Physics
How Gravitational Waves Reveal Merging Black Holes
Quick fact
The first gravitational wave detection (GW150914) was caused by two black holes merging about 1.3 billion light-years away, and the wave stretched and squeezed LIGO's 4-kilometer arms by only a fraction of a proton width.
Why this is interesting
You've probably seen pictures of black holes, but they are invisible. So how do we know they exist and that they sometimes collide in spectacular events?