| Terahertz (THz) | Nanohertz (nHz) |
|---|---|
| 1 Terahertz | 1 × 1021 nHz |
| 2 Terahertz | 2 × 1021 nHz |
| 3 Terahertz | 3 × 1021 nHz |
| 4 Terahertz | 4 × 1021 nHz |
| 5 Terahertz | 5 × 1021 nHz |
| 10 Terahertz | 1 × 1022 nHz |
| 20 Terahertz | 2 × 1022 nHz |
| 25 Terahertz | 2.5 × 1022 nHz |
| 50 Terahertz | 5 × 1022 nHz |
| 100 Terahertz | 1 × 1023 nHz |
| Reference | Terahertz (THz) | Nanohertz (nHz) |
|---|---|---|
| European mains electricity | 5 × 10-11 THz | 5 × 1010 nHz |
| Concert pitch A above middle C | 4.4 × 10-10 THz | 4.4 × 1011 nHz |
| An FM radio station | 0.0001 THz | 1 × 1017 nHz |
| A Wi-Fi band | 0.0024 THz | 2.4 × 1018 nHz |
| A desktop processor clock | 0.003 THz | 3 × 1018 nHz |
The terahertz is a unit of frequency equal to one million million hertz. Its symbol is THz. It occupies the gap between the highest radio frequencies and the lowest infrared light, a region that for most of the twentieth century was so hard to generate or detect that it was known as the terahertz gap.
The difficulty was practical rather than theoretical. Electronic oscillators run out of speed above a few hundred gigahertz because charge carriers cannot respond fast enough, while optical sources such as lasers become inefficient below the infrared. The band between them had no convenient technology on either side, and for decades it was the least explored part of the electromagnetic spectrum.
That changed with ultrafast lasers and photoconductive antennas, which generate terahertz pulses by illuminating a semiconductor with a laser pulse lasting a few femtoseconds. Terahertz time-domain spectroscopy now measures the response of materials across the band, and the technique has become a standard tool in materials science.
Security screening is the best-known application. Terahertz radiation passes through clothing, paper, plastic and ceramics but is stopped by metal and absorbed by water, and its photon energy is far too low to ionise anything, so it can image concealed objects without the risks of X-rays. Body scanners at airports work on this principle.
Molecular spectroscopy uses the band because many large molecules have rotational and vibrational modes here. Explosives and drugs have characteristic terahertz signatures, and the technique is used in pharmaceutical quality control to identify crystal forms that look identical by other methods. Art conservation applies it to see beneath paint layers without touching a canvas.
Astronomy observes at these frequencies from high, dry sites or from orbit, because atmospheric water vapour absorbs terahertz radiation strongly. Cold dust and molecular gas in star-forming regions radiate here, which is why observatories are built at high altitude in Chile and at the South Pole.
Communications research has begun to move into the band. Data rates rise with available bandwidth, and there is far more room above 100 gigahertz than below it, so laboratories have demonstrated wireless links carrying hundreds of gigabits a second at frequencies approaching a terahertz. The obstacle is that water vapour absorbs these waves strongly, limiting a link to a few hundred metres in clear air and much less in rain. That confines the technology to short hops: connections between rooftop antennas, links inside a data centre, or the last stretch between a lamp-post and a building rather than anything resembling a broadcast.
One terahertz equals one million million hertz, 1000 gigahertz, or a wavelength of about 0.3 millimetres.
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.