Are forces really carried by virtual particles?

1. Jun 23, 2006

Robert100

In the "Beyond the Standard Model" forum there is a thread entitled "What causes gravity???"

In the discussion Yogi writes:

What is the thinking among most physicists today on this issue? Are virtual particles considered certainly real, and the correct explanation for the 3 basic non-gravity forces? Is Yogi's point of view considered a mainstream minority position, or is it unique to him? When I see statements without citations I don't know how to evaluate the claims.

Robert

2. Jun 24, 2006

Meir Achuz

Virtual particles are an artifact of perturbation expansion, as one method of calculation. I think that too many students start to think of them as real. I would rate Yogi's opinion as a mainstream minority position with which I agree.

3. Jun 25, 2006

Farsight

I don't think the problem is limited to students. All too often I look at some paper talking at length about some mystery, but the opening paragraphs skip oh so quickly over a presumption involving particles. You really start to notice this after a while. It's like people presuming cents to be the "messenger particles" of money instead of a calculation quantum.

4. Jun 26, 2006

ZapperZ

Staff Emeritus
But you find this stranger than the classical "fields"? Think about it. You'll notice that this is as strange, if not STRANGER, than quantum fields.

I'm usually amazed when people try to either dismiss, or justify dismissing, quantum fields has being nothing more than mathematical artifact, without realizing that the VERY same argument can be made of the beloved classical fields. If anything, I have more of a justification to dismiss classical fields due to their shorcoming in making all of the predictions that we have verified so far in QED. For some odd reason, this point has been overlooked.

In condensed matter physics, we deal with many of these "quanta" fields that mediate many kinds of interactions. While there may be just 3 (or 4 depending if you buy gravitons), in condensed matter, we have phonons, spinons, magnons, polarons, axions, chargons, holons, etc.. etc. All of them, in one way or another, mediateds many different kinds of interactions. Are these "real"? How do you judge such a thing, and what makes you can tell? You just can't base this on simply a "matter of tastes" or "personal preference", which frankly, is what most of these types of discussion has been based upon. But how about using the criteria that THEY WORK! One may not realize it, but claiming "It Works!" is a freaking big deal in physics. You get a lot of recognition and funding when you can show that a theory or description actually matches very well with experiments. It's what most of us physicists life for!

So when questions like this are asked, I would like to ask something back: When you "accept" something to be "real" or "valid", especially in physics, what criteria do you use? Do you pay attention to experimental verifications that agree with a certain description, or do you only accept things that sit well with your "world view", which is what I call as a matter of tastes? Or have you even though about such a process on how you actually make your decision? I ask this because I'm almost sure that if one applies the same logic to object against "virtual particles", one could easily use that to object against classical fields also. So now what?

Zz.

5. Jun 26, 2006

I think ZapperZ's comments fit well into a classic misunderstanding I've often pointed out to people. Science doesn't say 'it *is* this', it says 'it *looks* like this'. Essentially this means - 'Science' is a collection of models, some backed by experiment, some waiting to be backed. If a model fits the theory, it works and therefore 'it *looks* like the model'.

6. Jun 26, 2006

Farsight

Please don't put words into my mouth Zapper. I've never mentioned fields. It's the constant reference to particles that I find strange. The phonons, spinons, magnons, polarons, axions, chargons, holons you mention all carry particle baggage. Look, this is only wikipedia, but note the quote:

http://en.wikipedia.org/wiki/Spin-charge_separation

In condensed matter physics, the spin-charge separation is an unusual behavior of electrons in some materials under some special conditions that allows the electrons to behave as a bound state of two independent particles, the spinon and the chargon.

Successful use of quanta isn't a problem. But it's like trying to understand money by fixing on the cents in your calculator. We have to look beyond that, without dismissing it as crackpot country.

