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LIGO vs. LISA: the race for gravitational waves

Gravitational waves were first detected on Earth. But soon the search continues in space. This Tuesday the satellite LISA Pathfinder will be switched off. Her data will live on in a new experiment.

If it wasn't for Albert Einstein, we may never have started looking for gravitational waves. Einstein predicted the existence of gravitational waves about 100 years ago. But it was September 2015 before the world saw the first evidence of them.

Physicists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) gave out a worldwide shout when they spotted them. LIGO has two detectors or antenna, each 3000 kilometers apart, with one in the US state of Louisiana and the other in Washington.

The detectors consist of L-shaped vacuum tubes, four kilometers long. LIGO uses lasers to measure with high precision the distance between hanging mirrors in the tubes. It's an international project - some of LIGO's technology was  also developed by scientists at the Max Planck Institute for Gravitational Physics in Potsdam and Hannover/Germany.

And since their first discovery in 2015, the scientists have made two further discoveries of these much sought-after gravitational waves.

Deutschland Max-Planck-Institut Gravitationswellen (picture-alliance/dpa/M. Hanschke)

This is, how researchers at the Max-Planck Institute for gravitational physics visualize gravitational waves.

But what are gravitational waves? Well it's thought gravitational waves occur when two black holes collide in deep space. If we can prove their existence, we may be able to prove a whole lot more about our universe as well. These discoveries were likely to have taken place between 1.3 and 3 billion light years away from Earth.  

Green light for a space lab

But this American Earth-based detector is not alone. The European Space Agency (ESA) has been working on its own plan to collect even more precise measurements with a space-based satellite laboratory.

LIGO's success has given the Europeans renewed impetus. It was decided this year that ESA's plans will in fact go ahead - possibly by 2034.

The project is called Evolved Laser Interferometry Space Antenna / New Gravitational Wave Observatory (eLISA/NGO) - or just LISA for short.

The core of the observatory will consist of three satellites at a distance 2.5 million kilometers apart. They will be connected by a single laser beam, which will measure the distance exactly. If there's a gravitational wave, the distance between them will change. And the detectors will precisely record the pattern of the wave.

LISA Pathfinder (ESA/ATG Medialab)

LISA Pathfinder was only testing the detector design. A laser beam measured the distance between two small cubes.

Successful test in space

The first step to ESA's space-based observatory has been taken. LISA Pathfinder, which was a test-phase mission, ends on July 18, 2017, having shown its technology works in space. The satellite did not detect any gravitational waves, but its technology may be used on its big sister, LISA.

Scientists behind LISA Pathfinder limited the test phase to much shorter distances than 2.5 million kilometers. It was a self-contained experiment in which a laser measured the distance between two cube objects, with an edge length of 46 millimeters, floating in microgravity. The two cubes were just 38 centimeters apart. And instead of measuring gravitational waves, the detectors measured the impact of micrometeoroids, like tiny grains of dust, on the cubes as they set the cubes in motion.

In the end, LISA Pathfinder showed its instruments were five times more accurate than the scientists had hoped. The scientists are satisfied that the instrument is suitable for the coming LISA mission.

For now, though, it is curtains for LISA Pathfinder: mission complete. It will leave its current earth orbit before entering a so-called graveyard orbit around the sun.

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