Gravitational Waves
When two black holes spiral together, they shake spacetime itself, sending ripples outward at the speed of light. On 14 September 2015, LIGO felt one — a stretch of 4-km arms by 1/10,000th the width of a proton. The signal is a chirp: as the holes spiral in, the orbit speeds up, so the wave's frequency and amplitude both sweep up — then a violent merger and a fading ringdown.
how to hear two black holes collide a billion light-years away
General relativity predicts that accelerating masses radiate gravitational waves — propagating distortions of the spacetime metric that stretch space one way and squeeze it the other, transverse to their travel. They're feeble: only the most violent events in the universe make detectable ones.
A binary of two compact objects loses orbital energy to this radiation, so the orbit shrinks and speeds up — a runaway inspiral. The wave's frequency is twice the orbital frequency and evolves as:
f(t) ∝ (t_merge − t)^(−3/8)
while the amplitude grows with frequency. The result is the unmistakable chirp you see scrolling above — rising tone, rising loudness, then a sharp merger spike, then a damped ringdown as the newborn black hole settles to a smooth Kerr shape (an exponentially decaying sinusoid at its quasinormal-mode frequency).
The single most important number extracted is the chirp mass:
ℳ = (m₁ m₂)^(3/5) / (m₁+m₂)^(1/5)
It sets how fast the chirp sweeps, so reading the sweep rate weighs the black holes from the waveform alone. GW150914 was a 36 M☉ and 29 M☉ pair merging into a 62 M☉ hole — the missing 3 solar masses were radiated as gravitational waves in a fraction of a second, briefly outshining all the stars in the observable universe combined (in gravitational-wave power).
Why it's a revolution: every previous window on the cosmos used light (or neutrinos). Gravitational waves are a fundamentally new sense — they pass through everything, carrying pristine information from regions no light escapes. Since 2015, LIGO/Virgo/KAGRA have logged hundreds of mergers; the 2017 neutron-star merger GW170817 was seen in gravitational waves and light, launching "multi-messenger astronomy," confirming gravitational waves travel at c, and showing that colliding neutron stars forge the universe's gold and platinum. (Nobel Prize 2017.)