In 2017, detectors picked up a gravitational wave from the collision of two neutron stars. Just 1.7 seconds later, telescopes saw a flash of gamma-ray burst from the same spot. That tiny delay became a treasure for physicists.
It allowed a test of whether gravitational waves and light travel at the same speed. If the speeds differed, the time gap would have been larger. The authors explored what happens if special terms that break spacetime symmetry are added to the equations of gravity. Such additions are like invisible cracks in the smooth fabric of the universe. They could make gravitational waves behave oddly: either all waves slow down equally, or different types spread apart in speed, like light in a crystal.
Using this event and the first direct observation of gravitational waves, GW150914, from black holes, scientists calculated limits for such "cracks." It turned out the additions must be vanishingly small — on the order of 10⁻¹⁴ or even smaller. Otherwise, we would have noticed a mismatch in the signals. This is like trying to hear a whisper at a rock concert: any noticeable distortion would immediately give itself away. The work outlines the boundary up to which Einstein's standard gravity remains unshaken.
Gravitational waves from GW170817 and light from the gamma-ray burst traveled for 130 million years, yet arrived just 1.7 seconds apart — as if two runners started at the same time and, after a marathon around the Earth, finished with a gap the thickness of a hair.