Cursed Units

A collection of cursed scientific units. What If? article: Dimensional analysis: Distance in astronomy: Pulsar scintillation: Background music was me improvising on a piano. Some small corrections: the distances between the planets in the graphics should be between their centres, and the wording when describing the pulse delays is backwards at one point. If you know any other cursed units, share them in the comments! Here’s some more (look for the hearted comments to see the original posts)... Noise in a voltage signal is measured in V/√Hz (volts per square root hertz). You would expect the amount of data produced by a particle collider over a time period to be measured in bytes or number of collisions, but apparently it’s measured in fb^-1, inverse femtobarns, a barn being roughly the cross-sectional area of a uranium nucleus. Other variations of kWh from commenters: kWh/annum contains seconds, hours and years all mashed together. Also lightbulb power consumption is sometimes displayed in kWh/1000h which, if I’m not mistaken, is Watts. This is probably to avoid confusion with the other use of watts which is to measure lightbulb brightness, which would all be fixed if anyone cared about lumens or candela. There’s an alternative to SI units called CGS, centimetre-gram-second. In CGS, units of charge are derived, and because they really shouldn’t be derived there’s multiple conflicting standards. One of these is to define charge using the statcoulomb, equal to 1 g^(1/2) cm^(3/2) s^(-1). Multiplication and division of units is all well and good, but it breaks down when you’re working with units on logarithmic scales, like pH, decibels, or stellar magnitude. Sheet resistance, the electrical resistivity of rectangular sheets of materials, is measured in Ohms per square. Not square metres, not square inches, just square. This is because resistance across a rectangle is proportional to length and inversely proportional to width, so it depends on the aspect ratio but not the size. A barrer is unit of gas permeability used for membranes and contact lenses, and it is equal to 10^(-10) cm^3(STP) cm / (cm^2 s cmHg). The 4 occurrences of ‘centimetres’ can’t be canceled right away, because they all mean different things. This is now my favourite unit. Other bizarre combinations of units mentioned by commenters: thermal effusivity is measured in J/(m^2 K s^(1/2)), a Jansky in radio astronomy is 10^(-26) W/(m^2 Hz), and electrical capacitance is measured in Farads which are kg^(-1) m^(-2) s^4 A^2. A lot of commenters suggested various imperial units. To me these aren’t cursed in a particularly deep way, since they just involve a conversion factor. I’ll give a special shout-out to acre-feet though. Some people have questioned why I didn’t write nm/√nm as just √nm. It’s the same reason we don’t write the Hubble constant in Hertz – you lose all information about how it was measured and what it represents. Other commenters explained that the 2 astronomical units is because the distance between the measurements is twice the distance from the earth to the sun. I know that; I’m jokingly questioning why the definition of the parsec doesn’t have the factor of 2.