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Question about Virtual Particles

  1. Apr 16, 2015 #1
    Wikipedia says that virtual particles can not be observed, they are a handy concept for understanding what happens in quantum interactions, annihilating each other before they can be detected as real particles. However it also says that under certain circumstances, if they are moved apart from each other quickly enough, they can turn into real particles instead of annihilating each other. As I understand this is the principle behind black holes Hawking Radiation, where one of the pair particles falls into the hole while the other one can escape it.

    My question is: Has the event of a virtual particle turning into a real detectable particle ever been experimentally observed?
     
  2. jcsd
  3. Apr 16, 2015 #2

    bhobba

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    No - because virtual particles don't actually exist. They are simply an artefact of the mathematical methods used called perturbation theory and something called a Dyson Series:
    http://en.wikipedia.org/wiki/Dyson_series

    Thanks
    Bill
     
  4. Apr 16, 2015 #3
    Admittedly I've never looked into Hawking Radiation. What would it mean then if it was actually observed? A quantum gravity behavior of the field which is just pictorially explained as virtual particles going astray, but not quite?
     
  5. Apr 16, 2015 #4

    bhobba

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    I am not sure of the point you are trying to make. But in the theory virtual particles lead to effects - that doesn't make them real.

    In fact there is another formulation called lattice gauge theory were they are absent and allows theoretical predictions. Trouble is it can only be done on a computer and hasn't as yet achieved the accuracy of the usual method.

    Thanks
    Bill
     
    Last edited: Apr 16, 2015
  6. Apr 16, 2015 #5
    I'm not trying to make a point, if I had one I wouldn't ask a question. Even Wikipedia manages the topic in this way and this virtual pair production becoming real is popular: https://en.wikipedia.org/wiki/Hawking_radiation

    If the effect of the - unreal - virtual particle is for it to become real, one wonders about the reality status of the former a bit more, if the explanation is left at that. So I was asking what the field-theoretic approach actually means in this case.
     
  7. Apr 16, 2015 #6

    phinds

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    Hawking has stated that the whole "virtual particle" thing is not what actually happens in Hawking Radiation but rather is just a way to describe in English what can really only be described accurately in math. It's sort of an analogy, not a description of reality.
     
  8. Apr 16, 2015 #7

    bhobba

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    If you do a search you will find many threads on this forum explaining virtual particles are simply a mathematical artefact. They do not appear in other methods. They are not the cause of Hawking radiation.

    Thanks
    Bill
     
  9. Apr 16, 2015 #8
    Wiki also says:

    "The longer a virtual particle exists, the more closely it adheres to the mass-shell relation. A "virtual" particle that exists for an arbitrarily long time is simply an ordinary particle.
    However, all particles have a finite lifetime, as they are created and eventually destroyed by some processes. As such, there is no absolute distinction between "real" and "virtual" particles. In practice, the lifetime of "ordinary" particles is far longer than the lifetime of the virtual particles that contribute to processes in particle physics, and as such the distinction is useful to make."
     
  10. Apr 16, 2015 #9

    bhobba

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    Last edited: Apr 16, 2015
  11. Apr 16, 2015 #10
    Just to report that I already read those threads long before this one. I know about it. But since this conception is so pervasive, even on wiki etc... it does confuse me. For example that above Wiki quote is pretty bewildering, it seems to positively contradict everything this forum usually says.
     
  12. Apr 16, 2015 #11

    phinds

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    Yes, because Wiki is wrong. I don't know why more knowledgeable people don't get on there and fix it but they don't, or if they do, the ignorant come back and "fix" it back.
     
  13. Apr 16, 2015 #12

    bhobba

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    Anything written about QFT outside a QFT textbook is almost certainly WRONG. It is not an easy area and explaining without the proper technicaluties invariably leads to inaccuracies.

    Thanks
    Bill
     
  14. Apr 16, 2015 #13
  15. Apr 16, 2015 #14
    That wiki passage doesn't cite a specific source. It would be interesting to know where they got it from.
     
