Today I read in the
Scientific American:
"According to quantum theory, even a perfect vacuum is not truly empty; it is filled with fluctuations as a result of the Heisenberg uncertainty principle. The fluctuations take the form of pairs of virtual photons. These photons are called virtual because, in an uncurved spacetime, far from any gravitational influence, they appear and disappear restlessly, remaining unobservable in the absence of any disturbance.
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Now, could somebody please explain to me how we know that there are virtual photons? I never knew Physics could be so imaginative.
....and I definitely have the wrong username.
Thanks to Eien_menu, Wertperch, Iwhosawtheface and Fruan for helping to shed some light on the issue.