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B Something from nothing

  1. Oct 12, 2016 #1
    Why do some physicists talk about particles popping into existence from nothing. The nothing they refer to is a vacuum, but the vacuum exists in this universe so must contain the fabric of the universe, it must contain gravity, so obviously it is a something rather than a nothing.

    Kaku and Krauss are 2 I can think of that have talked about this.
     
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  3. Oct 12, 2016 #2

    Vanadium 50

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    Because Kaku will say anything if he thinks it will bring in a buck. Real physicists say no such thing.
     
  4. Oct 12, 2016 #3
    So do you think most physicists would say that these particles DO come from something?
     
  5. Oct 12, 2016 #4

    phinds

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    You misunderstand. He is saying that there ARE NO "particles" of this sort.

    You are talking about the so-called "quantum foam", which is a highly speculative extension of quantum mechanics.

    Also, there is no such thing as "the fabric of space". This is a made-up pop-sci concept that evolved from something casual that Einstein said.

    No, it is NOT true that "it must contain gravity". Space-time contains gravity only if there is matter present. There are models that describe a universe with no matter but they are not the ones we live in. In the one we live in, there is nowhere that has no gravitational field but the existence of a gravitational field has nothing to do with the quantum foam concept.

    DO NOT think that you are learning actual physics by reading pop science books, ESPECIALLY ones by Kaku.
     
  6. Oct 12, 2016 #5

    OCR

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    Well, Kaku "might" say nothing... but he'd still want 50 cents...[COLOR=#black].[/COLOR] :oldlaugh:
     
  7. Oct 12, 2016 #6

    But I assume what they are talking about has been observed in labs and in those labs there is gravity, there is the universe and what we exist in is made from something so it's not possible for there to be nothing in the universe. Or is just a theory?

    Im not a fan of Kaku, and other physicists talk about it too.
     
  8. Oct 12, 2016 #7

    phinds

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    "nothing" is an English language word, not well defined in physics. You need to be more specific.
     
    Last edited: Oct 13, 2016
  9. Oct 12, 2016 #8

    phinds

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    Why do you assume that? If it had been observed, do you think I would have said it is highly speculative?
     
  10. Oct 12, 2016 #9
    Thought you meant quantum foam is speculative.

    Aren't they referring to virtual particles which have been observed?

    They argue these particles come from nothing. But something must be causing them so they can't be coming from nothing.
     
  11. Oct 12, 2016 #10

    phinds

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    Yes, that is what I meant because that is what you were talking about.

    virtual particles are a mathematical fiction and have no physical existence so it would be REALLY hard to observe them.

    SInce they don't exist, it's meaningless to argue about what the come from. Also, you need to stop saying "they ... ". You are making factual claims with no citations other than mention of useless pop-sci books.
     
  12. Oct 13, 2016 #11
    In fact nothing has never been observed.

    Physics ultimately comes down to observation and measurement, in what units would nothing be measured and if you did measure a lump of nothing you then have a measure of something, ie a nothing measured is by definition a something.
     
  13. Oct 13, 2016 #12

    vanhees71

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    Where did they do this? I guess some popular-science book, and that's bad, because I'm sure both of them know better! There's nothing popping in and out of existence from the vacuum. By definition the vacuum is stable under time evolution, and it's Poincare invariant.
     
  14. Oct 13, 2016 #13

    A. Neumaier

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  15. Oct 13, 2016 #14
    Here is a simple (and grossly oversimplified) picture of how you get something from nothing. Take a vacuum state vector |0> and project it onto a basis that permits multi-particle states |s_i> where the state s_i consists of multiple particles in specific states. Then we get

    |0> = sum(s_i) |s_i><s_i|0>.

    (<s_i|0> is essentially a vector-coupling coefficient analagous to a Clebsch-Gordon coefficient.)

    An observer looking for s_i will then find it with relative frequency |<s_i|0>|^2. And this can be non-zero whenever the net quantum numbers of s_i all add up to those of the vacuum. For example, a particle/anti-particle pair with opposite momentum and vanishing net angular momentum. (Note that s_i is a non-space-time state, whereas a space-time observer will have to project s_i into a space-time frame.)
     
  16. Oct 14, 2016 #15

    A. Neumaier

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    This implies already that the inner product with the vacuum state is zero. Thus the observer will find s_i with probability zero.

    Note that a particle-antiparticle pair cannot decay into the vacuum or be created from it, because of energy conservation.
     
    Last edited: Oct 14, 2016
  17. Oct 14, 2016 #16
    Not so. It's a standard vector-coupling to a composite state that is indistinguishable from the vacuum (by which I mean it has the quantum numbers of "nothing") when seen only as a composite state.

    That assumes positive energy states only. That may be true for what an observer sees, but they may be seeing only part of s_i and the negative energy part may not be accessible to the observer, just implied by the vacuum requirement.

    Anyway, let's not take this notion too literally in a context of current theory. I was only trying to give an idea of how the concept of "something from nothing" could arise. Clearly, it is a challenging idea for our current knowledge.
     
    Last edited: Oct 14, 2016
  18. Oct 14, 2016 #17

    A. Neumaier

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    A k-particle state is orthogonal to j-particle state unless k=j. Since the vacuum is a 0-particle state it is orthogonal to any multiparticle state!

    And in relativistic quantum field theory, all states have nonnegative energy.
     
    Last edited by a moderator: Oct 14, 2016
  19. Oct 14, 2016 #18
    Only if they are states in the same basis. Nature knows nothing about an observer's basis. Any vector in one basis can be projected to a superposition in another. I made this projection explicit in my first post.

    Yes, but if we are going to imagine "something from nothing" we will inevitably run up against existing theories that will have to be modified. We already know this from trying to bring gravity into QFT.

    Even if we require that any observer can see only nonnegative energy, we must still understand that a universe with net vacuum quantum numbers has no observer except as an internal part of the system.
     
    Last edited by a moderator: Oct 14, 2016
  20. Oct 14, 2016 #19

    Nugatory

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    This might be a good time to mention the Physics Forums rule restricting discussion to currently accepted science.

    Also, please do remember that is a B level thread started by a poster who has been misled by the pop-sci treatment of QFT.
     
  21. Oct 15, 2016 #20

    A. Neumaier

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    But inner products are independent of the basis. And zero inner product in one basis (hence in all) implies zero probabilities.
     
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