Definition
Gravitational waves are ripples that travel at the speed of light when a massive object accelerates quickly and makes spacetime, the fabric of the universe, tremble. Einstein's general theory of relativity first predicted them, and they stretch and squeeze space itself.
Ripples on a Cosmic Trampoline
Picture a tightly stretched rubber sheet. Put two bowling balls on it and spin them around each other. The sheet shakes hard, and round ripples spread outward.
Space is a lot like that rubber sheet. Einstein explained that anything with mass bends spacetime. When two incredibly heavy objects, like black holes, spiral around each other or crash together, they send out waves that make spacetime itself ripple across the whole universe. Those are gravitational waves.
These waves don't push through matter the way a gust of wind does. They stretch and squeeze the space that matter sits in. So the distance between objects grows in one direction and shrinks in the perpendicular direction, again and again.
How to Catch a Wobble Smaller Than an Atomic Nucleus
Every gravitational wave detected so far began tens of millions to billions of light-years away. By the time one reaches Earth, it is so faint that a detector arm 4 km (about 2.5 miles) long changes in length by only about one-thousandth the width of a proton.
To measure this, scientists built giant detectors called LIGO. Each one is a huge L-shaped vacuum tunnel. Laser beams are sent down both arms at once, bounce off mirrors at the ends, and come back. Then the light waves are overlaid and compared.
When a gravitational wave passes through Earth, one arm gets stretched by a tiny amount while the other gets squeezed. By measuring that difference precisely, scientists picked up a wobble in space far smaller than an atom. In 2015, they made the first direct detection in human history of gravitational waves from a black hole collision.
To Be More Precise: A New Sense for the Universe
To be more precise, discovering gravitational waves completely changed how we explore the universe. Until now, we mostly relied on electromagnetic waves, or "light," such as visible light, radio waves, and X-rays, seen through telescopes.
But light has a weakness. Thick clouds of interstellar dust and gas can block or scatter it. Gravitational waves, on the other hand, interact with matter only very weakly. They pass through space with almost no absorption or scattering, so they carry information from deep in the universe in a relatively intact form.
Even the merger of black holes, where not even light can escape, can be glimpsed through gravitational waves. Waves produced in the universe's earliest moments haven't been detected yet, but if we find them, they're expected to tell us about those first moments. That's why astronomers call gravitational wave detection a historic leap: in addition to our "eyes" on the universe, we gained a new "ear" to hear the universe breathe.
Common misconceptions
Gravitational waves are electromagnetic waves or vibrations of matter drifting through space.
Gravitational waves aren't particles of matter or light spreading out. They are the stretching and squeezing of spacetime itself, the very thing matter sits in, being passed along.
Where you meet it
When two black holes, each dozens of times heavier than the Sun, spiral wildly around each other and merge into one, they blast out powerful gravitational waves.
When two neutron stars, each as heavy as the Sun but crushed down to the size of a city, collide, they send big ripples through spacetime that travel all the way to detectors on Earth.
In one sentence
Gravitational waves are ripples in spacetime itself, sent out at the speed of light when huge cosmic objects accelerate.