Science, like so many other things, has tipping points. Figure out how to isolate and identify one virus and you’re on your way to bagging them all. Spot a single exoplanet and within a few years, you’ll have thousands. The same is true with the cosmic phenomenon known as gravitational waves. It was in 1915 that Albert Einstein first predicted that the fabric of spacetime itself could be set to rippling by a powerful enough force, but it wasn’t until 2015 that the first waves were spotted. Three months later, it happened again.
Now, investigators from the Laser Interferometer Gravitational-wave (LIGO) Observatories in Hanford, Wash. and Livingston, La., have announced a third detection. This one fills in a significant gap left by the first two breakthroughs and takes the science a big step forward.
In theory, anything can set off a gravitational wave. Roll a pair of bowling balls into each other on a trampoline and you could feel the collision radiating across the surface. A pair of pebbles would have a similar effect, but it would be too small to detect.
On the scale of the universe—which is roughly 46 billion light years in all directions—it would take bowling balls of a considerable size to create a disturbance that would generate a similar ripple noticeable on Earth. There aren’t many things with that kind of mass out there, but black holes will do nicely.