Tests of General Relativity with GW150914

Just six years into his career, Dr Walter Del Pozzo has achieved what many academics fail to accomplish in a lifetime: the Italian-born astrophysicist has produced out-of-this-world evidence to corroborate the general theory of relativity in a way that has never been done before.

The post-doctoral researcher in gravitational wave physics has ‘seen’ the merging of two binary black holes (BBHs) and the so-called ringdown of a third in a system detected by the LIGO Scientific Collaboration – and found no violations of Einstein’s 1915 theory that gravity is a distortion of space-time by massive objects.

The result of this research, conducted with colleagues across the world, is a paper entitled ‘Tests of General Relativity with GW150914’, of which Walter was one of the main editors. It was published recently in Physical Review Letters, the world’s leading physics letter journal, garnered many citations and won Walter the College’s Paper of the Month award for May.

The paper follows up the detection of gravitational waves (GWs) from a pair of colliding black holes, news of which made global headlines earlier this year.

‘When you have two objects – two black holes – merging, there is a moment in space-time when gravity is at the strongest it will ever be in the foreseeable future,’ explains Walter, who took up his Birmingham post three years ago. ‘In that moment, our understanding of gravity is put to its strongest test because that is when if there are small deviations from that which we think describes gravity, in other words GR, they will be amplified. So the observation of binary black holes (BBHs) merging is the best place to test whether our understanding of gravity is correct. And this is what the paper deals with.’

So what, exactly, did the researchers observe?

‘What we saw was a pair of black holes, with masses 35 and 30 times the mass of the Sun, that were orbiting around each other just before the merger at half the speed of light. They formed a third black hole with a mass of 62 times that of the Sun. Each of these black holes was a few hundred kilometres wide.’

Walter and his fellow researchers from the LIGO and Virgo collaborations – about 1,000 in all, mainly from the US, Europe and India – devised a series of tests aimed at detecting anything that didn’t follow GR.

‘We used six of these tests in this paper,’ says Walter. ‘There are essentially two ways you can do these tests; one of which is the agnostic way, where you relax as many assumptions as you can about GR and its requirements, or you assume something else other than GR and look for the presence of these extraneous ingredients. For example, GR predicts very precisely the evolution of the orbit. It does that through some very defined relations. So one of the tests we did was to relax those relations and learn from the data how much they could vary without contradicting them.

‘Within our experimental uncertainty, we found no evidence for a violation of GR. In other words, relations are satisfied to a reasonable degree. This is the first time this has been done. Something similar has been done before but not even close to the level of gravity that we’re dealing with in this system. It’s not comparable to what’s been done previously because until now the best test of GR has come from the binary pulsar system. It’s an exquisite test of GR, but these two pulsars are orbiting each other at about 1000km per second. So there’s a factor of 1000 more dynamics. There is no other system in the Universe that can reach such energies and velocity.’

Did Walter expect the result the research delivered?

‘Not really,’ he admits. ‘I’ve spent most of my career so far preparing for this, but the reality is a different story. The system that we saw was far from what we expected to see. Another thing we were able to see in this system was the ringdown of the final black hole, which is formed in a very perturbed state and needs to settle to equilibrium. To do this it needs to get rid of excess energy and it does so by ringing, in the way that glass rings if you tap it, at a very specific frequency – and we found evidence for that very specific frequency. That was one of the most surprising things: the ringing of a black hole was something we hadn’t prepared for.’

Walter, who is moving back to the University of Pisa, where he took his first degree, acknowledges the research as a major career highlight. ‘This could be the apex of my career, but you never can tell because the Universe is full of surprises. We’ve not yet pinpointed anything that violates GR, but you never know what the Universe is going to throw at us. So I’m going to continue to work in the same field – and will still collaborate with academics from Birmingham – and I’m looking forward to the unexpected.’