Answerer

Hi guys, lets get straight to the point, we all know virtual photons carry mediate the electromagnetic field and that it cannot be observed experimentally. So, is there any apparent reasons why we can't observe any virtual photons(not real photons)? Or do they truly exist in the first place?

Oxymoron

Heisenberg's uncertainty relation in its usual form states that

u(x)*u(p) >= h

where u() is the uncertainty in a value, x is the position of a particle, p is its momentum and h is a constant. Some physical jiggery-pokery turns it into

u(E)*u(t) >= h

where E is the particle's energy and t is time. This says that if a particle of energy e is created, then if it vanishes before the time interval u(t) then it would be as if it had never existed. Voila! Virtual particles that appear and disappear without violating the conservation of energy.

Answerer

Well, I do know about the fact that virtual particle does not violate the conservation of energy. But thats not my question, my original question is about the reason behind our failure to observe or discover virtual photons experimentally or whether virutal particle do exist. I really hope that someone will have some answers here.

Corona688

They can't be observed, because they're virtual; they're not really there, in a physical sense... you CAN convert them to real photons, though, by adding energy.

Shake an electron, for example; real photons come flying out of it's virtual cloud. Some say that that 'charge' in a physical sense is simply the ability to make virtual photons.

Morat

We can observe virtual particles. Just not directly. There is a difference.

The Casimir effect, for instance, is one way of indirectly observing virtual photons.

I believe, but don't quote me on this, that they cannot be directly observed for the usual reasons. The act of observing one either poofs it away, or forces it to become a real photon.

muon

Yeah, this is basically right. The *effects* of virtual photons have been observed many many times. The Casimir effect is an example, as are the anomalous magnetic moment of the muon and the atomic Lamb shift.

What more do you want, though? If you were to observe a virtual photon directly, by definition it wouldn't be virtual, would it?


(Seriously, that's almost the definition of "virtual photon".)

Answerer

Ok, I think I am going to understand what you guys are trying to say. Is it because the act of 'direct' measurement intervene with the wavefunction of the virtual photons that cause it to become 'real' or 'disappear' that make us unable to observe it directly? Or am I misinterpreting all your explanations wrongly?

Corona688

That sounds right. On the quantum level, *ALL* observations involve interfering with what you're measuring afterall.

joedad

It confuses me as well.

From here

http://www2.slac.stanford.edu/vvc/theory/virtual.html