eli5: If absolute 0 is only theoretical, how do we know what it is in Celsius and fereignheit


eli5: If absolute 0 is only theoretical, how do we know what it is in Celsius and fereignheit

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Why is it theoretical? Something that cannot be achieved does not make it theoretical. We’ve never seen the core of the sun – but we don’t think it is “theoretical” – we’re pretty sure it exists.

While we haven’t reached absolute zero, and we never will, we do know right around where it will be. That’s why we set the Kelvin scale to start at that number and have it go up at the same rate as Celsius (a one degree increase in Celsius is equal to a one Kelvin increase), just plus or minus 273.15. We also know the conversion rate to go from any temperature scale to another, so we can just convert there.

Absolute 0, or 0 K (Kelvin), is well defined. It is theoretical in that sense that no matter can achieve this temperature due to quantum mechanics, only become really close. There is no negative temperature in the Kelvin scale, thus absolute zero is at 0 K (hence the name).

1 K is defined to be the same as 1 °C, but the Celsius scale chooses a different point zero, which is where water freezes. Having the same scale, and water freezing at around 273.15 K (at 1 atm) makes absolute zero in the Celsius scale -273.15 °C. Fahrenheit chooses a different scale than Kelvin/Celsius and also a different point zero. But due to the same reasons as to why Celsius has negative temperatures, Fahrenheit also has absolute zero as a negative value.
As for the physics, only the definition of Kelvin with absolute zero at 0 K really makes sense. The scale in all cases is arbitrary, but we chose the scale of Celsius because it was widely adopted.

“Only” does not belong in front of “theoretical”.

We know how other temperatures act relative to absolute zero. We cannot get things to it but we can get things very close.

The simplest way to predict it is to get a chamber full of helium. Start it at 20 celsius. Heat it to 100 celsius and see how much it expands.

It expands by about 27%, so we know that 100c is 127% as hot as 20c. So, 20-(100×80/27) is absolute zero.

The rate of temperature change is measurable and consistent enough that we can extrapolate what would happen past our ability to measure. This lets us “solve for 0” on the temperature scale, and so we know 0 Kelvin would happen at -273.15 Celsius.

Temperature is a measure of the average kinetic energy of the atoms or molecules in the system. The absolute temperature is proportional to the average translational kinetic energy of the atoms of molecules. We define 0 Kelvin as the temperature where kinetic energy is 0.

There is a formula for calculating temperature from the kinetic energy, for gas this formula is ½m<v2> = (3/2)kBT

Thermal energy is like kinetic energy, but on a microscopic level. You can measure that energy in different materials at different temperatures, and extrapolate when it will be zero. If you have enough measurements in different materials at near-0°K, you’ll eventually hit a pretty accurate estimate of how much colder you need to go for that energy to reach zero.

Celsius and Fahrenheit aren’t based off of Kelvin, Kelvin is based off of Celsius.

Celsius and Fahrenheit are based off of the freezing and boiling points of water (for Celsius) and brine (saltwater, for Fahrenheit).

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.


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.

If I tell you a woolly mammoth weighs 1200 kilograms, you can convert that to pounds because the formula to convert between those units is well defined. The mammoth is entirely theoretical, at no point did you need to weigh a physical mammoth, in fact they are long dead.

0 Kelvin is a known point on the temperature scale that we just happen to think cannot possibly exist in reality, but we know how to convert between Kelvin and Celsius and Fahrenheit using basic formulae so I don’t see what you think would be difficult about that.