Gravitational Waves: Theory, Astrophysical Sources, and Detection

How they are generated

Because gravity couples to all forms of mass and energy, any accelerated mass can in principle produce gravitational waves. In practice, detectable waves require enormous masses moving at relativistic speeds or large changes in the system's quadrupole moment. The most promising and well-observed sources are compact, rapidly orbiting systems whose motion is not spherically symmetric.

Astrophysical sources

• Close binary star systems, especially those in which the stars are compact objects.
• Binaries of compact remnants such as white dwarfs, which can produce continuous or long-lived signals in some cases.
• Pairs of neutron stars, whose inspiral and merger emit strong, short-lived bursts and—when they collide—can be accompanied by electromagnetic radiation.
• Coalescing black holes, which are among the loudest gravitational-wave sources and played a central role in the first direct detections.

Observation and significance

Indirect evidence for gravitational waves existed before direct measurement, for example from orbital decay in binary pulsar systems. The first unambiguous direct detection was recorded on 14 September 2015, confirming Einstein’s prediction and opening a new observational window on the universe. Modern detectors measure tiny fractional changes in distance (strain) produced by passing waves, allowing astronomers to study phenomena—such as compact-object mergers—that are difficult or impossible to observe with light alone.