Some 31 million miles from Earth, three pairs of 4-pound cubes of gold and platinum alloy sit in space. Each set floats inside one of three protective spaceships fixed in an equilateral triangle measuring up to 3 million miles to a side. Lasers between the blocks detect incredibly tiny stretching and shrinking movements in space-time.
It sounds like a futuristic instrument, and in a sense it is. Scientists don't expect the project, which will detect gravitational waves, to launch until 2034. But researchers from the European Space Agency have been testing a model ship—a single experimental corner of the triangle. And, in a paper in the journal Physical Review Letters, they report that the extremely sensitive technology on board is working according to plan.
Called the Laser Interferometer Space Antenna Pathfinder, the spaceship clocks in at around 7.5 feet in diameter and launched in December 2015, into an orbit around the sun about a million miles from Earth (92 million miles from the sun). It carries the cubes without ever touching them—shielding them from solar radiation and wind while they fall freely through space unmoved except by the force of gravity. To stay in position around the blocks without bumping them, the LISA Pathfinder fires microthrusters that exert a force so gentle it's like that of a falling snowflake. Only under these conditions will the antenna be able to record the wildly small measurements it needs to make. The final device will have to detect changes in the distance between the metal cubes in the three separate spaceships down to the picometer—some 60 times smaller than the tiniest atom—and their relative acceleration to the femtonewton—a millionth of a billionth of the acceleration of gravity on Earth.
Researchers chose to build the cubes from gold and and platinum to make them easier to handle. The metallic blend keeps these test masses, as they're called, dense and small. "The bigger the object, the more difficult it is to measure its acceleration," says Stefano Vitale of the University of Trento in Italy, the lead investigator behind this stage of the project. The alloy also doesn't react to magnetic forces, which could overpower the gravity readings. "And also it looks good," he adds.
The trial antenna holds two cubes, separated by 38 centimeters. "It can't detect gravitational waves but it can detect disturbances," says Vitale. "We're testing our ability to put two test masses in space and create an environment without disturbances—a force equal to the weight of a bacteria on your hand would disturb it."
Researchers and engineers designed the ship to minimize electrical and other influences on its contents. They also balanced everything onboard to ensure that nothing generated any confounding gravitational forces of its own.
In this peaceful state, the antenna will detect the tiny shifts caused by gravitational waves, a phenomenon that Albert Einstein predicted a century ago and that was observed for the first time in February by the Laser Interferometer Gravitational-Wave Observatory, which has two detectors, in Livingston, Louisiana, and Hanford, Washington. When these faint ripples in space-time pass by, they will disturb the metal blocks in a way that the lasers between them can detect. The space-based antenna will measure a different, much lower frequency than LIGO can and it will have the sensitivity to see back to within several hundred million years of the Big Bang, much earlier than the ground-based observatory does.
"LIGO is looking at high energy, and we're looking at low energy, in terms of frequency," says astrophysicist Paul McNamara, the lead project scientist for the pathfinder project, likening the difference to that between X-ray and infrared light. The waves LIGO detected came from the collision of black holes each 93 miles in diameter and half the mass of the sun. The space-based antenna will measure universe-sized objects as massive as 10 million suns.
These are measurements that cannot be made from the planet. "You may think that you're sitting still in your office, but the ground is moving beneath you," says McNamara. "There is no quiet place." Even in the middle of North America, detectors this sensitive can pick up far-off earthquakes and the surf crashing on the coasts. "It would be shaking like crazy if we tried to make the low frequency measurements on the ground," he says.
The other advantage of the antenna will be its massive size, between 2.6 to 13 times the distance from the Earth and the moon, depending on the final design. As McNamara puts it simply, "you have to go to space to do it."
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