Lord Kelvin · Absolute temperature
Is there a coldest possible temperature?
That time, that place
Thermometer scales are human choices. Celsius divides freezing and boiling water into a hundred; Fahrenheit uses other anchors.
So zero means nothing in particular — just where water freezes. Is there a real zero, a temperature you cannot go below?
Why this question
The clue was in gases. Cool a gas and it shrinks, at a steady rate — the same amount of volume per degree.
Extend that straight line and it reaches zero volume somewhere near minus 273 Celsius.
The volume never actually reaches zero; the gas liquefies first. But it was a sign that below that point there is nothing.
What was found
Kelvin set that point as absolute zero and proposed counting temperature from there. Water freezes at 273 kelvin.
This is not one more scale. It is an answer to what temperature is: how vigorously the particles move, with absolute zero the point where that motion is least.
Laboratories now come within a billionth of a degree of it. They never arrive — and that they cannot is itself a conclusion of thermodynamics.
Zero was a human choice, and nobody knew whether there was a floor
Volume-versus-temperature lines for different gases converge on one point
Temperature acquired a definition, and thermodynamics an absolute scale