LISA Pathfinder free fall test beats expectations

Femto-gravity at Lagrange point makes space a good place to look for gravitational waves

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European Space Agency (ESA) boffins are tossing hats in the air as data from their LISA Pathfinder mission suggests its gravitational wave detection kit is going to live up to expectations.

In the popular mind, LISA – the Laser Interferometer Space Antenna – has been upstaged by the Laser Interferometer Gravitational-wave Observatory (LIGO) “we've found gravitational waves!” result earlier this year, but the situation is more nuanced.

The full-scale LISA project would not only provide a platform for independently replicating the LIGO discovery, it would offer up different observational possibilities.

Before that happens, the mission scientists need to prove their technology will be viable. Hence the smaller-scale LISA Pathfinder project that the ESA is celebrating.

The team in charge of the program have published a paper in Physical Review Letters in which they say the Pathfinder's two gravitational masses have successfully been put through a “sub-femto g” free fall.

The two gold-platinum 2kg masses are the key instrument that would look for gravitational waves, so they need to be free from local gravitational effects, as far as is possible.

As they say in the abstract of the paper, the value measured in the free-fall experiment beats the LISA Pathfinder requirement by “more than a factor of 5”, and is “within a factor [of] 1.25 of the requirement for the LISA mission”.

In lay terms, that means the test masses were nearly motionless with respect to each other, experiencing a relative acceleration lower than “one part in ten millionths of a billionth of Earth's gravity”, an ESA statement said.

As a result, above a very small noise floor, if the test masses experience a measurable relative acceleration, it's likely to be the momentary squeezing of space as a gravitational wave passes by.

The LISA Pathfinder is orbiting at the L1 Lagrange point, a spot where the gravity of Earth and Sun cancel each other out – which is why the mission was able to achieve its femto-gravity.

As well as offering a platform to independently verify the LIGO results, the ESA says LISA will be able to detect gravitational waves emitted across a broader frequency spectrum than the roughly 100 Hz signals spotted by LIGO.

The ESA says supermassive black hole mergers, for example, are expected to result in lower frequency oscillations than LIGO measures. ®


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