Gravitational Waves: A new era of Astronomy

11 December 2018

Interview with 

Shane Larson, Northwestern University

Gravitational Waves

Binary Stars

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Predicted over 100 years before they were detected for the first time in 2015, gravitational waves won Nobel Prizes for their discoverers and further validated Einstein and his General Relativity theory that said they should exist. Since 2015 the detectors - long tunnels resembling giant letter Ls - have continued to operate and the scientists working on them have been picking through the cosmic hubbub they've recorded. And from that data they've been able to tease out evidence of further gravitational wave events corresponding to pairs of black holes merging and neutron stars colliding. It's ushering in a new era of astronomy. This week four more events were announced, and Chris Smith caught up with team member Shane Larson to hear how it’s going…

Shane - When stars reach the ends of their lives, they become these very compact stellar skeletons called a black hole: An object which is so gravitationally strong that nothing can escape it. These stellar skeletons, they lie around in the cosmic graveyard together and sometimes they find each other. They orbit around each other and merge to form a bigger black hole. And we can detect those with these gravitational wave detectors LIGO and Virgo.

Chris - Now you made the first announcement of this happening back in 2015 - 2016 and since your first observation run you've added another detector now, so you're actually detecting with three different observatories aren’t you?

Shane - Yeah. So the network right now is the LIGO observatories. The two of them in the United States one in Hanford, Washington and one in Livingston, Louisiana. And then the European gravitational observatory called Virgo which is outside Pisa, in Italy.

Chris - And how do these detectors do what they do?

Shane - We call them observatories because we're doing astronomy, we're observing the universe, but they are not telescopes in the sense that we're used to thinking about astronomy. They're laser interferometers. So we basically use lasers. There are these gigantic Ls and the lasers go in two directions down what we call the arms. And these gravitational waves, when they come through the detector, they change the lengths of those arms and we can sense that by timing, how long it takes the laser to go down to the end of the arm and back again.

Chris - Given that these changes are going to be really subtle, excruciatingly tiny changes in the lengths of those arms, which are kilometers long aren’t they, on these observatories. How do you actually detect a difference that minute?
Shane - Gravitational waves, they warp the shape of the detector in a very definitive way. They tend to take one of the arms and make it longer at the same time that they take the other arm and make it shorter. And so we look for that consistent pattern between the two arms to be confident that we're seeing something from this cosmic event.

Chris - How do you actually know that that corresponds to a couple of black holes that were twirling round each other now coalescing to make a big black hole?

Shane - Everything that generates gravitational waves makes a very specific pattern of waves depending on the mass, as well as how that mass is moving relative to other things around it. Black holes look different than, say, neutron stars because black holes behave differently when they get close to each other than neutron stars do.

So we look at the shape of the waves and the shape of the waves encodes in it what made the waves, and so this is very analogous to the way we do astronomy. Traditionally, when you look at a star through a telescope the light encodes all the information about the star. When I look at the light from a star I can tell how hot it is based on the colour of the light. I can tell how fast the star is rotating based on whether or not the colour of the light is shifted towards the red or shifted towards the blue, right? All of that information is encoded in the bit of information that we get. And our job as astronomers, whether were astronomers who use light or astronomers who use gravitational waves, is to understand how to extract that information from the waves that we get.

Chris - And how far away are the detections that you're describing in the new papers?

Shane - The new detections are some of the most distant gravitational wave sources that we've detected so far. They're, kind of, 3-4 billion light years away from Earth.

Chris - That’s quite a long way isn't it? Now, some people though, actually quite reputable people, have said that actually you've got your sums wrong and they've been a number of headlines saying that, actually, what you're seeing isn't gravitational waves it's actually noise in your apparatus or some other explanation for the signals you're seeing. Have they got a point?

Shane - The core root of all science is intense scrutiny by our colleagues and our understanding of our data in response to that scrutiny. We are very confident that the detections that we've made are all detections, for a variety of reasons. We have multiple detectors that all see the same thing. And now with the release of this new catalog we have 10 different binary black hole events that all agree with the behaviour for black hole gravitational wave emission.

So we're confident that these events are definite astrophysical events. There's an additional measurement that we made last summer, where we detected the gravitational waves from a binary neutron star merger. So a neutron star is a slightly different kind of stellar skeleton. It's not quite as dense as a black hole but it still results from a star dying. And what's important about that detection is, we detected it both in gravitational waves and with telescopes. So we have extraordinary confidence in that detection because we've confirmed it using very traditional astronomy methods as well.

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