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Europe's LISA Pathfinder mission explained

Zulfikar AbbanyJune 6, 2016

The LISA Pathfinder satellite is Europe's moonshot mission in the international race to detect gravitational waves. Has the team behind it made a breakthrough in just three short months? Here's the lowdown.

LISA Pathfinder
Image: AEI/MM/exozet

The European Space Agency (ESA) says its LISA Pathfinder satellite is "paving the way for future missions." But future missions for what, exactly? The answer has a whole lot to do with gravitational waves.

These two words may fill you with a world of confusion, but hang tight, because they can also open up a universe of possibility. Luckily, Professor Karsten Danzmann, a co-principle investigator of the LISA Pathfinder mission, spoke to DW to help break the science down to size.

First, some background: LISA Pathfinder is a research spacecraft with a separable propulsion module. It was launched in December 2015 from Europe's spaceport in French Guiana and reached an operational orbit, about 1.5 million kilometers (930,000 miles) from Earth, in late January. Its mission began on March 1.

The main objective of the mission is to demonstrate the "key technologies that could be used for a future mission to detect gravitational waves" in space, providing a closer view than an Earth-based observatory, such as the setup used by scientists in the LIGO project.

LISA Pathfinder puts two test masses in a near-perfect gravitational free-fall to control and measure their motion.

"There's really nothing like [LISA Pathfinder]," said Danzmann, who also serves as director of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) and the Institute for Gravitational Physics at Leibniz Universität Hannover.

LISA Pathfinder is an ESA mission, which also carries a NASA payload
LISA Pathfinder is an ESA mission, which also carries a NASA payloadImage: Getty Images/ESA

"What you need to detect gravitational waves is the measurement of the distance between two free bodies," Danzmann explained. "Now, obviously, on Earth you have a problem with gravity because there are no free bodies. The best thing you can do on Earth is suspend the mirrors [used in an interferometer] to make them almost free in one direction. But only at high frequencies."

And the thing is, excitement usually comes at low frequencies.

"That's where the most spectacular waves are expected. So you have to go into space," Danzmann said. LISA Pathfinder is acting as "a gravitational wave detector with free-flying masses."

There is still one drawback to LISA Pathfinder, Danzmann admits: it has short arms.

"The only thing we don't have is the long arms that are required to detect low frequency gravitational waves," Danzmann said. "But those cost money, and if you only have one satellite, you can't have a long arm."

Hold on. Short arms, long arms…what are they?

Gravitational waves stretch and squeeze space by a fractional amount, and the absolute amount that you detect depends on the length of your measurement system. Got it?

Okay. And the interferometer thing?

An interferometer is a device developed in the late 1880s, used in many fields of science and engineering. The team behind LIGO (Laser Interferometer Gravitational Wave Observatory) says they "work by merging two or more sources of light to create an interference pattern, which can be measured and analyzed."

LISA Pathfinder
The LISA Technology Package core assembly, hosting the two test masses and, in between, the interferometerImage: ESA/ATG Medialab

LISA Pathfinder also uses an interferometer. The name LISA in fact stands for Laser Interferometer Space Antenna.

So why detect gravitational waves? Is it just to prove Einstein right?

Albert Einstein predicted gravitational waves in his general theory of relativity more than a hundred years ago. Since then, physicists have been trying to prove they exist. Just this February, scientists working on the LIGO project claimed they had detected gravitational waves emitted from two colliding black holes.

But do such discoveries matter to us on Earth?

"We're not doing this to invent new frying pans," Danzmann quipped. "We're doing fundamental research, so I can't tell you what the applications will be. But then again, when the first transistor was discovered nobody had a clue what it was good for, and when the first computer was built the world market for computers was estimated at four pieces. So I'm rather optimistic that over the next few years there will be some benefit."

Surely there will be, even if only from the technology that LISA Pathfinder is testing in space.

Then there is the pure science aspect.

"We know almost nothing about the universe. More than 99 percent of the universe is dark and will never be observed with light or any other type of electromagnetic radiation," Danzmann said.

Prof. Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) and Institute for Gravitational Physics at Leibniz Universität Hannover
Karsten Danzmann, LISA Pathfinder, co-principle investigator: "We're the only player in the game!"Image: picture-alliance/dpaJ. Stratenschulte

But everything interacts via gravity. And since everything that interacts via gravity and forms structure emits gravitational waves, scientists hope that this dark part of the universe will one day be lit up for us. They don't know what exactly what is lurking way out there, but they believe it will be very different than what we know now.

International competition is instense. Is Europe in ahead with LISA Pathfinder?

"Europe is not just ahead," Danzmann said, "we are the only player in the game." There is the occasional observation from here on the ground, but they tend to be of "light objects in our backyard."

"Even the LIGO event which they saw happened at a distance of 1.3 billion light years. Cosmologically this is still our backyard," he says. "Whereas the objects we're after with the space detectors, they are at the beginning of space and time. Some day we [hope to] listen to the big bang itself."

Enough with the suspense. Has the European team made a breakthrough?

We'll all just have to wait until they tell us on Tuesday, when LISA Pathfinder scientist Dr. Martin Hewitson and others invites Reddit users to "ask them anything." As always, we'll have the news that day as it breaks.