The Hidden Science of the Missing Gravitational Waves

Space should be churned up like a speedboat-filled lake, crisscrossed by gravitational waves rushing at the speed of light in every direction. That’s because any kind of acceleration, of any kind of mass, will produce a gravitational wave. When you whoosh your arm through the air, you are launching a gravitational wave that will travel forever. The Earth produces gravitational waves as it orbits the sun. So do black holes that twirl around or crash into each other.

Every accelerating mass produces a signal, and all those signals should add together into a detectable background.

So where is it? Scientists have been trying to tune in to the staticky drone of gravitational wave background noise for years. An experiment that uses the timing of distant pulsars has been running for over a decade, searching for the portion of the background due to pairs of supermassive black holes. But they haven’t heard a peep.

Then, early this year, the Laser Interferometer Gravitational-Wave Observatory (LIGO) achieved a positive detection of a single gravitational wave event, resulting from the merger of lighter, stellar-mass black holes. The more subtle mission of the pulsar timing experiments and their search for background seemed to get drowned out. They have, after all, produced a null result.

But sometimes silence speaks volumes.

Gravitational waves come in different frequencies, just like light waves. Their frequency is based on their motion—objects in a year-long orbit, no matter their mass, will make waves with the same frequency (though lighter objects will produce a lower-amplitude wave).