  16. Apr 16, 2015 #15
    Yes.

    http://arxiv.org/abs/1105.4714

    One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. While initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences, for instance producing the Lamb shift of atomic spectra and modifying the magnetic moment for the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed if it might instead be possible to more directly observe the virtual particles that compose the quantum vacuum. 40 years ago, Moore suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. This effect was later named the dynamical Casimir effect (DCE). Using a superconducting circuit, we have observed the DCE for the first time. The circuit consists of a coplanar transmission line with an electrical length that can be changed at a few percent of the speed of light. The length is changed by modulating the inductance of a superconducting quantum interference device (SQUID) at high frequencies (~11 GHz). In addition to observing the creation of real photons, we observe two-mode squeezing of the emitted radiation, which is a signature of the quantum character of the generation process.
     
  17. Apr 16, 2015 #16

    bhobba

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    Incorrect - as many threads on this forum explain - including our FAQ.

    Its not just this forum:
    http://physics.stackexchange.com/questions/4349/are-w-z-bosons-virtual-or-not/22064#22064
    For anyone still tempted to associate a physical meaning to virtual particles as a specific quantum phenomenon, let me note that Feynman-type diagrams arise in any perturbative treatment of statistical multiparticle properties, even classically, as any textbook of statistical mechanics witnesses.

    More specifically, the paper http://homepages.physik.uni-muenchen.de/~helling/classical_fields.pdf shows that the perturbation theory for any classical field theory leads to an expansion into Feynman diagrams very similar to those for quantum field theories, except that only tree diagrams occur. If the picture of virtual particles derived from Feynman diagrams had any intrinsic validity, one should conclude that associated to every classical field there are classical virtual particles behaving just like their quantum analogues, except that (due to the lack of loop diagrams) there are no virtual creation/annihilation patterns. But in the literature, one can find not the slightest trace of a suggestion that classical field theory is sensibly interpreted in terms of virtual particles.

    The reaon for this similarity in the classical and the quantum case is that Feynman diagrams are nothing else than a graphical notation for writing down products of tensors with many indices summed via the Einstein summation convention. The indices of the results are the external lines aka ''real particles'', while the indices summed over are the internal lines aka ''virtual particles''. As such sums of products occur in any multiparticle expansion of expectations, they arise irrespective of the classical or quantum nature of the system.

    Now can we move on please.

    Thanks
    Bill
     
    Last edited: Apr 16, 2015
  18. Apr 16, 2015 #17

    bhobba

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    Its a common misconception even amongst those that actually know QFT. But misconception it is.

    Thanks
    Bill
     
  19. Apr 17, 2015 #18
    So you have two answers to your question:

    No based upon semantic objections to your question.
    Yes from a well respected group of physicists, who made the observation themselves.

    You choose.
     
  20. Apr 17, 2015 #19

    bhobba

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    I choose logic from understanding what virtual particles are - as per the link I gave.

    But if you don't agree feel free to contact its author Professor Neumaier about it:
    http://www.mat.univie.ac.at/~neum/

    Thanks
    Bill
     
    Last edited: Apr 17, 2015
  21. Apr 17, 2015 #20
    This is the most complete answer so far. But the comments on it... There doesn't seem to be a definitive nail on the issue, at least for someone who is still slightly outside of the field. Here is a chat log where it's dissected:

    http://chat.stackexchange.com/rooms/5906/discussion-between-arnold-neumaier-and-user1247

    It seems very similar to the interpretational issues of QM. I guess that for you it's not. But it's hard to see. For example, the argument that "a single Feynman diagram is meaningless so virtual particles are unphysical" isn't definitively convincing: the diagrams could be seen as a weak form of superposition (okay they're not "states"), but we don't observe the pure state wavefunction either, only the measurement. So while classical fields are completely deterministic (and their Feynman diagrams with only tree levels remain formal) quantum theory is peculiarly probabilistic and involves superpositions, which we usually talk about as being "real".

    The other argument about virtual particles being absent in Lattice theories is more convincing. But this could be seen as just a different viewpoint that a theory offers, while virtual particles retaining value in the continuum picture.

    The main problem is that it's not just profane people that allow this idea, but even scientists in that very field. The CERN website says it. So since you put the issue at the level of something uncontroversial and established, it's confusing. The other threads about virtual particles don't go into this detail. Maybe we should talk about this more, not less. I don't agree with craigi in that many scientists have often been wrong anyway: it's called a "argument from authority" fallacy.
     
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