Has there been any new evidence on the existence of Gravitons? My
Physics Professor told us that physicists think that they exist, but
have not actually seen them yet. Why is this?
There is indirect evidence for gravitational radiation. The most recent
Nobel prize in physics was given for the experiments that uncovered this
evidence. The evidence comes from measuring the spin of an object
called a "pulsar", which is supposed to be a spinning compact mass -
a neutron star, perhaps. The pulsar orbits around a companion star
that is nearby. The energy of the system was observed to decrease
slightly over a time of several years. The decrease appeared to be
consistent with the amount of decrease predicted by general relativity
from the radiation of gravitational energy.
We believe that if gravitational energy is radiated, then it must
be radiated in discrete quanta. These quanta would be "gravitons".
There is no direct evidence for gravitons to date.
Jack L. Uretsky
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Update: June 2012