I’m not even trying to suppress a little glow of delight that I was the one who got to ask Stephen Hawking, a month before the announcement that gravitational waves (GWs) have been detected, when or if that would happen. That isn’t to claim any great prescience; it was an obvious thing to ask, not least because the most sophisticated GW detectors on the planet, the Advanced LIGO facilities in Washington and Louisiana, had just switched back on after both being upgraded. And the fact that Hawking replied, during the Q&A of his BBC Reith Lectures at the Royal Institution in London, that GWs would probably be seen in the next five years, implies that he was as unaware as I was that one had already been detected.
It still staggers me that these ripples in spacetime, caused by incredibly violent astrophysical events such as a merger of two black holes, were just waiting to be seen the moment Advanced LIGO (the Laser Interferometer Gravitational-Wave Observatory) turned its two detectors back on last September. They were only doing a test run, for goodness’ sake, and there it was: a 0.2-second chirp in the data, which at first the scientists couldn’t believe was the genuine item. When I spoke to LIGO’s executive director David Reitze at the beginning of last year, he said that he expected that it would take 2-3 years after completing the upgrade to bring Advanced LIGO to its design specification. I don’t think even he had any real expectation of such an amazing result so soon.
And after all, it is beyond imagining. Over a billion years ago, well before any multicellular creatures existed on Earth, two black holes crushed together, and the ripples that squeeze and stretch space by less than a trillionth of an inch have been steadily crossing the cosmos ever since, occasionally passing through stars as if they weren’t there. In the meantime we evolved, Einstein devised the theory of general relativity, we built these multi-kilometre-sized instruments to sense such minute changes in the travel distance of light beams, and then we flipped the switch just as the wave arrived. Of course, the implication is that they must be happening all the time, all over the universe. Still, I think Reitze’s alleged comment on hearing the news—“Holy shit, what is this?”—rather understates the matter.