14.2.4—The lowest possible temperature is zero kelvin on the thermodynamic

Absolute zero is zero kelvin, the lowest limit of thermodynamic temperature; it corresponds to −273.15 °C.

It is a limiting state, not simply “no motion” in every quantum description. Use it as the zero of the absolute scale.

Cooling from 300 K to 150 K halves the absolute temperature even though Celsius readings do not behave as a ratio scale.

Negative Celsius temperatures can be physically valid, but temperatures below 0 K are not reached in the ordinary thermodynamic scale.