This was part of the old "General" section, but seems rather more geeky than confusing:
- In relativistic calculations, the speed of light is often set to the unitless number 1, as this simplifies equations (for example, E=mc2 becomes E=m). This makes time and space use the same units, so that a light-year actually does measure time as well as space (it's equivalent to one year). It's doubtful that this justifies any of the other examples on this page.
- One of the major, earth-shattering revelations of relativity was that spatial and temporal dimensions are, in some sense, the "same thing". One can, in fact, use units of time and distance interchangeably, and be correct — i.e. you can talk about "years of distance" or "kilometers of time" and be technically correct. If you work with relativity a lot, you'll get pretty used to using just "year" as a unit of distance — which is perfectly correct.
- You could also use distances to measure time — which isn't commonly done, because a kilometer of time, for example, is a very small amount of time — around about 1/300,000 of a second. But "kilometers of time" are seen in long-distance telecommunications, where signals take noticeable time to propagate, and even "millimeters of time" become important in designing circuits that switch billions of times per second, such as those in a modern CPU.
- With some mucking around with unit conversions it is possible to measure mass in units of distance. Starting from kilograms, multiply by G (the gravitational constant) then divide by c2. This leaves you with your mass in meters, which will be very small (the Earth's mass is about 9 millimeters).
- Incidentally, this is the formula for a Schwarzchild radius (i.e. where a black hole's event horizon is). So a black hole with a mass equal to Earth's would be 9 mm in radius, or 18 mm across. As a quick approximation, a black hole's radius is 3 km for every Solar mass it has. A theoretical black hole with a mass equal to the mass of the observable universe would have a radius about equal to the radius of the observable universe (black holes become progressively less dense as they become heavier).
- Inverted with some obfuscated measurements like the becquerels per diopter, which is just a roundabout way to describe meters per second. The becquerel measures frequency, with n becquerel meaning "n per second", while the diopter is used for lenses, with a n diopter focusing parallel light rays 1/n meters away from it. Technically, that means "becquerel per diopter" is "(1/s) per (1/m)", or m/s.
Tamalesyatole
There is an Isaac Asimov-Black Widowers-Short Story about not knowing what temperature certain chemical reaction must be held, 40°F or 40°C.
Turns out that it really doesn't matter, since -40°F=-40°C
...ya llegó.
Regarding the "absolute zero" entry, don't really see how that's fully justified, unless the physical laws of that universe allow negative kinetic energy. You can have sub-zero degrees Celsius and Fahrenheit because they are based on phase change, but absolute zero is a complete lack of kinetic energy, meaning all particles in the sample do not move, at all, in any direction. To go below zero Kelvin would be to experience negative kinetic energy, or some sort of anti-movement (not simply a lack of movement or movement along a negative vector) and is sufficiently abstract and alien to the way our universe works that trying to justify it with methodology based on the physical laws of our reality just smacks of [didn't do the research] to my mind. YMMV.
Still Forecharmer
Where should we add the Dollar=Cent
video? I know it was refrenced in this XKCD Alt Text.
Regarding the following bulletpoint:
- At least in Germany, instead of saying "kilometre per hour", many people tend to use the abbreviation "km/h" in everyday conversation. But only the letters are said out loud, not the slash. If this unit wasn't so ubiquitous, you could easily mistake this for "kilometre multiplied by hour".
You're misunderstanding what he's saying. Km/H is kilometers divided by hours, but if you leave out the "/" it means Kilometers Hour, Km H, or Km*H (because two variables without any other notation implies multiplication.)
The problem with this bullet point isn't that he's wrong, but that it's not necessarily true that the speakers are expressing the literal mathematical expression; in English at any rate it'd just be an abbreviation for the phrase "kilometers per hour" without any loss of meaning or understanding since the operation is baked into the extended phrase. Of course, I don't know German and it may well be that KMH is meaningless in that language and it can only mean anything if it's using a literal math formula.
I would like to comment on the distinction between "metre" and "meter", as they have very specific SI meanings.
A "metre" is the SI term used for the unit of length. (In American English, this is often spelled "meter", but this is not the SI term). A "meter", on the other hand, is used for measuring instruments (except those with names deriving directly from "metre", so, for example, "metre stick" and "newton meter"). These are the SI terms so I have corrected them when not In Universe. (It's not a matter of American or British English; it's simply a matter of using technical terms).