Is there any experimental data supporting the existence of gravitons? If there is, how fast
do these wave/particles travel? Are they similar to photons?
Asked by: John Shultz
Answer
Gravitons are indeed very similar in concept to photons. Both represent the quantized field
of a fundamental force: photons the electromagnetic field and gravitons the gravitational
field. Both are forces with infinite range -- hence both involve massless particles that
travel at the speed of light. The field equations governing the propagation and interaction
of these particles are different, of course, but the basic properties I just described are
the same.
Since these are quantum objects governed by quantum field theory they have no 'action at a
distance.' This is by design. Quantum field theory is what's called a local theory: all
interactions take place at a particular space-time point. This can be represented by
space-time diagrams that look (reassuringly) like particles bouncing off one-another (the so
called 'Feynman diagrams'). This is no accident -- gravitons and photons are constructed to
mediate forces. They travel from one of the interacting particles to the other, carrying
information about the electromagnetic or gravitational field disturbances at the speed of
light. Ultimately, all action in this formalism is local (and hence less troubling
philosophically). In short, the quantum description of the fundamental forces is
designed to do away with action at a distance.
Of course we know gravity exists -- so if you believe that gravity is described by a quantum
field theory we've 'detected' gravitons. But then that's not too satisfying. We can 'prove'
the existence of quantized photons by many different experiments, such as the photoelectric
effect. Unfortunately all such 'quantum gravity' detection experiments have not been
successful because gravity is much, much weaker than the electromagnetic force. Hence
detecting a single graviton is a real challenge!
P.S. Some people like to say that action at a distance lives on in the phenomena of
collapsing wave-functions. This kind of quantum mechanical process is not describable by a
quantum field theory (the effects are instantaneous, supposedly) so it cannot be local in the
sense I described above. You can read more about this on a previous PhysLink question:
about the concept of non-locality.
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '