Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Entangled particles

  1. Jul 21, 2017 #1
    I have some questions about quantum entanglement

    1 - Why is this phenomenon considered so surprising, if it just states conservation of quantities (e.g. spin, momentum) and we are already accostumed with conservation of quantities in classical physics

    2 - Suppose we have, say, a hydrogen molecule, which means we have two entangled electrons. Suppose further we do not have made yet any measurement of the spin of any of the two electrons, but we know one has to be up while the other is down.

    Given that, we separate the two hydrogen atoms... and now we measure the spin of one of the electrons to be up... the other will certainly be down.

    Now... Why is this considered bizarre? Why can't we just realize that the two electrons had their spins already set up in the state we later would find them to be in? To be more precise, I will give an example

    Before destroying the molecule:
    Electron 1 has spin up in z direction
    Electron 2 has spin down in z direction

    Of course, calling "Electron 1" and "Electron 2" is just to keep things clear, afterall they are indistinguishable particles.

    Suppose further we don't know these are their spin states. When the electrons got separeted, it seems obvious that their spin states will keep in that way. In other words, there is nothing bizarre happening, it is just we did not know what their spin state were.
  2. jcsd
  3. Jul 21, 2017 #2


    User Avatar
    Gold Member

    Tio, I don't know this stuff well enough to give you a solid explanation but I assure you that your understanding of what entanglement is is a MIS-understanding. You are saying that the situation is exactly analogous to putting a red sock in one box and a green sock in another box and then separating the boxes without knowing which is which. As soon as you open either box you know what the color of the sock in the other box has to be. That is not how entanglement works. I'll leave it to one of our more knowledgeable members to give you an explanation but it has been discussed here many times so a forum search would serve you well.
  4. Jul 21, 2017 #3
    Thanks phinds. So dogs are joining the forums too... :smile:

    You got my point. Good to know that Quantum Entanglement does not work the way I thought.
  5. Jul 21, 2017 #4


    User Avatar

    Staff: Mentor

    Google for "Bell's theorem" and also check out http://www.drchinese.com/Bells_Theorem.htm, which is maintained by our own @DrChinese.

    If you measure at in-between angles as well as exactly opposite angles, quantum mechanics predicts results that are different from what you would get if the spins were set when the pair was created. The differences are subtle but detectable; and the experiments have confirnpmed that quantum mechanics is correct and the "set at creation time" model is not.
  6. Jul 22, 2017 #5
    It depends on how you measure: if the measurements bases are parallel, you get opposite spins, like you say. If the bases are not perfectly parallel, that is not guaranteed. So it depends partly on how you measure. And since the 'first' basis determines the direction of the (measured) spin-up (down), it determines the direction of the 'second' electron's spin-down (up). And vice versa. And that is precisely the effect of conservation in this case.

    If you keep the detector angles parallel, the measured spin values should always be opposite, which is not the case with fixed opposite spins (hidden variables), for they could misalign with the detector angles.

    Correct me if I'm wrong.
    Last edited: Jul 22, 2017
  7. Jul 22, 2017 #6
    Since we do not know exactly where an electron is at any given point(only about a 95% chance of where it is) we can only predict its whereabouts in relation to another electron, just like you would say a person is short COMPARED to another person and like we would say the mass of a proton is 1 using RELATIVE atomic mass. It seems to me to be the same principal and since an electron will commonly not orbit in the exact space as another electron (common sense because they are of the same charge) thats when we say the electron has an opposite spin, AGAIN in relation to another electron and as for the concept being amazin I must emphasize it is because the study of electrons has interested physicists all over the world just take a look at the double slit experiment

    And as for "just accepting" in science nothing is "just accepted" everything is tried and tested since scientists of old just accepted false facts modern scientists dont "just accept" anything such a way of thinking is completely against science and does not exist in the modern science realm.
    I hope this answer helped and please note I am also open to reform and rebuke.
  8. Jul 22, 2017 #7


    User Avatar
    Gold Member

    @thejosh I cannot see what bearing your post has on this thread. You have said nothing about entanglement which is the subject of this thread and the double slit experiment has nothing to do with entanglement.
  9. Jul 22, 2017 #8
    Are you sure about that? Quantum amplitudes work just fine.....
  10. Jul 22, 2017 #9


    User Avatar
    Science Advisor
    Gold Member


    What is the z direction but an arbitrary direction? And yet for 2 spin entangled electrons, they will be opposite in spin in any arbitrary direction. Such electrons will maintain this state regardless of distance. You might then deduce that their spin measurements are actually predetermined in all arbitrary directions, so as to make your idea work out. I.e. that they are fully independent and maintain no ongoing physical relationship as their separation increases.

    But that precise idea - that the spins were preset - fails to account for the actual observed statistical relationship between entangled electron spins. Bell wrote about this in 1964, and until you read and understand that, it will not make a lot of sense. But the short version is that your idea is not only in disagreement with the predictions of quantum mechanics, it is falsified by experiment. See for example:

    Bell's Theorem with Easy Math
  11. Jul 23, 2017 #10
    @phinds I am sorry if my post caused confusion but I merely wanted to point out some of the reasons why electron behaviour is considered amazing in modern science and the best example I could think of was the double slit experiment but thanks for the feed back I will attempt to make more precise answers in the future.:sorry:...
  12. Jul 23, 2017 #11
    Basically, the entangled situation is not like your having a red stick and a blue stick in the box, but not knowing which is which. Such a situation can exist in QM, but is not the general situation.

