Gravitational Wave Detection

Gravitational waves are detected by measuring the minute changes they induce in the relative lengths of perpendicular arms of giant laser interferometers, a feat first achieved by LIGO in 2015.

Definition

Gravitational-wave detection is the measurement of the strain, the fractional change in distance, produced by a passing wave, accomplished by laser interferometry over kilometre-scale baselines on the ground, by planned space interferometers, and by timing arrays of millisecond pulsars at very low frequencies.

Scope

This topic covers the principle of interferometric detection, the response of an L-shaped interferometer to wave strain, the dominant noise sources (seismic, thermal, and quantum shot noise) and the techniques used to suppress them, the global detector network (LIGO, Virgo, KAGRA) and planned space and pulsar-timing observatories, and the matched-filtering data analysis used to dig signals out of noise.

Core questions

• How does a laser interferometer convert spacetime strain into a measurable signal?
• What noise sources limit sensitivity and how are they overcome?
• How are weak signals identified within detector noise?

Key concepts

• Laser interferometer
• Strain sensitivity
• Seismic and thermal noise
• Quantum shot noise
• Detector network and triangulation
• Matched filtering

Key theories

Interferometric strain measurement

A passing wave changes the lengths of two perpendicular interferometer arms oppositely, shifting the interference of recombined laser light, so that the measured phase shift is a direct readout of the gravitational-wave strain.

Matched-filter detection

Because expected waveforms can be computed in advance, signals far below the noise are extracted by correlating the data against banks of theoretical templates, the technique that confirmed the first black-hole merger.

Clinical relevance

Detection technology defines what gravitational-wave astronomy can observe: ground-based interferometers cover the audio band of stellar-mass mergers, planned space missions will reach lower frequencies for massive black-hole binaries, and pulsar-timing arrays probe nanohertz waves from supermassive black-hole pairs, together spanning the gravitational-wave spectrum.

History

Joseph Weber's resonant-bar attempts in the 1960s spurred the field; Weiss conceived the interferometric approach in the early 1970s, and after decades of development LIGO achieved the first direct detection in September 2015, an achievement recognized by the 2017 Nobel Prize to Weiss, Thorne, and Barish.

Key figures

• Rainer Weiss
• Kip Thorne
• Barry Barish
• Ronald Drever

Seminal works

• abbott2016
• saulson1994

Frequently asked questions

How can detectors measure a length change smaller than an atomic nucleus?

By using kilometre-long arms, high-power stabilized lasers reflected thousands of times, and extreme isolation from seismic and thermal disturbance, interferometers sense the differential arm-length change of order 10^-18 metres that a gravitational wave produces.

Why are several detectors needed rather than one?

A network confirms that a signal is astrophysical rather than local noise and, by comparing arrival times across widely separated sites, locates the source on the sky, which is essential for pointing telescopes for multi-messenger follow-up.