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Are virtual particles really there?

by wangyi
Tags: virtual paticles, zee
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kexue
#55
Dec3-10, 06:58 PM
P: 196
Lo and behold, Michael Peskin answered me, too!!! Physcists are nice people!

His answer
I am not sure what you mean by "fictitous".

Light gives a good example. Light is part of electromagnetism. It is carried by photons, individual particles that move from place to place at the speed of light. Photons can be created and detected individually, so
I assume that you consider them "real".

Another part of electromagnetism is the Coulomb potential. A negative charge is attracted to a positive charge at a distance. In elementary
physics, we say that the positive charge sets up an electric field, and the negative charge experiences a force when it interacts with this electric field. The positive charge receives an equal and opposite reaction force. 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.
The virtual photon carries

Energy < (momentum) x c

so formally it has negative mass. There is even a sense in which it
is transferred instantaneously or even goes backward in time, although other electrodynamic effects add to this one so that there is no violation
of causality.

The virtual photon is not a real particle, but it is certainly real, in the sense that the electron really does change its momentum in the encounter.

I hope that this makes the nature of a "virtual particle" clearer. To
learn more, I recommend the beautiful book by Richard Feynman: QED, the
Strange Theory of Light and Matter (Princeton U. Press, 1988).

Best wishes, Michael Peskin
kexue
#56
Dec3-10, 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.
tom.stoer
#57
Dec4-10, 02:46 AM
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So how would you now summarize (in your own words) the meaning of "the existence of virtual particles"?
kexue
#58
Dec4-10, 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.

Whether virtual particles are real or not is a moot question.

Here's the idea. In quantum mechanics nothing is really real unless you can observe it or measure it. In order to be observable, a particle has to have some minimum amount of energy for some minimum amount of time; this comes out of the uncertainty principle that says the product of those two things has to be bigger than a certain number.

So it's possible to conceive of a particle whose energy is not big enough or whose lifetime is not long enough to permit a true quantum measurement, but still both of them could be greater than zero. The world could be full of such particles, and the measurements would never show it.

Well, quantum field theory takes those particles seriously. It says they interact with observable particles, for example they make the electron which emits and absorbs them a bit heavier, and a bit more sluggish in motion, than it would be if they didn't exist.

Furthermore, QFT says that the virtual particles are the ones that carry the forces. For example with photons, the "real" photons make light, and other forms of electromagnetic radiation, but the virtual photons carry the electric force; a charged particle is charged because it emits virtual photons. And the other bosons, that carry the weak and strong forces, behave the same way. Real particles interact with each other by exchanging virtual bosons.

This is the story quantum field theory tells, and the justification, the reason you should at least consider beliving in it, is that it makes fantiastically correct predictions. That bit above where I said that interacting with virtual particles made the electron sluggish? It's called the anomalous moment of the electron, and the prediction, based on virtual particles, matches experiment to six decimal places.
Or more shortly those of Curtis Callan
The "virtual" particle is real enough, since its existence leads to perfectly measurable effects on "real" particles with which it interacts.
As I said I'm just a learner of quantum field theory, I have to rely on the judgement of others at my stage of knowledge.
nismaratwork
#59
Dec4-10, 03:55 AM
P: 2,284
Quote Quote by kexue View Post
My own words would be those of selfadjoint, I guess, which I already quoted in post 8 of this thread.



Or more shortly those of Curtis Callan


As I said I'm just a learner of quantum field theory, I have to rely on the judgement of others at my stage of knowledge.
One last time:

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?
kexue
#60
Dec4-10, 04:12 AM
P: 196
One last time:

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?
Nismaratwork, if Frank Wilczek is inclined to include that concept of virtual particles in his inventory of reality, l'm inclined to do the same.

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.
nismaratwork
#61
Dec4-10, 04:15 AM
P: 2,284
Quote Quote by kexue View Post
Nismaratwork, when Frank Wilczek is inclined to include that concept of virtual particles in his inventory of reality, l'm inclined to do same.

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, as proposed by selfadjoint so eloquently.
You're only hurting yourself here, but that's your choice... good luck.
TrickyDicky
#62
Dec4-10, 04:26 AM
P: 3,038
Quote Quote by kexue View Post
My own words would be those of selfadjoint, I guess, which I already quoted in post 8 of this thread.