    Let's consider the electrons as distinguishable; the fact that they are not doesn't change much to the issue of entanglement ‑ we could also simply consider two different particles, which can be entangled too. With these two particles, both of spin 1/2, you very might well have a situation (let's call it “state +-”) with, say, particle A having spin +1/2 (along the z axis) and particle B having spin -1/2. Or alternatively (“state -+”), particle A having -1/2 and particle B +1/2. But QM tells us that the system can also be in any linear combination of these two states. This is a kind of mix of state +- and state -+, but only kind of. It's not a situation that is one of states +- or -+, but that we don't know. It is a state that has it's own distinct properties. In particular, following the way you do the mix of +- and -+ (the linear combination), you may obtain a state in which the total spin following another axis, say the y axis, is +1, 0 or -1; or again, some combination of these. So even if you mix +- and -+ in equal proportions, there can be different ways to mix them.

    This is the theory. The Bell theorem is about some of the consequences of this theory that can be tested, and have been, showing that the entanglement cannot be described just as a mix.
  13. Jul 29, 2017 #12
    Is it possible to have sets of entangled particles in sufficient number to transfer information with a useful bandwidth?
    If so, it seems that one half could be carried into space and allow instantaneous communication between Earth and an extraterrestrial vessel.
    Mars rovers could be controlled from Earth in "real time".
    Perhaps a future starship could venture to other systems with continual and immediate communication?
    Could this really be possible?
    If so, it feels like it violates some basic tenants of space-time.
    Please pardon my ignorance.
    I find this fascinating, but it's way over my head.
    I expect you'll let me know if this is inappropriate speculation for this thread.
  14. Jul 29, 2017 #13


    User Avatar
    Gold Member

    There IS no information transfer in any meaningful/useful sense with entanglement so bandwidth is an irrelevant concept here.
  15. Jul 29, 2017 #14
    I infer from your response that that the particle states can be observed, but not controlled.
    Did I mention that this is over my head?
    Thanks for your tolerant response.
  16. Jul 29, 2017 #15


    User Avatar
    Gold Member

    Sorry about that. When I've answered the same question 50 times (not your fault) and the latest questioner clearly hasn't done much if any reading on the matter (this part IS on you) I get a little snippy and I should not. I've been smacked about the head and shoulders for this and I do try to remember not to do it. I actually had a MUCH worse response, which I edited, so I thought I had gotten rid of the snippiness.
  17. Jul 29, 2017 #16
    Apparently my reply conveyed something I didn't intend.
    You have no reason to apologize for anything.
    I honestly appreciate your helpful response.
  18. Jul 29, 2017 #17


    User Avatar
    Science Advisor

    For socks the result is not surprising because you tacitly assume some form of realism (the socks had their colour all the time; measuring it just exposes this fact) In QM this is highly non-trivial.
  19. Jul 30, 2017 #18

    David Lewis

    User Avatar
    Gold Member

    Using that analogy, then what does counterfactual definiteness mean? That if you find a red sock, by sole virtue of the fact that it's red, the one you can't see must be green?
  20. Jul 30, 2017 #19


    User Avatar

    Staff: Mentor

    CFD is a somewhat weaker claim, namely that the other sock has some color even though we haven't measured it and might never measure it. That claim, along with the the other conditions of the problem, allows us to conclude that the other sock is green.

    I must confess that I have never found a useful distinction between "realism" and "counterfactual definiteness", other than that people argue a bit less about the meaning of the latter term than the former. As far as I can tell, @haushofer's statement works just as well if written as "For socks the result is not surprising because you tacitly assume some form of counterfactual definiteness (the socks had their colour all the time; if you measure it you just expose this fact)" without losing anything important.

    (Haushofer may have a different perspective - if so, listen to him not me)
  21. Aug 2, 2017 #20


    User Avatar
    Science Advisor
    Gold Member

    The bizarre nature of entanglement comes from a contradiction between (1) the uncertainty principle, and (2) the speed of light being an absolute limit for information travel.

    If you assume both (1) and (2), then knowing everything about the history of a particle and what it has interacted with in the past cannot allow you to predict the position and momentum of that particle better than the Heisenberg uncertainty limit.

    However, a separated pair of particles A and B with very strong entanglement may have strong enough position and momentum correlations, so that measuring A allows you to predict the position or momentum of B better than the Heisenberg limit. These particles may even be separated by a sufficiently vast distance that light cannot pass between them in the time it takes to do measurements.

    Since this is a contradiction, either (1) or (2) must be false (or both).

    In short:
    Entanglement is weird because it throws into question common assumptions about reality that seem true in everyday life, but are actually contradictory.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Entangled particles
  1. Entangled particles (Replies: 3)

  2. Entangled particles (Replies: 2)

  3. Entangled particles (Replies: 8)