How Gravitational Waves are Detected

The LIGO detector is an instrument that uses laser pulses to measure minute ripples in space-time caused by gravitational waves.

LIGO aerial view
The Livingston LIGO observatory with two 4 km arms

What are Gravitational Waves?

Gravitational waves are ripples in the fabric of space-time caused by the acceleration of an asymmetric mass. Space-time is stretched and compressed in the direction of travel of the waves. Interestingly, if the mass has perfect spherical or cylindrical symmetry, gravitational waves will not be generated.

In order for LIGO to detect a gravitational wave, it has to be strong enough to be distinguishable from background noise. LIGO has detected waves from a distant cataclysmic merger of two black holes. The energy of the merger was great enough to be detectable 1.3 billion light-years away. Over this distance, the effect of the waves diminishes considerably and LIGO must be capable of detecting tiny changes in the length of the arm: about 1 x 10-18 meters, equivalent to about 1/1000th the diameter of a proton.

The Detectors

Gravitational waves are detected at a pair of identical LIGO installations (Laser Interferometer Gravitational-Wave Observatory) that collaborate to rule out local effects. Each observatory has two perpendicular arms 4 kilometres long that enable the detection of gravitational waves.

The first detection was made on September 14th, 2015.

LIGO distance
The locations of the Livingston and Hanford LIGO detectors.

How does LIGO Detect Gravitational Waves?

The key to the instrument is that when a gravitational wave passes by:

  1. The speed of light is constant
  2. The distance over which light travels is altered by gravitational waves
    – therefore:
  3. The time-of-flight of a light pulse between two points in space will vary
  4. The variation is amplified by increasing the distance over which the pulse travels

In the detector, a laser light source is split and sent along each arm and the returning beams are configured to such that the recombined waves cancel each other out. When a gravitational wave passes through, the arms of the detector will no longer be equal and the recombining waves will no longer cancel completely. The detector will see the recombined beam becoming brighter as a gravitational wave stretches or compresses the surrounding space-time.

How are Subatomic Distances Measured?

High measurement resolution is achieved by extending the time of flight of the split beams within the detector. The longer a light wave spends travelling, the wider the gap between the phases of the recombined beams, the higher the measurement resolution can become.

To make the travel time for the beams long enough to get the required resolution, mirrors are used to cause the beam to travel the length of the arms 280 times, equivalent to 1,120 km.

A video showing how the detector works

A video showing the merger of the two black holes detected by LIGO



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