Aerial view of the Virgo gravitational-wave detector, situated at Cascina, near Pisa (Italy). Virgo is a giant Michelson laser interferometer with arms that are 3 km long, and complements the twin 4 km LIGO detectors. These detectors are sensitive to tiny changes in distance, which are a function of gravitational wave amplitude across a specific frequency range. (NICOLA BALDOCCHI / VIRGO COLLABORATION)

LIGO’s Lasers Can See Gravitational Waves, Even Though The Waves Stretch The Light Itself

If you think about the way a gravitational wave detector works, you might encounter a paradox. Here’s the solution.

One of the greatest scientific achievements in all of human history was at last achieved just a few years ago: the direct detection of gravitational waves. Although they were an early prediction teased out of Einstein’s General Relativity put out all the way back in 1915 it took a full century for them to be directly discovered.

The way we accomplished this dream is through a remarkable design shared by the LIGO, Virgo, and KAGRA detectors:

• splitting light so that it travels down two mututally perpendicular laser-arms,
• reflecting that light back-and-forth multiple times in rapid succession,
• and then recombining the beams to see an interference pattern.

When a sufficiently strong gravitational wave passes through with the right frequency to be detected, the arms alternately expand and contract, altering the interference pattern. But won’t the light expand and contract, too? The surprising answer is “no,” and this is the reason why.