Modern astrophysics rests on Albert Einstein's twin theories of relativity. Special relativity describes physics at high velocities and is built on the postulate that the speed of light in a vacuum, about 299,792,458 m/s, is the same for all observers. General relativity, Einstein's theory of gravity, reframes gravity not as a force but as the curvature of spacetime produced by mass and energy, an insight grounded in the equivalence principle that gravitational and inertial mass are indistinguishable and that an accelerating frame looks the same as a gravitational one.
One dramatic prediction of general relativity is gravitational waves, ripples in spacetime produced by accelerating masses. They were first detected directly in 2015 by the LIGO observatory, which observed the signal from a binary black hole merger. In 2017, the LIGO/Virgo collaboration together with electromagnetic observatories detected GW170817, a neutron star merger that also produced a kilonova, marking the birth of multi-messenger astronomy, in which light, gravitational waves, neutrinos, and cosmic rays are observed together to study extreme events. Cosmic rays themselves are high-energy particles from space striking Earth's atmosphere, while neutrinos are nearly massless, weakly interacting subatomic particles that can travel across the cosmos nearly undisturbed.
General relativity also predicts gravitational lensing, the bending of light by massive objects that can act as cosmic lenses, and a more compact version called microlensing, in which a brief brightening of a star reveals a compact foreground object. Such effects are now essential tools for mapping dark matter. More speculatively, the theory permits exotic solutions like wormholes, hypothetical spacetime shortcuts that could in principle connect distant regions of the universe, though none has ever been observed.