The nanohertz is a unit of frequency equal to one billionth of a hertz. Its symbol is nHz. One nanohertz is one cycle per thousand million seconds, which is a little under thirty-two years, so a single oscillation at this frequency takes longer than most careers.
Almost nothing uses it, and then in 2023 it became one of the most discussed units in physics. Several pulsar timing array collaborations, working independently on four continents, reported evidence for a background of gravitational waves in the nanohertz band. The signal appears to come from pairs of supermassive black holes orbiting each other in the centres of merged galaxies across the whole history of the universe.
The measurement technique is remarkable. Millisecond pulsars are neutron stars that spin hundreds of times a second and emit a beam that sweeps past the Earth with extraordinary regularity, rivalling atomic clocks. A gravitational wave passing through the galaxy stretches and squeezes the space between the Earth and each pulsar, changing the arrival times of the pulses by a few hundred nanoseconds. By watching dozens of pulsars for two decades and looking for a specific pattern of correlation between them, astronomers detect waves whose wavelengths are measured in light years.
No instrument could work any other way at these frequencies. A detector must be comparable in size to the wavelength it seeks, and a nanohertz gravitational wave has a wavelength of tens of light years. The galaxy itself is the apparatus, and the pulsars are its markers.
The unit appears elsewhere only in the slowest of natural cycles. The precession of the Earth's axis takes about 26,000 years, roughly 1.2 nanohertz. Orbital variations in eccentricity, obliquity and precession that pace the ice ages, the Milankovitch cycles, run from about 0.5 nanohertz down to a tenth of that.
Below the nanohertz the concept of frequency becomes strained, because nothing has been observed long enough to see a full cycle, and the value is inferred from theory rather than counted.
One nanohertz equals 0.000000001 hertz, one cycle per thousand million seconds, or 0.001 microhertz.
| Unit | Symbol | 1 Nanohertz equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Microhertz | µHz | 0.001 µHz | 1000 nHz | Nanohertz to Microhertz |
| Millihertz | mHz | 0.000001 mHz | 1000000 nHz | Nanohertz to Millihertz |
| Hertz | Hz | 0.000000001 Hz | 1000000000 nHz | Nanohertz to Hertz |
| Kilohertz | kHz | 1 × 10-12 kHz | 1000000000000 nHz | Nanohertz to Kilohertz |
| Megahertz | MHz | 1 × 10-15 MHz | 1 × 1015 nHz | Nanohertz to Megahertz |
| Gigahertz | GHz | 1 × 10-18 GHz | 1 × 1018 nHz | Nanohertz to Gigahertz |
| Terahertz | THz | 1 × 10-21 THz | 1 × 1021 nHz | Nanohertz to Terahertz |