You must be very busy preparing for the launch – thanks for taking the time to talk with us. LISA Pathfinder will be testing the concept of gravitational wave detection, can you explain exactly what these are?
Often we think of space as a big empty vacuum, but you can also think of it as a space-time – a four- dimensional fabric with both length, breadth, height and time, that pervades the universe. According to the ideas of Isaac Newton, if something happens somewhere in space – say something explodes or two stars merge together – then instantly we would know that the gravitational field has changed. We have what we would call ‘instantaneous action at a distance.’ But then along came Einstein, and he said that information cannot travel faster than the speed of light: one of the outcomes of the general theory of relativity, which is a hundred years old this year, is that there has to be something to carry the information of gravity. When you do the mathematics, this seems to have a wave-like nature, and these are what we call gravitational waves – they’re essentially carrying the information of gravity throughout the universe.
When we think of waves in water moving up and down, that’s a ‘dipoles’ wave, but gravity’s a bit more complex – its waves are ‘quadrupoles’. One way of thinking about that is to imagine a ring – in one half of the wave cycle it gets taller and thinner, and in the second half-cycle it gets shorter and fatter. There’s a continuous stretching and squeezing as it travels through space, which in turn changes the fabric of space-time itself.
So what sort of objects do generate these gravitational waves?
They’re produced by pretty much anything – you, me, cars, planes. But we’re just tiny, and space is very ‘stiff’ – it really doesn’t like to deform and so while we do produce gravitational waves, they’re minuscule in terms of amplitude. Basically we’re looking for violent things – to get a signal that’s strong enough to measure on Earth, or in orbit near Earth, it really has to be a very big, violent event in the universe. Waves are created by an acceleration of mass, so if you think of two objects orbiting each other you can have an angular acceleration – if you have two stars orbiting and you’re viewing them from a long way away then at some times you see the two stars separated, with one gravity field, and at other times they’re roughly in line with each other.
Think of the electromagnetic spectrum and you have radio telescopes and X-ray telescopes – very different instruments but they’re still measuring parts of the electromagnetic spectrum. It’s just to do with wave frequency, and with gravitational waves it’s exactly the same. With Earth-based detectors you’re measuring high-frequency waves from things that oscillate hundreds or thousands of times a second, while with space-based detectors you may look at things that change over thousands of seconds – six orders of magnitude difference in frequency.
Ground-based gravitational wave experiments like LIGO in Louisiana, USA, and VIRGO near Pisa in Italy are looking for events involving stellar-mass objects – things like neutron stars, supernovae and stellar black holes, up to about ten times the mass of the Sun. But up in space we’re hoping to look for the really big guys – things like galaxies smashing into each other, 10-million-solar-mass black holes and stuff like that.
How do we detect them and why do it in space?
The most popular solution is to use laser interferometry – you shine light out along two ‘arms’, reflect it off a flat surface, and compare the phases of the outgoing and returning laser light waves from each direction. If a passing gravitational wave causes the kind of cyclic distortion in space-time we’d expect, then the phases should shift as the distance along the arms alters.
Doing it in space has many advantages, but the biggest is the frequency range. Most of the things we’re looking at on the ground are ‘burst sources’ that appear for a tiny fraction of a second and then are gone – that could be a supernova, or the coalescence of the very moment of merger between neutron stars or small black holes. So you turn on your detector and hope something happens, and if it does then you do science on the thing you’ve measured. In space, the lower frequencies we can detect mean we could track these kind of events for years, as they slowly spiral in towards a merger. Another big advantage is we could measure waves from white dwarf binaries – pairs of compact stellar remnants with about the mass of our Sun, which are much more common than the heavier systems. We think there could be tens of millions of these in the galaxy – too many to distinguish from each other. In general they’d create a background noise, but there are some systems we already know about close to Earth – we can predict the exact signature of waves they should be producing according to general relativity, and we can look for that and hopefully verify that they’re behaving as predicted by Einstein. You can’t do that from the ground because these low-frequency signals would be drowned out by things like cars driving by, clouds moving overhead, and even ocean waves. On the ground you’re really limited to high frequencies and can’t hope to detect waves of, say, ten hertz (cycles per second) and above.
Another big difference is arm length. On the ground there are obvious limits to how big you can make your detector, and how weak the waves are you can detect. The big space-based LISA detector for which LISA Pathfinder is a first step could be anything between one and five million kilometres (between 621,370 and 3.1 million miles) long. All you’re limited by is the number of photons you can detect at the end of the path, and you can just increase that laser power if you want to go further.
So how exactly will LISA Pathfinder take that first step?
