Why a falling spaceship could help us understand the universe

By Rachel Feltman

An artist's impression of LISA Pathfinder. Picture / ESA-C.Carreau.
An artist's impression of LISA Pathfinder. Picture / ESA-C.Carreau.

Just four months after scientists announced the first detection of gravitational waves - ripples in space time that could help us learn about mysterious objects such as black holes - researchers from the European Space Agency have taken a vital step towards catching some for themselves.

Today, the team behind the Laser Interferometer Space Antenna (LISA) Pathfinder mission, the prototype of a spacecraft designed to detect gravitational waves from space, announced a record-setting free-fall test.

The test, described in a Physical Review Letters study, showed that the spacecraft's golden innards are capable of experiencing the closest thing to free fall ever observed in a man-made object.

"With LISA Pathfinder, we have created the quietest place known to humankind. Its performance is spectacular and exceeds all our expectations by far," Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics and director of the Institute for Gravitational Physics at Leibniz University, in Hanover, Germany, said.

Everything in space, including the International Space Station, is essentially in free fall, falling rapidly under the force of (almost) nothing but gravity. But most objects aren't quite in literal free fall. Lots of things can give satellites and other space objects tiny pushes in one direction or another. Even sunlight exerts physical force on objects it touches.

But as you might recall from our coverage of the first detection of gravitational waves, sensing them requires an entirely unnatural level of stillness and quiet.

Gravitational waves are the ripples in the pond of space time. The gravity of large objects warps space and time, or "space time" as physicists call it, the way a bowling ball would change the shape of a trampoline as it rolls around on it. Smaller objects will move differently as a result - like marbles spiraling toward a bowling-ball-sized dent in a trampoline. Large objects should all produce detectable gravitational waves, but giant black hole collisions and other space disasters send them out like beacons.

Because these waves can pass through physical objects without changing (unlike light) they could be used to carry remarkably pristine data about these events back to detectors on Earth. And they could be used to study "dark" objects, such as black holes, that don't emit light for us to detect with traditional telescopes.

But in the grand scheme of things, these waves are tiny. Any minute vibration from other forces could muck up a scientist's data. That's why LISA has to be oh-so-perfectly quiet and still.

To attain actual free fall in space, LISA - a proof-of-concept for a trio of spacecrafts that the ESA hopes to launch in a couple of decades - uses an array of thrusters to keep a pair of 4.6cm gold and platinum alloy cubes virtually immune to the effects of anything but gravity. Electrodes sense their position relative to the position of the outer spacecraft, telling the thrusters when to fire and create tiny forces to negate everything but gravity's pull.

When the LISA observatory mission launches in 2034, lasers will detect the tiny movements that occur between the two cubes despite this free-fall correction. Those movements will be the result of gravitational waves. The researchers believe the spacecraft's current sensitivity has surpassed the level needed to detect black hole mergers anywhere in the universe.

"The measurements have exceeded our most optimistic expectations," project scientist Paul McNamara said in a statement. "We reached the level of precision originally required for LISA Pathfinder within the first day, and so we spent the following weeks improving the results a factor of five."

- Washington Post

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