About three billion years ago, two dead, far-off stars, each dozens of times more massive than our sun, spiralled into one another. The resulting collision sent ripples through the fabric of space. In January, those ripples finally reached the Earth, where they were detected by two enormous, high-tech instruments in rural parts of the US.
And the shape of the ripples – the "sound" the black holes made colliding – has demonstrated yet again that Albert Einstein's 102-year-old theory of general relativity – which describes how gravity works – stands up even under the most extreme circumstances.
This is the third confirmed detection of gravitational waves by the Laser Interferometer Gravitational-wave Observatory (LIGO), a pair of 4-kilometre-long detectors, one in Louisiana and one in Washington state. The first detection of two colliding black holes was announced in February last year; the second was announced in June. There was another signal, but it wasn't clear enough for scientists to say with confidence that it was a real detection.
The confirmation of the existence of gravitational waves, predicted by Einstein, was the biggest discovery in physics since the Higgs boson. It changed the face of astronomy, opening up the possibility of a whole new way of looking at the universe – by using gravitational waves instead of light.
Graham Woan, a professor of astrophysics at the University of Glasgow, told BuzzFeed News that this third detection is exciting in three ways. Most obviously, it makes it more likely that these collisions are relatively common in the universe – LIGO has detected about one for each month it's been running, having shut down for improvements for most of 2016.
"It’s another source," he said. "Another part of the population [of black holes]. What will emerge over the next few detections is the shape of the population – how heavy they are, how far away."
The first detected collision was between two very large black holes, totalling about 65 times the mass of the sun. The second pair were much smaller – roughly 20 times. This one is in between the two, about 50 solar masses. The number and type of black holes, and when they were formed, can tell us interesting things about the early years of the universe, and how stars form and die.
The second exciting aspect to the discovery is the fact there is a hint in the data that one of the black holes may not have been spinning in the same way as it was orbiting. "It's just a hint," Sheila Rowan, director of the Institute for Gravitational Research at the University of Glasgow, told BuzzFeed News. "But it may be that the spin of at least one of the black holes wasn't aligned with the overall motion of the pair."
If you picture the solar system, the Earth and all the planets orbit in the same direction around the sun, all on the same plane, and they all spin around their axis in the same direction too. That's because they formed together out of the same spinning disc of dust and gas when the solar system was new.
If one of the black holes isn't spinning in the same way as the other, or on the same plane as their orbit around each other, then that's evidence that the two stars that made the black holes didn't form together but wandered alone through space and bumped into each other later. "It’s the kind of info that gives us hints about how these systems formed," says Rowan. "It can tell us about what's happening out there – how these systems came together.
"If one or both aren't aligned, that’s a hint that they might have formed separately and formed a pair later on." With just one observation, it doesn't say much about how common that is, but as LIGO detects more it will paint more of a picture.
The third reason for excitement is that, because this collision was so far away – three billion light years, more than twice as distant as either of the earlier detections – it provides a neat test of one of Einstein's predictions.
Shorter-wavelength light travels faster than longer-wavelength, when it's moving through something other than a vacuum. That's what happens in a rainbow – the bluer end of the spectrum travels faster than the redder end, so it curves more as it passes through a raindrop or a glass prism. It's a phenomenon called "dispersion". But Einstein's theory said that couldn't happen with gravitational waves – they all ought to travel at the same speed, the speed of light, regardless of wavelength.
"These signals have travelled for a significant fraction of the age of the universe," says Woan, so if there was even a tiny bit of dispersion – if shorter wavelengths were just a little bit faster than longer ones – they "would have arrived enormously earlier". But that's not what was seen; instead, the evidence is that they all travel at exactly the speed of light, just as Einstein predicted.
"We keep prodding Einstein around to see how he holds up," says Rowan. "This is a different way to test it, but it's held up again." In a way, she says, people are hoping that these extreme tests will reveal some tiny discrepancy from Einstein's predictions, because that could hint at some new form of physics that could help with the remaining big questions, such as how to understand gravity from a quantum-mechanical point of view.
"Every time we do one of these tests, we chase general relativity further along," she says. "If at some point we do see some variation that would be incredibly interesting, because it would throw up potentially new forms of physics that we haven't got." But so far, Einstein's equations have taken everything physicists can throw at them.
Rowan says that as more and more of these black-hole pairs are discovered, the picture will become clearer. "This discovery narrows down our prediction on how common these events are," she says. "Potentially [after LIGO is upgraded again] we could be seeing one a day.
"In 18 months we've gone from not knowing that these pairs existed to starting to see a population of them. [Gravitational waves] are becoming a real tool for astronomy."