What are gravitational waves? | Science News


[chirp] That sound signals two black holes colliding in space. Not as loud as you’d expect for a monster merger, but back here on earth, that little chirp represents something revolutionary — the first direct evidence for gravitational waves. Two giant detectors that make up the Advanced Laser Interferometer Gravitational Wave Observatory (or LIGO for short) picked up waves from the cataclysmic collision of two black holes about a billion light-years away. The discovery opens up a new window on the cosmos much like the invention of the telescope did. A century ago, Albert Einstein predicted that big things, like planets, bend the fabric of spacetime. In Einstein’s universe, gravity isn’t a force — at least not in the way we typically think about forces. It’s just what happens when stars, planets and moons follow those curves. When an object — any object — accelerates, it produces ripples in the fabric of spacetime, known as gravitational waves. These energetic ripples radiate out and compress and stretch that fabric similar to how sound waves vibrate the air. You produced some gravitational waves this morning when you got out of bed, just really wimpy ones. Even two black holes orbiting around each other produce gravitational waves that are too weak for us to detect here on Earth. But when big things like black holes or neutron stars, slam into each other they produce a veritable earthquake in spacetime. Scientists have tried to detect waves from such explosive collisions for decades, but picking up the signal is like trying to measure a one millimeter change in a stick that the thousand light-years long. The recently upgraded LIGO has finally succeeded. LIGO splits a laser beam between two four-kilometer-long perpendicular tubes. The beams bounce off mirrors hundreds of times. When the beams recombine, they cancel each other out, but when gravitational waves from merging black holes hit Earth, they ever so slightly squeeze one tube and stretch the other, which causes the beams to wobble in and out of alignment. That signal crescendos as the black holes merge. If you converted the signal to a sound, this is what you would hear: [chirp] Evidence of gravitational waves not only means that Einstein was right, it also allows us to observe the universe in a completely new way. Right now, we peer out into the cosmos through telescopes the record light or other forms of electromagnetic radiation. Detecting gravitational waves means we can study space by listening to gravity itself. [music]