HOW ARE GRAVITATIONAL WAVES DETECTED?

As gravitational waves pass, they stretch space in one direction and squeeze it in a perpendicular direction, then alternate, repeatedly. The effect felt on Earth of the waves from a black hole merger is extremely small, typically a change in the length of a body by a mere billion billionth of its size. Consequently, the only way to detect such a small effect is with a big ruler. Enter the Laser Interferometer Gravitational Wave Observatory ( LIGO) – a 20th- Century technological marvel. At Hanford in the state of Washington is a four-kilometre ruler made from laser light. Three thousand kilometres away at Livingston, Louisiana, is an identical ruler. Each site actually consists of two tubes 1.2 metre in diameter, which form an L-shape down which a megawatt of laser light travels in a vacuum more empty than space. At each end the light bounces off 42kg mirrors, suspended by glass fibres just twice the thickness of a human hair and so perfect they reflect 99.999 per cent of the light. It is the Lilliputian movement of these suspended mirrors that signal a passing gravitational wave.

LIGO splits laser light into two and sends it down each arm, where mirrors bounce it back to a point where the light is re- combined. If the crests of the two waves coincide, the light detected is boosted. If the crest of one coincides with the trough of the other, the light is cancelled out. Consequently, LIGO is sensitive to changes in the length of one arm relative to the other of a fraction of the wavelength of light. A lot of ingenuity is expended in getting that measurement down even further to a hundred-thousandth the diameter of an atom.

At 5:51am EDT on 14 September 2015, first in Livingston, then 6.9 milliseconds later in Hanford, the rulers repeatedly expanded and contracted by a hundred-thousandth the diameter of an atom marking the first ever direct detection of gravitational waves.