Long Summary
Gravity, the fundamental force that governs the universe, was once thought by Newton to act instantaneously across space. However, Einstein’s 1915 theory of general relativity redefined gravity as distortions in space-time that propagate as gravitational waves at the speed of light. This shift in understanding raised the essential question of how fast gravity actually moves.
Detecting gravitational waves proved to be a monumental challenge, requiring the development of advanced instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO). By measuring infinitesimal distortions in spacetime caused by passing gravitational waves, LIGO and related facilities around the world aimed to capture these elusive signals. The breakthrough came in 2015, when gravitational waves from colliding black holes were successfully observed, providing direct evidence for these ripples in the fabric of space-time.
To precisely measure the speed of gravity, scientists needed a celestial event emitting both gravitational waves and electromagnetic radiation simultaneously. Such an opportunity arose in 2017 when two neutron stars collided, producing detectable gravitational waves followed by a gamma-ray burst. The near-simultaneous arrival of these signals after traveling 144 million light-years allowed researchers to compare their speeds with extreme accuracy.
Analysis showed that gravity and light traveled nearly at the same speed, with any difference being less than one part in a quadrillion. This remarkable result confirmed Einstein’s prediction that gravitational waves propagate at the speed of light. Minor timing differences were accounted for by the physical processes occurring just before and during the neutron star collision, offering a consistent explanation for the observed slight delay of electromagnetic signals.
This discovery marked a milestone in multi-messenger astronomy, showcasing how combined observations of gravitational and electromagnetic waves can unlock profound insights into fundamental physics. It solidified general relativity’s standing as the best description of gravity and opened new avenues for exploring cosmic phenomena. The precise measurement of gravity’s speed not only affirms our theoretical understanding but also enhances our capability to probe the universe’s deepest mysteries.
