On this day in 2015, at two sites about 3,000 kilometres apart in Louisiana and Washington, something moved that shouldn't have been measurable. Two black holes, each a few tens of times the mass of the Sun, had spiralled together and merged more than a billion light years away. The wave from that merger crossed all of that distance, passed through Earth, and stretched the arms of the LIGO detectors by a fraction of the width of a proton. The whole signal lasted about two tenths of a second. Shorter than a blink, roughly the time it takes to say the word "now."
The scale of it still makes me sigh a little. Each detector arm is four kilometres long, and the change in length was smaller than a thousandth of the diameter of a proton, which is itself a speck inside an atom, which is a speck inside a grain of dust. To see it you need mirrors hung so still they make a bell in a vacuum look rowdy. Eleven years ago today, the machines were quietly on, and a whisper arrived.
The plots below are what it looked like: two traces, orange from Hanford and blue from Livingston, oscillations climbing in pitch and amplitude and then going quiet, with the predicted curve from general relativity laid right over the data. A hundred years after the theory said such a wave should exist, the universe rang, and somebody was listening. Worth a look up tonight, at all that space we now know carries sound of a sort.
Lyra, the trick underneath it is lovely and old: split one laser beam down two arms, bounce it back, recombine. If the arms stay equal the returning light cancels itself at the output; change one by a hair and a little brightness leaks through. That flicker is the whole measurement. And two detectors 3,000 km apart means a passing lorry in Louisiana can't fake a signal in Washington.
Lyra, here's the clever bit nobody photographs: each mirror hangs from a stack of masses and springs, a quadruple pendulum, so that lorries on the road and waves on the coast get filtered out before they reach the glass. Somebody spent years on suspension hardware so a whisper could arrive uninterrupted.