Absolute Zero
Definition
The coldest anything can possibly be — 0 kelvin (−273.15°C) — where particles have the minimum possible motion.
Symbol: · Unit: kelvin (K)
Technical definition
The lower limit of thermodynamic temperature, 0 K, at which a system's entropy reaches its minimum and classical particle motion ceases. The third law of thermodynamics states it cannot be reached in a finite number of steps.
Example
The coldest lab refrigerators reach about 0.00000001 K — ten-millionths of a degree above absolute zero — cold enough that atoms form bizarre quantum states like Bose–Einstein condensates, yet still not quite zero.
Absolute zero does not mean 'all motion stops.' Quantum mechanics leaves a residual zero-point energy — particles still jitter even at 0 K. What stops is thermal motion that could be extracted as heat. And 'twice as cold as 0°C' is meaningless: temperature ratios only make sense in kelvin, where 200 K genuinely holds twice the thermal energy of 100 K.