Gravitational waves were first detected by LIGO #OTD in 2015, 100 years after they were predicted by Einstein.

This discovery, announced the following February, marked the beginning of gravitational wave astronomy.

The description of what was discovered and how it happened fits in a short post. It is simultaneously so amazing and so preposterous that some people who hear it will want to spend the rest of their lives understanding how it could be true and what we can learn from it:

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 Over a billion years ago, in a distant galaxy, two massive black holes spiraled into each other.

Eight years ago, here on Earth, the resulting ripples in spacetime made some mirrors move about 1/1000 of a proton radius.

Scientists predicted this minuscule effect, then precisely measured it by bouncing lasers back and forth along the arms of a pair of 4km-long interferometers.