gravitational waves

Gravitational waves are ripples in the curvature of spacetime itself, set off when massive objects accelerate. Just as a stone dropped in a pond sends rings spreading outward, two stars whirling around each other stir the fabric of spacetime and send waves rippling away at the speed of light. As a wave passes, it gently stretches space in one direction while squeezing it in the perpendicular direction, then reverses, over and over.

Einstein predicted them in 1916, soon after completing general relativity, by noticing that his field equations allowed wave-like solutions much as the equations of electromagnetism allow light. For decades many doubted they were real rather than a mathematical artefact, and even Einstein wavered. The effect is staggeringly weak: a strong wave changes the distance between two points by less than a thousandth the width of a proton over a kilometre, which is why it took a century to catch one directly.

These waves carry energy away from their source, which slowly drains the orbit of a binary system and makes the two bodies spiral closer together. This steady, measurable loss was first confirmed indirectly in the 1970s by watching a pair of dead stars wind inward exactly as predicted, earning a Nobel Prize and giving strong confidence that the waves were genuine long before they were heard.