How can laser interference reveal a gravitational wave?

Gravitational-wave observatories such as LIGO compare extraordinarily small changes in perpendicular directions. A laser beam reaches a beam splitter, which sends light into two arms. Mirrors return the light so the beams recombine. The detector measures their interference, which depends on their relative optical phase: how the crests and troughs of one wave align with the other.

A passing gravitational wave can produce different changes along the two arms, depending on its direction and polarization. In a simple, slowly changing Michelson model, a differential single-pass arm change ΔL changes the round-trip phase by 4πΔL/λ, where λ is the laser wavelength. The factor includes the outward and return paths. It describes a phase comparison, while the local speed of light remains the same.

For an illustrative wavelength of 1,000 nanometers and ΔL of 1 picometer, the phase change is about 0.0000126 radians. Dividing that differential length by a nominal arm length of 4,000 meters gives a differential strain of 2.5 × 10⁻¹⁶. These selected numbers explain the calculation; they are not the size of a typical detected LIGO signal or its sensitivity.

Real observatories add optical cavities, suspended mirrors, vacuum systems and active control. Ground vibration, thermal motion and instrumental disturbances can also affect the measured phase. Scientists calibrate the response, monitor those disturbances and compare observations across separated detectors. A changing interference signal alone does not establish a gravitational wave. The scientific feat combines an extremely sensitive instrument with careful evidence about what produced its output.