Cursed Units

A collection of cursed scientific units. Cursed Units 2 is now out! Watch it here: What If? article: Dimensional analysis: Distance in astronomy: Pulsar scintillation: Background music is me improvising on a piano. More info: #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. Some 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 included (I also know why it doesn’t, this joke wasn’t communicated very well). If you know any other cursed units, share them in the comments! Some of these are in Cursed Units part 2, but here’s some more... 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. 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. 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.