Are All Photons Truly Virtual?

  • #1
Hill
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My understanding is (was) that "virtual particles" is a computational concept used in perturbation calculations in QFT e.g. in Feynman diagrams. This understanding is in conflict with the following note in Quantum Field Theory for the Gifted Amateur by Tom Lancaster and Stephen J. Blundell:
All photons that we detect actually interact with electrons in detectors such as the eye. They must all then, in some sense, be virtual! How can this be? We know that particles that are offshell have the range over which they can propagate limited by the extent to which they’re off-shell. If we see photons that have travelled from distant stars they have to be pretty close to being on-shell. We’ve seen before that when a particle is on-shell we hit the pole of the particle’s propagator. Therefore photons from Andromeda, visible on a moonless night, must be so close to the pole that there can’t be any observable effects from being off-shell. (p.348)
and again,
we know that virtual photons (and remember that all photons are, to some extent, virtual photons) couple to a fermion line at both ends of their trajectory. (p.362)
Is it so? Can't similar reasoning be applied to other particles?
 
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  • #2
Hill said:
Is it so?
I would say this source is taking a viewpoint that is, to say the least, not the usual one. The usual viewpoint is that, since the photons we see coming from distant objects are, as far as we can detect with our most accurate instruments, on shell, we should treat them as on shell--i.e., real, not virtual.

Also, the whole concept of "virtual particles" comes from perturbation theory (which is also where things like the propagators that are talked about in the quote you gave come from). But perturbation theory is just an approximation. There is no good reason to treat it as fundamental and to claim that, because perturbation theory says that only a photon that travels an infinite distance and is never detected is truly on shell, we should treat that as actual fundamental truth.

Hill said:
Can't similar reasoning be applied to other particles?
No. Even if the reasoning were valid for photons, photons get absorbed when they are detected. Particles like electrons, protons, neutrons, atoms, molecules, etc. don't. So the reasoning doesn't apply to them since it depends on photons being absorbed when they are detected.

That said, what I said above applies just as much to other particles as to photons. The electrons, protons, etc. that we actually detect are, as far as we can tell with our most accurate instruments, on shell just as the photons that we detect are. So the experimental facts are that we detect real particles, not virtual particles or "almost real but still a little virtual" particles. And perturbation theory is an approximation for all particles, not just photons.
 
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  • #3
PeterDonis said:
Even if the reasoning were valid for photons, photons get absorbed when they are detected. Particles like electrons, protons, neutrons, atoms, molecules, etc. don't. So the reasoning doesn't apply to them since it depends on photons being absorbed when they are detected.

That said, what I said above applies just as much to other particles as to photons. The electrons, protons, etc. that we actually detect are, as far as we can tell with our most accurate instruments, on shell just as the photons that we detect are. So the experimental facts are that we detect real particles, not virtual particles or "almost real but still a little virtual" particles. And perturbation theory is an approximation for all particles, not just photons.
Thank you. Seems to me then that that reasoning still could be valid for gluons. Are all gluons virtual?
 
  • #4
Hill said:
My understanding is (was) that "virtual particles" is a computational concept used in perturbation calculations in QFT e.g. in Feynman diagrams. This understanding is in conflict with the following note in Quantum Field Theory for the Gifted Amateur by Tom Lancaster and Stephen J. Blundell:

and again,

Is it so? Can't similar reasoning be applied to other particles?
A similar note appears in the Griffiths book on particle physics:
Actually, the physical distinction between real and virtual particles is not quite as sharp as I have implied. If a photon is emitted on Alpha Centauri, and absorbed in your eye, it is technically a virtual photon, I suppose. However, in general, the farther a virtual particle is from its mass shell the shorter it lives, so a photon from a distant star would have to be extremely close to its “correct”mass; it would have to be very close to “real.” As a calculational matter, you would get essentially the same answer if you treated the process as two separate events (emission of a real photon by star, followed by absorption of a real photon by eye). You might say that a real particle is a virtual particle which lasts long enough that we don’t care to inquire how it was produced, or how it is eventually absorbed.
 
  • #6
The statement is 100% correct, and 100% useless, and not intended to enlighten. In a less charitable moment, I'd characterize it as "look how clever I am!" You can see other examples of this behavior here and elsewhere.

A real photon is a quantization of a plane wave solution. A plane wave is an approximation, and this approximation has no beginning or end. One could say photons, therefore "don't exist", but in the same sense that frictionless planes, stretchless ropes, perfect spheres, etc. don't exist. Like these, real photons are useful idealizations.

This can be countered with "But you'll might make a mistake in the 65th decimal place." And I would say this is exactly how seriously to take this argument.
 
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  • #7
Hill said:
Seems to me then that that reasoning still could be valid for gluons.
Meaning, because we never observe gluons? (Or quarks, for that matter?) Not really, because the reason why we can't observe quarks and gluons directly, the short range behavior of the strong interaction, also means perturbation theory doesn't work. And if perturbation theory doesn't work, the whole concept of "virtual particles" goes out the window anyway.

Hill said:
Are all gluons virtual?
No. See above.
 
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