If space-time can curve, then it can also wiggle. Spotting those wiggles, turns out, is really hard.
A hundred years ago or so, Einstein published his General Theory of Relativity which said that space and time aren’t just the background arena for stuff in the universe to do things in — space-time *is* the stuff of the universe. It curves and interacts with matter and energy. As the great physicist J.A. Wheeler put it, “Matter tells spacetime how to curve. Spacetime tells matter how to move.”
Along with all the other mind-bending predictions of General Relativity came one elegant prediction: space-time can wiggle. Energetic events in the universe should create gravitational waves that propagate outwards across the cosmos, similar to the way electromagnetic waves (or, as call them, ‘light’) are emitted by accelerating electrons.
The prediction was easy, but a quick calculation showed a bit of a problem with detection. These gravitational waves are tiny. Like, really small. Brain-breakingly weak. So weak, it took a hundred years to catch one moving through a pair of insanely sensitive detectors called LIGO.