Gravitational Waves Explained

What Are Gravitational Waves?

In general relativity, gravity is the curvature of spacetime caused by mass and energy. When massive objects accelerate, they create disturbances in this curvature that propagate outward as waves, much as a boat moving through water creates waves on the surface. These gravitational waves carry energy away from the source, traveling at the speed of light through the vacuum of space.

As a gravitational wave passes through a region of space, it stretches and compresses that region in a characteristic pattern. Space is alternately stretched in one direction and compressed in the perpendicular direction, then the process reverses. This stretching and compression is typically described using two polarization modes, called plus (+) and cross (x), which differ in their orientation by 45 degrees.

The amplitude of gravitational waves is described by a dimensionless quantity called the strain, denoted h, which represents the fractional change in distance between two points. Even the strongest gravitational waves reaching Earth produce strains of only about 10^-21, meaning that a kilometer-long ruler would change length by only about one-thousandth of the diameter of a proton. This extraordinarily tiny effect is why gravitational waves were so difficult to detect.

Sources of Gravitational Waves

Gravitational waves are produced by any mass that accelerates, but detectable waves require enormous masses moving at relativistic speeds. The strongest sources include: binary systems of compact objects (neutron stars or black holes) spiraling toward each other and eventually merging; asymmetric supernova explosions; and potentially the rapid rotation of deformed neutron stars (pulsars with mountains or other asymmetries on their surfaces).

The most powerful gravitational wave events are binary black hole mergers. As two black holes spiral inward, they orbit each other faster and faster, emitting gravitational waves of increasing frequency and amplitude. This characteristic signal, called a chirp, rises in pitch over seconds or minutes before culminating in the merger itself. The merger can radiate gravitational wave energy equivalent to several solar masses, making these events briefly the most powerful energy emissions in the universe.

Neutron star mergers also produce strong gravitational waves with an important bonus: they emit electromagnetic radiation (light, radio waves, gamma rays) as well. The first observed neutron star merger, GW170817 in August 2017, was detected simultaneously by LIGO, Virgo, and dozens of electromagnetic telescopes worldwide, inaugurating the era of multi-messenger astronomy.

How LIGO Detects Gravitational Waves

LIGO uses a technique called laser interferometry. Each LIGO detector consists of two perpendicular arms, each 4 kilometers long, arranged in an L shape. A laser beam is split and sent down both arms simultaneously, bouncing off mirrors at the far ends and returning to a detector at the junction. If both arms are exactly the same length, the returning beams interfere destructively and no signal is detected.

When a gravitational wave passes through the detector, it stretches one arm and compresses the other (and then reverses). This creates a tiny difference in the arm lengths, which shifts the interference pattern and produces a detectable signal. The length change LIGO must measure is extraordinarily small, typically less than 10^-18 meters, about one-thousandth the diameter of a proton.

To achieve this sensitivity, LIGO employs a suite of sophisticated technologies including quantum-limited laser systems, active seismic isolation that reduces ground vibrations by a factor of a billion, ultra-pure mirrors suspended as pendulums to minimize thermal noise, and signal recycling techniques that effectively increase the laser power circulating in the arms. The two LIGO detectors are located 3,000 km apart (in Hanford, Washington, and Livingston, Louisiana) so that genuine gravitational wave signals can be confirmed by their simultaneous detection at both sites.

The First Detection and Beyond

On September 14, 2015, both LIGO detectors recorded a gravitational wave signal lasting about 0.2 seconds. The signal, designated GW150914, matched the predicted waveform for two black holes, with masses of approximately 36 and 29 solar masses, spiraling together and merging to form a single black hole of about 62 solar masses. The missing 3 solar masses were radiated away as gravitational wave energy. The event occurred about 1.3 billion light-years from Earth.

This detection was announced on February 11, 2016, and earned the 2017 Nobel Prize in Physics for Rainer Weiss, Kip Thorne, and Barry Barish. Since then, LIGO and its European counterpart Virgo have detected dozens of gravitational wave events, including mergers of neutron stars, mergers of a neutron star with a black hole, and numerous binary black hole mergers of varying masses.

The gravitational wave catalog has revealed populations of black holes with masses that were unexpected from electromagnetic observations alone. Some of the detected black holes have masses of 50, 60, or even 80 solar masses, larger than what standard stellar evolution models typically predict. These observations are driving new understanding of how massive stars live and die, and whether some of these black holes formed through successive mergers of smaller ones.

The Future of Gravitational Wave Astronomy

Gravitational wave detection is still in its early stages. Planned upgrades to LIGO and Virgo, along with new detectors like KAGRA in Japan and LIGO-India, will increase sensitivity and improve the ability to locate sources on the sky. The space-based detector LISA (Laser Interferometer Space Antenna), planned for launch in the 2030s, will detect gravitational waves at much lower frequencies than ground-based detectors, opening access to massive black hole mergers at the centers of galaxies, thousands of compact binary systems in our own galaxy, and potentially signals from the early universe.

Pulsar timing arrays, which monitor the precise arrival times of radio pulses from dozens of millisecond pulsars, are sensitive to gravitational waves with periods of years to decades. In 2023, several pulsar timing collaborations reported evidence for a gravitational wave background, a low-frequency hum of gravitational waves permeating the universe, likely produced by the combined effect of millions of supermassive black hole binaries throughout cosmic history.