Gravity keeps the same speed as light
The neutron-star merger GW170817 and its gamma-ray burst arrived within about two seconds after 130 million years of travel. Gravity and light share a speed.
Established2 min read
On 17 August 2017 LIGO and Virgo heard two neutron stars collide. 1.7 seconds later Fermi saw a gamma-ray burst from the same event. Over 130 million light-years, a 1.7-second gap means the speeds agree to about one part in 10¹⁵.
That single delay killed a pile of modified-gravity theories that wanted a slower or faster graviton. General relativity’s prediction — gravitational waves at c — is now a measured travel time, not only a symmetry.
Caveats
- The seconds of delay may include the engine of the burst, not only a speed difference. The limit is therefore an upper bound on any difference.
Sources
- 1Abbott B. P. et al.. Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger. The Astrophysical Journal Letters 848, 2017.