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Insights Vacuum Fluctuations in Experimental Practice - Comments

  1. Jan 19, 2017 #1

    A. Neumaier

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  2. jcsd
  3. Jan 19, 2017 #2
    Sir, you write:
    "what fluctuates in the experiment is the electro-optical signal detected, not the vacuum."

    Sir, by what experimental/observational means can we discern that the fluctuations are not part of the Vacuum?

    It's one thing to say that "vacuum fluctuations" are an unsupported assertion, but it's another thing to be able tor rule them out, particularly by experimental means. How can this "unsupported assertion" be ruled out, experimentally?
     
  4. Jan 20, 2017 #3

    A. Neumaier

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    Claims require proof, otherwise they can be ignored. That's the rule in science. Nothing is ruled out in science; the demarcation line between science and speculation is the proof, not the disproof.

    The electro-optical signal is the only thing measured, and it exhibits fluctuations. Thus they are fluctuations of the signal, not of the vacuum.
    To understand what this means consider fluctuations of a visual signal seen on an oscilloscope in an ordinary experiment, and someone claims that these are a visual proof of fluctuations of the vacuum. Nobody would take such a claim serious without a proof.

    To argue that fluctuations of the vacuum are measured one would have to give theoretical evidence that the signal fluctuates in the same way as the vacuum. Lack of this evidence is enough to reveal the claim as pure speculation.
     
  5. Jan 20, 2017 #4
    Thanks Arnold!
     
  6. Jan 22, 2017 #5

    mfb

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    Nice article!
    Sometimes we would be happy to have a 10% accuracy...
     
  7. Jan 22, 2017 #6

    A. Neumaier

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    I know. I was thinking of optimistic figures (low lying baryon spectrum to ##1-9##%), and deliberately used the vague expression ''few''.

    As you probably know (I write this for @atyy who thinks no continuum limit is needed), a lot needs to be done to get values that can be compared with experiment, not just calculations on a fixed lattice. From the paper just cited:
    The final results for the baryon masses in the infinite volume limit, together with their error margins, are given in Table 1 on p.38.
     
  8. Jan 22, 2017 #7
    I've been reading up on QFT itself and I am beginning to see where Neumaiur is coming from. Still not sure I have everything straight though. Actually positive that I don't lol, the majority of the math itself isn't the issue though. I'm currently on the Ultraviolet cutoffs and the imaginary i. Though I still need to work on the LSZ reduction formula.

    Point being is from what I've gotten so far I can see the validity behind this article. I've gotten far enough to realize what I thought of as vacuum isn't what I thought
     
    Last edited: Jan 22, 2017
  9. Jan 23, 2017 #8

    PeterDonis

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    Moderator's note: some off topic posts and responses have been deleted.
     
  10. Feb 20, 2017 #9
    I think virtual particles are OK in diagrams if they help solve the problem. But it is not OK to let virtual particles break the fourth wall, as is done with Hawking radiation. One virtual particle has positive energy and the other has negative energy. Hawking radiation relies on Maxwell's demon to ensure that the negative energy virtual particle always goes into the black hole and the positive energy virtual particle always escapes.
     
  11. Feb 20, 2017 #10

    mfb

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    The derivation of Hawking radiation does not use virtual particles at all. That is just a pop-science description that leads to misconceptions.
     
  12. Feb 20, 2017 #11

    Nugatory

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    We have another live thread on this misconception right now: https://www.physicsforums.com/threads/how-does-hawking-radiation-work.904630/
    The links in the first two replies are worth reading.
     
  13. Mar 6, 2017 #12
    I don't think that this question has been answered fully.

    Can you give us an example of an experiment(thought experiment) as YOU would imagine it, such that it would provide sufficient proof for vacuum fluctuations being "real" and being "directly" detected according to your standards? (You are allowed to use futuristic devices which do not exist yet, but may in the future)

    edit: In particular. What kind of experiment would you propose/imagine for -> "...one would have to give theoretical evidence that the signal fluctuates in the same way as the vacuum. "


    As far as i am concerned, real and direct is quite a mouth full. Special relativity for example is a nice and elegant theory which is capable of predicting some of our experiences of the world quite accurately as long as there is no gravity involved.
    In the end however, it is just a method to predict experiences. There are other theories/methodologies which have the equivalent predictive power to SR. We simply cannot know if spacetime out there is as we imagine it to be. Nor can we know _directly_ if particles or fields are "real" as we imagine them to be.
    All we CAN know is that by following the methodologies of the according/more successful theories, we can predict our experiences more accurately than following less successful theories.

