SHAKE your fist and you shake the universe. Just by moving the mass of your hand back and forth, you are sending out ripples in space and time – pieces of travelling gravity that distort everything they meet.
The wobble in your hand’s gravitational influence spreads out in all directions, moving at the speed of light. Because these waves are actually changes in space-time, they warp any object they meet, alternately squashing and stretching it in different directions.
But only a little bit. Astrophysicists reckon that to spot the gravity waves from events such as black hole collisions, detectors might need to spot distortions of about one part in 1021 – equivalent to a change in the distance from London to New York by the width of a uranium nucleus. Seeing the gravity waves created by the inflating universe will be even trickier.
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To reach such astonishing sensitivity, physicists and technicians across the world have spent more than 20 years developing laser interferometers that will (hopefully) give us our first glimpse of gravity waves.
The interferometer has two arms at right angles to each other, and the idea is to compare the lengths of these arms using a laser. A laser beam splits in two where the two arms join, and part of the beam races up each arm. The light bounces off suspended mirrors at the ends of the arms, and returns to the splitter, where the two beams are recombined.
Each beam of laser light is coherent, with all peaks and troughs in phase, so when recombined the crests of one may coincide with the troughs of the other, and they cancel each other out. When the beams combine at a light detector, the detector will see nothing. But when a gravitational wave comes by, it briefly alters the length of the light’s path, stretching one arm and squashing the other. This changes the relative phase of the two beams. They no longer cancel perfectly, and your detector registers some light. Time to crack open the champagne.
The longer the arms, the more sensitive the instrument. So the latest generation of gravity wave detectors are huge machines. They are sited in Germany, Japan, the US and Italy. The italian detector, VIRGO, is a giant with 3-kilometre arms, but it is dwarfed by LIGO, the Laser Interferometer Gravitational-Wave Observatory, which has 4-kilometre-long arms.
LIGO actually comprises two observatories. One is in Livingston, Louisiana, and the other is at Hanford in Washington State, 3000 kilometres away. This is the best way to tell if an observation is valid: genuine gravitational waves should affect both Livingston and Hanford. Any spurious disturbance, such as a ground tremor or glitch in the electronics, will hit only one site.