How to surf gravitational waves

Black holes colliding
Credit: © Multi-university SXS (Simulating eXtreme Spacetimes) project
Trous noirs en voie de collision
  • Trous noirs en voie de collision
  • Ondes gravitationnelles
  • LIGO - Observatoire Hanford
How to surf gravitational waves

Long ago in a galaxy far away, two black holes caught in a gravitational dance merged, causing a disturbance in the force... the gravitational force, that is. For a brief moment, their collision released 50 times more energy than all the stars in the observable universe. About 1.3 billion years later, the gravitational tsunami reached the shores of our planet.

On September 14, 2015, an international team of researchers succeeded in detecting a gravitational wave for the very first time!

The announcement, on February 11 of this year, took the scientific community by storm—and with good reason. More than “simply” observing the existence of gravitational waves, it confirmed a heretofore untested prediction made by Einstein in his general theory of relativity; it was also the first time the merger of binary black holes had been observed—a major technological triumph that opened a brand new window for studying the universe.

A one hundred-year wait

In 1915, Albert Einstein revolutionised science by describing gravity, not as an attractive force between two masses, but as a deformation of spacetime caused by masses, which affects neighbouring masses. So the Moon orbits Earth because Earth’s mass curves spacetime around it, which alters the Moon’s path: picture a rolling golf ball that follows an undulating terrain.  However, as masses move in space, the curvature they create follows them. In an article he published the following year, Einstein predicted that extremely massive accelerating objects should generate waves of spacetime that would propagate at the speed of light.

A ground-breaking detection

A century later, this prediction was validated by two LIGO (Laser Interferometer Gravitational-wave Observatory) detectors in the United States. The discovery was made by timing the distance travelled by a laser beam, along two “L” shaped pipes, each 4 km long and oriented in an east-west and north-south direction. The same signal was observed 7 milliseconds later by the second LIGO detector, which confirmed that a gravitational wave, originating in a region beyond the Large Magellanic Cloud, in the southern hemisphere, had passed through Earth.

At its maximum, the gravitational wave altered the length of the pipes by one thousandth of the diameter of a proton! At our distance from the origin, 1.3 billion light-years away, the wave was quite small, but it’s worth a Nobel prize to anyone who learns how to surf one.

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