by Clifford Will

Clifford Will (http://www.phys.ufl.edu/~cmw/) is Distinguished Professor of Physics at the University of Florida and Chercheur Associé at the Institut d’Astrophysique de Paris. Until the end of 2018, he is Editor-in-Chief of CQG.

According to general relativity, the gravitational interaction is propagated as if the field were massless, just as in electrodynamics.   Thus the speed of gravitational waves is precisely the same as the speed of light, a fact spectacularly confirmed when gravitational waves and gamma rays from the binary neutron star merger event GW170817 arrived within 1.74 seconds of each other, even after traveling for 120 million years.

But some modified gravity theories propose that the field could be massive, so that gravitational waves might propagate more slowly than light, and with a speed that depends on wavelength.   The shorthand term for this is a “massive graviton”, although quantum gravity plays no role in this discussion.  This is entirely a classical phenomenon.

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