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How big is too big?

  1. Jul 16, 2006 #1
    i'm reading Quantum: A Guide For The Perplexed at the moment. There's a bit in it about a group who managed to show double slit quantum interference with 'buckyballs' (i think they're carbon atoms) which are naturally a lot larger than electrons, so i was wondering - does anyone know if there's a limit to the size of the particles used to show the interference? is there some kind of size limit to quantum effects?
     
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  3. Jul 16, 2006 #2

    vanesch

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    That's the 10^6 $ question :smile:

    In fact, there does not need to be any upper limit. There are essentially two different kinds of views on quantum theory: those that claim that there is a "macroscopic" limit (with projection), and those that don't. This is just to say that it is *conceivable* that quantum interference is _potentially_ thinkable for arbitrary large systems.

    Each time someone talks about "the wavefunction of the universe" he's assuming that nothing is too big. But in fact, we don't know whether that's sensible or not.
    The reason is that there are effects (decoherence effects) which make it harder and harder to observe quantum interference effects on large scales (decoherence is entirely explained itself also as a quantum effect).
     
  4. Jul 17, 2006 #3
    so does quantum theory itself prevent us from seeing it's effects on our sort of scale?
     
  5. Jul 17, 2006 #4
  6. Jul 17, 2006 #5

    vanesch

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    Yes. It is called decoherence. Have a look at:
    http://plato.stanford.edu/entries/qm-decoherence/

    The essence is this: when a quantum system has two components which could show interference (its state is |a> + |b> say), and these two components interact differently with some part of the environment, then the environment entangles with the two components. Now, the environment being a complicated system with a lot of uncontrolled degrees of freedom, the state is now |a>|envA> + |b>|envB> and every attempt to do an interference experiment will now not have an amplitude <a|a> + <b|b> + 2 Re{<a|b>} ; where the last term is the interference term which indicates quantum effects, but rather:
    <a|a><envA|envA> + <b|b><envB|envB> + 2 Re{<a|b><envA|envB>}

    and due to the complexity and the uncontrolledness of the environment, in most cases, <envA|envB> ~ 0, so this suppresses any observable interference. The reason is that in most basis, envA and envB are essentially random vectors of high dimensionality, and the in-product of random vectors of high dimensionality is usually very close to 0.
     
  7. Jul 17, 2006 #6
    Farsight: I'm not sure, I can't remember who the experiment was attributed to.

    vanesch: thanks for the link, but i don't understand the bit underneath that you wrote (i haven't done any advanced physics) but thanks anyway.
     
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