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New (apparent) confirmation of virtual particles.

  1. Nov 19, 2011 #1
    Using a simulated rapid-moving mirror, scientists provided evidence the existence of virtual photons by coaxing them out of a vacuum by means of the dynamical Casimir effect:

    http://www.world-science.net/othernews/111117_casimir.htm


    As a layman, I will reserve judgement until the experts review the results. I would suggest, however, as a VP skeptic, that the procedure seem to me to be pumping a lot of energy into a system that might produce photons by driving quantum fluctuations 'over the top', so to speak, and that the fact the photon pairs matched predictions for VP might only show that the fluctuations match the bundle of attributes of photons at the moment of production, not that there were actual photons there.
     
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  3. Nov 20, 2011 #2

    ZapperZ

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    Then you should write a rebuttal to the paper, which has been https://www.physicsforums.com/showpost.php?p=3620514&postcount=141". And you have a daunting task to boot, because you are trying to falsify the whole concept of Cassimir forces and, indirectly, Quantum Field Theory.

    You may be skeptical of it, but you are using it in your modern electronics, whether you realize it or not.

    Zz.
     
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  4. Nov 20, 2011 #3
    Where am I 'trying to falsify the whole concept of Cassimir forces'?

    Unless you mean there is an ex cathreda dictum that Cassimir force—>VP. That's an unassailable argument I cannot defeat.

    The Cassimir force(s) is a fact. Even the Wikipedia article would be rigorous enough for most people to establish that.

    Anywho, I didn't notice the paper was already noted on PF. Nov 17th. Just wanted to bring it to people's attention, and offer an informal comment on the matter. I've noticed there is no dearth of VP skeptics in the past here that definitely do not take the unparsimonious, VP-ex-cathedra position lying down, and that may be expert enough to put my layman's feeble rebuttal into more rigorous form, or replace it with something else, or play devil's advocate or whatever.
     
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  5. Nov 20, 2011 #4

    alxm

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    http://www.nature.com/nature/journal/v479/n7373/full/nature10561.html" the actual Nature paper.

    As far as the debate on the 'reality' of virtual particles goes, this does not change anything. It's just yet another correct QED result. Nobody seriously disputes that QED works, or that the vacuum exists, or that it behaves quantum mechanically. I don't agree with the view stated in the article's abstract:

    But in the main text they switch to the term "vacuum fluctuations", which is a less contentious term, IMHO. The non-relativistic, non-quantized-field analogue of the Casimir effect is the London force, which is also often described as quantum mechanical "charge fluctuations" (even though it's time-independent). Similarity, it can also be calculated through perturbation theory invoking "virtual" excited states of the non-interacting atoms. Although people do not tend to say the London force "proves these virtual excited states exist" or that the London force is 'mediated' by them.

    Naturally you never measure the atoms in actual excited states (presuming they started out in their ground states). Nor have I ever seen someone invoke the "time-energy uncertainty principle" to explain why they're not. And I think it's quite possible that those virtual excited states could "become real" in some extreme circumstance, although it'd certainly be extremely difficult to get an electronic excitation out of such a weak force.

    Quantize the field, take relativity into account, and you get additional effects at short range, which is what the Casimir effect is. Quantizing the field and introducing relativity causes a measurable physical difference, and requires a somewhat different approach, but I don't see how that justifies a different conception of what virtual states are.

    I suspect it's all just because 1) popular accounts tend to talk of virtual particles in this way, and 2) A lot of physics students graduate to QED directly from basic non-relativistic theory, and miss how a lot of the same formalism (e.g. path integrals, second quantization) is applicable to ordinary non-rel QM.
     
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  6. Nov 20, 2011 #5
    What about PDFs and deep inelastic scattering during proton collisions? You can do flavor tagging of the resulting jets to know what virtual particles you scatter off in the proton. Certainly the jets produced in your detector show that there is some reality associated to virtual particles, aside from experiments involving the Casimir effect (ok, I guess this only proves vacuum fluctuations).
     
  7. Nov 20, 2011 #6

    qsa

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    That brings another question to my mind. if there is a difference between VP in vacuum and VP that are responsible for transmiting force. if you do that in the later case for gravity then you will have anti-gravity machine, that would make Mr. Spock very happy :biggrin:
     
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  8. Nov 20, 2011 #7

    ZapperZ

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    What did you just say? Did you even READ the actual paper, rather than just the news report of it?

