Ripples in spacetime that propagate at the speed of light, carrying information about cosmic events across the universe.
Gravitational waves are ripples in the fabric of spacetime itself, predicted by Einstein's General Relativity. When massive objects accelerate, they create disturbances that propagate outward at the speed of light, stretching and compressing space as they travel.
The amplitude of gravitational waves decreases with distance (1/r), while the strain is proportional to the second time derivative of the quadrupole moment of the source. This means only the most violent cosmic events produce detectable signals.
Imagine dropping a stone in a calm pond. The ripples spread outward, causing the water surface to rise and fall. Gravitational waves work similarly—but instead of water, they ripple through spacetime itself, stretching and compressing the universe as they pass.
The most powerful gravitational wave sources are binary systems of black holes or neutron stars spiraling toward each other. As they orbit, they lose energy through gravitational radiation, causing their orbits to decay and eventually merge in a catastrophic event.
This equation describes the power radiated in gravitational waves by a binary system. The closer the objects get (smaller separation a), the more intensely they radiate, leading to an inspiral that accelerates until merger. LIGO has detected dozens of such events, confirming Einstein's predictions.
Like ice skaters pulling in their arms to spin faster, binary black holes spiral inward as they lose energy. The gravitational waves they emit are like the energy the skaters lose to friction—each wave carries away a bit of orbital energy, causing the black holes to spiral closer together.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) made history in 2015 by detecting gravitational waves for the first time. Using 4-kilometer arms with laser beams, LIGO measures changes in length smaller than a proton's diameter as waves pass through.
This equation shows how a gravitational wave with strain h causes a path length difference ΔL in an interferometer with arm length L and laser wavelength λ. The longer the arms and the shorter the wavelength, the more sensitive the detector becomes.
Imagine measuring the distance to the Moon with a laser. If the Moon moved by a single hair's width, you'd never notice. But LIGO does something even harder—it detects changes smaller than a proton by comparing laser beams traveling in perpendicular directions, looking for tiny timing differences.