How LIGO made waves in astronomy

LIGO directly detected gravitational waves for the first time, but how did they do it?
26 February 2016

Interview with 

Professor Norna Robertson, CalTech, and Professor Sheila Rowan, University of Glasgow


One of the predictions Einstein made that hadn't yet been observed were the gravitational waves, and this is what the LIGO team found this month. Graihagh Jackson was there for the announcement, and spoke to scientist Professor Norna Robertson about how they did it...

Norna - The event that we say was two black holes which were orbiting each other and they're also moving towards each other because they're losing energy as they orbit each other and so they speed up and go round and round, faster and faster until they finally merge. And it's that final inspiral and merger, which happens in a fraction of a second, that produce a big burst of gravitational waves.

Graihagh - And that's rippled across the universe to us.  How long does it take to reach us though?

Norna - The event happened something like one billion years ago, a billion light years away, and it's take all that time to ripple across space towards us and pass through the Earth on September 14th, 2015.

Graihagh - It's quite remarkable really, isn't it?

Norna - It is remarkable.  It's wonderful.  There have been many, many people involved in developing detectors and developing all the analysis techniques and, for all of us, this is really a momentous occasion.

Graihagh - Alongside Norna, something like a thousand scientists across 16 countries have been working together for 25 years!  And, like Norna, Sheila Rowan from Glasgow University has spent her whole career searching for them...

Sheila - I wanted to be a scientist and wanted to be a physicist, I think, since I was about nine years old.  When I was young, I couldn't think of anything more exciting to do in life than spend it studying these big questions and the universe.  When you go out and you look up, where did it all come from?  What's out there?  How far does it go? And I've been lucky enough that I've been able to spend my life working in this area and doing that.

Graihagh - Lucky enough to also see all her hard work come into fruition.  But how did LIGO detect them?

Sheila - When they're produced, of course, there's a huge amount of energy as two black holes collide but then that's got to spread out and travel across the universe.  So, by the time it gets to us here on earth, it's a tiny signal and that means it's hard for us to build instruments that are sensitive enough to do that.  And the way we do it is we take light from a laser, we split that laser light into two and we send it out along two four-kilometer long paths.  It hit mirrors at the end of those paths, those mirrors send laser light back, the light then adds up again there, and whether it adds up so that you get a bright spot or whether it cancels itself out and you get a dark spot, depends on how far the light has travelled on that four kilometer path.  Now what a gravitational wave does is it changes the lengths of the arms, the paths that light has travelled and, fundamentally, it does that by shaking the mirrors that we've put down.  The trouble is it doesn't shake them very much - it shakes those mirrors by about 1/10,000 of the size of a proton inside an atom.

Graihagh - So how would you ever measure that?

Sheila - It's a big challenge and that's one of the reasons it's taken decades of work to do this, and there are various things that are key.  One incredibly important thing, of course, is to take those mirrors that the gravitational wave's going to shake and make sure that nothing else shakes them.  So, we couldn't just sit them on the ground because the ground moves all the time.  It shakes due to far away earthquakes, it shakes just due to people driving cars past, so we can't do that.  Instead what we do is we take the mirrors and we actually hang them...

Graihagh - Now this isn't how you'd hang a mirror on the wall - no siree.  Because a gravitational wave passing through would move a mirror by less than the width of a proton and all this other stuff that Sheila mentioned: seismic activity, cars even, would move the mirrors and could give us a false positive.  So how do you make a motionless mirror - I hear you ask?  One of the key things is what you hang the mirror with.  LIGO have use ultra high tech glass or silica to hang it because silica molecules don't wobble around too much.  You can think of this as kind of like the fanciest shock absorbers around.  This makes the mirror almost motionless.  The final key component is the fact that there are multiple devices that record the movement of the hundreds of components that all connect to the mirror.  Knowing how much these various bits of machinery move it means that with great precision they can account for these tiny movements. Now that they've made these motionless mirrors and even detect one gravitational wave, when will they detect the next one?

Sheila - We don't know the answer to that yet.  We do have more data, we just haven't had time to look in there yet and see what's in there.  So, we don't know, you'll have to wait to hear back from us but we promise we're looking hard.

Graihagh - Watch this space then?

Sheila - Or as my colleague in Glasgow often says "watch this space time".


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