Last edited: Jun 26, 2006
7. Jun 26, 2006

nrqed

However, "virtual particles" arise from applying *perturbation theory* and giving an interpretation to each term in the expansion in terms of the exchange of virtual particles. It's not clear to me that virtual particles would have any place in our understanding at all if people had been able to solve non-perturbatively field theories. It is in *that* sense that I personally feel that they are pure mnemonics.

8. Jun 26, 2006

ZapperZ

Staff Emeritus
What did you mean by "giving an interpretation"? Doesn't this mean as something similar as putting a "face" into some form of reality?

We don't need to invent virtual particles simply as a way to "visualize" perturbation expansion. People have been doing that quite well without it. Furthermore, a perturbative mathematical expansion does not require the HUP as the underlying physics. However, to what extent does the concept of virtual particles allows for predictions and theories BEYOND just perturbative expansion? Does the existence of virtual strange quarks in a proton that carry a fraction of the proton spin make it real and simply goes beyond just a mathematical mnemonics? And a classical field isn't?

Again, use the same rationality and you'll see that practically all of what you accept can fall under the same umbrella. This has been my point all along.

Zz.

Last edited: Jun 27, 2006
9. Jun 26, 2006

ZapperZ

Staff Emeritus
Then you have appeared to have read about something without knowing where it came from.

All of these "particles" came out of QUANTUM FIELD THEORY. The classical field that you have so ingrained in you are replaced in quantum theory with such "particle" field! So I didn't put any words into your mouth. It isn't my fault that you just didn't realize what you were invoking.

Why were you quoting me all this? I used to study spin-charge separation, so were you trying to educate me in what was going on there? Besides, the quote you gave me points out exactly what I've been trying to stress - that without such a model, you have no tools to come up with the scenario of spin-charge separation. Go beyond your beloved Wikipedia and look at the PHYSICS of such a thing and tell me that this isn't "real"?

And get this, before you can look BEYOND it, you have to UNDERSTAND what it is FIRST! So don't accuse me of dismissing something when you are doing it yourself, and all of this based on what, what you read in Wikipedia? You are able to make all of these conclusions based on what you read on the Web? And this doesn't scare you at all?

Zz.

10. Jun 27, 2006

CarlB

I find these arguments by various posters along the line of the above rather convincing. The implication is that it is at least conceivable that one could do without the gauge bosons. It's not like quantum mechanics still has its "locality" virginity intact.

So a fundamental theory need explain the elementary fermions and their interactions, nothing more -- provided one includes the gauge bosons as "interactions". This makes me like the density matrix formulation of quantum mechanics even more.

It makes my head spin. Or at least it makes me think of spin. Thanks.

But are there any papers in the refereed literature (or even arXiv) which describe this sort of thinking? It seems obvious, so it should be out there, at least in passing reference.

Carl

Last edited: Jun 27, 2006
11. Jun 27, 2006

Staff Emeritus
I have been trying to follow up some of Julian Schwinger's old ideas, but am being held back by a lack of free papers on the web. As I understand his "source theory" idea he was tryinng to build up QFT from phenomenological data without introducing unobserved things like the quantum vacuum and virtual particles. In order to avoid the vacuum he had to account for the Casimir effect, the great example of a physical effect allegedly due to the vacuum and virtual particles. He had an approach to doing that - accounting for Casimir without vacuum - and some of that is on the web.

12. Jun 27, 2006

CarlB

You can drop by your local physics library and download the source theory papers off of the Physical Review, where they were mostly published., or at least where most of the ones I read were from. Here's a start:
http://prola.aps.org/abstract/PR/v152/i4/p1219_1

I see that getting these papers from PRL is expensive. Of course if you're affiliated with a US university, they are probably available for free at the library. My local university, Washington at Seattle, is kind enough to allow visitors into the library. They have terminals where you can download the papers. The terminals have ports where you can insert a "flash memory" or flash disk, (USB electronic disk drive) and thereby walk away with an electronic copy. In fact, at the Physics library at the University of Washington, you can plug your memory stick into the monitor, courtesy of cables that I donated.

