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Nature of Quantum mechanics

  1. Jul 13, 2015 #1
    I'm having difficulty gauging what the accepted interpretation of the quantum nature of matter is.

    On the one hand I have been taught that properties of a particle are always definite, but due to the quantum nature of existence we cannot measure several properties to 100% accuracy at once.

    On the other hand I have read that the quantum nature of a particle means that it can teleport, which could lead to light speed violation (i know it was disproved, but only by means of averaging).

    Can someone provide a brief explanation of which view is correct, or whether both are?
     
  2. jcsd
  3. Jul 13, 2015 #2
    I'e studied a lot of quantum mechanics, and teleporting is not a quantum property. Did you mean to say tunnelling?
     
  4. Jul 13, 2015 #3
    Sorry, yes, quantum tunneling, mixed it up with quantum teleportation.
     
  5. Jul 13, 2015 #4

    ZapperZ

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    This is not true. QM doesn't really prohibit you to determine the value of any property to arbitrary accuracy. It depends on your instrument. However, QM does indicate your ability and accuracy to determine the value of the corresponding non-commuting observable! Position and momentum are an example of such non-commuting pair. The more accurate you determine the position in a single measurement, the less accurate you are able to predict its momentum, because the momentum spread that it could acquire will be increasingly large.

    Using the corrected post for "tunneling" instead of "teleporting", I am not sure what issue you have with this, since you didn't exactly asked a question about this. Tunneling phenomena are well-established. We even use it in electronics. The issue of whether a signal can travel faster than c during tunneling is still being debated, and will require plenty more experimental evidence to sort it out.

    Zz.
     
  6. Jul 13, 2015 #5
    Let's focus on this question then - Is it accepted that particles have definite properties?

    If a particle has definite properties but we can not measure them simultaneously because they are non-commuting then how is it possible that a particle may tunnel?
     
  7. Jul 13, 2015 #6

    ZapperZ

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    I do not understand your definition of "definite property". Please note that there is something called "realism", which is that even before measurement, a system is already in some "definite" state. A measurement simply measures such a state. QM does not appear to follow that. If you do a search on realism (even browse through the last few pages of the Noteworthy Papers thread), you will see several discussions and publications on the test of classical realism in quantum systems. All these experiments point to these system as violating "local realism" principle.

    But this is more of an issue of quantum superposition, than with "tunneling".

    Zz.
     
  8. Jul 13, 2015 #7

    stevendaryl

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    The formalism of quantum mechanics associates a "state" with a system. This state evolves deterministically with time and allows one to compute probabilities for the results of various measurements. The quantities that we normally think of as "properties" of a particle: its position, its momentum, its energy, its angular momentum, etc., are only probabilistically described by the state.
     
  9. Jul 13, 2015 #8
    Can you explain to me then - I understand that a wavefunction collapses when it is observed, does it also collapse for a particle when it collides with another particle. (I am trying to gauge the concept of observation now).
     
  10. Jul 13, 2015 #9

    ZapperZ

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    This is not that easy.

    First of all, the "collapse" of the wavefunction is for a specific observable. So if you measure, say, the angular momentum along a certain direction (call it the z-direction), then the value or projection of the angular momentum in that direction will be determined. However, the angular momentum in the perpendicular direction (xy-plane) still remains undetermined and continue to be in a superposition. So just because you made ONE measurement and "collapsed" the wavefunction into a particular definite state, it doesn't mean that ALL other observables are also equally determined. So the wavefunction doesn't collapse for all other observables automatically.

    Secondly, if the particle collided and THEN you make a measurement of something on the second particle that tells you a property of the first, then yes, you have made a measurement. If both particle remained in the system and there were no measurement made, then you have a 2-particle system that is still undetermined. Remember, a conductor consists of gazillion of electrons continually colliding with each other. That entire system is quantum mechanical, despite the numerous collisions that it undergoes.

    Zz.
     
  11. Jul 13, 2015 #10
    Thanks, I guess what I'm getting at is it seems that for something to be observed it requires an entity with intent to do so for it to happen, which seems odd. Then the fact that an observer can pass on information about what they have observed to another individual even though the other they did not observe the collapse also seems odd.

