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Barry Barish helped turn LIGO from an interferometer proposal into an operating large-scale gravitational wave observatory. He directed its construction and early operation and established the broader LIGO Scientific Collaboration.^[ref]

The scientific architecture combines sensitive laser interferometers at widely separated sites. A candidate astronomical signal must be distinguished from local disturbances, using its relative arrival times and its consistency with the detectors' responses. This made instrumental performance, calibration and cross-site comparison inseparable from the eventual measurement.^[ref]

A binary black hole is a binary system whose two components are black holes. Gravitational waves carry away orbital energy and angular momentum, gradually reducing the separation; sufficiently close systems progress through inspiral, merger and the settling of the final black hole. The increasing orbital angular velocity produces a signal whose gravitational-wave frequency rises, called a chirp. In GW150914, two LIGO interferometers recorded a signal sweeping from about 35 to 250 hertz in September 2015. Comparing that measured gravitational wave strain with general relativity waveforms inferred the component masses and the merger's properties. This permits the study of systems that need no luminous surrounding gas to be detected.^[ref]

A gravitational wave is a propagating disturbance of spacetime curvature produced by changing mass-energy multipole moments. In general relativity, its two transverse gravitational wave polarizations alternately stretch and squeeze freely falling separations. The leading radiation from a slowly moving source is governed by its mass quadrupole moment, rather than by its total mass alone.

The experimental breakthrough was GW150914: observed on 14 September 2015 and published on 11 February 2016. The LIGO Scientific Collaboration and Virgo Collaboration reported a merging pair of black holes, using the two LIGO detectors at Hanford and Livingston; Virgo was being upgraded and did not observe this event.^[ref]

The 2017 Nobel Prize in Physics recognized Rainer Weiss, Barry C. Barish and Kip S. Thorne for contributions to LIGO and gravitational wave observation. Rainer Weiss developed interferometric detection and its noise analysis; Barry C. Barish led the project into construction and operation; Kip S. Thorne developed its theoretical and astrophysical foundations. The experiment depended on a large collaboration.^[ref]

GW150914 is the first directly detected gravitational wave, interpreted as a binary black hole inspiral, merger and ringdown. Its discovery paper reports a peak projected gravitational wave strain of approximately $10^{-21}$ and a combined matched filter signal-to-noise ratio of 24. The inferred source-frame component masses were $36_{-4}^{+5}$ and $29_{-4}^{+4}$ solar masses, with 90 percent credible intervals. These are model-dependent estimates from a detector network.^[ref]

The interesting measurement is a chain: curvature changes relative light travel times, wave interference turns those changes into a photodetector signal, calibrated records from separate sites constrain a common source, and statistical inference tests competing explanations. The following original models explain that chain; the plotted time series are stipulated calculations, never measured GW150914 data.

Numerical relativity computes solutions of the Einstein field equations when an analytic solution or a small-perturbation approximation is insufficient. A 3+1 decomposition of spacetime separates initial spatial geometry and matter from their subsequent evolution. Initial data must satisfy the Hamiltonian constraint and momentum constraint, while the lapse function and shift vector specify the coordinate evolution.

The calculation also needs boundary conditions and a numerical evolution formulation. Resolution studies, constraint monitoring and comparison with known solutions test whether the computed result approximates the intended spacetime. Binary black-hole mergers provide an observational application: numerical waveforms connect strongly dynamical geometry with interferometer signals.^[ref] Extracting a physical wave requires controlling the frame and propagation model as well as solving grid equations.