Or more shortly those of Curtis Callan


As I said I'm just a learner of quantum field theory, I have to rely on the judgement of others at my stage of knowledge.
So now you admit that since post #8 (writen by a PF poster btw) you had your answer, and actually all of the responses you have gathered via e-mail are in essence a retelling of things that have been answered to you in this thread by tom and nismaratwork among others.
I really don't know what your problem is but it seems to go beyond what can be solved in a forum.
tiny-tim
#63
Dec4-10, 04:32 AM
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Quote Quote by kexue View Post
Lo and behold, Michael Peskin answered me, too!!! Physcists are nice people!
Peskin is not talking about the same virtual particles as you are.

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 ghost-like. 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!
kexue
#64
Dec4-10, 05:08 AM
P: 196
Peskin is not talking about the same virtual particles as you are.


Tiny-tim, 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.
haushofer
#65
Dec4-10, 05:51 AM
Sci Advisor
P: 910
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?
kexue
#66
Dec4-10, 06:00 AM
P: 196
Quote Quote by haushofer View Post
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?
Excellent question!
unusualname
#67
Dec4-10, 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 ;) )
ConradDJ
#68
Dec4-10, 07:07 AM
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P: 302
Quote Quote by tom.stoer View Post
I think the existence of virtual particles is more a question about "existence" than about "virtual particles"... My conclusion is that this discussion is an apparent problem as the whole context isn't well defined ontologically and it therefore does not really make sense to talk about the "existence" of virtual particles.

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.
jtbell
#69
Dec4-10, 08:39 AM
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Quote Quote by Born2bwire View Post
But if we keep posting in threads on virtual particles so that they stay at the top then we virtually have a virtual sticky.
I herewith take this virtual sticky across the event horizon, thereby converting it into a real sticky.
nismaratwork
#70
Dec4-10, 11:23 AM
P: 2,284
Quote Quote by kexue View Post


Tiny-tim, 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.
You lack the basic understanding of perturbation theory it seems, to understand that the responses you've garnered are not in support of what I'll laughingly call your thesis.
tom.stoer
#71
Dec4-10, 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
kexue
#72
Dec4-10, 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:

I'm a physic student with a quick question.

Are virtual/ off mass particles really out there, do they really exist or are they just mathematical artifacts of perturbation theory and thus fictious?

I would be very grateful for any answer.
the first very short answer from Steven Weinberg
They are, in your words, mathematical artifacts of perturbation theory. SW
the other from David Politzer
This is precisely the kind of question you should be asking as
you're learning about relativistic quantum mechanics. And in
trying to find answers, you'll surely learn quite a bit of
physics.

However, as you work on it, I suspect that the more of the
physics you understand, the less relevant the initial question
becomes.

I do not mean to be cryptic just for its own sake, but, in
finding answers, you'll first find that there are problems with
the question.

In particular, the most problematic words and concepts in your
note to me are: exist, fictitious, and real.

You probably already know that there is some quantum funny
business that concerns the relation of energy uncertainties and
time intervals (or at least those are the terms that are usually
used). So is your question just a matter of degree? Are rho
mesons real particles? Or are they just mathematical artifacts
whose role is to simplify our account of the behavior of "real"
particles? How about the rho''? Pions? W's? Protons are known
to live much longer than the current age of the universe. But
what if they're unstable, too? Are quarks mathematical
artifacts? Some people would insist yes. But where does that
get them? Some people will tell you that you can't construct a
gauge invariant, correct state of a single electron. In some
sense they're right, and one can imagine structuring all physical
calculations in terms of space-time correlations between gauge
invariant sources. But insisting on that is just plain foolish.

In practice, you will be in a better position to confront and
answer questions of the sort you asked me all by yourself once
you have mastered the rudiments of the theoretical calculations
under discussion and (very importantly) how those calculations
are used to confront the physical (i.e., real) world.

If you really and absolutely believe that quantum mechanics has
anything to do with uncertainties and probabilities and that wave
functions collapse upon the making of observations (as opposed to
all of those being conveniences for us who are not all that
smart after all), then you believe that quantum mechanics doesn't
apply to you. However, the only argument in favor of that
proposition is that it makes you comfortable. There is not a
shred of experimental evidence. Rather, there is an enormous
body of evidence to the contrary.

We are all Schroedinger's cats!

D.P.


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