It is not at all unpleasant walking around a campus as an amateur. You probably are as interested in the subject as anyone there, you will find that you belong. If you care what other people think you might try dressing a bit nattier than I do, as I was once asked at the Mathematics library if I was there to fix the copier. [see picture at http://www.brannenworks.com ] It was all I could do to suppress the natural reply, "no, I'm here to break it". But with a flash memory, there is no reason to wear out the copier and bend up a bunch of 2" thick 50-year old compilations of obsolete physics papers.
[/edit]

I am a true fan of Schwinger, and I think that it would be time well spent for any physicist, whatever the level, to spend a day downloading and reading Schwinger papers. The papers by Kimball Milton are of course the place to start as they include a complete set, if I recall. I should also mention that Milton is likely to give you advice by email for that sort of thing.

A few weeks ago I started a website devoted to Schwinger's "Measurement Algebra", http://www.MeasurementAlgebra.com and I will eventually get around to putting up a description of all his papers on these various subjects. The best source for this exquisitely elegant introduction to quantum mechanics is his book, "Quantum Kinematics and Dynamics" that is still printed and available at low cost.

My feeling is that Schwinger was exactly on the right path when he started out QKAD, but then he went from a density matrix formulation to a standard state vector formalism and from there ended up with what everybody else had, but with a slightly more complicated (and therefore less convincing) way of doing it. Oh, and I started a website devoted to the density matrix formalism too, but there are very few papers written in it that I have found so far, so I am writing one up: http://www.DensityMatrix.com

In addition to the brilliance of his own papers, Schwinger had a list of remarkable students. One thing I would like to say is that it seems that Schwinger had a good influence on the students who were around him because they seem to be very nice people who are working on interesting things and willing to talk about it with others. I've corresponded with LP Horwitz, who shares an interest in the Schwinger Measurement Algebra.

Also, the general idea that Schwinger was pursuing, that of trying to eliminate the infinities by stepping back to a more phenomenological approach, has been pursued recently by more modern authors. In particular, you might enjoy the ideas in:

Stefanovich (also see his online book on arXiv)
http://arxiv.org/abs/hep-th/0503076

I wish I had my copy of Peskin and Schroeder with me, there's a standard result of QFT where one finds that the diagrams that have no exterior lines can be ignored without modifying physical results. If I recall, they cancel out because of a division but it's been years since I looked at it. But if my memory serves me well, that one result is source theory in a nutshell.

Carl

Last edited: Jun 27, 2006
13. Jun 27, 2006

CarlB

You can drop by your local physics library and download the source theory papers off of the Physical Review, where they were mostly published., or at least where most of the ones I read were from. Here's a start:
http://prola.aps.org/abstract/PR/v152/i4/p1219_1

I am a true fan of Schwinger, and I think that it would be time well spent for any physicist, whatever the level, to spend a day downloading and reading Schwinger papers. The papers by Kimball Milton are of course the place to start as they include a complete set, if I recall. I should also mention that Milton is likely to give you advice by email for that sort of thing.

A few weeks ago I started a website devoted to Schwinger's "Measurement Algebra", http://www.MeasurementAlgebra.com and I will eventually get around to putting up a description of all his papers on these various subjects. The best source for this exquisitely elegant introduction to quantum mechanics is his book, "Quantum Kinematics and Dynamics" that is still printed and available at low cost.

My feeling is that Schwinger was exactly on the right path when he started out QKAD, but then he went from a density matrix formulation to a standard state vector formalism and from there ended up with what everybody else had, but with a slightly more complicated (and therefore less convincing) way of doing it. Oh, and I started a website devoted to the density matrix formalism too, but there are very few papers written in it that I have found so far, so I am writing one up: http://www.DensityMatrix.com

In addition to the brilliance of his own papers, Schwinger had a list of remarkable students. One thing I would like to say is that it seems that Schwinger had a good influence on the students who were around him because they seem to be very nice people who are working on interesting things and willing to talk about it with others. I've corresponded with LP Horwitz, who shares an interest in the Schwinger Measurement Algebra.

