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Entanglement Question

  1. Apr 29, 2015 #1

    what is the best way to understand entanglement ? I just wanna know your opinions on the subject, because I'm a bit confused

    There are people who say that "must be some another dimension in which information travel from one to other particle" and others who say this proves that "Distance is an illusion" lol ? There's even a video about it
  2. jcsd
  3. Apr 29, 2015 #2


    Staff: Mentor

    Its nothing like that.

    Entanglement is simply what separates standard probability theory from QM:

    All this stuff like EPR etc is, is a different type of correlation than you find in standard probability theory.

  4. Apr 29, 2015 #3
    thanks bhobba, you always helping!!
  5. Apr 29, 2015 #4
    The truth is, we don't fully understand entanglement yet. The general gist of what you're talking about isn't all that crazy.

    You might be interested in this:

  6. Apr 29, 2015 #5


    Staff: Mentor

    We do as, for example, the link I gave explains.

    Its just that it implies a different type of correlation as per Bells Theorem that is counter intuitive to everyday experience.

  7. Apr 30, 2015 #6
    calling distance an illusion because of the experiment that maybe we don't know yet, yes It's 100% CRAZY.
  8. Apr 30, 2015 #7
    This stuff is really more Beyond the Standard Model, or Quantum Foundations and to some extent General Relativity than Quantum Physics, but one thing we know for certain is that space and time are not as they seem intuitively. You can call it crazy, but it is supported by experiment and by our best theories.
  9. Apr 30, 2015 #8
    I'm calling crazy the video saying "distance is an illusion" (whatever that means), which has nothing to do with entanglement. I'm not calling entanglement 'crazy', which is just a tool of the universe that we don't know how it happens, yet. that's all
  10. Apr 30, 2015 #9
    Is it this?

    If so, it's totally fine. It's just many of the most notorious physicists discussing the Bohr-Einstein debates and the Bell Tests. It's all well accepted mainstream quantum mechanics.

  11. Apr 30, 2015 #10


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    Science Advisor

    I'd be interested in knowing if you found the post Entangled States are like Unitary Matrices useful or not. It covers how operations on maximally-entangled two-party states act isomorphic to a unitary matrix w.r.t. how they get operated on by either side independently.
  12. Apr 30, 2015 #11
    I think
    Totally fine? So answer me. what does means "Distance is an illusion"? It makes no sense. If it's something that no one understands how it works. why someone make claims without evidence? btw the guy who created this video, ins't a physicist. he is a new age follower.
    As bhobba said, we do understand a lot of the experiment , It just that is counter intuitive to everyday experience.
    Last edited: Apr 30, 2015
  13. Apr 30, 2015 #12


    Staff: Mentor

    Its a bit better than many popularisations but totally fine - no I wouldn't say that. For example they talk in slogans like the world isn't there unless you look - which isn't what QM says - its actually silent on it if you don't observe.

    Always be wary of popular accounts - take what they say with a grain of salt - come here for what's really happening - but your thinking cap will need to be on because easy this stuff aren't - but it has to be said understandable with effort.

  14. Apr 30, 2015 #13


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    Gold Member

    Conclusions of that video are not correct. First of all in science nothing is proved - only verified or falsified. And second even now experiments have not yet falsified local realism.
  15. May 4, 2015 #14
    In order to understand the meaning of entanglement, let me give you an example: "We have 6 marbles then we put them in two boxes. So give one box to Bob and the other to Alice, Alice goes to New York and Bob goes to Paris. They don't know how many marbles there are in their boxes. they open their own boxes when they arrive to their destinations. Then Bob opens his box and looks at 4 marbles so he understands instantly there are 2 marbles in Alice's box". However it can not be a process as a traveling information, so we can't define the" velocity" quantity for this knowing. But it's just a classical example and difference phenomenon exactly happens in quantum mechanics.
  16. May 4, 2015 #15
    That's fine from a QM perspective. The picture in QFT is more complex, but it becomes really interesting when we start to look for a quantum theory which is compatible with General Relativity, where it should be no suprise that space and time take on a counterintuitive form.
  17. May 4, 2015 #16
    yeah, It's more complex in QFT but more interesting.
  18. May 7, 2015 #17
    This is a bad example. It only shows a situation that is easily explainable with hidden variables and does not violate Bell's inequality.

    Here is the example that I would give:
    Imagine you have three doors, and behind each you have a hidden binary value. If someone told you that no matter which two you open, they will show opposite values to each other, you'll say this does not make sense.
    Well, QM tells you something like that. It does not go as far as to say they will be 100% opposite, but it can say that the probability for different values is 75% for example, and this is still just as insane. Any classical or "hidden variable" explanations that do not involve a value behind some door changing because of you opening another door can only get to 66%.
    It's a bit hard to think in probabilities for such simplified example, so you have to extend it a bit. Instead of just one set of three doors, you can have 100 or 1000 sets each of 3 doors.
    QM additionally muddles the water by saying you can never open the third door in a set after having opened the first two.
  19. May 8, 2015 #18
    Wouldn't it be that having opened the first two doors you will find nothing behind the third?
  20. May 8, 2015 #19
    Not really. The three doors represent three possible angles of measuring polarization, for example. Opening two represents measuring the polarization on two different angles on each of two entangled particles. You can not have a set of three entangled particles, so you just can not "open three doors".
    [EDIT: I think you can have a set of three or even more entangled particles, but only with other kinds of entanglement than assumed here - i.e. with a known sum of spins, instead of the equal spins we need here.]

    On one particle you can only measure one angle and then its state has been changed. Measuring more than once on the same particle does not count as opening a door because its result is after the changes caused by the first measurement, and is thus irrelevant.
  21. May 9, 2015 #20
    I think the OP question was if entanglement proves that "distance is an illusion", .. this question makes no sense, even "distance is an illusion" makes no sense, could someone explain to me?
  22. May 10, 2015 #21
    well, entanglement is a sort of non-local interaction. its a far cry from saying distance is an illusion, but i hope you can see how it at least points in that general direction :p
  23. May 10, 2015 #22
    From a third party point of view there is clear space between the entangled particles. The book-keeping will demand that they are space-separated on measurement (think consevation of momentum). Without measurement though, I wonder how it would appear from the particle's point of view.?
  24. May 10, 2015 #23


    Staff: Mentor

    Why do you think it must be viewed as particles with a definite position separated by 'clear space'?

  25. May 10, 2015 #24
    Because when they are measured they are found to be space separated and the later they are measured the farther part they are found to be. Is that the wrong way to think about them?
  26. May 10, 2015 #25


    Staff: Mentor


    You are falling into the trap of ascribing properties when not observed.

    Of course the first position measurement will break entanglement.

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