Taking the search into space: LISA pathfinder will contains technology that will underpin the future LISA missions.
© ESA/C. Carreau
Looking for gravitational waves on the ground comes with some natural limitations. Like, how long can you make the detectors arms? And what can be done about the fact that, gravitationally speaking, the Earth is a rather noisy place?
Well, there are ways to get around the arm length issue. Engineers can cleverly use mirrors to increase the distance the laser beam travels, giving them a much longer ‘virtual length’. To give you a sense of what can be achieved using the best mirrors, Advanced LIGO (Advanced Laser Interferometry Gravitational-wave Observatory) has arms four kilometres long, but its mirrors effectively bounce the beams back and forth about 280 times, giving its arms a ‘virtual length’ of 1,120 kilometres.
That might sounds like a long way, but to answer the Universe’s biggest questions, you need a much bigger detector…one with arms millions of kilometers long… and it has to be located in a gravitationally quiet place. To do that, you have to take the detector into space.
Currently slated for launch in 2034, the European Space Agency’s (ESA) planned Gravitational Wave Observatory mission, called the Laser Interferometer Space Antenna (LISA), is an attempt to do just that. It will consist of a constellation of three identical spacecraft, flying in formation to create a detector with ‘arms’ 2.5 million kilometres long. The distance between the spacecraft will be precisely monitored to detect passing gravitational waves.
Because its arms are so long, LISA will be able to detect gravitational waves emitted by some of the Universe’s earliest and biggest black holes, thousands of binary star systems and maybe even from the Big Bang itself. It is hoped that LISA will extend our capabilities to ‘listen’ to new kinds of dark phenomena in the Universe – perhaps even shedding light on the mysterious dark matter that makes up some 80 per cent of all the matter in the Universe.
Before the LISA can be launched, the technology underpinning it must be developed and tested. Launched in 2015, LISA Pathfinder – containing major payload contributions from the UK – is a huge step towards that goal.
The LISA Technology Package (LTP) aboard Pathfinder is a mini LISA arm, instrumented with much of the same technology that will one day help LISA ‘listen’ to the gravitational rumblings of the Universe.
At the heart of the LTP are two test masses (identical 46mm cubes) that are kept in a near-perfect free-fall, under the influence of gravity alone. These two test masses need to be unaffected by external forces(in space even something as seemingly feeble as the pressure exerted by sunlight could mask the gravitational wave signals) so that they remain almost motionless relative to one another while the spacecraft keeps its position around them. This is how future detectors will measure any changes in position caused by a passing gravitational wave.
The UK has played a significant role in developing and supplying the laser interferometer metrology and the test-mass charge management system being used in LTP to demonstrate the LISA measurement concept. The UK optical bench and readout system can detect exceptionally tiny changes (as small a few millionths of a millionth of a meter) in the separation of the test masses. UK scientists are also helping to analyse the data returned during LISA Pathfinder’s mission.
And the data returned has surpassed all expectations: during the mission so far, LISA Pathfinder has demonstrated that the two cubes are free falling through space, under the influence of gravity alone, five times more precisely than originally required.
With all of this work, it looks like gravitational wave astronomy has a bright future – and you can be sure that wherever it goes next, UK scientists and engineers will be at the heart of it.
Download our infographic for more information on how LISA and LISA Pathfinder will detect gravitational waves.
If you would like to know more about gravitational waves and the UK’s contribution to gravitational wave research, visit our dedicated page.