Also, the general idea that Schwinger was pursuing, that of trying to eliminate the infinities by stepping back to a more phenomenological approach, has been pursued recently by more modern authors. In particular, you might enjoy the ideas in:

Stefanovich (also see his online book on arXiv)
http://arxiv.org/abs/hep-th/0503076

I wish I had my copy of Peskin and Schroeder with me, there's a standard result of QFT where one finds that the diagrams that have no exterior lines can be ignored without modifying physical results. If I recall, they cancel out because of a division but it's been years since I looked at it. But if my memory serves me well, that one result is source theory in a nutshell.

Carl

14. Jun 27, 2006

Staff Emeritus
Thanks so much for the info & links, and I will follow it up as best I can, but I have to disabuse you on one point. I have no "local physics library"; I live in the academic boonies. I once obtained an old copy of the PR at my public library through inter-library loan, and it was kind of a hassle. I got a phone call when it came in (it was from Marquette U. in Milwaukee), and I had to dash in right then because they only had it for a day, copy the pages, and hand it back to them. I can do that, but gee, the web is so much kinder!

And the link you provided is an example of what I was talking about. Anything with prola.aps in its URL is for sale to non-APS members, not free. So it is a temptation to pay up and join to get the 10 or whatever it is free downloads that members are entitled to.

I'm away from home and my copy of P&S right now myself but I do have a hazy recollection of the passage you mention. I'll look it up when I get back.

The biography paper says that measurement algebra gives you Dirac theory, and that Schwinger used it (MA) for his introductory QM courses. Does it now handle QFT? Added - I see from your website that it has been modified to use quaternions. Is this for real or just the usual hopeful quaternion monster?

Last edited: Jun 27, 2006
15. Jun 27, 2006

Perturbation

$$\langle\Omega |T\left\{\psi (x_n)\cdots\psi (x_1)\right\}|\Omega\rangle =\lim_{T\rightarrow\infty (1-i\epsilon )}\langle 0 |T\left\{\psi (x_n)_I\cdots\psi (x_1)_I\right\exp\left[{\textstyle -i\int^T_{-T} d^4x H_I}\right]\}|0\rangle}\left(\langle 0 |\exp\left[{\textstyle -i\int^T_{-T} d^4x H_I}\right]|0\rangle\right)^{-1}$$

The diagrams in the numerator exponentiate. Those in the denominator exponentiate as a load of vacuum diagrams which cancel the vacuum diagrams of the exponential in numerator. Presumably that's what you're referring to, or to LSZ reduction maybe.

16. Jul 4, 2006

Staff Emeritus
Looking at this again, at home, I think it was actually the first introduction of the Ward identities that I was thinking of. You get them essentially by playing around just with the external lines.

17. Jul 15, 2006

Farsight

18. Jul 16, 2006

japam

virtual meson

are not supposed that , for example, the strong force is originated by the Yukawa virtual meson (originated from neutron and protons), that attracts the protons in the nucleus in such a way that they appear atracting one on another?
or this is a too old version that has been changed, (by which new theory?)

19. Jul 17, 2006

ZapperZ

Staff Emeritus
It's the old version. The strong force is not mediated by any meson.

Zz.

20. Jul 17, 2006

Staff: Mentor

The force between protons and neutrons in a nucleus is a "residual" of the basic interaction between quarks. It's somewhat like the force between atoms in a solid or liquid (the van der Waals force) being a "residual" of the basic electromagnetic interaction between the electrons and nuclei.

Nevertheless, I think I remember from my undergraduate nuclear physics course many years ago that in some situations you can model the interaction between nucleons as if it were due to pion exchange, with fairly good results.