Most complicated experiment ever hopes to detect gravitational waves for the first time

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A proposed experiment hopes to find gravitational waves, which are hypothetical fluctuations in the fabric of spacetime. All it needs is three satellites orbiting the Sun, and lasers capable of detecting movements as tiny as a trillionth of a meter.

Albert Einstein first predicted gravitational waves in the early twentieth century. An outgrowth of general relativity, gravitational waves are incredibly tiny perturbations in the fabric of the universe, but on big enough scales it's possible to detect their presence. The major sources of gravitational waves are any binary systems that include a super-dense object like a white dwarf, neutron star, or black hole.

The sole indirect observation of the waves came in 1974, when astrophysicists discovered a pair of dead stars that were losing energy in a way that could only be explained by gravitational waves. That discovery ultimately earned the 1993 Nobel Prize in Physics, but the search for these waves has been at a standstill ever since.


That's where the Laser Interferometer Space Antenna, or LISA, enters the picture. The experiment hopes to directly observe gravitational waves for the first time. But, because we don't have any neutron stars handy, we're going to have to look for almost infinitesimal measurements of gravitational wave activity, and even to find that much movement will require some extraordinary steps.

Here's what LISA entails. Three spacecraft would be placed in orbit around the Sun, located in the Earth's orbit about twenty degrees behind. The three spacecraft would form a giant triangle connected by laser beams. Inside each spacecraft would be a cube made of platinum and gold that would be allowed to float freely. Gravitational waves would occasionally pass through the craft and cause the cubes' positions to change slightly. The lasers would be able to pick up on these movements and, just like that, you've directly observed gravitational waves. Easy, right?


The problem is that the movement caused by a gravitational wave would be incredibly minute. The spacecraft would all be about five million kilometers apart, and the movement caused by a passing wave would be only about 1 picometer, a distance that's 100 million times smaller than the width of a human hair. Since a picometer is a trillionth of meter, we're talking about a distance that's five quintillion times smaller than the distances between the satellites.

Even if you have lasers that are that impossibly accurate, there are still complicating factors. After all, these spacecraft will move around for reasons entirely divorced from any gravitational wave activities. The lasers themselves will create tiny amounts of noise in the measurements, and those could easily be mistaken for gravitational waves.


LISA's developers at the Jet Propulsion Laboratory have spent years diligently creating the tools to cancel out that noise and account for these movements. Their work has certainly impressed people - the U.S. National Research Council gave the project a high recommendation for funding earlier this year - but even if LISA can get up into space and start looking for waves, the task will remain immensely challenging.

JPL physicist Bill Klipstein explains:

"In order to detect gravitational waves, we have to make extremely precise measurements. Our lasers are much noisier than what we want to measure, so we have to remove that noise carefully to get a clear signal; it's a little like listening for a feather to drop in the middle of a heavy rainstorm."


So then, the task of directly detecting gravitational waves has gone from utterly impossible to vastly improbable. As these things go, that's a massive breakthrough.

[Physical Review Letters]