Einstein's theories still making ripples

08 February 2016

Interview with

Christopher Moore, University of Cambridge

A century or so ago, Einstein suggested that the fabric of space should be globeperiodically punctuated by gravitational "ripples" produced by massive objects like black holes. And although all of his other theories have so far stood up to scientific scrutiny, no one has yet managed to find these gravitational waves. But, recently, there's been growing excitement across the scientific community and on social media about these ripples. But what are these waves, how are we hunting for them, and, if we find them, what can they tell us about the Universe? Georgia Mills took a trip to the Institute of Astronomy, in Cambridge...

Christopher - My name is Christopher Moore and I'm a student at the Institute of Astronomy in Cambridge and I'm studying for my PhD, and I work on gravitational waves.

So gravitational waves are typically very, very small ripples in the gravitational field, or space-time, and they travel through the universe at the speed of light.  So the gravitational field is something like the sun - you can imagine the sun as being a very heavy object sitting on a rubber sheet and the sheet bends in towards the sun and that curvature, that bending of the sheet, is what the gravitational field really is.

Georgia - Rather than thinking of gravity as a pulling force, you can think of it as curvature of the fabric of the universe - what we call space-time. To help me out, Chris had an idea for a demo we could do to visualise this...

Right, so let's give this a go.  I've brought along some props - I couldn't find a rubber sheets, I found my housemates bed sheet.  I'm sure she won't mind my borrowing it.  I've got grapefruit to represent the sun and I've got some planets here as well - some chocolates.

So here we go.  So we've pulled out the sheet, as tight as possible, and it's formed a straight surface.

Christopher - It should be infinite really but we haven't got an infinite bed sheet - so a large flat sheet.

Georgia - Right.  So this represents space time...

Christopher - Yes...

Georgia - I'm going to dump a sun in it. Right - the grapefruit, or sun, has pulled down the rubber sheet, or space time, and now it's formed a sort of dip in the middle.

Christopher - Yes, so the sun (the grapefruit) is in the middle of our sheet, and the sheet is bending in toward the middle of the sun and this represents the gravitational field of the sun.

Georgia - So let's test out this gravitational field with the planets... I'm going to roll all these planets along the sheet and see what happens...

Christopher - That's better.  You got nearly two orbits there.

Georgia - So some of our planets shot off into the abyss but some of them did some quite nice orbital shapes around our sun and eventually went into the middle.

Christopher - So the idea is, if you were to roll the marble fast enough across the sheet but not directed towards the sun, it should roll round and round the centre - the centre being the grapefruit - round and round and this is called an orbit.

Georgia - This curvature of space time means that planets are caught in the gravity of larger objects and they form orbits.  Our demo isn't perfect - the friction of the sheet means our planets actually slow down and fall into the centre.  Luckily for us, space is free of these frictions so our planet isn't spiralling towards a fiery inferno.  So can we use this demo to visualise gravitational waves?

Christopher - So, in our demo we had a large grapefruit as the sun and a very small round chocolate sweet as our planet.  If instead we had two grapefruits - so two large objects going round each other; as they move they constantly change the shape of the rubber sheet.  And if you looked at this from a long, long way away, you'd see small changes in the fabric of the sheet - small changes in the space time - rippling out from the centre of this system and these ripples would be the gravitational waves.

Georgia - When massive objects interact, they cause ripples along the space time which, in theory, could be detected by us.  Gravitational waves were predicted by Einstein almost 100 years ago so, have we managed to find them yet?

Christopher - No, we haven't.

Georgia - Well, how are we going about trying to find them -  I'm assuming scientists are trying their hardest?

Christopher - There's a number of ways you can do it.  The most popular approach is to hang a couple of mirrors a few kilometers apart and to try and measure the distance between these mirrors using lasers.  If a gravitational wave were to go through, between your mirrors, it would stretch and compress the space in between your mirrors and your mirrors would move.  And you'd be able to measure this as a change in the length of the distance in between your two mirrors.

Georgia - Isn't there a risk that all this other kind of stuff can get in the way?

Christopher - That's what makes it so difficult.  So the gravitational waves are extremely weak and if you build these two mirrors on the Earth, the signal is likely to be swamped by all other sorts of noise - wind, weather, seismic waves, earthquakes, all that sort of thing.  So that's what makes it hard to do.

Georgia - This challenge has been met head on by the scientific community.  There are several detectors up and running, including a-LIGO in America.  To avoid all of this chaos on Earth's surface, a giant L shape with vacuum chambers 4 kilometers across has been built.  While the E-LSA project aims to solve this problem by heading into space but there's one big question we haven't answered yet... Why do we actually want to find them?

Christopher - Up until now, all the astronomy that we have been able to do is using light or electromagnetic waves.  The systems that give off gravitational waves in large amplitude; the sorts of systems that we might be able to detect are things like: two neutron stars or two black holes in a very tight orbit and these are not the sort of systems that are ideal to study using traditional electromagnetic telescopes.  So these systems - it's much better to hunt for them using gravitational waves than it is using telescopes.

Georgia - So, if I understand it correctly, this would be kind of like another way of doing astronomy - as different from light is from sound.

Christopher - Yes, that's a very good analogy.  It's a completely new way of doing astronomy.  So you can think of normal telescopes as being your eyes and gravitational wave astronomy as being your ears and we're trying to listen to the universe as well as look at it.

Georgia - And I've being doing a little bit of searching about the this and the word "inflation" has come up a little bit in relation to gravitational waves - what's this about?

Christopher - So inflation is a theory that says the universe underwent a very rapid period of expansion back just after the big bang or very early on.  This process can produce gravitational waves; these are very, very low frequency gravitational waves with very, very long wavelengths, and you can hunt for these today by looking for particular patterns in the cosmic microwave background radiation.  So that's one way of hunting for very low frequency gravitational waves.

Georgia - So gravitational waves can be used to look at - I say current - more recent cosmological events like black holes and what neutron stars are doing but they also might provide a way to peer into the very ancient history of the universe?

Christopher - Absolutely, yes.  That's one possible way we can look back to the very early stages of the universe.

Georgia - Provided they're there?

Christopher - Provided they're there.  We have very good reason to think that they are there.

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