# Gravitational waves detected. Einstein was right … again

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity

As if celebrating the 100th birthday of general relativity weren’t enough, the LIGO-Virgo collaboration has provided “the icing on the cake” with today’s announcement of the first direct detection of gravitational waves. At press conferences in the USA and Europe, and in a paper in Physical Review Letters published afterward, the team announced the detection of a signal from a system of two merging black holes.

The signal arrived on 14 September, 2015 (its official designation is GW150914), and was detected by both the Hanford and Livingston advanced detectors of the LIGO observatory (the advanced Virgo instrument in Italy is not yet online). It was detected first by an algorithm for low-latency searches for generic transient signals, and confirmed by later analyses using matched filtering methods. Interestingly, the detection was made during the late stage of an engineering run, prior to the start of the official “science” run, but the detectors were running smoothly and quietly at the time. The signal-to-noise ratio from matched filtering was 24, yielding a significance greater than $5 \sigma$. In fact, the signal is so strong that it can easily be seen by eye in the data (after suitable band-pass filtering to suppress such things as instrumental lines in the noise). The signals in the two detectors are even related by a minus sign, consistent with the relative orientations of the arms of the two instruments, and they arrived within about 7 milliseconds of each other, well within the 10ms light-travel-time between the two sites.

The signal is a quintessential inspiral-merger-ringdown waveform, of a kind displayed by every relativist who has ever given a talk about gravitational waves. From now on we can replace our theoretically generated waveforms with one from real data! [Illustration by C M Will]

The detected merger is really quite remarkable: two black holes of mass $36^{+5}_{-4} \,M_\odot$ and $29 \pm 4 \,M_\odot$ at a distance of $410^{+160}_{-180}$ Mpc (corresponding to a redshift of about 0.1). The final black hole mass is $62 \pm 4 \, M_\odot$ with a spin parameter $0.67^{+0.05}_{-0.07}$. This is the first detection of a stellar-mass black hole binary, and the discovery already implies a revision in the estimated rates for such systems to $2 - 400 \, {\rm Gpc}^{-3} \, {\rm yr}^{-1}$.

Two new tests of general relativity were done, one verifying that the post-Newtonian coefficients in the expansion of the phasing of the waves show no evidence of a deviation from GR, and another showing no evidence for an anomalous dispersion of the gravitational wave signal, as would occur if the putative “graviton” were massive. In the latter case, the lower bound on the Compton wavelength of the graviton was found to be $10^{13}$ km.

In the coming days and weeks, numerous items of currency and bottles of wine will surely change hands, as various bets and pools are paid off. I was convinced that the first detection would be a binary neutron star merger, but luckily I didn’t back my prediction up with cash or spirits.

More importantly, relativists and astrophysicists can now stop talking about the “coming era” of gravitational-wave astronomy, and actually start doing it!

For more depth, read our collection of gravitational waves papers in Classical and Quantum Gravity