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Absolute zero is a term used in science to define a point at which the motion of particles which constitutes heat is minimal. That temperature is -273.15°C, -459.67°F or zero kelvins1. It was the Irish physicist Sir William Thompson, 1st Baron of Kelvin (1824 - 1907), who worked out this point that is so important for the survival of anything.
Anders Celsius decided the point at which water froze was the ideal base for his scale of temperature. Kelvin decided to maintain the Centigrade scale but make the starting point the coldest temperature possible to achieve. Thus absolute zero is 373.15°C lower than water's boiling point2.
At Absolute Zero strange things are hypothesised to happen to matter. It is known that the hotter a substance is, the more energy it has and the more the molecules jiggle. The colder the temperature, the slower the molecules move and at Absolute Zero the molecules do not vibrate at all and the matter contains minimal energy.