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Quantum entanglement and black holes

  1. May 28, 2008 #1
    I am a total novice when it comes to quantum mechanics, but I was reading up on the phenomenon of entanglement and a question arose. Theoretically, could someone take two entangled particles and put one in a black hole, using the other to observe what happens to it? Wouldn't this violate the axiom that no information can escape a black hole? Do you have any ideas about what would happen in this theoretical situation? Have scientists ever discussed this idea or am I totally off-base? Hopefully this is not too stupid a question.

    Moderators: if this is better suited for the Quantum Mechanics forum, feel free to move it!
  2. jcsd
  3. May 28, 2008 #2


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    Entanglement violates a lot of things, so I don't see a problem violating the event horizon.
  4. May 29, 2008 #3
    Since entanglement doesn't communicate information, the event horizon is a bit irrelevant.
  5. May 29, 2008 #4


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    You can't send information via qauntum entanglement, so you wouldn't be violating anything to do this.

    Someone once told me this great entanglement analogy, I'll modify it for this situation. Say you have a pizza that is half Supreme and half Hawaiian. Someone cuts in in half and puts in it two closed boxes, one for you and one for your friend without telling you which half is in which. The two boxes are now 'entangled', each contains a mixed Supreme/Hawaiian state.

    Now that you have provisions you decide to go on a journey into a black hole, while your friend stays behind. Once you have passed through the event horizon* your friend gets a bit peckish and opens his pizza box. He has the Hawaiian, and also now knows that you have the Supreme. Does this mean that information has passed out of the event horizon, since an outside observer now knows something new about something inside the Hole? The answer is no, since you couldn't use this to send a message from inside the hole. There is no way for you to send a message that yes you have the Supreme, but they accidentally put anchovies on it.

    *It is actually a non-trivial problem that as far as an external observer goes, you will never actually be observed to pass the event horizon in the first place. It actually takes an infinite amount of time for you to do this seen by an outside observer. Notwithstanding this issue, even if they could see you pass through the event horizon, it is still impossible to transmit information via entanglement.
  6. May 29, 2008 #5
    Thanks for the responses. The pizza analogy was helpful.
  7. May 29, 2008 #6
    @ Wallace...

    WADR, your pizza analogy implies hidden variables and Bell's inequalities do a pretty good job of ruling those out. As such, I don't think a pizza can capture the essence of quantum entanglement. Sadly, I do not have a better analogy to offer...

    Though, if an entangled particle fell through an event horizon and then a measurement was made upon it, that measurement would fix one of the variables of the companion particle outside the black hole. But since the information about the measurement made within the event horizon can never be brought outside for correlation, passage though the event horizon effectively destroy the entanglement, which leads to a loss of information.

    You also wrote:

    So, a large enough star collapses from within, forming a black hole. As result of all of the information that we can observe from prior to the collapse is present on or above the surface of the event horizon “forever” after the collapse (including all of the information we knew about the interior of the star prior to the collapse – mass, spin and charge). And any additional mass that gets trapped appears to take forever to fall into the back hole, at least from an outside observer’s perspective. Why can't the perspective from the mass falling through the event horizon be ignored (since there is no way to know if it really passed all the way becasue information can’t pass back out telling us that it did)? If so, then can we say that mass does not fall through the event horizon at all but rather falls onto the surface of the event horizon? As such, no information is ever lost, it’s just red-shifted until the red-shift of surrounding space becomes greater; at which point the information is radiated back into space via Hawking... This also would make the statement that "black hole entropy is proportional to surface areas instead of volume," obvious.

    Ps. @ cesiumfrog

    WADR as well, quantum entanglement most certainly does communicate information. It simple does not do it in any useful way that is faster than the speed of light.
    Last edited: May 30, 2008
  8. May 30, 2008 #7


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    The black hole is irrelevant -- you can't use one to 'observe' the other under any circumstances.
  9. May 30, 2008 #8
    Again, WADR, you certainly can use one to observe the other as long as the speed of the "act of observing" does not violate the speed of light. And since getting information out from within an event horizon would clearly require FTL travel, it seems that the black hole is relevant.
    Last edited: May 30, 2008
  10. May 30, 2008 #9


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    The pizza thing is obviously a simple analogy, designed only to demonstrate the key misunderstood point about quantum entanglement, which is that you cannot transfer information using entangled particles. This has already been pointed out by myself, cesiumfrog and Hurkyl. I agree with Hurkyl, the black hole thing is really a red herring in the discussion, since you can't pass information via entanglement in any case, so the black hole event horizon question is moot.

    Entanglement allows you to know something about a particle an an arbitrary distance from you in an instant, however this is not information transfer, since you can't send or receive a message via this process, that is the crucial point.
    Last edited: May 30, 2008
  11. May 30, 2008 #10
    And as I have pointed out to you, Hurkyl and cesiumfrog, you all seem to have a common misunderstanding about what entanglement is... Entanglement allows you to know that an entangled partner knew something about its partner particle an arbitrary distance from one another in an instant. However, you will not be able to show that this occurred in a manner that violates the speed of light.

    If entanglement did not carry information and/or if entanglement did not transfer information, there would be no need of such a concept QM. The fact is that entanglement does carry information and does transmit information, it just does it in a way that forces us to "observe" this information transfer at slower than light speed.
    Last edited: May 30, 2008
  12. May 30, 2008 #11
  13. Jun 2, 2008 #12
    I'm not sure if I like the pizza analogy, and here is why:

    1. It implies that the two particles are only fragments (halves) of a single particle, which is not the case.

    2. It implies that the two particles had been observed originally and then concealed (the person who cut the two pizza halves observed them in order to separate them according to style and place them in the boxes to conceal their identity). This is of course not the way entangled particles work. The observation does not take place until after they are separated.
  14. Jan 11, 2012 #13
    Could the information be teleported out by entanglement?
  15. Jan 12, 2012 #14
    And I think the pizza is both hawaiian and supreme until the box is opened.
  16. Jan 12, 2012 #15


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    It was my understanding that Quantum Entanglement had not been conclusively shown to contain or not contain hidden variables. Is this incorrect?
  17. Jan 12, 2012 #16


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    Only if there's a cat in there with it.
  18. Jan 12, 2012 #17
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