system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : If we never manage to create a true absolute zero, how do we know that it sit exactly at -273.15 °c instead of ,uh, -273.69 or something else?
gpt
Background: As temperature drops, the pressure that a fixed volume of gas exerts on its surroundings drops. As temperature increases, pressure increases.


Basically, the relationship between pressure and temperature of a gas is really well understood and well defined. We can take various pressure measurements at various temperatures and plot them on a graph. That plot shows a perfect straight line. We then just follow that line down the temperature axis until we see where pressure drops to 0. You can't have a pressure below 0, and so at that point we know the atoms of the gas have basically stopped moving, which we define as Absolute 0.