Did colliding black holes cause gravity waves?

Toby - Sure, absolutely. So gravitational waves are ripples in space time that get formed when very massive objects move in, for us, an incredibly dramatic way. So they were first observed in 2015 from a pair of black holes, a billion light years away. So a billion years ago, these very massive black holes, which were many times the mass of the sun, collided with each other and set off these ripples. They travel for a billion years and then they stretch space and time as they pass. So on Earth, this very amazing experiment called LIGO in America measured this distortion in space and time as the wave rippled past. And that was the first time these things have been seen. So since then, we've seen quite a lot of similar mergers. So we now have the technology to see that. But this new discovery in June was a completely different class of black holes producing quite different ripples. So these are super massive black holes. Super massive black holes live at the centres of galaxies, and they're sort of amazingly millions or even billions of times the mass of the sun. When galaxies merge, these super massive black holes also come into orbit around each other because each galaxy will have one at its centre. They'll start orbiting and they produce a signal. And it's that, that's been measured really amazingly in June.

Chris - Andrew?

Andrew - One of the things I think's really interesting about this LIGO experiment, you described these distortions in space and time, is to quantify how big that distortion is. Because I think I read that the LIGO experiment is the most extremely small disturbance, detectable over a very large distance. Yes.

Chris - How big is it?

Toby - So for the first event discovered it was unbelievably tiny. So what it does is it stretches. So you should think of a fractional change, a percentage change in length, and the percentage change in length I think is about a billionth of a billionth of a percent. So it's absolutely tiny, I mean, it's unbelievable that we can measure, I mean I can't measure them. These brilliant experimental physicists have developed the technology to measure it's taken decades.

Chris - Tony, why is it important that we can even do this? What are gravitational waves, why did they win a Nobel Prize for Kip Thorne and others and effectively validate what Einstein was saying? Why is this useful? What sort of new vista does it open in the universe for us?

Tony - Well, it really is a new window on the universe. I mean, previously, we've looked at the universe, we've looked at the light that we see from distant stars and distant galaxies, and we can read off the behaviour of the universe from that. But now we can essentially hear the universe as well because these gravitational waves, they're travelling long distances and they're feeling the size and shape of space and time itself. So they're really a new window or we can now hear what space and time is doing.

Chris - How do we know where they've come from though? Because if they're tiny and they're travelling just uninterrupted across the universe, when we detect them, how do we know which way they've come from?

Tony - We see the light as well, or in this case gamma ray. So very high frequency light that we can't actually see, but we can detect.

Chris - So you see a light source and you also see some gravitational waves issuing from the same part of space. And you say, well, they're probably connected?

Tony - So these events can be caused not just by black holes, but also by neutron stars. And in the case of neutron stars, we will see those light signals as well. So we can detect those.

Chris - But what about his super massive black holes? How do we see that happening then?

Tony - That is very much a sort of rumbling background. So it's not so directional. It's a rumbling background. So what we use are these special objects that are out there called pulsars. These are stars which in a way they pulsate with a very regular time interval.

Chris - These are Jocelyn Bell Burnell's little green men. But they were what we thought were aliens and turned out to be stars sending out absolutely regular pulses of information.

Tony - Exactly, and those pulses, they're so regular that we can really use them to see how sort of time is changing due to the space, the distance.

Chris - So they’re like metronomes in space. And as space wobbles with the waves coming through, you can see that wobble goes off kilter and that tells you that a gravitational wave has distorted space time in that sort of neighbourhood.

Tony - You're expecting the pulse to come at one particular time, and then there's a slight time change then you can sort of detect that, and that's what you're looking for. But the really interesting thing I think about this event, this new measurement, all the talk has been that it does come from these super massive black holes, but this is where you can really start to test different physics because it might not be from super massive black holes. It might be from some really funky stuff in the early universe, right? So for example, there can be things called phase transistors, which is where almost the properties of matter change in the early universe. And you get these bubbles of new phases forming. You can even get signals of exotic theories like string theory that are sort of imprinted on this signal. And this signal allows us to probe that, that's why it's so exciting.

Chris - How are you going to get to the bottom of that one Toby?

Toby - Through time <laugh>. So these measurements, they've actually been measuring these pulsars, which are unbelievable objects. So they're rotating typically hundreds of times a second. And they're neutron stars. So they're the mass of the sun giving out these pulses, but they're accurate like atomic clocks. So when you look at them over years, they hold their regularity of their pulses as well as atomic clocks. And that's what's allowed them to see these tiny distortions. And as they measure for longer and longer, they see it more accurately.