First LIGO, now LISA: finding gravitational waves in space
We've picked up gravitational waves on Earth – now, astrophysicists intend to do the same from space. Belinda Smith reports on the first steps to a space-based detector.
Overnight, a spacecraft 1.5 million kilometres from Earth quietly released two metal cubes – and in doing so, passed a major milestone in the mission to pick up gravitational waves from space.
The European Space Agency’s LISA Pathfinder, which launched on 3 December 2015 from the Guiana Space Centre and slotted into orbit on 22 January, is a proof-of-concept mission to prove that two masses – in this case, a pair of identical 46-millimetre gold-platinum cubes – can fly through space, untouched but shielded within the spacecraft, and be “linked” by a network of lasers.
From LISA Pathfinder’s launch and its six-week journey to its work site, each cube was held in place by eight “fingers” pressing on its corners. Two weeks ago, the fingers peeled back, and two rods pressed gently on opposite sides of the cubes to hold them in place.
And this week, the rods retracted, leaving the cubes floating freely inside the centre of the spacecraft.
As the masses in the Pathfinder orbit the Sun, they are, in effect, in freefall. And for the next week, scientists will gently prod them into place using electrostatic forces.
“Only under these conditions is it possible to test freefall in the purest achievable form. We can’t wait to start running experiments with this amazing gravity laboratory,” says European Space Agency project scientist Paul McNamara.
The scientific payload, which was more than a decade in the making, will kick in next week. The team will stop manoeuvring the cubes; instead, they will ensure the spacecraft is centred on one of the masses, and use the lasers to measure any shifts between that mass and its sister.
They expect to run experiments for several months to determine how accurately the cubes can be positioned relative to each other – to the order of a millionth of a millionth of a metre.
If LISA Pathfinder is successful, we are a step closer to a full gravitational wave observatory – eLISA – planned to be launched in 2034.
Rather than two masses housed in the one unit, eLISA will comprise masses linked by lasers in spacecraft millions of kilometres apart.
As gravitational waves stretch the space-time between the masses, lasers pinging between them will be ever-so-slightly out of sync – much in the way that the four-kilometre arms of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) stretched in September last year, thanks to a cataclysmic collision between of two black holes 1.3 billion light-years away.
Space observatories such as eLISA will measure gravitational waves in the millihertz range, inaccessible to detectors even as sensitive as Advanced LIGO, and perhaps will uncover how massive black holes form, grow and merge.
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