Gravitational Waves: How LIGO Listens to Spacetime

In September 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a signal that lasted 0.2 seconds. Two black holes — one of 36 solar masses, one of 29 solar masses — had merged approximately 1.4 billion light-years away, converting roughly 3 solar masses of rest energy directly into gravitational waves. The amplitude of the resulting spacetime ripple when it reached Earth was approximately 10⁻²¹ — a strain that caused the 4-kilometre-long laser arms of the detector to stretch and compress by a distance of 10⁻¹⁸ metres.

This is one-thousandth the diameter of a proton. LIGO measured it.

The detection, made on 14 September 2015 and published in Physical Review Letters on 11 February 2016 under the names Barry Barish, Kip Thorne, and Rainer Weiss (who shared the 2017 Nobel Prize in Physics for the achievement), was not just the first direct detection of gravitational waves predicted by Einstein’s general relativity a century earlier. It was the first observation of a binary black hole system of any kind, and it demonstrated that the universe contains black holes of this mass class in merging binaries — something no one had observed before, by any means.

Key parameters

Parameter	Value
LIGO arm length	4 km
Strain sensitivity	~10⁻²³ /√Hz at 100 Hz
Mirror mass	40 kg fused silica
GW170817 distance	~130 Mpc
First detection (GW150914)	14 Sep 2015
LISA arm length (planned)	2.5 million km

The Physics of Gravitational Waves

General relativity describes gravity not as a force but as the curvature of spacetime produced by mass and energy. Accelerating masses produce ripples in this curvature that propagate outward at the speed of light — gravitational waves. Unlike electromagnetic radiation, gravitational waves interact extremely weakly with matter (the gravitational coupling constant is 10³⁶ times weaker than the electromagnetic coupling). They pass through the Earth, through the Sun, through entire galaxies almost without interaction.

This is what makes them scientifically valuable: a gravitational wave carries unmodified information about its source, regardless of intervening matter. It is also what makes them extraordinarily difficult to detect.

The strain $h$ of a gravitational wave is defined as the fractional change in the proper distance between two freely falling test masses:

$h = \frac{\Delta L}{L}$

where $\Delta L$ is the change in separation and $L$ is the initial separation. For the first LIGO detection (GW150914), h ≈ 10⁻²¹. At LIGO’s 4 km arm length, this corresponds to ΔL ≈ 4 × 10⁻¹⁸ m.

The amplitude of gravitational waves falls as 1/r with distance, unlike electromagnetic intensity which falls as 1/r². This more favourable distance scaling means that doubling the detector sensitivity doubles the observable volume of the universe in all directions — an eightfold increase in the number of sources accessible.

How LIGO Works

LIGO is a Michelson interferometer operating at a scale and sensitivity level that took decades of incremental engineering advances to achieve. Two perpendicular arms, each 4 km long, are evacuated to below 10⁻⁹ torr — among the largest ultrahigh-vacuum systems in the world. A laser beam (Nd:YAG, 1064 nm, initially 200 W, increased to over 200 W at the beam splitter after power recycling) is split and sent down both arms, reflected by 40 kg fused silica mirror test masses suspended on multi-stage pendulum isolation systems, and recombined at the beamsplitter.

When no gravitational wave is present, the arms are tuned to destructive interference at the output photodetector — the “dark port.” A gravitational wave passing through the detector stretches one arm while compressing the other, producing a differential phase shift that causes a small amount of light to leak to the dark port. The photodetector measures this fringing signal.

The noise budget is the defining technical challenge. Every vibration — traffic, ocean microseismic noise, thermal fluctuations in the mirror coating, quantum shot noise in the laser — appears as a spurious signal indistinguishable from a gravitational wave. LIGO’s performance is characterised by its amplitude spectral density (noise floor) as a function of frequency:

• Seismic noise dominates below ~10 Hz and is suppressed by a five-stage active-passive isolation system (HAM seismic isolation + Hydraulic External Pre-Isolator)
• Thermal noise from mirror coating Brownian motion dominates in the 10–200 Hz range and is minimised by operating mirror suspensions at room temperature with extremely low-loss coatings
• Quantum shot noise (photon counting statistics) dominates above ~200 Hz and is reduced by increasing laser power and by squeezing the quantum state of the light (quantum squeezing, implemented in Advanced LIGO)

The installation of quantum squeezed light at LIGO in 2019 reduced shot noise by a factor corresponding to 15 dB of squeezing, improving sensitivity at high frequencies by approximately 30% with no change to laser power. This is one of the first practical applications of quantum optics techniques to a precision measurement instrument.

