Astronomy
Using Pulsar Timing Arrays to Detect Gravitational Waves
Quick fact
Pulsar timing arrays are sensitive to gravitational waves with wavelengths of light-years, which is why they require monitoring for over a decade. In 2023, several collaborations announced the first evidence of a background hum of such waves, likely from supermassive black hole mergers.
Why this is interesting
You've probably heard of using lasers to detect ripples in spacetime, but did you know that a network of dead stars can do the same thing? How can we 'listen' to the universe's deepest hum using cosmic clocks?
Read the full explanation
Understanding Using Pulsar Timing Arrays to Detect Gravitational Waves
Imagine you're on a boat in a calm sea, and a large wave passes beneath you. The boat rises and falls slowly. Now imagine that the sea is spacetime, and the boat is a pulsar—a rapidly spinning neutron star that emits precise beams of light like a lighthouse. When a gravitational wave passes, it stretches and squeezes the space between the pulsar and Earth. For the pulsar's regular 'tick' to show a tiny change—a few billionths of a second—the pulsar must be incredibly stable. Millisecond pulsars rotate hundreds of times per second, and their pulses arrive with such precision that they act as cosmic clocks. By observing many pulsars across the galaxy, we form a 'pulsar timing array' that can pick out the pattern of a passing gravitational wave. The key is to look for a correlated shift in the arrival times of pulses from different pulsars, depending on their direction in the sky. That correlation is the fingerprint of a gravitational wave.
A deeper explanation
The underlying principle is Einstein's general relativity, which predicts that accelerating masses produce ripples in spacetime—gravitational waves. While LIGO detects high-frequency waves (100 Hz) from stellar-mass black holes using kilometer-long laser interferometers, pulsar timing arrays target wavelengths of light-years and frequencies of nanohertz. These waves are generated by supermassive black hole binaries (each millions to billions of solar masses) orbiting each other in distant galaxy mergers. The Earth and the pulsars serve as free-falling test particles. When a gravitational wave passes, the proper distance between Earth and a pulsar changes, altering the pulse arrival time. The signal is stochastic—a combination of waves from many sources—and appears as a common, isotropic noise. The Hellings-Downs curve describes the expected correlation of timing residuals between pulsar pairs as a function of angular separation; detecting this specific curve is the smoking gun. Arrays like NANOGrav, PPTA, and EPTA have been collecting data for over 15 years, and in 2023 they announced strong evidence for this background. This validates a core prediction of relativity on cosmological scales, and opens a new era of low-frequency gravitational wave astronomy, potentially revealing the mergers of the most massive black holes in the universe.