| Millihertz (mHz) | Terahertz (THz) |
|---|---|
| 1 Millihertz | 1 × 10-15 THz |
| 2 Millihertz | 2 × 10-15 THz |
| 3 Millihertz | 3 × 10-15 THz |
| 4 Millihertz | 4 × 10-15 THz |
| 5 Millihertz | 5 × 10-15 THz |
| 10 Millihertz | 1 × 10-14 THz |
| 20 Millihertz | 2 × 10-14 THz |
| 25 Millihertz | 2.5 × 10-14 THz |
| 50 Millihertz | 5 × 10-14 THz |
| 100 Millihertz | 1 × 10-13 THz |
| Reference | Millihertz (mHz) | Terahertz (THz) |
|---|---|---|
| European mains electricity | 50000 mHz | 5 × 10-11 THz |
| Concert pitch A above middle C | 440000 mHz | 4.4 × 10-10 THz |
| An FM radio station | 1 × 1011 mHz | 0.0001 THz |
| A Wi-Fi band | 2.4 × 1012 mHz | 0.0024 THz |
| A desktop processor clock | 3 × 1012 mHz | 0.003 THz |
The millihertz is a unit of frequency equal to one thousandth of a hertz. Its symbol is mHz. One millihertz is one cycle every thousand seconds, a little under seventeen minutes, so the unit describes things that repeat slowly enough to watch.
The lower-case m matters. A millihertz is a million times smaller than a megahertz, and the two are distinguished only by the case of the first letter. This is one of the few places in the SI where a typing error changes a quantity by six orders of magnitude, and it is a recurring source of confusion in documents that lose their capitalisation.
Seismology is the unit's natural home. A large earthquake sets the whole Earth ringing like a bell, and these free oscillations have periods of minutes to about an hour, placing them between roughly 0.3 and 5 millihertz. The gravest mode, in which the planet alternately expands and contracts as a sphere, has a period near twenty minutes. Measuring these frequencies is how the deep structure of the Earth was mapped before seismic tomography.
The Sun oscillates in the same range. Helioseismology studies pressure waves trapped inside the Sun, whose strongest signal is a five-minute oscillation at about 3.3 millihertz. Tracking millions of these modes reveals the Sun's internal rotation and the depth of its convection zone, information no telescope can obtain by looking at the surface.
Ocean tides sit far below even this. The principal lunar semidiurnal tide repeats every twelve hours and twenty-five minutes, which is about 0.022 millihertz, and the whole family of tidal constituents that tide tables are built from occupies the range below a tenth of a millihertz.
Engineering meets the unit in the sway of tall buildings and long bridges, whose fundamental modes fall between about 100 and 500 millihertz, and in the slow control loops of heating systems and chemical plant, where a process may take many minutes to respond to a change.
One millihertz equals 0.001 hertz, one cycle per 1000 seconds, or 1000 microhertz.
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.