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Question

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: http://www.physlink.com/ae133.cfm
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley


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