One of the most profound predictions of Albert Einstein's General Theory of Relativity was the existence of gravitational waves. For a century, they were only an indirect prediction, but in 2015, they were directly detected for the first time, opening an entirely new way to observe the universe.
According to General Relativity, mass and energy curve the fabric of spacetime. When massive objects accelerate, they create disturbances—ripples—in this fabric that travel outwards at the speed of light. These ripples are gravitational waves.
Only the most extreme, cataclysmic events in the cosmos can produce gravitational waves strong enough for us to detect. These include:
Gravitational waves are incredibly weak. By the time they reach Earth, the stretching and squeezing of space is on a scale thousands of times smaller than the nucleus of an atom. Detecting this requires an astonishingly sensitive instrument called a laser interferometer.
For centuries, all of astronomy was based on observing light (electromagnetic radiation). Gravitational wave astronomy is a completely new sense. It allows us to "hear" the universe and observe phenomena that are completely dark and invisible, like the merger of two black holes.
What is a gravitational wave?
What kind of cosmic event was the first to be detected via gravitational waves?
How does an instrument like LIGO use lasers to detect the passage of a gravitational wave?