    At what point do you bring in physics, rather than just semantics?

    Again, look in QFT. Then look at the physics we use to describe the semiconductor material that you use in your electronics. Tell me that the description of QFT (which makes use of such virtual particle interactions) are not used to accurately describe the behavior of these material.

    Now THAT is a "FACT".

    And since when is Wikipedia "rigorous"? You learn about all this through Wikipedia? Are you serious?

    Zz.
     
  9. Nov 20, 2011 #8

    alxm

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    I'm not a particle physicist, but I'm pretty sure you're still not measuring any virtual particles. You're measuring a scattering process, and you are visualizing the outcome of that scattering process in terms of the virtual-particle contributions used to describe the interaction.

    In my field (QC), we'll often say things like how a particular multi-electron system's ground state has a large contribution from the so-and-so (excited-state Slater) determinant. Which does not mean the ground state isn't the ground state. Or that we view the thing as being temporarily excited by "quantum fluctuations". It comes about because the states we're talking about are non-interacting. Since there's 'obviously' no such thing as a "non-interacting" system, these states are regarded as fictional. (In this context) It's just a convenient basis, since it's solvable and happens to become "real" eigenstates once the interaction is negligible.

    Swap an electronic ground-state for the vacuum's ground state, and suddenly we've got these virtual particles which are "popping in and out of existence". (Or just a "vacuum fluctuation"), and suddenly it's all mysterious and deep, rather than a superficial description of your mathematical description? As I said, I don't get it.

    I don't know QCD (or have much reason to), so I can't say a lot about the specifics there. But there's certainly reality to the quantized field. I'm not calling into question QED or QFT or the Standard Model. As I see it, there's reality 'behind it' in the sense that yes, you can use these non-interacting states to calculate an interaction. And yes, you can visualize and rationalize and discuss results in those terms. But it's another thing to say "This is the mechanism through which the interaction happens", rather than "how we describe it". That implies to me that the existence of virtual particles is a necessary postulate of QFT.
     
  10. Nov 21, 2011 #9
    I just was yesterday directed to http://www.staff.science.uu.nl/~hooft101/gthpub/GtH_Yukawa_06.pdf" [Broken] after asking a question about renormalization here at PF. In paragraph one it says: virtual particles, particles that are annihilated very shortly after being created.

    And yes, that is what they of course are. So could you guys just spare us that 'it is just popular talk for the masses' myth?

    Or let me ask this: what are the guys doing at CERN? Where are all these particles they are looking for? How and where does these often very massive particles live that they are searching for??

    QFT is a relativistic local theory. Of course do the intermediate states in interactions have a different interpretation in relativistic theories than in non-relativistic theories.

    Basically, as has been stated many times before, the whole distinction into 'real' and 'virtual' particles is a bit erroneous in QFT. Every particle that does not lived forever, that has been annihilated and absorbed, is not on-mass shell, is not a 'real' particle.
     
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  11. Nov 21, 2011 #10

    cgk

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    That is precisely the point. The common view that virtual particles are "real" is a semantic, which is not backed by any proper physics. So one should *not* claim that physics proves the existence of virtual particles, because it is simply not true. Virtual particles represent terms in a perturbation series, nothing more. That point is very obvious point in interacting non-relativistic quantum mechanics (e.g. quantum chemistry), where one could apply the very same interpretation to the calculations as is done in QED, with the same validity, (including virtual excited states, vacuum fluctuation etc), just to arrive at something which is way over the top and could be interpreted in a much simpler fashion otherwise. For this reason no one would even consider claiming the physical reality of, say, virtual orbitals, just because they are used in accurate QC calculations and those calculations agree perfectly with experiment. But somehow doing the very same thing in QED does not seem far off anymore to some people.

    alxm gave an different obvious example of that: The non-relativistic Casimir effect is nothing but the London dispersion force and it needs neither a QED vacuum nor relativity nor virtual particles for its accurate calculation[*]. This generally comes as a big surprise for people who view the force between metal plates as a proof of QED vacuum forces or virtual particles: That this very force can be calculated from non-relativistic quantum mechanics[*] without any reference to a QED vacuum. In fact, if you're doing it for molecules, you can easily obtain the 1/R^6 dispersion behavior with any quantum chemistry program by just telling it to make a CCSD(T) potential curve.

