The landmark detection of gravitational waves—first confirmed over a century after Albert Einstein's prediction—has revolutionized cosmology. With 11 events observed to date, these spacetime ripples have deepened our understanding of the universe. But if gravitational waves arise from gravity itself, are they immune to its influence, unlike other cosmic phenomena?
In general relativity, gravity isn't a traditional force but a curvature of spacetime caused by matter and energy. Einstein's field equations describe how mass-energy distribution warps spacetime geometry, dictating the motion of objects within it. By analyzing the local matter-energy content, physicists can precisely model these spacetime distortions.
The nature of a particle—whether elementary, composite, or massless—doesn't exempt it from gravity's grasp, as energy itself curves spacetime (with mass as one form of energy). Even photons, with zero rest mass, bend along curved paths, as seen in gravitational lensing, where light from distant sources is deflected by intervening massive objects.
Gravitational waves follow suit. Like photons, they are massless, propagate at the speed of light in vacuum, and carry energy—key factors that curve spacetime according to general relativity.
Similar to light, gravitational waves have wavelengths, energies tied to wavelength and amplitude, and experience redshift from the universe's expansion.
LIGO and Virgo have detected 11 gravitational wave events from black hole or neutron star mergers. The nearest, at 100 million light-years, shows clear stretching from cosmic expansion upon reaching Earth.
This redshift confirms gravitational waves are shaped by spacetime's curvature and the universe's expansion. Another key observation: the 2017 neutron star merger GW170817, which produced both gravitational waves and a gamma-ray burst arriving on Earth within 2 seconds—after 100 million light-years.
Related: How do gravitational waves escape from black holes?
Over 30 million seconds per year, this near-simultaneous arrival pins the speeds of light and gravity to within 1 part in 1015, proving they encounter identical delays from spacetime curvature.
Near the massive host galaxy of GW170817, both signals followed the same geodesics through intense gravitational fields to reach us.
Observations thus show gravitational waves redshift from expansion, trace photon paths, suffer equivalent delays and energy shifts in curved spacetime, just like other massless waves.
In quantum gravity theories, gravitons mediate gravitational interactions. Their self-interaction—evident in wave interference—aligns with general relativity's nonlinear nature, where waves scatter rather than pass freely.
This graviton-graviton scattering hints at a deeper quantum framework for gravity.