    Say that you are outside of a system, is that system in a superposition of states, even if particles in that system are colliding?
     
  12. Jul 13, 2015 #11

    Nugatory

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    You may have missed the significance of what ZapperZ said in post #9 above:
    If we just consider a single spin-one-half particle, the states "spin up" and "superposition of spin left and spin right" are exactly the same state, just written differently, in the same way that "moving northeast" and "moving northwards and eastwards" are the same vector except written differently. (Any other spin directions will work the same way - for example "spin left" and "spin right" are both superpositions of "spin up" and "spin down").

    If we measure the spin of a particle along the vertical axis and we get spin up, we say that the wave function has collapsed into the spin-up state so it's no longer in a superposition of spin-up and spin-down... but it's still in a superposition of spin-left and spin-right, as well as all other (non-commuting) observables except the specific one that that we measured, spin on the vertical axis.
     
  13. Jul 13, 2015 #12

    ZapperZ

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    Yes, that was what I've been telling you with the conduction electron example.

    Zz.
     
  14. Jul 13, 2015 #13

    bhobba

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    QM is silent on the issue - in some interpretations they do in others they don't.

    In interpretations where it has definite properties its got to do with lack of knowledge about initial conditions. But to understand it you need to delve into their details eg:
    http://arxiv.org/abs/quant-ph/0611032

    Thanks
    Bill
     
  15. Jul 14, 2015 #14

    bhobba

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    It doesn't - all an observation is, is an interaction - no intent, concious observer, information passing etc, required - well in the vast majority of interpretations anyway - there are fringe ones that do.

    Basically QM is a theory about observations that occur in a common-sense classical world - its not quite as weird as some popularisations will give the impression it is eg conciousness is not required, particles are not in two places at once - all the really weird stuff you have likely read is just populist sensationalism.

    If you want to see what QM is REALLY about see:
    http://www.scottaaronson.com/democritus/lec9.html

    That's right - it simply an extension of ordinary probability theory. Popularisations wont tell you that however because its rather ho hum.

    Thanks
    Bill
     
  16. Jul 14, 2015 #15
    Not quite as wierd but still wierd enough.
     
  17. Jul 14, 2015 #16
    Is it not like saying:

    Basically Special Relativity is a theory about observations that occur in a common-sense classical world - its not quite as weird as some popularisations will give the impression it is eg length contraction is not required, particles do not time dilate - all the really weird stuff you have likely read is just populist sensationalism.

    But since in reality length contraction and time dilation occurs.. then why couldn't particles be in two places at once.. etc. also occur in QM?
     
  18. Jul 14, 2015 #17

    bhobba

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    It isn't. Its a theory about space-time geometry implied by space-time symmetries.
    http://physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

    Unlike QM it objectively exists independent of observation, just like Euclidean geometry objectively exists.

    Since its silent on what's going on when not observed you can likely come up with an interpretation where that's true. But what you would gain with such a weird view has me beat.

    Thanks
    Bill
     
    Last edited: Jul 14, 2015
  19. Jul 15, 2015 #18
    It, in fact, depends on the interpretation of relativity. If you follow Lorentz, you have no "time" dilation but a distortion of clocks, and not "length" contraction but a distortion of rulers, above things not more strange or weird than the well-known from everyday life distortion of usual meters by temperature.
    So, how strange the universe looks depends on the interpretations we give to physical theories. In the case of relativity we, obviously, prefer a quite mystical interpretation where "space" and "time" themself are influenced and become a "curved spacetime".
     
  20. Jul 15, 2015 #19

    bhobba

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    Ilja is correct. Lorentz Aether Theory (LET) is also a valid interpretation of SR not based on space-time geometry. But its still objective.

    Don't agree GR in its usual interpretation is 'mystical' - but that is way off topic for this thread.

    Thanks
    Bill
     
  21. Jul 18, 2015 #20
    Bill, you say that all that QM is is an observation which is an interaction.

    How do you exactly define "interaction" ?

    What interacts in QM?
     
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