The Chirp

Gravitational waves compress and stretch spacetime by infinitesimal amounts. When GW150914 arrived, LIGO's 4-kilometre laser arms shifted by less than one-thousandth of a proton's diameter.

Raw detector readings resembled pure noise - the signal remained invisible to direct observation. Two detectors, 3,000 kilometres apart, recorded near-identical noise patterns 6.9 milliseconds apart. That delay is how long the gravitational wave took to travel from Livingston, Louisiana to Hanford, Washington.

LIGO employs matched filtering methodology. Einstein's equations precisely predict gravitational wave characteristics for merging black hole pairs. The distinctive "chirp" signal - frequency and amplitude increasing together - follows exact mathematical patterns determined by the masses and spins of the merging objects.

By correlating predicted templates against noisy measurements, LIGO isolates buried signals. The process is like listening for a specific bird call in a crowded forest - if you know what it sounds like, you can find it even when everything else is louder.

GW150914's correlation strength reached a signal-to-noise ratio of 24 - meaning the chance of random noise producing this match was less than 1 in 200,000 years of continuous data. The signal was real.

Two black holes - 36 and 29 solar masses - spiralled inward over millions of years. During their final fractional second, orbital speed approached half light-speed, completing dozens of cycles before merger.

The resulting 62-solar-mass object released three solar masses as gravitational energy - more than 10⁴⁷ joules - momentarily outshining all observable stars combined.

After travelling 1.3 billion years while expanding with the universe, the wave reached Earth on 14 September 2015.

LIGO uses laser interferometry. A single laser beam splits and travels down two perpendicular 4-kilometre arms. Mirrors reflect the beams back and forth about 280 times, effectively creating 1,120-kilometre paths.

When the beams recombine, they interfere. If both arms are exactly the same length, they cancel perfectly. A gravitational wave passing through stretches one arm while compressing the other - the interference pattern changes.

The sensitivity required to detect GW150914 means isolating the detector from every conceivable noise source: seismic activity, thermal fluctuations, quantum shot noise, even traffic on nearby roads. The detectors are among the most sensitive instruments ever built.

As two black holes spiral closer, they orbit faster. Faster orbits produce higher-frequency gravitational waves. The amplitude also increases as the black holes approach merger.

The result is a distinctive signal: frequency and amplitude rising together over a fraction of a second. When converted to sound (the actual signal is below human hearing), it sounds like a bird chirp - hence the name.

The exact shape of the chirp encodes information about the black holes: their masses, spins, orbital parameters, and distance. Matched filtering doesn't just detect the signal - it measures the event.