    [*] as long as you are not in the R -> 0 limit.

    So you think you can lecture a quantum chemist on electronic structure theory? Maybe you should have a look on what practioners of this field do before you go any further in that.
     
  12. Nov 21, 2011 #11

    Ken G

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    This is just not an issue that is resolvable, because there is no claim in physics that is not semantic, unless it looks like "experiment A will give result B." That's it, that's all that isn't semantic in physics. Because "semantic" means "conveys meaning", and meaning is not something that physics tests. So you say that no "proper physics" backs that virtual particles exist, but I can just as easily say that no proper physics backs that electrons exist, or that spacetime curvature exists, etc. etc., and the reason is very simple: proper physics is not about "backing what exists." That's not the purpose of physics, because regarding things as real, along the way of doing physics, is a device used by physics, not an end result of physics.

    Indeed, no one should think of physics as "the means to find out what exists." Instead, we should think of it as "the means to enter into certain pretensions about what exists in order to gain understanding of whatever actually exists, which is not understandable except via this device." So in that light, we may gain understanding by regarding virtual particles as existing in some pretend sense (different from the pretend sense in which real particles exist, hence "virtual"), and we may also gain some understanding by pointing out the ways in which we should not take their existence too seriously (in short, the reasons we should take the existence of real particles somewhat more seriously than virtual particles). No argument should look like "here's why virtual particles do exist", or "here's why virtual particles don't exist", because the place where these things exist is demonstrably only in physics books. So instead, it should look like "here's why community A gets value in regarding these things as in some sense existing, in context B" and "here's why community C gets value in rejecting the idea that these things exist, in context D." That's physics, that's just how it works, it's no different from arguing over whether spacetime curvature exists. It's just that for some reason, this basic structure of how physics works is obvious for Newtonian and Einsteinian gravity, but in virtual particles, it gets all controversial!
     
    Last edited: Nov 21, 2011
  13. Nov 21, 2011 #12

    alxm

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    You quote this, why? Is it because you don't think anyone who's actually a particle physicist would agree with what I'm saying? Because they certainly http://www.mat.univie.ac.at/~neum/physfaq/topics/virtual1" [Broken].
    The full quotes is: "The contributions of virtual particles (particles that are annihilated very shortly after being created) also tend to divergent expressions."

    I don't see where t'Hooft actually said they were 'real' things, rather than a mathematical abstraction. Which ones would he mean, by the way, the ones with infinite energy, or the ones in the terms with finite energy, or both?
    I didn't say that. I said it's jargon used to describe perturbation calculations, which in popular context tends to be given a literal physical interpretation, presented as physical fact, 'proved' by the calculations being correct. When in fact it is at best a philosophical question whether or not they 'exist'.
    They're looking for particles predicted by the Standard Model, etc. This is a complete straw-man, given that I already explicitly said I don't dispute the results of the Standard model.

    I'm afraid I can't answer "How and where does they live?", because I have no idea what you're talking about.
    What do you mean "of course"? There's no obvious reason why that'd be the case. How does the inclusion of SR in a physical theory support a particular philosophical interpretation of it?
    Besides which, you have lots of relativistic field theories and methods which do not make any use of virtual particles at all. So if that's what's 'real', you'll have to explain why different realities exist alongside each other.

    I summarized perturbation theory, roughly as: You neglect an interaction because it creates a mathematically-intractable problem, and then re-introduce that interaction in terms of the fictional non-interacting states.

    But you'd like to add that once it's applied to a relativistic theory, it's not just a convenient way of solving a mathematical problem, but 'proof' that the interaction physically occurs through a sequence of smaller interactions (some of which have infinite energy and should therefore be neglected(!)), that just happened to sum up in a way that happens to correspond mathematically to the method we used elsewhere, where it was just fictional.

    How fortunate!
     
