Michelson Interferometer
Definition
A beam-splitter and two mirrors that detect absurdly tiny distance changes — LIGO is a giant Michelson that 'hears' black holes.
Technical definition
A split beam travels two arms and recombines; a path difference ΔL shifts fringes by ΔL/λ. LIGO's 4 km arms detect ΔL ≈ 10⁻¹⁹ m.
Example
The 1887 Michelson–Morley experiment used this setup to hunt the 'aether wind' — finding nothing, which paved the way for relativity.
Common confusion
LIGO doesn't photograph gravitational waves — it measures the arm-length wobble they cause via fringe shifts. The interferometer is a ruler, not a camera.