| Nanohertz (nHz) | Gigahertz (GHz) |
|---|---|
| 1 Nanohertz | 1 × 10-18 GHz |
| 2 Nanohertz | 2 × 10-18 GHz |
| 3 Nanohertz | 3 × 10-18 GHz |
| 4 Nanohertz | 4 × 10-18 GHz |
| 5 Nanohertz | 5 × 10-18 GHz |
| 10 Nanohertz | 1 × 10-17 GHz |
| 20 Nanohertz | 2 × 10-17 GHz |
| 25 Nanohertz | 2.5 × 10-17 GHz |
| 50 Nanohertz | 5 × 10-17 GHz |
| 100 Nanohertz | 1 × 10-16 GHz |
| Reference | Nanohertz (nHz) | Gigahertz (GHz) |
|---|---|---|
| European mains electricity | 5 × 1010 nHz | 0.00000005 GHz |
| Concert pitch A above middle C | 4.4 × 1011 nHz | 0.00000044 GHz |
| An FM radio station | 1 × 1017 nHz | 0.1 GHz |
| A Wi-Fi band | 2.4 × 1018 nHz | 2.4 GHz |
| A desktop processor clock | 3 × 1018 nHz | 3 GHz |
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.
The gigahertz is a unit of frequency equal to one thousand million hertz. Its symbol is GHz. It is the range of modern wireless communication and of the clock inside every current computer, and almost nothing in it was in everyday use before the 1990s.
Wi-Fi occupies two main bands, at 2.4 and 5 gigahertz, with a newer band near 6. The 2.4 gigahertz band is crowded because it is licence-free almost everywhere and shared with Bluetooth, cordless phones and microwave ovens, which operate at 2.45 gigahertz because water molecules absorb energy efficiently there. The 5 gigahertz band offers more channels and higher rates but is absorbed more strongly by walls, so it covers a smaller area.
Mobile telephony spans the range. Earlier generations used bands below 1 gigahertz for their long reach into buildings and across countryside, while newer allocations at 1.8, 2.6 and 3.5 gigahertz carry more data over shorter distances. The higher a frequency, the more bandwidth is available and the less it penetrates, and every generation of network design is a negotiation between those two facts.
Computer clock speeds crossed into gigahertz around the year 2000 and stalled near four a few years later. The limit is thermal: power dissipation rises steeply with frequency, and beyond about four gigahertz a conventional silicon processor cannot shed the heat. Performance gains since then have come from adding cores, widening instruction issue and improving memory hierarchies rather than from raising the clock.
Satellite links, radar and radio astronomy all work here. Weather radar typically runs between 2.7 and 5.6 gigahertz, air traffic control radar in similar bands, and the microwave background radiation that fills the universe peaks near 160 gigahertz, well above the range of terrestrial communication.
Millimetre-wave systems at 24 gigahertz and above are used for automotive collision-avoidance radar and for the highest-capacity mobile network bands. Their very short wavelengths allow small antennas and enormous data rates but are blocked by almost anything, including rain and foliage.
One gigahertz equals one thousand million hertz, 1000 megahertz, or 0.001 terahertz.