Pathfinder really is just a technology demonstration. This isn’t a detector itself, but the big issue has always been how you go about measuring a million- kilometre (621,370-mile) length with picometre (one trillionth of a metre) accuracy. Measuring the small distance over a long arm is theoretically fine, but the question is, what do we measure? We need something that’s susceptible to the fluctuations in space-time, but is isolated from the fluctuations of the Solar System and the spacecraft.
So LISA Pathfinder is a satellite with two gold- platinum tubes called ‘test masses’ floating freely inside: these tubes should be susceptible only to fluctuations in space-time – not to local factors like the pressure of radiation from the Sun, or from the environment of the spacecraft itself – factors like its thermal and magnetic properties. To make useful measurements, we have to keep these masses isolated from any external forces down to something like a quadrillionth of Earth’s gravity. Pathfinder is going to test whether this kind of measurement is actually possible.
How will the spacecraft do that?
Well you start with a very dense test mass. Force equals mass times acceleration, and we want to keep acceleration extremely low, so if you have any external forces, they’ll have less effect on a heavier mass. Gold-platinum alloy has a density of about 20,000 kilograms per cubic metre (1,250 pounds per cubic foot) so it’s ideal. What’s more, if you get the ratio of metals in the alloy just right, you get very close to zero magnetic properties. Then you put the test mass in a spacecraft, and you have to design a spacecraft that also won’t affect the measurement. We’d love to take this test mass and have it floating in space on its own, but we need this little shield called a spacecraft.
So for example there are no magnetic materials on board – everything’s made of titanium rather than stainless steel. We have to keep the test mass in a high vacuum because even though space itself is a vacuum, the inside of a satellite can be quite dirty as it starts to leak gas after launch. Even an atom bouncing off our test mass could completely wipe out our chance of measuring gravitational waves. We also have to be careful with the spacecraft’s temperature – unlike most satellites, if we want to use a system it has to be powered constantly rather than turned off and on when needed. This ensures the thermal properties are the same throughout the mission. The gravity of the satellite itself also has
to be balanced so that the mass is only affected by external gravity. We have a very detailed computer model of the spacecraft with every instrument box in it, and we have to optimise the locations to keep the gravity even. We position the boxes near the test mass to an accuracy of about 100 microns (one tenth of a millimetre, or 0.004 inches). We have to model the mass and position of the internal cables – and very close to the centre, we even have to take cable ties into account.
How does LISA Pathfinder actually work?
With LISA Pathfinder everything’s in the one spacecraft – effectively it’s one ‘arm’ of a very small interferometer complete with two test masses and a laser. What we’re testing is whether can we make the measurement in the first place – can we build something that will be sensitive to gravitational waves when we build the larger version of it?
To actually measure gravitational waves, you’d need the two separate arms. You’re essentially counting the ‘ticks’ of a laser light going along two different arms, and because you’ve got two arms you can cancel out a lot of the laser noise to make accurate measurements. But our arm is just way too short – we really need them to be more than a million kilometres (621,370 miles) long to detect the kind of waves we’re looking at.
And a significant issue with Pathfinder is that our two test masses lie on the same line – if our primary test mass starts to drift towards one side then the whole spacecraft will follow, and the other test mass could hit the wall, and then it would no longer be free as it’s in contact with the spacecraft. We have to constantly apply forces to the second test mass to make sure that doesn’t happen, and it’s the force we have to apply to that second mass that’s our primary science output. That’s simply not an issue with the full LISA configuration, where the two test masses are floating in different spacecraft [with the laser interferometer in a third], so they can be allowed to drift in whatever direction they want and take the spacecraft with them.
How long will the mission last?
The mission has three months for the European measurement, and then three months for a US experiment that’s also on board. Beyond the six months, there’s a variety of ideas for extended missions, since this is one of the most sensitive instruments ever built. We could look at micrometeoroids striking the spacecraft, near-Earth objects such as comets and asteroids, perhaps even look at some alternative theories of gravity.
Can you tell us a bit about the long-term plans for a full-scale LISA mission?
The European Space Agency selected the second and third large missions in its Cosmic Vision programme about a year ago – the second is an X-ray telescope called Athena, while the third will study the universe using gravitational waves. The first thing we need to do is carry out an industrial study, after which we can propose what the mission looks like – what we call an architecture. At the moment the best one we can find is a LISA-like mission, using gold-platinum tubes and laser interferometry to measure the distances.
The planned launch date for the mission is 2034, but these missions take time to get up and running and to get the technology built and tested. In 20 years time, I hope we’ll be getting very excited!