Two pulsars sit somewhere on the sky. This explorer sweeps the angular separation between them and computes, live, the exact correlation astronomers expect between their timing wobbles if a gravitational-wave background really fills space - the same Hellings-Downs curve real pulsar timing arrays compare their data against.
Published figures: NANOGrav's 15-year data set timed 68 millisecond pulsars for gravitational-wave-induced deviations of tens to a few hundred nanoseconds, reporting a stochastic background amplitude of about 2.4e-15 at a one-year reference frequency (2023, with EPTA, PPTA, and CPTA). The correlation curve shown here is the real Hellings-Downs formula (Hellings and Downs, 1983), computed exactly for the angle on the slider.
Drag to orbit and scroll or pinch to zoom. Scrub the separation slider, press Play sweep, or reveal the correlation-curve chart.
Pulsar Timing Array 3D Explorer
This browser explorer sweeps the angular separation between a reference pulsar and a probe pulsar and computes, in real time, the exact correlation their timing wobbles should share if a gravitational-wave background passes through - the Hellings-Downs curve real pulsar timing arrays like NANOGrav compare their data against. Drag the separation slider from 0 to 180 degrees and watch the correlation number rise, dip negative, and climb back up.
Millisecond pulsars spin hundreds of times a second and arrive with clockwork regularity. A passing gravitational wave stretches and squeezes spacetime, nudging each pulse's arrival time by a few tens to a few hundred nanoseconds. A single pulsar cannot tell a real signal from ordinary noise, but a whole array of them can: pulsar pairs that are close together in the sky should wobble in step, pairs on opposite sides of the sky should wobble in a different, predictable pattern, and the shape of that pattern versus angular separation is the Hellings-Downs curve.
- Scrub the separation slider from 0 to 180 degrees, or press Play sweep to watch it cycle automatically
- Read the live-computed correlation number and whether the pair is positively correlated, anti-correlated, or near zero
- Reveal an inline chart of the full correlation curve with a marker showing exactly where the current angle sits
- Drag to orbit the scene, scroll or pinch to zoom
- Runs fully in the browser with the vendored three.js engine - no account, no upload
| Figure | Published value |
|---|---|
| Hellings-Downs correlation at 0 degrees separation | 0.500 (exact) |
| Hellings-Downs correlation at 180 degrees separation | 0.250 (exact) |
| NANOGrav 15-year data set | 68 millisecond pulsars, timed 15+ years |
| Pulse-arrival timing precision | microsecond-level; gravitational-wave residuals of tens to a few hundred nanoseconds |
| 2023 stochastic background amplitude | about 2.4e-15 at a one-year reference frequency |
| Likely source | a population of inspiraling supermassive black-hole binaries (roughly 100 million to 10 billion solar masses) |
The 2023 announcement - made jointly by NANOGrav, the European Pulsar Timing Array, the Parkes Pulsar Timing Array, and the Chinese Pulsar Timing Array - reported the first evidence that pulsar-pair correlations across the sky follow this predicted shape, widely read as evidence for a background of gravitational waves at nanohertz frequencies, far below anything LIGO can detect. For the mechanism behind a single pulsar's clockwork beam, see the Neutron Star Pulsar Explorer. For the much higher-frequency, kilohertz gravitational-wave chirp LIGO detects from merging stellar-mass black holes, see the Gravitational Waves Explorer - a completely different instrument, frequency band, and source population from the nanohertz background this page explores.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached.
This is an educational approximation, not a live pulsar catalog or a gravitational-wave detection - the highlighted pair's angular separation is exact and the correlation number is computed live from the real published Hellings-Downs formula, but the other pulsars scattered around the scene are an illustrative array, not NANOGrav's real 68-pulsar sky pattern, and no gravitational-wave signal or noise is simulated.
Frequently Asked Questions
What is a pulsar timing array?
A pulsar timing array is a network of millisecond pulsars, timed for years with microsecond precision, used together as a galaxy-scale gravitational-wave detector. NANOGrav's 15-year data set times 68 such pulsars.
What is the Hellings-Downs curve?
It is the expected correlation between the timing wobbles of two pulsars as a function of their angular separation in the sky, if a gravitational-wave background is really passing through. The curve equals 0.5 at 0 degrees separation, dips negative for pairs roughly 50 to 100 degrees apart, and rises to 0.25 at 180 degrees. This scene computes it live from the exact published formula (Hellings and Downs, 1983).
What did NANOGrav actually announce in 2023?
NANOGrav, together with the European, Parkes, and Chinese pulsar timing arrays, reported the first evidence that real pulsar-pair correlations follow the Hellings-Downs shape - read as evidence for a stochastic background of gravitational waves at nanohertz frequencies, likely from a population of inspiraling supermassive black-hole binaries.
How is this different from LIGO detecting gravitational waves?
LIGO detects a brief kilohertz-frequency chirp from one merging pair of stellar-mass black holes using laser interferometers on Earth. A pulsar timing array instead looks for a steady nanohertz-frequency background using pulsars themselves as galaxy-sized clocks - a completely different frequency band, instrument, and source population.
Are the 20 pulsars in the scene a real sky map?
No. They are an illustrative scatter used to give the array a sense of scale. Only the highlighted gold-and-magenta pair's angular separation is exact - it always equals the slider's value - and the correlation number for that pair is computed live from the real formula, not simulated or invented.
Does this page simulate an actual gravitational-wave detection?
No. It displays the deterministic EXPECTED correlation curve that a real detection is compared against. It does not add noise, simulate a measurement, or claim to detect anything - it is an educational approximation of the underlying published mathematics.