Recent gravitational wave detections have allowed physicists to confirm with greater and greater precision what Einstein predicted over 100 years ago in the theory of general relativity: that gravity does not act instantaneously as Newton thought, but instead propagates at the speed of light.
"The speed of gravity, like the speed of light, is one of the fundamental constants in the Universe," Neil Cornish, a physicist at Montana State University, told Phys.org. "Until the advent of gravitational wave astronomy, we had no way to directly measure the speed of gravity."
Over the past few months, physicists have made very rapid progress in bounding the speed of gravity using gravitational wave observations.
Initially, the first LIGO detections of gravitational waves constrained the speed of gravity to within 50% of the speed of light.
In a paper published last week in Physical Review Letters, Cornish and his coauthors Diego Blas at CERN and Germano Nardini at the University of Bern have combined the first three gravitational wave events reported by the LIGO and Virgo collaborations, allowing them to improve the original bounds to within roughly 45% of the speed of light.
Just two days later (and after the physicists mentioned above wrote their paper), another paper was published in The Astrophysical Journal Letters by the LIGO and Virgo collaborations, whose authors are affiliated with nearly 200 institutions around the world. By using data from the gravitational waves emitted by a binary neutron star merger detected in August, they were able to constrain the difference between the speed of gravity and the speed of light to between -3 x 10-15 and 7 x 10-16 times the speed of light.
