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The existence of point particles and an infinite universe

  1. Jul 6, 2013 #1
    It seems to me that the question as to whether the universe is infinite or not carries the same validity as the question as to electron, quarks, etc. being infinitesimal or otherwise stated being modeled as point particles. It seems to me that these two quandaries are linked and perhaps can justify one another.
     
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  3. Jul 6, 2013 #2

    phinds

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    Is there a question here or did you just need to get that off your chest?

    If you are asking if your idea is valid, I don't see how they have anything at all to do with each other.
     
  4. Jul 6, 2013 #3
    I suppose a question I could form to go with my statement would be relative to what are point particles considered to be infinitesimal?
     
  5. Jul 6, 2013 #4

    Nugatory

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    In our experiments to date, there is no detectable spatial extent or evidence of internal structure. So the best answer to your "relative to what?" question is "relative to the limits of our measurements".

    It is not a priori impossible (although also not likely) that observations at higher energies than we can reach today might give us a different answer. Unless and until that happens, there's not a lot of point in wondering whether we're dealing with point particles or non-point particles whose behavior is in every respect indistinguishable from point particles.
     
  6. Jul 7, 2013 #5
    The best and most objective answer, indeed. But I wouldn't say true point-like particles exist, because if you can magnify them with some quantum microscope than they are not points at all.
     
  7. Jul 7, 2013 #6

    Bill_K

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    If it's the best and most objective answer, what grounds do you have for disagreeing with it?

    Quantum microscopes do not exist - when you invent one, be sure and let us take a look. Until then, experiment and theory indicates that elementary particles are indeed point-like.
     
  8. Jul 7, 2013 #7
    You're wrong, quantum microscopes do exist:
    http://physicsworld.com/cws/article/news/2013/may/23/quantum-microscope-peers-into-the-hydrogen-atom
    http://www.newscientist.com/article/mg21829194.900-smile-hydrogen-atom-youre-on-quantum-camera.html
     
  9. Jul 7, 2013 #8

    Bill_K

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  10. Jul 7, 2013 #9
  11. Jul 7, 2013 #10

    Bill_K

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    An elementary particle is an excitation of a quantum field, but it is created at a point and annihilated at a point.
     
  12. Jul 7, 2013 #11
    Yes. There is no all-embracing classical-like concept. I have yet to find what the op was attempting to say.
     
  13. Jul 7, 2013 #12
    I believe your 'question' does have some importance.

    At some high energy (some small distance) scale, if particles cease to become point like (which I expect at some scale they will) then we have to abandon the use of quantum field theory as we know it.

    We would require something else to describe what is going on. This isn't too surprising since incorporating gravity into our current theory of elementary particles is rather difficult.

    So, understanding how particles behave at this small distance may also tell us about gravity at very small scales and perhaps information about inflation etc. who knows.
     
  14. Jul 8, 2013 #13
    I'm not so sure, they have just begun with this technology, I wouldn't be surprised if they will be able to detect and actually see an electron directly.
     
  15. Jul 8, 2013 #14
    I wonder what is below quantum level. if particles cease to become point-like (which would happen eventually), than we would have sub-quantum field and sub-quantum physics.
    And we will know what and how are created those point-like particles?
     
  16. Jul 8, 2013 #15

    Bill_K

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    I guess you haven't given it much thought in that case. The size of an atomic orbital is about 10-8 cm, while the electron is already known to be pointlike to less than 10-17 cm, or nine orders of magnitude smaller. Resolving power is directly proportional to energy, so you would need to use a probe whose energy was nine orders of magnitude greater, at least in the 10 TeV range.
     
  17. Jul 8, 2013 #16
    Hmmm. I just realized that I don't quite understand what "point-like" actually means wrt quantum systems. Electrons, for instance.

    An electron is never localized to a exact known location (for it would have infinite impulse uncertainty, thus infinite impulse, thus infinite energy... not good).

    We have an experimental proof that this happens - we are quite sure electorn-degenerate matter in white dwarfs and neutron stars exists. The matter where electrons are forced into ever smaller spatial regions, and they do acquite energy which resists further cramping.

    So, an electron is never in a point. So any other particle.

    Am I missing something?
     
  18. Jul 8, 2013 #17

    Bill_K

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    Yes. This has nothing to do with the Heisenberg uncertainty principle, and the particle does not need to be "localized". The particles we talk about in ordinary Schrodinger quantum mechanics are also point particles. Likewise for the Dirac Equation.

    The issue is whether the particle has internal structure, that is, does an electron have parts. Is it extended in space, does it have internal degrees of freedom - can it come apart, can it vibrate, etc. If the answer was yes to any of these, you'd have a hard time writing down a theory that was Lorentz invariant. String theory says yes, ordinary quantum field theory says no.
     
    Last edited: Jul 8, 2013
  19. Jul 8, 2013 #18
  20. Jul 9, 2013 #19
  21. Jul 9, 2013 #20
    But if you are able to magnify electron even though with so much energy, than it is not a point-like particle!?
     
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