# On the scale of frequencies

This is just the inverse of time. The standard unit, Hertz, is equal to once per second. There's an implicit one of what ? question involved in that: when the answer to that is cycles of some repeating system, especially when that repeating system is rotational, there is some sense in distinguishing the quantity from an inverse time. Rates of rotation, in particular, are better described as angle/time: the standard notion of frequency is implicitly one turn divided by the period of the repeating process, but commonly ends up being handled in terms of the result of dividing this by radian.

aHz

Hubble's constant, which guages the rate of expansion of the universe, is arouhd 2.27 atto Hertz (albeit more usually expressed in kilometres per second per mega-parsec, for which it's in the low seventies, or kilometres per second per light year, for which it's in the low twenties). This isn't really the frequency of anything but, if it were, the period would be around 14 giga years – the age of the universe.

fHz

The central bar of the Milky Way galaxy spins at 1.8 femto Hertz; the Sun orbits the galactic centre at 0.14 fHz and the spiral arms (pressure-waves, in the inter-stellar gas, that trigger star-formation) circulate round the galaxy at about 0.63 fHz. The last few million years of Earth's history have seen a cycle of glaciation with a frequency of roughly 0.3 fHz. The Earth's orbit's eccentricity fluctuates, due to gravitational perturbations from other planets, with a frequency of about 77 fHz.

pHz

The axis about which the Earth spins precesses, tracing out a cone at a frequency of about 1.2 pico Hertz. The Earth's orbit's elliptical axis rotates, resulting in a relative frequency of around 1.4 pHz in the relationship between seasons and orbit. The angle between Earth's spin axis and plane of orbit varies (between 67.9° and 65.5°) with a frequency of 0.77 pHz.

nHz

The Sun follows a roughly eleven-year cycle of variation in activity; at solar minimum it has few sun-spots and little dramatic activity; activity increases from there to a solar maximum with many sun-spots and much activity, then decreases again to another minimum after roughly eleven years – so with a frequency of 2.88 nano-Hertz. The planets orbit The Sun with frequencies: 132 nHz (Mercury), 52. nHz (Venus), 31.6875361 nHz (Earth), 16.85 nHz (Mars), 2.671 nHz (Jupiter), 1.072 nHz (Saturn), .38 nHz (Uranus), .191 nHz (Neptune).

µHz

At solar minimum, coronal mass ejections happen about once per week – at a frequency of about 1.7 µHz; at solar maximum, the rate goes up to about twice per day – or more – or a rate of 23 µHz. In the early twenty-first century, humanity as a whole was launching about one rocket bound for space each week.

mHz

The ISS orbits Earth with a period of 90 minuts, i.e. a frequency of 5/27 mHz.

Hz

Standard long-playing vinyl records ran on a 100/3 RPM turn-table; this turns at a rate of 5/9 turns/second. There are roughly forty lightning flashes on the Earth per second (or 44 times per minute – sources conflict).

kHz

The rotors in the GE90 jet-engine spin at 10,400 RPM, i.e. 0.173 kHz.

MHz

In the part of CERN's new (due open in May 2008) Large Hadron Collider specialized for study of the beauty quark, LHCb, the frequency of proton-proton interactions shall be 40 MHz – 40 million collisions per second.

GHz

Electromagnetic radiation with a frequency of 1 GHz has a wavelength of one light nanosecond, which is roughly one foot. Mobile telephones using GSM (Global System for Mobile communications) operate at frequencies of 0.85, 0.9, 1.8 and 1.9 GHz (hence wavelengths of about 35, 33, 17 and 16 cm; a.k.a. 14, 13, 6½ and 6¼ inches). Higher frequency electromagnetic radiation, with frequencies from around 3 to around 300 GHz, are generally described as microwave (despite actually having wavelengths of a millimetre and more, rather than microns). The processing rhythm of modern computers ticks along at a few GHz; I'm typing this (in 2017) on a computer (from 2009) whose four processors each run at 1.3 GHz (when they're busy; they spend most of the time idle).

THz

Infrared radiation has frequencies in the range from around 0.3 to around 400 tera Hertz (and micron-scale wavelengths).

PHz

Visible light is electromagnetic radiation with frequencies roughly in the range from 0.4 to 0.75 peta Hertz; ultraviolet light has frequencies in the range from there up to around 30 PHz. Electromagnetic radiation at higher frequencies gets described as X-ray or gamma ray; the distinction is mostly based on what processes (is believed to have) generated them.

Written by Eddy.