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Are virtual particles really there? 
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#55
Dec310, 06:58 PM

P: 196

Lo and behold, Michael Peskin answered me, too!!! Physcists are nice people!
His answer 


#56
Dec310, 07:30 PM

P: 196

And yes, now it is 100 percent clear to me how the Coulomb potential works!
A bit strange that PF could not answer me that, but instead you people confused me here a fair amount. 


#57
Dec410, 02:46 AM

Sci Advisor
P: 5,451

So how would you now summarize (in your own words) the meaning of "the existence of virtual particles"?



#58
Dec410, 03:21 AM

P: 196

My own words would be those of selfadjoint, I guess, which I already quoted in post 8 of this thread.



#59
Dec410, 03:55 AM

P: 2,284

The Coulomb Potential is real, the way it's described is through Perturbation Theory which uses virftual particles to describe those internal lines on the diagram. It's just a tool to describe former however, not a reality causing it... how hard is that to grasp? 


#60
Dec410, 04:12 AM

P: 196

When you consider them as just a tool, thats's fine, too. I only wish that on the coming 'virtual particle threads' on this forum, the answers, especially coming from science advisors or mentors would be a bit more even, taking into account also the more orthodox view, the view which has been summarized by selfadjoint so eloquently. 


#61
Dec410, 04:15 AM

P: 2,284




#62
Dec410, 04:26 AM

P: 3,043

I really don't know what your problem is but it seems to go beyond what can be solved in a forum. 


#63
Dec410, 04:32 AM

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HW Helper
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P: 26,148

He is not talking about the myriad of virtual particles in perturbation theory (internal lines in a Feynman diagram). He is not talking about quantum field theory at all. He is simply describing transfer of momentum which of course is real! He says In quantum theory, the interaction of a quantum particle (e.g. an electron) with the Coulomb field extracts a definite quantum of momentum from the positive charge and transfers it to the negative charge. To describe this transfer of momentum, we say that a "virtual photon" passes between the positive charge and the electron.That's fine. Everyone agrees that a field is real, even though it's ghostlike. It has energy, it has momentum, it has various other attributes. And when it gives momentum to a particle, clearly it loses momentum, and that loss (or gain) of momentum is a genuine change in a genuine real physical attribute of the field. Peskin is simply saying that the momentum of a field is real, and therefore any change in momentum is also real, and if quantised can be considered as a particle. (similar to visualisation of real photons as "condensing out" of the electromagnetic field) This has nothing to do with quantum field theory. It does not even have anything to do with ordinary quantum theory, except for his stipulation that the momentum must be quantised (which makes it not only real, but also capable of being considered a particle) in other words, his description of the reality of this transfer of momentum stands perfectly well on its own, and the quantisation can then be added or not added to it, according to taste. Your virtual particles, in all their infinite glory, appearing in each interaction with all possible momentums and all possible displacements (including far into the past and future) have nothing to do with what Peskin is describing. Finally, you might like to ask Peskin whether he takes the same view of the virtual electrons that participate in that Coulomb interaction after all, they participate in it on exactly the same basis as the virtual photons do, and there's twice as many of them! 


#64
Dec410, 05:08 AM

P: 196

Tinytim, Nismaratwork, Trickydicky and whoever else, you claimed over and over again on this forum that virtual particles are just mathematical devices, fictious, unphysical, even silly, they only exist in the mathematics and so forth were your claims. Some people, Witten, Wilzcek, Callan, Peskin, Zee, selfadjoint and myself see it differently and include that concept of virtual particles in their inventory of reality. You do not, or do you? You do not have to like it that people have different opions than yours, but you could at least respect it. 


#65
Dec410, 05:51 AM

Sci Advisor
P: 912

Nice thread. Somehow I like these discussions which don't change a bit of the calculations you're doing, but puts them in a different framework (another such a question is about the meaning of diffeomorphism invariance. Guaranteed to have 100+ posts :P)
Another question which I now have when reading this thread is the following: Imagine that virtual particles are indeed the result of the fact that we can't analytically write down the generating functional Z[J]. If we could, we wouldn't need perturbation theory and virtual particles wouldn't be there. But imagine then that it is really impossible to solve for Z[J] analytically (I mean, by now we simply think we don't have the right mathematical tools to do it, right?). Would that change the "ontology" of virtual particles? 


#66
Dec410, 06:00 AM

P: 196




#67
Dec410, 06:26 AM

P: 661

It's always amusing to read that someone thinks they finally understand the ontology of nature. (And, yes, I know, it's even more amusing when someone suggests a new crazy scheme ;) )



#68
Dec410, 07:07 AM

PF Gold
P: 302

Yes, certainly. But there's a reason this question comes up again and again. Suppose we restate it in more useful terms rather than ask whether "virtual" particles are "real", we should ask, why is it that what happens is so accurately described by this bizarre kind of perturbation theory used in QFT? That's a question we don't know how to answer, but there's no reason to think it's not worth asking. No doubt we'd be much better off, theoretically, if "the whole context" of this discussion were "well defined ontologically." We're just not there yet. So  whatever's "really there"  how is it that it looks just as though an infinite number of interactions were taking place all the time "behind the scenes", so to speak? As though "everything that can happen, does happen" in some sense? Surely this is telling us something important about the world, about what "physical existence" means, that hasn't yet been clearly conceptualized, even though we have a mathematical description that works extremely well. After all, the only reason we have a Quantum theory is because Max Planck wasn't satisfied with inventing a heat equation that fit the observations very accurately. He wanted to understand why that particular equation worked  what it was telling us about the underlying nature of things  which led to the discovery of the quantum of action. Over a century later Planck's question is still outstanding, but it's still the right question, IMHO. 


#69
Dec410, 08:39 AM

Mentor
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#70
Dec410, 11:23 AM

P: 2,284




#71
Dec410, 11:48 AM

Sci Advisor
P: 5,451

Let me summarize why the question is problematic:
1) in QFT perturbation theory results in the invention of virtual particles; why do we not invent virtual apples in Newtonian theory of gravity? 2) w/o perturbation theory nobody would care about virtual particles 3) the discussion shows that especially the Coulomb potential and the virtual particles related to it are gauge dependend, so cannot be "real" in the sense that everybody has the same understanding; a "virtual photon" in Coulomb gauge and in axial gauge are two different "things" 4) in physics it's always difficult to explain what "is real" and "why it is the way it is"; what we can do is to calculate experimentally testable phenomena, but not an ontology 


#72
Dec410, 03:51 PM

P: 196

Since this virtual sticky thread became real sticky now, I like to contribute with two more answers I just received to my question which was the following:



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