Why Can't Detected Particles Be Virtual?

[Bobhawke]

On the wikipedia page about virtual particles it says:

"If a single particle is detected, then the consequences of its existence are prolonged to such a degree that it cannot be virtual."

Could someone explain to me why this is true, ie why is it that if we detect a particle it cannot be virtual?

 

[Tac-Tics]

On the wikipedia page about virtual particles it says:

"If a single particle is detected, then the consequences of its existence are prolonged to such a degree that it cannot be virtual."

Could someone explain to me why this is true, ie why is it that if we detect a particle it cannot be virtual?

As I understand it at least, virtual particles are, by definition, particles which pop in and back out of existence between classical states. You shoot two classical electrons at each other. They exchange a photon and their momenta are affected. We collect the electrons a short while later when they hit a detector.

But the photon never hits a detector. The photon is never measured. It was not something we "put into" the system, not was a it a by product. It's a ghost. In fact, the electrons didn't even exchange a single photon. All possible interactions between the two electrons occurred, weighted by their appropriate amplitudes.

 

[Naty1]

Read a little further in the Wikipedia article and it also says:

There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields. As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles.

So your quoted statement while accurate is subtle and easy to misinterpret. In classical physics the expalantion makes no sense.

An alternative way to think about it is that virtual particles appear in pairs which cancel...if you are able to detect one of the pairs, it's already real and it's partner annihilated...an example is right outside the horizon of a black hole...one particle is absorbed and disappears and presto chango it's partner becomes real and emitted as a bit of energy...

 

[Vanadium 50]

An alternative way to think about it is that virtual particles appear in pairs which cancel...

I don't think that's a very good way to think about it. If an electron and a neutrino scatter through the exchange of a virtual W, there is no "pair".

 

[yiannis235]

You have to learn the meaning of off-shell and on-shell particles.A particle on the mass shell satisfies the relation p^2=m^2 where p and m are the momentum and the mass of the particle.The propagator of a particle is 1/(p^2-m^2) (the denominator at least).virtual particles are of-shell and that is why they don't propagate...