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

Detecting Macroscopic Entanglement With the Bell Inequality

  1. Oct 15, 2008 #1
    So first off, Ill admit Ive forgotten most of the finer details of the bell inequality so I apologize if Ive gotten something wrong.

    From how I remember it, the bell inequality is a test of any local hidden variable theory vs. QM. It uses the fact that the inequality measured if a classical, local hidden variable theory were true is necessarily different than the inequality given by QM.

    As far as I know, the nature of decoherence is somewhat of a mystery. Personally, I like Everett's interpretation that decoherence is just an illusion caused by the entanglement of a macroscopic object (the observer) with the measured quantum object. I was thinking of this and the other theories (Penrose's specifically which I really dislike) and it seems to me like these are more than just interpretations. Although MWI is clearly an interpretation, Everett's view of decoherence should be testable.

    So heres the experiment I thought of that should test it:

    Put a photon into an equal superposition between up and down spin (or polarization w/e). Have a detector in a vacuum (space) measure this photon. Then have the detector shoot a new photon with identical spin to the measured photon, alone with a tennis ball with some property determined by the measured spin (e.g. angular momentum, speed).

    Now do the Bell inequality test on the photon and the tennis ball (repeated many times of course). The detectors must be isolated enough from the first detector that no information about the projectiles can reach them (or else the systems wave function would collapse).

    If a macroscopic object is capable of entangling with a quantum object, the measured inequality should be that predicted by QM. However if some theory like penrose's is right, the tennis ball would not be capable of entangling with the photon and the measured inequality would be that of a hidden variably theory.

    Did I miss something or is this actually testable?
  2. jcsd
  3. Oct 15, 2008 #2


    User Avatar
    Science Advisor
    Gold Member

    Are you proposing to test something from Penrose or from Everett? The consensus has always been that there is no test possible of MWI (although this is a subject that gets attention).

    As far as I know, progressively larger objects are capable of being entangled. I am speaking of atoms, and I do not think there is any particular cutoff. A tennis ball might be a little tough as its entropy is so high it would be hard to nail down the state sufficiently to perform any kind of test. In fact, a high-efficiency detector would itself be about the macroscopic limit (pretty much by definition).
  4. Oct 15, 2008 #3
    The rules of physics give us conservation laws that demand two electrons in on a atom with a specific combined spin that in a pronominal that cause them to jump off simultaneously will do so conserving spin in a predictable way.
    Likewise, when a single photon converters into two photons via SPDC.

    Exactly what physics rule of conservation do you hope to use that would apply to a photon and a tennis ball wrt their spins??
  5. Oct 20, 2008 #4
    seriously? thats your only comment? First of all, isnt spin conservation just conservation of angular momentum? Anyway, its not really important. quick solution:
    have some other objects in an isolated vacuum that are used to conserve all conserved quantities. e.g. a photon with opposite spin and some object with opposite angular momentum.

    I really dont understand why you would bring this up unless Im misunderstanding your comment.. Theres just so many possible ways to fix it, and my proposed theory was far from specific...
  6. Oct 20, 2008 #5
    yea I know there are no tests possible, but then I thought of this and dont see any immediate problems with it. In theory at least it seems like a testable difference between the two theories, as hard as it may be to do in practice.

    If the two states of the tennis ball used for the entanglement are significantly different (direction of momentum/angular momentum?), shouldn't it be easy to test?
  7. Oct 21, 2008 #6
    Are you kidding !?
    You say that your “ proposed theory was far from specific”
    But complain that I expect such a minor specific detail as to how you expect to make the spin values of a photon and a tennis ball the same!

    If there are “just so many possible ways to fix it”
    just name one that might even come close to giving a tennis ball the same spin as any one photon.
  8. Oct 21, 2008 #7

    Ken G

    User Avatar
    Gold Member

    To clarify this issue, perhaps it would be of use to note that "gedankenexperiments" have a strange limitation in quantum mechanics. Normal gedankens say, "of course we cannot actually do this, but we can conceptualize the result anyway, and we should not encounter an inconsistency". In other words, consistency must extend not only to experiments that can actually be done (technologically speaking), but also to those that cannot. However, in quantum mechanics there is yet another possibility-- that there might be more than just technological limitations, there might be fundamental limitations in our ability to extract scientific information from reality (due to certain assumptions we make before we even set out to do science).

    So we must distinguish gedankens that generate observables that are difficult to actually obtain, from those that do not generate observables at all. On the observable side of the issue, the debate in this thread might be about the question: when an electron interacts with a tennis ball, is angular momentum conserved?

    I think the answer must be that it is, in any QM interpretation (I don't know Penrose's). The reason is that spin is a QM notion, but it is defined by behaviors that do cross the QM/classical "boundary". In other words, it is already a QM notion that survives the classical filter we always apply, and that's why the CI includes the notion of spin as an observable operator. Also, the CI includes a "correspondence principle", which says that classical conservation laws must also be statistically conserved in QM, insofar as the conservation laws can be applied to QM notions that survive coupling to classical objects (which all QM notions do, by definition). So I do think that an electron can be said to confer its spin to a tennis ball if the electron spin flips in the interaction, even though such a tiny effect would be unmeasurable, because one could imagine aggregating vast numbers of the same effect until it was measurable. That's the correspondence principle. But what I don't see is why Penrose's picture would say anything different. I don't know that picture, but I doubt Penrose would build a picture that was not compatible with the correspondence principle.

    But it sounds like you are saying that the only thing that would be different would be the concept of entanglement, not the outcome of any actual measurements. That's a bird of a different feather, for now we are in danger of leaving the realm of gedankens that actually connect with observations, and enter into the realm of gedankens that may not be doable-- not because of technological limitations, but because of fundamental limitations in the way humans can interact with and learn about reality.
    Last edited: Oct 21, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook