Looking into space with gravity

The telescopes that the early pioneers of astronomy were using all relied on visible light reaching the Earth from far away in space. But telescopes can see in other ways too. In the last few years, an even bigger step forward has ushered in a new era in astronomy with the use of gravitational waves to see deep into space. LIGO, the Laser Interferometric Gravitational-Wave Observatory picks up ripples in the fabric of space made by massive objects like black holes, enabling us to study things in a whole new way. Ed Daw, from the University of Sheffield, is a professor of gravitational waves and dark matter, and spoke to Chris Smith about this heavy subject...

Ed - It's a pretty hard thing to explain, so I'm going to use an analogy; and because I'm from Sheffield, the analogy is going to involve snooker. Because as you know, we have these big snooker competitions at the Crucible every year. When you watch a game of snooker, what you're watching is balls roll around on a green-based flat table with slate under it, right? And so there's an idea from a snooker game that the motion of the balls doesn't really disturb the table. In fact the table's designed to be a very passive object. Now it turns out that that's kind of like classically, traditionally, people thought about space and time. People thought that space and time was a theatre in which objects did their things.

However Einstein showed that, in fact, the real world behaves a little bit more as if the snooker table, instead of being made of slate with felt on top of it, was actually made of stretch rubber. Now when you think about it, if you had a snooker game on a stretch rubber sheet things would be very different; so for example, if I put a ball down on the table, on my stretch rubber table, it would cause the sheet to become distorted. And so the background space-time in the same way is perturbed by the presence of matter, in the form of things like stars and even more exotic objects like black holes that you've mentioned. Now gravitational waves are a consequence of more complicated motion of objects. So back to the snooker table, imagine you have two snooker balls orbiting around each other. Now I know that doesn't happen with real snooker balls, but we don't have gravity in snooker...

Chris - Depends who's playing!

Ed - ...so let's just extend it a little bit, right? So as the snooker balls rotate around each other, what are they going to do to the rubber table? They're going to make waves on it. And those waves are the analogy, direct analogy, of gravitational waves, and those are the things we've learned to detect with LIGO.

Chris - So Ed, if I may ask then: I detect a game of snooker by watching where the balls go. How do you detect your gravitational game of snooker? How do you pick up where a black hole is going?

Ed - The first thing is to detect the waves at all. Now because they're very, very tiny... I've said it's a rubber sheet, but actually spacetime is much stiffer than rubber, so the motions you're looking for are very tiny and subtle. So the detector in a nutshell consists of two ordinary pendulums - actually more than two, let's just pretend it's two - with masses on the end, separated by some distance. And when a gravitational wave comes through, both of the pendulums starts swinging; but it turns out they swing in such a way that the distance between them oscillates. And by using a laser detector you can detect the change in the distance between your two suspended bodies.

Chris - How big a difference are we trying to detect here?

Ed - If you have two of these pendulums separated by kilometre, the change in their separation moves by less than a thousandth of the diameter of a proton.

Chris - How on earth can you detect that?

Ed - By using very, very sophisticated, highly controlled laser metrology methods. You basically set up resonant cavities. The actual masses aren't actually ordinary pendulums, they're made out of mirrors that reflect infrared light, and then you make a resonator out of the two mirrors and you build up laser intensity between the mirrors; and when they start to oscillate, that causes the properties of the built up light between the mirrors to start oscillating as well, and you can amplify the effect of the tiny motion using the properties of the resonator.

Chris - What can this tell us, Ed, that we can't learn by using, say, the Hubble Space Telescope?

Ed - Well it turns out that much like many objects on earth don't actually admit very much light, there are plenty of objects probably in the universe that don't emit very much light either, or for that matter any other kind of electromagnetic wave. And those objects are therefore - with ordinary telescopes or other electromagnetic wave sensors - those objects are actually invisible. So we might have thought that there might be a lot of black holes in the universe before we had our gravitational wave detectors, but there are whole classes of objects, black holes for example, that it turns out were utterly invisible to all of our electromagnetic detectors, which have now been revealed by what happens when black holes collide, which is they emit some gravitational waves for a short time which are picked up by the gravitational wave interferometers in LIGO and Virgo.

Chris - How far across the universe will these gravitational waves propagate so we can detect them?

Ed - The one we detected very recently, the latest detection, which was from an object that happened last May, in May 2019, was 17 billion light years away. The source was that far away. So it's quite awesome how far away these objects are.