Frank Wilczek on Virtual Particles and Summing diagrams....

[asimov42]

TL;DR

Wilczek seems to think they're real.

Hi all, - an initial apology - there are a large number of threads on virtual particles on the site, and I apologize for adding another one. I had two questions - on a related note, the guidance provided by @A. Neumaier's FAW on virtual particles has been highly valuable for a novice .

1) Upon doing a bit more reading I found this letter that was posted here on PF (written by Frank Wilczek) about the reality of virtual particles:

It comes down to what you mean by "really there". When we use a concept with great success and precision to describe empirical observations, I'm inclined to include that concept in my inventory of reality. By that standard, virtual particles qualify. On the other hand, the very meaning of "virtual" is that they (i.e., virtual particles) don't appear *directly* in experimental apparatus. Of course, they do appear when you allow yourself a very little boldness in interpreting observations. It comes down to a matter of taste how you express the objective situation in ordinary language, since ordinary language was not designed to deal with the surprising discoveries of modern physics.

This honestly bothers me - as a Nobel physicists, he's implying that he takes virtual particles as being real (although, yes, there's a mixed message - but "inventory of reality" is odd phrasing to me). In light of @A. Neumaiers FAQ this seems ridiculous - does anyone know of other quotes form Wilczek where he makes his position more clear?

2) Since I am not yet at the level where I can write down all the Feynman diagrams for a specific event - is there always an amplitude associated with an ingoing leg reaching the outgoing leg unchanged ... as in, since were summing over all paths (before renormalization, should there not always be one term that defined the event "nothing happened" ... hope that somewhat clear.

 

[Vanadium 50]

You are trying to learn physics by grabbing quotes from famous people. You will never succeed doing this.

The most basic property of real things is that they cvan be counted. Virtual particles cannot.

 

[PeterDonis]

I found this letter that was posted here on PF

Link, please?

 

[PeterDonis]

is there always an amplitude associated with an ingoing leg reaching the outgoing leg unchanged ... as in, since were summing over all paths (before renormalization, should there not always be one term that defined the event "nothing happened"

Yes. In fact, not only is that term always there in the perturbation series, it's always the leading term--the first one in the series. That's because it corresponds to a diagram with zero vertices and zero loops.

 

[asimov42]

Link, please?

Hi @PeterDonis - apologies, the link is here:
https://www.physicsforums.com/threads/are-virtual-particles-really-there.75307/post-3030488

 

[asimov42]

Yes. In fact, not only is that term always there in the perturbation series, it's always the leading term--the first one in the series. That's because it corresponds to a diagram with zero vertices and zero loops.

Thanks, I realized I should have been a bit more careful and said "an incoming leg reaching an outgoing leg unchanged".

 

[PeterDonis]

the link is here

Ok, so this isn't an actual paper or article by Wilczek, it's just what someone said Wilczek said when they asked Wilczek about virtual particles. So it's not really an acceptable source for PF discussion.

Note, btw, that in the quote from Wilczek's Nobel lecture in that same post, he says "speaking loosely" when talking about virtual particles.

As a matter of terminology, the term "real", without qualification, does not have an accepted scientific definition. It's a label that people like to use, but it doesn't correspond to any actual theoretical concept in any scientific theory.

 

[asimov42]

Ok, so this isn't an actual paper or article by Wilczek, it's just what someone said Wilczek said when they asked Wilczek about virtual particles. So it's not really an acceptable source for PF discussion.

Note, btw, that in the quote from Wilczek's Nobel lecture in that same post, he says "speaking loosely" when talking about virtual particles.

Thanks @PeterDonis - apologies, it's not a scientific source, so probably not valid to discuss. I just found it odd that in the letter he seems to give much more weighting to the 'reality' of the idea.

 

[PeterDonis]

I just found it odd that in the letter he seems to give much more weighting to the 'reality' of the idea.

Scientists will often say things in informal contexts that they know they would never get away with in an actual peer-reviewed paper. "Informal contexts" here includes pop science books published by reputable publishers; a letter (was it really a letter? I didn't get that from the post you linked to, all the poster said was that he "asked" Wilczek) is even more informal than that.

 

[asimov42]

Scientists will often say things in informal contexts that they know they would never get away with in an actual peer-reviewed paper. "Informal contexts" here includes pop science books published by reputable publishers; a letter (was it really a letter? I didn't get that from the post you linked to, all the poster said was that he "asked" Wilczek) is even more informal than that.

Indeed, it was apparently a letter - the poster (in that thread) had sent a bunch of email messages to various leading physicists asking a one line question about whether virtual particles are "really there" or just math... essentially. Perhaps he (Wilczek's) meant informally that it was useful in his mind to think of them as real - no way to know.

 

[asimov42]

Thanks, I realized I should have been a bit more careful and said "an incoming leg reaching an outgoing leg unchanged".

@PeterDonis, sorry - if I might just bother you for one more second - for, e.g., Coulomb scattering of a proton off another proton, the leading diagram would just be two lines I presume? Then further terms would be added for the interaction starting with a virtual photon line?

 

[PeterDonis]

for, e.g., Coulomb scattering of a proton off another proton, the leading diagram would just be two lines I presume?

Yes. That's the "nothing happens" case.

Then further terms would be added for the interaction starting with a virtual photon line?

Yes. Think of the terms as ordered by the number of vertices. The "nothing happens" diagram is the only one with zero vertices. The "single virtual photon line" diagram, the first one with an actual interaction in it (i.e., virtual photon with a vertex on each proton line) is the main one with two vertices (but not the only one--you should be able to find two others). Then you have diagrams with four, six, eight, etc. vertices (the four vertex diagrams include the first diagram with a loop in it).

Note, btw, that I'm only considering QED diagrams here, i.e., I'm treating the protons as elementary particles with no internal interactions (it's more common to consider electrons for this since they don't have any internal interactions at all, whereas protons do, they're just not relevant until you get to significantly higher energies).

 

[asimov42]

Note, btw, that I'm only considering QED diagrams here, i.e., I'm treating the protons as elementary particles with no internal interactions (it's more common to consider electrons for this since they don't have any internal interactions at all, whereas protons do, they're just not relevant until you get to significantly higher energies).

Gotcha! Thanks @PeterDonis , very helpful as usual.

 

[PeterDonis]

You're welcome!