The GWTC Catalogue: What Has Been Found

The Gravitational-Wave Transient Catalog (GWTC) is the running record of confirmed gravitational wave detections by the LIGO-Virgo-KAGRA network. As of GWTC-3 (released 2021, covering O1-O3 observing runs), the catalogue contains 90 confirmed events.

Binary black hole mergers (BBH): The vast majority — 83 events. These dominate the catalogue because BBH systems are the loudest gravitational wave sources, producing signals detectable to cosmological distances. The mass distribution reveals an unexpected abundance of black holes in the 20–50 solar mass range, above the “pair instability gap” predicted by stellar evolution theory — a result with significant implications for the formation channels of these systems.

Binary neutron star mergers (BNS): Two confirmed events, including GW170817 — by far the most scientifically rich gravitational wave event detected.

Neutron star-black hole mergers (NSBH): Two events confirmed in O3.

GW170817 deserves special mention. On 17 August 2017, LIGO and Virgo detected a 100-second gravitational wave chirp from two merging neutron stars. 1.7 seconds later, the Fermi Gamma-ray Burst Monitor detected a short gamma-ray burst (GRB 170817A) from the same sky direction. Within hours, dozens of electromagnetic observatories identified an optical transient — a kilonova — in the galaxy NGC 4993, 40 Mpc distant. The simultaneous multi-messenger detection confirmed that binary neutron star mergers are the source of short gamma-ray bursts, directly measured the Hubble constant (H₀ = 70⁺¹²₋₈ km/s/Mpc, consistent with both Planck CMB and distance ladder values), and provided direct evidence that heavy element nucleosynthesis — the production of gold, platinum, and other r-process elements — occurs in these mergers. The strontium signature in the kilonova spectrum was detected by ESO’s X-Shooter spectrograph: the gold in your jewellery was forged in a neutron star collision.

LISA: The Space-Based Observatory

Ground-based interferometers like LIGO are limited to frequencies above approximately 10 Hz by seismic and gravity gradient noise. An entirely different class of gravitational wave sources — supermassive black hole mergers, extreme mass ratio inspirals, galactic binaries — emits at frequencies of 10⁻⁴ to 10⁻¹ Hz, inaccessible from the ground.

The Laser Interferometer Space Antenna (LISA), an ESA-led mission selected for the L3 slot in the Cosmic Vision programme and targeting launch in the mid-2030s, will operate as a space-based interferometer with arm lengths of 2.5 million km — 625 times LIGO’s arms, enabling access to the millihertz frequency band.

Three spacecraft in a triangular formation will exchange laser beams across the 2.5 Mkm arms, tracking the separation between free-falling test masses aboard each spacecraft. The LISA Pathfinder mission (2015–2017) demonstrated residual accelerations on the test masses of 1.74 × 10⁻¹⁵ m/s²/√Hz — exceeding the requirement by a factor of more than five, confirming the core technology readiness.

LISA is expected to detect tens of thousands of gravitational wave sources per year, including mergers of supermassive black holes to cosmological distances (z > 10), continuous emission from thousands of short-period binary white dwarf systems in the Milky Way, and the early inspiral signal from BBH mergers that LIGO will later detect — providing weeks of warning before the merger and enabling coordinated electromagnetic follow-up.

The third gravitational wave observatory of note is KAGRA in Japan, which uses cryogenically cooled sapphire mirrors to reduce thermal noise, and is part of the global network that provides sky localisation for electromagnetic follow-up. Its addition narrows the uncertainty ellipse in the sky position of detected sources from hundreds to tens of square degrees.

What Gravitational Waves Cannot Tell Us

Gravitational waves probe spacetime directly. They reveal masses, spins, orbital parameters, distance, and — through tidal deformability — the equation of state of neutron star matter. But they carry no information about chemical composition, temperature, or the electromagnetic environment of their sources. They are complementary to, not a replacement for, electromagnetic astronomy.

The multi-messenger era — gravitational waves combined with gamma rays, optical, X-ray, and radio observations — is the framework within which the most physically complete picture of compact object mergers can be constructed. GW170817 was its proof of concept. The catalogue that follows it, as LIGO sensitivity improves toward its A+ and Voyager design goals, will define a new field of observational astrophysics.

For the extreme objects that produce these signals, see neutron stars and pulsars: physics at the edge of matter.