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Is this true?

  1. Jul 19, 2004 #1
    Is this true???????

    I was explaining the impossibility of a complete vacuum to a friend when another friend overheard us talking. He then posed a claim that relativity says that "Something can't exist unless you see it". I have a very hard time believing this has any validity since the statement implies the possibility of something existing in a state of nonexistence...which is self-contradictory...anyway I could be wrong in my rejection of the idea. That is why I am asking you guys. Thanks
     
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  3. Jul 19, 2004 #2

    selfAdjoint

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    Your friend is thinking of a caricature of quantum mechanics, not relativity. One school of thought in QM makes the assertion that observation (aka measurement) is necessary for a properety of some quantum particle to "really exist". For example it is said that an electron in an atom does not have an orbit, because neither its position nor its momentum "really exist" except at points where it is measured. And even then they are uncertain!
     
  4. Jul 19, 2004 #3
    Right self-adjoint, but QM wouldn't claim that the electron didn't exist, only that certain properties of it aren't defined prior to measurement. So Imparcticle could tell his know-it-all friend what you said and that in any case the electron certainly exists.

    Am I correct

    Kevin
     
  5. Jul 19, 2004 #4

    selfAdjoint

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    You are, although critics have asked some supporters if the Moon exists when no-one is looking at it, and at least one of those supporters said "No!". But that's an extreme position, I don't think even a healthy minority of quantum physicists would agree.
     
  6. Jul 20, 2004 #5
    Regarding the moon

    Even if absolutely nobody is looking at the moon *now*, someone can still see its current state *later*, if they stand sufficiently far from the moon. But in order for them to see it *later*, it must interact with light *now*. And to do that, it must exist now. Unless that supporter is implying that the universe somehow knows that noone will ever ever ever see the moon's current state, and termporarily switches the moon out of existance..

    Hmm... could this also be argued for quantum particles?
     
  7. Jul 20, 2004 #6

    selfAdjoint

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    The ultra-radical version of the copenhagen point of view is that the universe is just that way. Things don't exist unless observed. Between observations you don't have spacetime events but non-spacetime "wave functions". Observation causes the wave function to "collapse" into the thing observed.

    I don't want to continue explicating this, or be forced into defending a position I don't believe, so let's just leave it that some people in the physics community do believe that.
     
  8. Jul 20, 2004 #7
    I heard Penrose mention that an experiment is planned to determine just how far these superpotions propagate before a reduction of state chooses one of the alternative in the wave function. For otherwise He seems to take the many worlds as a reduction to absurdity for quantum mechanics. And since no one observes mutli-universes or worlds, and since one would actually contradict the other, this superposition of states has to end somewhere up the change of larger scales.
     
  9. Jul 20, 2004 #8
    The problem I have with this point of view on QM is with gravity. If the said object doesn't exist when there are no observations, would it not make sense then that any and all gravitational influences it has are also in a state of flux? This surely would cause havoc. I may be wrong, but I think that alone tosses out the view that unless observed, it doesn't exist.

    Unless of course gravitational interaction counts as observation, but in that case, the particular view point can never be proven and is not a true scientific interpretation.
     
  10. Jul 20, 2004 #9
    Well, after I explained the subject of SelfAdjoint and others' replies, he said okay.
    Thanks for the replies.

    By saying "Things don't exist" you're acknowldgeing that something can (i.e., has the ability to) not exist. But, if something can not exist, then it can be in such a state of nonexistence. Notice, it must exist to be in any state let alone existence therefore contradicting the original idea!
    Did I interpret correctly? (I'm guessing no?) (I am not saying you believe this to be true)

    Wow, you mean there are superpositions of existence?
     
  11. Jul 20, 2004 #10

    selfAdjoint

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    Superpositions of the wave functions (or "in the state space"). These are real according to some, but according to a few do not become real until collapsed by observation. One interpretation is that the wave functions carry all the possible things you might know about a thing, and observation collapses that into the particular property you have learned.
     
  12. Jul 20, 2004 #11
    I see. So it is not so much a matter of it existing, but a matter of it's state of existence, right?
     
  13. Jul 21, 2004 #12
    existence

    I was reminded today, while thinking of this thread, of a bit I read in Dirac's QM book:


    "In the special case when the real dynamical variable is a number every state is an eigenstate and the dynamical variable is obviously an observable. Any measurement of it always gives the same result, so it is just a physical constant, like the charge of an electron."

    4th edition, page 38

    Now this seems to say that certain attributes, like charge, of an electron are definite, while position and momentum, for example, are not definite before measurement.

    Could some like an electron have a definite charge while not, in fact, existing?

    Kevin
     
  14. Jul 21, 2004 #13

    selfAdjoint

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    Every electron you see, when you see it, has the same charge. So it's constant. The point of the Dirac quote is that then the wave function of it is constant too ("just a number"), and doesn't have to collapse. Its probability distribution would be a (Dirac!) delta function.
     
  15. Jul 21, 2004 #14
    How do you know an electron is an electron and not a positron? (isn't that a probability for the wave function to incorperate?)
     
  16. Jul 21, 2004 #15

    selfAdjoint

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    You find a particle with mass energy 511 electron volts, and if it has a negative electric charge, it's an electron. If the charge is positive, it's a positron. Anything else, it's an unknown particle, notify the Nobel committee.
     
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