Michelix

Meet Michelix, a half-silvered plate who splits a laser beam in two and then counts tiny steps.

In a Michelson interferometer, one beam goes to a fixed mirror and the other goes to a mirror that can move. The beams come back and overlap, making a pattern of bright and dark rings called fringes.

Moving one mirror by Δd changes that beam's path by 2Δd, because the light travels there and back. Each time the path changes by one wavelength, one fringe passes. So Δd = mλ/2. With a red laser of 633 nm, counting 100 fringes means the mirror moved about 31.7 μm, less than half the width of a hair.

In 1887 Michelson and Morley used this set-up to look for an ether wind and found none, a key clue for relativity.

The LIGO detectors are giant versions with arms 4 km long. They can sense changes far smaller than a proton, enough to detect gravitational waves.