dark port
strain h(t)
drag to orbit · pinch or ⌘/Ctrl+scroll to zoom · zoom into the corner station
The chirp: as the black holes spiral in, the wave's frequency and strength sweep upward, then merger and ringdown. Watch one arm stretch while the other squeezes.
Slide exaggeration to "reality" — nothing visibly moves, yet the scope still reads the signal. That gap is the whole achievement of LIGO.
How LIGO hears spacetime
- A gravitational wave is geometry in motion. Einstein (1916): accelerating masses ripple spacetime itself. As a wave passes, distances stretch along one direction while shrinking along the perpendicular one, alternating at the wave frequency — watch the cyan ring of free test particles breathe into ellipses. Nothing pushes them; the space between them changes.
- An L measures exactly that. The two 4 km arms point along the stretch and squeeze directions, so a wave makes their lengths differ: ΔLx = +½hL, ΔLy = −½hL. A Michelson interferometer is the perfect differential ruler.
- The dark fringe is the trick. Laser light splits, runs both arms, reflects off the test-mass mirrors and recombines. The output port is tuned so the two beams cancel — the photodetector sits in darkness. The instant the arms differ, cancellation breaks and light leaks out. Leaked light vs time IS h(t). (See the dark-port inset brighten.)
- Folding and recycling multiply sensitivity. Each arm is a Fabry-Perot cavity: light bounces ~300 times, an effective path of ~1,200 km. A power-recycling mirror re-injects light heading back to the laser, building 20 W into ~750 kW circulating between the mirrors.
- The mirrors must be free particles. Each test mass is a 40 kg ultra-pure fused-silica cylinder hanging from glass fibres at the bottom of a four-stage pendulum, inside one of the largest vacuum systems on Earth. Above ~10 Hz it swings as if floating freely in space — only gravity (and gravitational waves) move it.
- The numbers are absurd. Real strain h ≈ 10⁻²¹: over 4 km that is ΔL ≈ 10⁻¹⁸ m — one ten-thousandth of a proton's width. This scene exaggerates the motion up to ~10²⁰×; at "reality" the animation is honest and nothing visibly moves.
- The chirp is the source's autobiography. Orbiting black holes radiate at twice their orbital frequency; losing energy, they spiral closer and faster, so frequency and amplitude sweep up — f ∝ (t_merge − t)^(-3/8) — until merger, then a ringdown as the new black hole settles. GW150914 (14 Sep 2015): 36 + 29 solar masses, 1.3 billion light-years, heard for 0.2 s. Here it is slowed ~40×; the audio follows the same sweep.
- Why two detectors? Hanford and Livingston sit 3,000 km apart. Only a real wave rings both within 10 ms — local trucks and earthquakes don't. The arrival-time difference also triangulates the source on the sky (now with Virgo and KAGRA).
Honest exaggerations: time slowed ~40×, arm motion scaled by the slider, beam drawn red though the real laser is invisible infrared, the 4 km arms compressed, photon pulses purely illustrative (real light crosses the arm in 13 µs).
Try: Replay the merger with sound on · slide exaggeration to reality · X-ray the corner station and follow the beam · watch the wave column arrive from the binary above · switch to the test wave and crank the amplitude.
Try: Replay the merger with sound on · slide exaggeration to reality · X-ray the corner station and follow the beam · watch the wave column arrive from the binary above · switch to the test wave and crank the amplitude.