Absolute zero

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Liquid helium at 4 kelvins
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Absolute zero is the theoretical temperature at which all thermal vibration ceases, about 273 degrees below zero Celsius (or 459 degrees below zero Fahrenheit). The Kelvin scale was defined to reflect this, with absolute zero being 0 K (zero kelvins, not zero *degrees kelvin). In practice, the laws of thermodynamics ensure that absolute zero can never be reached, although it can be closely approached as a limit with careful laboratory procedures. Quantum mechanics tells us that there is a finite amount of energy even at absolute zero, called zero-point energy. This is why liquid helium can't be frozen at normal pressures.

Through the general expansion of space, the Universe overall has cooled to just 2.7 kelvins. The superconducting magnets in the Large Hadron Collider are colder still, with operating temperatures of just 1.9 K[1] and the Herschel Space Observatory has to be cooled further to 1.4 K with 2,000 litres of liquid helium despite being in the depths of space.

Scientists have been able to cool a large group of molecules to 350 nanokelvins (i.e., 0.00000035 kelvins).[2] This is often described in a pop science way of describing the distance between absolute zero and room temperature as the distance between London and New York, with the lowest achieved temperature then being a hair's width.