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  14. Nov 21, 2011 #13

    alxm

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    You can say whatever you want. Actual philosophers of physics don't seem to agree with your position, since they've discussed http://www.springerlink.com/content/51r27u20u354mh5n/" quite a bit on whether "virtual particles are real", and neither side seems to agree with your idea that it's just as easy to say "electrons aren't real". (and they both make clear the mainstream position there, is that virtual particles aren't 'real' in the sense that I've been talking of)
     
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  15. Nov 21, 2011 #14

    Ken G

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    Then tell me: what experiment comes out A (to within any precision you like) if virtual particles exist, and not A if they don't? If you cannot give me such an experiment, then why do you think that physics adjudicates the issue of existence? And if you don't think that physics adjudicates the issue, that it is fundamentally an issue of philosophy of physics, then why do you think one answer or another is the correct one? Do you feel that philosophy generally is about correct answers? If any philosopher of physics thinks that physics adjudicates the question of what exists and what does not, they are a poor philosopher of physics indeed. If they feel that philosophy of physics adjudicates that issue, then they are not terribly clear on the history of philosophy.
     
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  16. Nov 21, 2011 #15

    alxm

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    I never said it did. I argued quite the opposite. They cannot be experimentally verified. Not because they can't be observed directly, but because the theory in which they exist does not actually postulate their existence.

    But an experiment can determine whether an electron exists or not. (I.e. test the hypothesis whether electrical charge is discretized or not) So by that standard they are not "just as real" in a physical sense as virtual particles are, which is what you claimed earlier.
    I do think it's a question of philosophy of physics, not physics. Which is exactly why I object to it being described as physics, and also object to the claim made in the article this thread is about, that the experiment 'confirms' their existence.
    Sigh. There's no point arguing with you - you didn't defend anything you said, and here you just went off on an attack against what you imagine they might've said in the papers I linked to, which you obviously didn't bother to look at.

    This is the quantum physics subforum. So I'm not going to debate philosophy. I was merely pointing out that these actual philosophers of physics, whatever they think about virtual particles, do not grant them the same ontological status as electrons. If you want to debate that, debate it with them, elsewhere.
     
  17. Nov 21, 2011 #16
     
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  18. Nov 21, 2011 #17
    Can you give me a reference for such calculation? I've always suspected this can be done without QED vacuum, but all textbooks I've read on QFT use QED vacuum as the starting point.
     
  19. Nov 21, 2011 #18

    cgk

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    yes, e.g.:
    http://prd.aps.org/abstract/PRD/v72/i2/e021301

    (I seem to remember that there also was a different, clearer article showing exactly the relationship between (QED) Casimir-Polder forces and (non-QED) London dispersion forces, but I can't find it at the moment. The only thing I can find are two experimental papers (which I can't access atm) which might give some hints:
    Appl. Phys. Lett. 93, 121912 (2008); doi:10.1063/1.2992030
    Appl. Phys. Lett. 92, 054101 (2008); doi:10.1063/1.2832664
    )
     
  20. Nov 21, 2011 #19

    cgk

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    I like your approach to physics, it is very effective. Maybe we should also introduce it into math; after all, all those pesky proofs are really annoying, so why don't we just hold a majority vote over whether or not a theorem is true? Or do you prefer an authority vote? You're not very consistent on that.

    Of course in chemistry that will not be necessary. Because, was we know, all theory of chemistry is well known, the problem is just that the equations are too complicated to ever be solved (Dirac, Nobel price in physics 1933). That is good, because it allowed some grandmasters of chemistry to find new favorite pastimes. For example, did you know that cancer can be avoided or even cured by just eating a few grams of vitamin C each day?(Pauling, Nobel Price in chemistry 1954, Nobel peace price in 1962). Doesn't make sense to you? Well, maybe that's some quantum interference gone wrong, because, as you know, things like consciousness, language, brain function etc are all consequences of quantum effects (Josephson, Nobel price in physics 1973), and those can easily be disturbed!
    That makes it even greater that we have all those authorities with big credentials telling us how the world works. And the best part? You got one for *every* viewpoint!
     
  21. Nov 21, 2011 #20
    Thanks I'll read it later. So what is the real smoking gun for QED vacuum? Would photon-photon scattering, if can be verified, be a decisive evidence?
     
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