Albert Einstein’s predicted gravitational waves were first detected experimentally by the American LIGO team earlier this year. LIGO (Laser Interferometer Gravitational-Wave Observatory) comprises two huge ground installations in the United States, in which hydraulic rams are used to stabilize the four-kilometer-long arms of the interferometers against terrestrial perturbations. A prodigious feat of engineering in itself.
In a news release, 7th June, 2016, the LIGO team congratulated the European Space Agency on the proof-of-principle of its space-based interferometer, Lisa Pathfinder (Laser Interferometer Space Antenna). Le Figaro’s senior science reporter, Tristan Vey, explains.
Lisa Pathfinder has shown that it is possible to study gravitational waves in space
Tristan Vey | Le Figaro | 8th June 2016
Two short months. It didn’t take long for the European satellite Lisa Pathfinder to demonstrate that the technologies required for the construction of a space telescope capable of observing gravitational waves, were already mature.
Gravitational waves, observed for the first time in February  by terrestrial detectors, are small ripples in space-time emanating from extremely energetic events, such as the fusion of black holes, or supernovæ. These waves, which propagate at the speed of light, cause space-time to vibrate like a jelly. Their passage thus has the (exceedingly small) effect of dilating and shrinking the distances between objects in their path. This can, for example, move earth and sun towards or away from each other by a fraction of a micron. By placing three satellites in orbit around the sun and measuring their relative positions, it should be possible to observe the passage of gravitational waves, and deduce valuable information about the cataclysmic events that produce them.
But can it be possible to maintain the satellites in constant relative position with an accuracy of some dozen picometers, being one millionth the thickness of a hair? The European Space Agency (ESA) has invested €500M in a demonstration satellite, Lisa Pathfinder, to find out. And it turns out that it is indeed possible.
On Tuesday, 7th June, ESA announced, accordingly, that the first 55 days of measurements had far exceeded the expectations of the scientists. In an article appearing in Physical Review Letters, the teams showed that the performance of the experimental craft was excellent. “The results are between five and seven times better than the benchmarks set by ESA”, enthused Antoine Petiteau, a researcher at the laboratory for astroparticles and cosmology (University Paris VII-CNRS), and head of France’s contribution to the mission.
Lisa Pathfinder began its measurements on the 1st March from a point of equilibrium between the earth and the sun, that is, at 1.5M km from the earth. The trick to ensuring that the satellite stays perfectly in place, consists in placing at its centre a small mass floating in the void. This is affected only by its weight, and is free of all interference from external sources: essentially the photons and charged particles emitted by the sun that strike the craft and constantly perturb its position. The floating mass provides a perfect reference point, and the position of the satellite is adjusted such that the mass remains immobile in relation to it.
But how to verify that the required degree of precision is achieved? A second, identical, free-floating mass is placed at 38cm from the first. “The two masses of course attract each other”, explains Antoine Petiteau. The [gravitational] attraction being in proportion to [the product of] their masses and the inverse of the square of the distance between them, — for those who have forgotten the Physics courses of their youth. “This is enough to move them closer by 20 microns per day.”
What interests the researchers, however, are other perturbations, much finer, that propel the masses at a speed in the order of one millimeter per year (ten times slower than continental drift). “Before launch, the small masses are kept in evacuated chambers”, explains Antoine Petiteau. “But the vacuum is not as good as that existing in space. When we open the chambers, the remaining molecules escape little by little. But they are still sufficient to strike the masses and move them by minute amounts.”
Other phenomena, linked for example to thermal fluctuations, also play a role. “We’re looking for the best ways to identify the causes of perturbations”, emphasizes the researcher. “This will be the subject of further articles in the coming months.”
However, the most important criterion has already been met: the stability of the satellite exceeds by a wide margin the specifications demanded by ESA. “The technological challenge is nevertheless not over”, warns Antoine Petiteau. It still remains to be demonstrated that we can use [on board] lasers to exactly measure the relative positions of the three satellites, and thereby detect any perturbations that betray the passage of gravitational waves.”
The discovery of the first gravitational waves at the beginning of the year [by the LIGO team], combined with this proof of principle, could accelerate Lisa Pathfinder’s calendar, which originally reached out to 2034. “ESA appears to want a final design by 2020, for a possible launch before 2030, perhaps 2029”, adds Antoine Petiteau. The cost remains highly speculative: of the order of €1bn. To be confirmed. §