eli5: If absolute 0 is only theoretical, how do we know what it is in Celsius and fereignheit
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There’s two ways to define Celsius.
Option one is the one everyone knows – water freezes at 0, and boils at 100. Nice and simple… But not very consistent, due to stuff like pressure getting in the way.
Option two is more complex, and depends on the precise definition of heat. We can define thermal energy as “the average kinetic energy of all the atoms in a system”, and then we can define temperature as a measure of thermal energy. At 0° C, we find that a gas has some amount of energy per atom. At 100° C, we find that it has a second amount of energy per atom. At 200°, we find that it has a third amount of energy per atom – and the difference between 0 and 100 is exactly the same as 100 to 200. We can then extrapolate down, to find that *zero* energy corresponds to a temperature of -273.15.
We have tried it with a lot of things, and it always extrapolates to that same number – -273.15°C. You can do the same process for Fahrenheit too, but you don’t *really* need to – the conversion between the two is the same for any temperature.
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This energy perspective also solves another big question – why can’t you go any colder? Going below absolute zero means you don’t just have no kinetic energy – you have less than zero kinetic energy. You aren’t just *not moving*, you’re *anti-moving*. This clearly makes no sense.
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