2016-05-31 - Einstein and Gravitons
An interesting comment (reader's letter) published in the May 2016 APS (American Physical Society) News Bulletin :
"... I believe it is of general interest to point out that Einstein did not believe that there are gravitons, even though, as is well-known, he was the one who proposed light quanta that were later called "photons."
In general relativity, the so-called gravitational force is not a true force, unlike the Lorentz force in electrodynamics, but a pseudo-force. This is because one can make a coordinate transformation that will eliminate the gravitational pseudo-force at a point, and indeed, as Fermi later showed, it can be made to vanish along an arbitrary world line. Now when a photon strikes an electron, as in the Compton effect, it gives the electron a kick, so to speak, or more technically, a momentum transfer, and hence it exerts a true force that cannot be eliminated by a coordinate transformation.
So if there were gravitons, they too would give kicks to particles they interact with, and hence would exert true forces. Thus if one wants to stick with the view about the gravitational interaction that emerges from Einstein's general relativity, one has to reject gravitons. This no-graviton view of Einstein could help to explain why there has been no success in the numerous, and mathematically impressive, efforts to quantize general relativity. For Einstein then, gravitational waves are classical waves that one should not attempt to quantize.
However, this is by no means the end of the story, because these classical gravitational waves that LIGO so remarkably detected are solutions to the linearized Einstein gravitational field equations. If one plugs these linearized solutions into the exact field equations, one finds there are true energy-momentum source terms that result due to the nonlinear structure of the exact equations. The LIGO theoreticians have yet to tell us what is the physical meaning of these classical quantities ? Do they really exist, are are they just mathematical artifacts ? If they do, can they eventually be detected as well ?
Frank R. Tangherlini - San Diego, California