we can figure out or understand, but, so far, we've done a pretty good job of explaining reality. Right now, we can already understand the workings of almost the entire universe. Yes, dark matter and dark energy remain enigmatic, we don't know what happens inside a black hole, and we don't know how to reconcile relativity with quantum mechanics, but these are things at the extremes.

    My guess is we will eventually be able to explain all phenomena in reality in a way that allows us to make very accurate predictions about them.
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  17. The Night Funky

    The Night Funky BMF Staff Member Moderator

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  18. Paladin

    Paladin Overjoyed Man of Liberty

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    They've demonstrated the effect using a satellite, but they have not used it for communication.

    I could well be wrong, but my understanding is that it simply isn't possible to communicate this way. However, it is possible to use this scheme to encrypt information in a way that makes interception impossible, because any interception of the key signal changes it.
  19. The Night Funky

    The Night Funky BMF Staff Member Moderator

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    The way I read the article, they've been able to do both. After all, if you're able to detect the result, then you're sending the message that the experiment worked.
  20. Paladin

    Paladin Overjoyed Man of Liberty

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    :chris:

    The information itself is not communicated through "spooky action at a distance," so is not faster-than-light. But the system does allow quantum-encrypted communications that can't be eavesdropped without detection.
  21. MikeH92467

    MikeH92467 RadioNinja

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    Anything that frustrated Einstein and nearly drove him to apoplexy is worth investigating. I also am math challenged (I can do arithemic quite well, anything more advanced...eh no) but I have found watching documentaries on quantum physics in general quite fascinating. Unfortunately, my fascination with it, does not mean I have any real grasp on it.:discuss:
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  22. Paladin

    Paladin Overjoyed Man of Liberty

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    "If you think you understand quantum mechanics, you don't understand quantum mechanics."

    - Richard Feynmann (or, at least, widely attributed to him)
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  23. Paladin

    Paladin Overjoyed Man of Liberty

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    I read Isaacson's biography of Einstein a few years back, and it had a very amusing story.

    Einstein was the rock star of physics, and a lot of the younger generation of physicists were intimidated by him, as he would often demolish their theories by coming up with some diabolical test case. He said to Neils Bohr something like this: suppose I have a box containing some fixed amount of energy sitting on a scale, and, by opening and closing a door on it very quickly, allow some energy to escape. Since I can know the time precisely and I can weigh the box before and after, haven't I now gotten around the Uncertainty Principle which says I cannot know both values with arbitrarily high precision?

    Bohr was deeply troubled by this, because he couldn't come up with an immediate explanation. He went home and suffered through part of a sleepless night and he came upon the answer. He approached Einstein the next morning, and very smugly explained...

    Einstein, when you allow the energy to escape, the box rises slightly on the scale. But if the box transitions between two heights in a gravitational field, the time measurement will become uncertain because of gravitational time dilation. You have failed to consider the effects of...(wait for it)...GENERAL RELATIVITY.

    :lol:

    I love that story.
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  24. Marso

    Marso High speed, low drag.

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    Those guys must have been great fun at parties...
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  25. The Night Funky

    The Night Funky BMF Staff Member Moderator

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    Al seemed to have a way with the ladies.

    [​IMG]