A four-kilometre ruler reads a fraction of a proton
LIGO’s arms are four kilometres long. A gravitational wave changes that length by a fraction of the diameter of a proton, a shift on the order of 10⁻¹⁸ metres.
A laser is split, sent down two perpendicular vacuum pipes, and brought back together. A passing gravitational wave stretches one arm and squeezes the other, by a strain of roughly 10⁻²¹ for the events that have been recorded. Over four kilometres that is a few billionths of a billionth of a metre. The light makes many trips, so the apparatus is longer than the building. The mirrors hang, isolated, in a vacuum.
Hanford and Livingston, about three thousand kilometres apart, both have to see it, within the ten milliseconds light needs to cross the United States. A local truck, a local earthquake, a local laser glitch does not make that appointment. The first recorded event, in 2015, matched the calculated ringdown of two black holes.
Caveats
- The proton comparison is a scale, not a particle LIGO is touching. The measured thing is a difference in arm length.
- Sensitivity is a curve, not a single number. Some frequencies are quieter than others, and the design keeps improving.
Sources
- 1Abbott B. P. et al.. Observation of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters 116, 2016.