Virtual particles

[wittgenstein]

Virtual particles
I'm confused. I have a degree in philosophy and physics is not my area of expertise. I have read about the subject so please don't give an answer fit for a 4th grader. What are virtual particles? Are they real or not? Are they some strange hybrid of reality and non-reality? And what about all this ' particles popping out of the void'? Does this mean that something can come from nothing?

[DrChinese]

Virtual particles
I'm confused. I have a degree in philosophy and physics is not my area of expertise. I have read about the subject so please don't give an answer fit for a 4th grader. What are virtual particles? Are they real or not? Are they some strange hybrid of reality and non-reality? And what about all this ' particles popping out of the void'? Does this mean that something can come from nothing?

You might want to check out: http://en.wikipedia.org/wiki/Virtual_particle

Then come back with more specific questions. They are "real" in many senses, but have a limited effective lifetime. Yes, they can be considered something out of nothing. Sort of. There is debate whether they should be considered "real" or mathematical devices that help to explain current physical theory. So be careful not to get hung up in ordinary definitions of words when ascribing properties to virtual particles.

Regardless, I would say the effects of virtual particles are quite real.

[Ben Niehoff]

Virtual particles and Feynman diagrams are conceptual tools for calculating scattering amplitudes. Physics only makes predictions about observable quantities. It does not discuss the philosophical question of "What is really there?"

If the Feynman diagram approach worked flawlessly for all 4 interactions, then we might have grounds to claim that the scenarios depicted in the diagrams are "what really happens". But as it is, I think we should hold out for a more complete theory.

[jtbell]

Are they real or not?

If they were "real", why would we call them "virtual"? :uhh:

I would say the effects of virtual particles are quite real.

(boldface added for emphasis)

To make this clear, this is different from the virtual particles themselves being real in an ontological sense.

[wittgenstein]

“If they were "real", why would we call them "virtual"? ”
Jtbell
That’s why I asked, “Are they real or not? Are they some strange hybrid of reality and non-reality?” If they were not real then one could say, “If they are not real, why would we call them virtual.” This leaves a hybrid real / not-real. I wanted to give all three possibilities so that the experts would have an exact question to answer. Mostly because all 3 answers (real, not real and real/not real) make no sense to me. But the experts have directed me to a better understanding.

[haushofer]

As far as I know virtual particles are a property of the fact that interactions are done perturbatively;eg, we can't calculate the interaction between 2 electrons EXACTLY. What we can do, is write down an expression which becomes more and more accurate after more terms. After a few terms in this expansion you encounter so-called "loops", and in these "loops" the momentum is not fixed: if you would associate a "particle" with these loops you would find that its energy is not fixed by the usual relativistic energy conditions.

What we measure, are the "external" particles which do obey the usual energy-relations: what comes in, and what comes out? These particles are called real. So in our case: the 2 electrons which come, "feel eachother", and are being repulsed as a consequence of that.

What we don't measure, are these loop-particles. They are a mathematical device of describing the interaction, but they don't obey the energyconditions and cannot be measured.