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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=mc[[superscript:2]] 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 c[[superscript:2]]. 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.
* In relativistic calculations, the speed of light is often set to the unitless number 1, as this simplifies equations (for example, E=mc[[superscript:2]] 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 c[[superscript:2]]. 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.
