THE WAVES OF REVOLUTION
Einstein was right about gravitational waves, and now science can prove it
In the two years since the twin LIGO observatories in the US made the first detections, gravitational astronomy had been handicapped. With only two observatories, it was impossible to pin down the origin of waves to less than a few hundred square degrees of sky.
Gravitational astronomy is most powerful when used alongside electromagnetic observations – from radio through visible to X-ray and gamma. But finding these counterparts has been impossible given a search field so broad. And so on 1 August the Virgo interferometer near Pisa, Italy, joined LIGO in its search for the waves.
Fourteen days later, they made their first joint detection, a pair of stellar-mass black holes merging. With three points of reference, the source was narrowed down to a mere 60 square degrees of sky, but the event was simply too dim to be picked up by any other telescope. But on 17 August, LIGO detected a wave created by colliding neutron stars for the first time, an event which should be accompanied by a hugely bright kilonova explosion. With Virgo’s help, the team narrowed the search area to 30 square degrees in the southern hemisphere. Dozens of the world’s best scopes – ALMA, VLT, the Hubble Space Telescope and many others – searched the sky for any sign of the kilonova. Before the end of the night they found a new point of light in the galaxy NGC 4993, the first electromagnetic counterpart to a gravitational wave.
After decades of trying, astronomers had finally managed to not only detect a gravitational wave, but been able to watch its visual counterpart for days after. Though the kilonova has faded, its legacy has not. The gravitational revolution of astronomy has begun.
Read our interview with the astronomer who was the first to see the light from the gravitational wave event on page 106