The discovery of gravitational waves – announced on February 11 – is an outstanding achievement. Personally, I think it is of greater importance even than finding the Higgs boson at CERN in 2012, which led to Nobel prizes for François Englert and Peter Higgs.
The existence of gravitational waves was predicted by Einstein’s theory of General Relativity. They were indirectly observed by two radioastronomers studying a binary pulsar discovered in 1974: the observations provided what was then by far the most rigorous test of Einstein’s theory. However, directly observing gravitational waves has been exceptionally difficult, requiring both exquisite precision and the ability to filter out all other confounding signals: hence the excitement that the same signal on 14 September 2015 appeared in two different detectors.
The discovery is also the best evidence yet of the actual existence of black holes. A couple of years ago I wrote an article on this blog about Why I don’t believe in black holes. The gravitational wave detections could therefore demonstrate that I was wrong, which is no big deal (especially since I ceased being a professional astronomer a long while back!). However, there is a small detail that indicates that the black hole interpretation might still be premature. According to the New Scientist, a gamma-ray burst appeared a mere half second later, and the chance of this being a random coincidence is about 0.2%. The problem is that such a burst is not supposed to happen with black hole mergers.
Gamma-ray bursts are themselves spectacular but enigmatic events, lasting from a few milliseconds to no more than a several minutes. (For an introduction to gamma-ray bursts, see here.) The origins of many remain mysterious, but neutron stars and black holes are usually implicated.
So if gamma-ray bursts aren’t meant to happen with black hole mergers, what could be happening? It’s perfectly possible that the only problem is that current models aren’t sufficiently realistic, and that with better data and better models it will be shown that the gravitational wave detection on 14 September 2015 was indeed of a black hole merger. Alternatively, the detection of a gamma-ray burst at the same time may indicate that it wasn’t a black-hole merger at all: perhaps instead, the gravitational waves were produced by the merger of two ultradense, ultracompact objects that were denser than neutron stars but not actually black holes. As I understand it – and I’m happy to be proved wrong – this may be a heretical view but it hasn’t actually been disproven.
What is clear is that gravitational wave astronomy has arrived. New detectors are needed, so that the location of the events themselves can be measured more precisely, and so that the association with other events, such as gamma-ray bursts, can be clarified. It’s an exceptionally exciting new field – and by far the best way to ascertain whether or not black holes actually do exist.
Andy Beardmore, a former colleague of mine at Leicester University and now part of the research staff there, comments on Facebook: “Theorists can predict anything once given an incentive – more than one paper has since appeared saying that, of course, you can get EM waves [eg gamma rays] from a merging black hole binary…“. He also recommended an article from the New Yorker magazine that’s a particularly detailed description of the discovery (here).