Around a billion light years ago, in a part of space that's nowhere near Earth, two giant black holes collided with one another. The scale of this collision is pretty hard to imagine: One of the black holes was around 29 times the mass of our sun, the other 36 times. "They released the same amount of energy as 8.5 billion trillion trillion Hiroshima nuclear bombs," explains Dr Alan Duffy, a professor of astronomy at Swinburne University. When they merged together the resulting black hole was 62 times the mass of our sun.
We know this because scientists detected it. Not using telescopes, but by shooting lasers along four kilometre tunnels as part of a massive physics experiment called the LIGO Collaboration. Stephen Hawking has called it a key moment a scientific history.
"The idea is that the lasers go up and down these tubes... They bounce off a mirror and come back, when they meet at the LIGO centre they should cancel out," Dr Duffy explains. But if one of the beams has travelled a slightly different distance, it means it has been stretched by the gravitational waves emitted from the black hole. "They won't perfectly cancel and you'll get a little bit of light hitting your detector," Dr Duffy says, which is exactly what the scientists at LIGO saw on 14 September, 2015.
Why is this discovery important? Well, two reasons. First up, we've never directly seen black holes before this. Scientists have been pretty sure they exist, but this is solid proof. "That in itself would be Nobel Prize winning," says Dr Duffy. "But, to put it in perspective, it won't even get a mention because of the enormity of the gravitational waves themselves."
Confirming the existence of gravitational waves matters because it proves what Einstein was predicting 100 years ago when he put forward his General Theory of Relativity. "In his theory he showed that spacetime—the actual thing that we live and move in—can ripple, just like when you throw a rock into a pond," Dr Duffy says. "You get the same effect if two massive objects, just say black holes, collide. That will actually cause ripples in spacetime."
Einstein thought we would never see these ripples because they'd be too weak to ever detect. But the fact LIGO was able to detect them gets even more incredible. Until just a few days before the waves were seen, LIGO was shut off being upgraded. "The facility turned on just days before. So these gravitational waves had travelled for hundreds of millions of years and hit us just a few days after we switched on our detectors," Dr Duffy says. The gravitational wave passed through Earth in the blink of an eye, and it stretched the detector by less than one thousandth of the size of the nucleus of an atom.
The detector is so powerful, the scientists had to make sure it hadn't been affected by things like the vibration from a truck driving past over a kilometre away. "I think even one of the teams had to worry about whether a lumberjack had felled a tree nearby," Dr Duffy says. "This is the level of detail you have to go to be sure that you're seeing these gravitational waves because it's just one of the biggest, most important discoveries in physics."
After the wave passed through Earth there was a frequency, or a ringing—the echo of the black hole crunching down on itself. This billion-year-old sound that has travelled across space and time, it's audible to humans. You can go and listen to it right now. "The most extreme collision we've ever seen in the universe has ended up turning out to be Middle C on a piano," Dr Duffy says.
Turns out the scientific community is better at keeping secrets than Beyonce, given that LIGO's findings were pretty much under wraps until they were published today in the Review of Physical Letters. Dr Duffy says the LIGO team asked every telescope on Earth to point towards the area in the sky where they knew the black holes collided, searching for more proof of the explosion. "Essentially, it's one of the most intense searches humanity has ever undertaken and it's managed to be done almost perfectly in secret... until now when we were 100 percent sure," he says.
Dr Duffy says the next step is detecting bigger and better black hole collisions. "Australia has a leading role in using distant stars, called pulsars, which actually detect far larger black holes merging," he says. "Tens of times of the mass of our sun is pretty big but we know that black holes can become millions or even billions times bigger than our sun. They just keep gobbling up matter." For now though the scientific community is celebrating one of the biggest discoveries ever. "It really means that we get to see into the universe with an entirely new sense," Dr Duffy says. "Sight, sound, touch, taste, smell—all those human senses use light. This is the first time that we'll be using gravity and it's an incredible moment."