Predicted by Einstein's general relativity in 1916 and first confirmed in 2015, gravitational waves have ushered astrophysics into a new era of cosmic observation. Advanced detectors like LIGO, Virgo, and upcoming KAGRA and IndIGO continuously monitor the skies for these spacetime ripples. But how exactly are they detected?
In 1916, Albert Einstein theorized that accelerating massive bodies produce spacetime disturbances that propagate as waves. It wasn't until September 14, 2015, that the LIGO collaboration detected gravitational waves from two black holes merging 1.3 billion light-years away. A second detection followed in December 2015, involving both LIGO and Virgo.
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Astrophysicists detect gravitational waves using interferometry, which analyzes interference patterns from coherent waves. LIGO and Virgo employ Michelson interferometers for this purpose.
LIGO's interferometer features two perpendicular arms of equal length, precisely tuned to multiples of the laser's wavelength, maintained in high vacuum. A laser beam splits at a beam splitter, travels down each arm, reflects off mirrors multiple times, and recombines to produce an interference pattern.
In the absence of gravitational waves, this pattern remains stable, confirming proper calibration. LIGO scientists spent over 40 years refining sensitivity to detect these faint signals, whose amplitudes are extraordinarily small.
Unlike electromagnetic waves that interact with particles, gravitational waves are distortions in spacetime itself. They exhibit quadrupole patterns, alternately stretching and compressing space in perpendicular directions based on polarization, distinct from monopolar or dipolar waves.
When a gravitational wave passes through LIGO, it stretches one arm while contracting the other—and vice versa—producing a distinctive oscillatory interference pattern.
LIGO detectors are positioned at angles and locations worldwide to ensure at least one captures any wave orientation optimally.
In vacuum, light travels at 299,792,458 m/s along the arms. A passing gravitational wave alters arm lengths, affecting light travel time: one arm lengthens, increasing time; the other shortens, decreasing it.
Though arm stretching might seem to preserve the interference unchanged, the key factor is propagation time, not wavelength. The wave temporarily modifies effective arm lengths, shifting beam arrival times and creating an oscillatory interference shift upon recombination.
Gravitational waves do shift light wavelengths—redshifting in stretched space, blueshifting in contracted—but light speed in vacuum remains constant at ~300,000 km/s regardless. Thus, propagation time through the arms is the critical measurable effect.
