Gravitational Waves

Ripples in the curvature of spacetime, propagating at the speed of light, produced when massive objects accelerate. Predicted by Einstein's general relativity in 1916; directly detected for the first time in 2015. The discovery opened an entirely new way of observing the universe — a "gravitational wave astronomy" that sees through dust, gas, and the early universe in ways electromagnetic radiation cannot.

Referenced in neutron-star (kilonova detections), black-hole (merger detections), and supernova (predicted but not yet detected from supernovae).

What They Are

In Newtonian physics, gravity acts instantaneously across space. Einstein's general relativity replaced this with the idea that mass curves spacetime, and that changes in mass distribution propagate as curvature waves at the speed of light. These are gravitational waves — not waves *in* space, but waves *of* space itself.

A passing gravitational wave stretches and squeezes distances transversely — alternating in two perpendicular directions (the two polarisations, called + and ×). The amplitude decreases as 1/distance (not 1/distance², unlike light intensity), making gravitational wave detectors sensitive to sources across vast distances.

The Detection Challenge

GW150914 — the first direct detection — changed the length of LIGO's 4-km arms by a thousandth of the width of a proton. Detecting this required laser interferometry with mirrors suspended on vibration isolation systems of extraordinary sophistication.

Development: Weber bars (1960s–70s, eventually discredited) → laser interferometers (prototypes from 1970s) → LIGO and Virgo (operational detectors) → confirmed detection 2015.

Key Detections

GW150914 (14 September 2015, announced 11 February 2016)

• Two black holes: 29 + 36 M☉ merging into 62 M☉
• 1.3 billion light-years away
• In the final 0.2 seconds: released more than 50× the luminosity of all stars in the observable universe combined
• Signal frequency rose from 35 to 250 Hz over 10 cycles (5 orbits) as merger approached
• 2017 Nobel Prize in Physics: Rainer Weiss, Kip Thorne, Barry Barish

GW170817 (17 August 2017)

• First neutron star merger detection — and the most scientifically productive gravitational wave event
• 70 telescopes and observatories observed the electromagnetic counterpart
• A gamma-ray burst (GRB 170817A) detected 1.7 seconds after the merger
• A kilonova (AT 2017gfo) confirmed: gold, platinum, and r-process elements synthesised in the merger — directly observed
• Constrained the speed of gravitational waves to within 1 part in 10¹⁵ of the speed of light
• Confirmed neutron star mergers as a primary gold factory

Gravitational Wave Background (2023)

• NANOGrav and collaborators announced detection of a stochastic gravitational wave background using pulsar timing arrays — 67 pulsars tracked for 15 years showing correlated timing variations
• Believed to originate from pairs of supermassive black holes across the universe — the "hum" of merging galaxy centres across cosmic time

Sources

Strongest known sources:

1. Merging compact objects — black hole binaries (most detections), neutron star binaries (most scientifically rich)
2. Core-collapse supernovae — if asymmetric; not yet detected
3. Rotating neutron stars — if deformed; not yet detected ("mountains" < 10cm high)
4. The early universe — inflation-era background; the theoretical holy grail

LISA — Space Detector

The Laser Interferometer Space Antenna (planned 2030s): three spacecraft in a triangular formation 2.5 million km apart. Will detect lower-frequency gravitational waves from supermassive black hole mergers and compact binaries across the universe — the gravitational wave equivalent of a radio telescope after optical astronomy.