# Virtual particles

I have a few questions.

Why is it that detection of a virtual particle would mean that it cannot be virtual? What is it exactly about the detection that destroys the virtual nature of a particle?

I have read in several places that the idea of virtual particles is just an artefact of perturbation theory - if we could solve things exactly we wouldnt need to speak of virtual particles. So does this mean virtual particles dont actually exist, but rather are a mathematical tool that is necessary in perturbation theory? Are there really particles flying about that violate the relativistic energy equation?

Thirdly, virtual particles can have an indefinite energy for a short period of time due to the energy time uncertainty relation. However in SR time is observer specific - does this then mean that in some frames of reference a particle is virtual, but in others it is not? Does this make sense?

tiny-tim
Homework Helper
faster than light?

Thirdly, virtual particles can have an indefinite energy for a short period of time due to the energy time uncertainty relation. However in SR time is observer specific - does this then mean that in some frames of reference a particle is virtual, but in others it is not? Does this make sense?

Hi Bobhawke!

I think that virtual particles travel faster than light …

(they certainly do "when two electrons exchange them")

if so, they would be faster than light for all observers.

Fredrik
Staff Emeritus
Gold Member
I have read in several places that the idea of virtual particles is just an artefact of perturbation theory - if we could solve things exactly we wouldnt need to speak of virtual particles. So does this mean virtual particles dont actually exist, but rather are a mathematical tool that is necessary in perturbation theory?
That's how I think of them, but I don't think that implies that they don't exist. If there are several possible mathematical models that predict the same outcomes of experiments, then who's to say that one of them is more "real" than the other.

bump!

Demystifier
Gold Member
1. I have read in several places that the idea of virtual particles is just an artefact of perturbation theory - if we could solve things exactly we wouldnt need to speak of virtual particles. So does this mean virtual particles dont actually exist, but rather are a mathematical tool that is necessary in perturbation theory?

2. Are there really particles flying about that violate the relativistic energy equation?
1. Yes.

2. No, at least not in the standard interpretation of quantum mechanics. However, in some versions of the Bohmian interpretation, something similar to that might have more sense.

George Jones
Staff Emeritus
Gold Member
does this then mean that in some frames of reference a particle is virtual, but in others it is not? Does this make sense?

A "particle" is virtual particle if it is off-shell, i.e., if

$$E^2 - p^2$$

does not equal the square of the rest mass of the particle. Since $E^2 - p^2$ is frame-invariant, this characterization is independent of frame.

A virtual particle is a particle that represents an internal line on a Feynman diagram. Although they are not constrained by the energy equation; P2 = -m2 , this doesn't mean they definatly are not on the mass shell, there energy can take any value.