Black Holes

3 of 11. We heard them. Arrow keys walk.

Instrument

Four kilometres, split,and a photon that knows both roads.

LIGO is not a telescope pointed at the sky. It is a ruler laid along spacetime. A laser is split down two arms. The arms are equal until a wave passes. Then they are not.

Aerial photograph of LIGO Hanford Observatory: two long perpendicular vacuum arms meeting at a corner station in the Washington desert.

Documentary image · NOIRLab / LIGO / NSF / AURA / T. Matsopoulos

LIGO Hanford Observatory, Washington. Two four-kilometre vacuum arms on the Columbia Basin.

LIGO-class interferometer, planPLAN · LIGO-CLASS MICHELSON INTERFEROMETERFabry–Pérot arm cavities · 1064 nm · strain exaggeratedlaserETM YETM Xbeamsplitter4 km4 kmΔL / L ≈ 10⁻²¹a fraction of a proton across four kilometresnot a photograph of Hanford or Livingston

Real strain is about 10−21. The shift drawn here is exaggerated so the geometry can be seen. Fabry–Pérot cavities multiply the effective path.

Hanford

LIGO Hanford · Washington

One L-shaped detector in sagebrush country. Two four-kilometre vacuum tubes. The same wave that reaches Louisiana reaches here milliseconds later, or earlier, depending on the sky.

Livingston

LIGO Livingston · Louisiana

A second, independent interferometer. Coincidence is the first filter: a real wave is a worldwide agreement, not a local tremor.

The network

Virgo · KAGRA · LIGO-India (coming)

More sites mean better sky location. Virgo in Italy, KAGRA in Japan. A detection is a collaboration, not a single machine’s opinion.

Hard · measured

What the detector actually records

The output is a time series of strain. Buried in it: the rumble of the ground, the grain of the laser itself, scattered light, a truck on a distant road. Matched filters (templates from general relativity) try on families of chirps until one fits the mess. A candidate is then followed by parameter estimation: which masses, which spins, which distance make the waveform fit.

Arm length 4 km (LIGO). Design strain sensitivity on the order of 10−23 / √Hz in the most sensitive band. Network detections are published in the Gravitational-Wave Transient Catalog (GWTC) via GWOSC.