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Virtual photons

I know what a virtual image is. I have been an optical engineer in a prior incarnation. But I do not know what a virtual photon might be.

For the speed of gravity determination they are using massive but highly sensitive devices. If something heavy moves, like a supernova, they might be able to get a measurement. I think they are waiting for a suitable event so they can get some numbers. Until then they are detecting neutrinos and doing Aetvos experiments with artificial satellites. Maybe we will be the generation that gets a clue.

I know what a virtual image is. I have been an optical engineer in a prior incarnation. But I do not know what a virtual photon might be.

In my layman's-level grasp of things (someone more qualified can always clean up my mess later on):

In the quantum scheme of things, a field is a region of space in which some property is altered. In the case of gravity, it's the geometric curvature. In the cases of electricty and magnetism, it's the permittivity and permeability, respectively.

When the object creating the fields is moved, it exchanges virtual particles--"vector bosons" of the force--with the surrounding space so the fields can change. (This gets around spooky "action at a distance" ideas of Newtonian/classical physics.) Virtual particles differ from their real cousins in that they're free. They don't really have to be accounted for in the mass/energy balance sheet of things because their energy is small and they don't exist for long. They're basically quantum hiccups.

Electric and magnetic fields are aspects of the force of electromagnetism; thus the vector boson is the photon. The presumed vector boson of the gravitational force is the graviton. Why it is presumed to propagate at photon speed, I'm not sure, but it probably has to do with relativity theory.

"For the speed of gravity determination they are using massive but highly sensitive devices. If something heavy moves, like a supernova, they might be able to get a measurement. I think they are waiting for a suitable event so they can get some numbers. Until then they are detecting neutrinos and doing Aetvos experiments with artificial satellites. Maybe we will be the generation that gets a clue."

The Speed of Gravity may not be found by pursuing General Relativity experiments with satellites in Space.

http://www.nasa.gov/centers/marshall/news/news/releases/2005/05-066.html

After a year in orbit, Stanford University and NASA's Gravity Probe B has detected no distortion of Space/Time. Results will be released in late 2006.

If you're a big fan of Einstein, better hold your breath.