    So to me, the question on if virtual particles are real or can be directly observed is nonsensical. What matters to me is, if the theory which is based on virtual particles and their fluctuations is capable of predicting what i will experience in the future more accurately or equally accurate as other competing theories.
     
    Last edited: Mar 6, 2017
  14. Mar 7, 2017 #13

    vanhees71

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    Since vacuum fluctuations are not existing (at least not accoring to standard relativistic QFT), you cannot even define an experiment to measure them. For the conception of measurements you need at least some model!
     
  15. Mar 7, 2017 #14
    I did not understand exactly, I'm sorry, but the topic is very interesting. For all I knew, the Casimir experiment would prove the existence of "a quantum vacuum." In addition, other authors, such as Feynman and Hawking argue explicitly a state of vacuum in which particles and anti-particles "virtual" is created and destroyed. In the second quantization are introduced proper operators of "creation" and "destruction. "
    Now the article casts doubt this description? How are and things?

    Thank you very much
     
  16. Mar 7, 2017 #15
    Can you show any textbook/peer reviewed article on QFT where they say that?
     
  17. Mar 7, 2017 #16
    This article for example:
    Particle Creation by Black Holes
    S. W. Hawking
    Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, England Received April 12, 1975

    https://www.brainmaster.com/software/pubs/physics/Hawking Particle Creation.pdf

    At page 202 :
    Just outside the event horizon there will be virtual pairs of particles, one with negative
    energy and one with positive energy

    but there are many other, very technical, and not simply informative, for example on the vacuum polarization, or the effect Lamb-shift etc.
     
  18. Mar 7, 2017 #17

    mfb

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    From Hawking about that picture:
    You can make up those things to describe something in English, but the actual calculations don't have any virtual particles or vacuum fluctuations.

    Same for vacuum polarization, Casimir effect, Lamb shift and everything else. Sometimes virtual particles are a nice (but not necessary) tool in calculations, but that is different from vacuum fluctuations.
     
  19. Mar 7, 2017 #18
    And all of them treat virtual particles as a mathematical tool, not physical 'reality' (whatever that means). And that is the whole point A. Neumaier is bringing up.
     
  20. Mar 7, 2017 #19
    does not convince me. So whatever it is nothing more than a mathematical tool. Even quarks have no physical reality, are only mathematical tricks, but then whatever. Even the light, we have only the equations of Maxwell there is no other reality than the Maxwell equations. Has anyone ever seen a "photon"? or a "quark"? What is the reality of a photon? Only mathematical tool...
     
  21. Mar 7, 2017 #20

    PeterDonis

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    Yes. We have detectors that can detect single photons (they're called photomultipliers). We ran experiments called deep inelastic scattering experiments in the 1960s where we shot high energy electrons into nuclei and watched them bounce off quarks; that's how the quark model was developed.

    But now ask the question: are photons and quarks "particles"? I would answer no: "particle" is just a mathematical tool we use to construct our models. Photons and quarks are photons and quarks. They are not little billiard balls or pointlike masses. But there are real things there that we see in experiments like the ones I mentioned above, and "photon" and "quark" are the names we give to those real things.

    So if you want to say that "virtual particles" are the name you will give to "whatever real thing it is that explains the Casimir effect, Hawking radiation, Lamb shift, etc.", sure, you can do that. But then you are using the term "virtual particles" to describe something that isn't a particle any more than photons or quarks are particles. The term "vacuum fluctuations" is really no better in that regard, because it invites the inference that whatever it is is fluctuating, like a wave--but it isn't a wave any more than it is a particle. It is something for which we have no intuitive analogy at all; it's not like anything we have an ordinary language word for.

    This is a major reason why many (like myself) keep emphasizing that to really understand physics, you have to use math, not ordinary language. There is no dispute about the math: we know how to construct mathematical models that make accurate predictions about all of the experiments I referred to above. The only disputes are about which ordinary language words are the "right" ones to use to refer to the physics; but that dispute is ultimately pointless because no ordinary language words are "right".
     
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