Absolute zero (−273.15°C or 0 Kelvin) is the lowest possible temperature, where atoms stop moving completely. But reaching it is impossible because of quantum mechanics—there’s always some energy left, even in the coldest systems. The coldest temperature ever achieved in a lab is 38 trillionths of a degree above absolute zero, inside a cloud of ultra-cooled atoms. At that temperature, matter starts behaving in strange, almost wave-like ways.