We’ve found gravitational waves. Hooray! But we want to find more, from different sources, all across the universe. That’s going to be possible thanks to the European Space Agency’s (ESA) incredible LISA Pathfinder mission.
Yesterday, this mission passed a crucial phase as it tests out a rather ingenious system that could eventually find more gravitational waves. The spacecraft is positioned 1.5 million kilometers (1 million miles) from Earth in the direction of the Sun, having launched on December 3, 2015. On board, it has two 46-millimeter gold-platinum cubes, which will be placed in the most precise freefall ever obtained.
What do we mean by this? Well, the cubes will be essentially left to “float” in space, inside the spacecraft. The spacecraft itself will fire thrusters ten times a second to prevent the cubes from touching the sides. A laser beam will monitor the distance between the cubes, down to a millionth of a millionth of a meter. Wowza.
A scaled-up version of this system could detect gravitational waves, it's hoped.
“This is why we sent the test cubes into space: to recreate conditions that are impossible to achieve in the gravitational field of our planet,” said Paul McNamara, ESA’s project scientist, in a statement. “Only under these conditions is it possible to test freefall in the purest achievable form.”
Yesterday, the cubes were released into freefall for the first time, with rods previously holding them in place. Over the coming weeks and months, beginning on March 1, the cubes will be intently monitored, to see how accurate such a system can be.
But they are separated by just 38 centimeters (15 inches), far too small a distance to actually measure any gravitational waves. To detect gravitational waves, like LIGO did, you need to be able to measure the tiny, tiny ripples in the fabric of space-time caused by a passing wave. LIGO achieves this by firing lasers down tunnels 4 kilometers (2.5 miles) long at right angles, and detecting any change in the time it takes for the laser to bounce off a mirror at the end and return to the start.
Shown is a white dwarf orbiting a pulsar, one possible source of gravitational waves. ESO/L. Calçada
LISA Pathfinder is instead a precursor to a much grander mission: eLISA (Evolved Laser Interferometer Space Antenna). It has been proposed as part of ESA’s Cosmic Vision Programme, and it will involve using three spacecraft positioned in a triangle, each separated by a million kilometers (620,000 miles). Like the cubes, a laser will be used to monitor the distance between the spacecraft – but the distance between them will mean eLISA is a billion times more sensitive than LISA Pathfinder.
The technology used on this mission could ultimately allow us to observe gravitational waves from supermassive black holes at the center of galaxies, and other distant cosmic sources. We might have to wait a while, though; eLISA has a tentative launch date of 2034, and it has been refused funding before.
But at the very least, this latest achievement is a cause for optimism, and the LIGO detection of gravitational waves should mean eLISA has a greater chance of going ahead. “Releasing LISA Pathfinder's test masses is another step forward in gravitational wave astronomy within this memorable month,” said Stefano Vitale of University of Trento, Italy, Principal Investigator of the LISA Technology Package, in the statement.