The Chirp

Gravitational waves stretch and squeeze space itself, but by impossibly small amounts. When GW150914 reached Earth, it changed the length of LIGO's 4-kilometre laser arms by less than one-thousandth the diameter of a proton.

The raw detector data looks like random noise. You genuinely cannot see the signal by eye. So how did LIGO find it?

LIGO uses a technique called matched filtering. Einstein's equations predict exactly what a gravitational wave should look like for any given pair of merging black holes. The characteristic “chirp” — a signal that sweeps up in frequency and amplitude — has a precise mathematical form.

By sliding this predicted template across the noisy data and looking for correlations, LIGO can extract signals buried deep in the noise. The match for GW150914 was so strong that the probability of it happening by chance was less than 1 in 200,000 years of data.

Two black holes, 36 and 29 times the mass of our Sun, had been spiralling toward each other for millions of years. In the final fraction of a second, they were orbiting each other at nearly half the speed of light, completing dozens of orbits per second before merging into a single black hole of 62 solar masses.

Three solar masses of energy — more than 10⁴⁷ joules — was radiated away as gravitational waves. For a brief moment, this event released more power than all the stars in the observable universe combined.

The wave then travelled for 1.3 billion years, stretching with the expansion of the universe, before finally reaching Earth on 14 September 2015.

• GW150914 Data Release — Original strain data and analysis notebooks
• Discovery Paper (arXiv) — “Observation of Gravitational Waves from a Binary Black Hole Merger”
• LIGO Sensitivity — How LIGO achieves sub-atomic precision