Absolute Zero
The Point at Which a Substance Is So Cold Its Atoms Cease Moving
Absolute zero has actually never been reached, not in nature nor in the laboratory. It may well be impossible to reach absolute zero and, even if it was achieved, it might go unnoticed since there is no thermometer that could measure it.
Measuring the temperature of something is, effectively, recording the average energy of the particles that make it up. Temperature indicates how quickly the particles are moving around. As an object is cooled, its molecules move around less and less. Theoretically, if an object is cooled enough then its molecules will stop moving completely and so it will have reached absolute zero.
The Kelvin Scale
The theory of absolute zero was first proposed in the 18th century by extrapolating a graph of temperature and energy to zero. Since energy rises steadily with temperature, the line connecting these two quantities can be projected backwards on a graph to find the temperature at which energy reaches zero. This point lies at -273.15 degrees Celsius or -459.67 degrees Fahrenheit.
It was Lord Kelvin who proposed a new temperature scale that started from absolute zero. The new scale effectively took the Celsius temperature chart and shifted it so that instead of water freezing at 0 degrees Celsius, it freezes at 273 kelvins.
Today it is the Kelvin scale that is most commonly used to measure temperature.
Absolute Zero
It is impossible to know what absolute zero would feel like. The coldest natural temperature that has ever been experienced on Earth was -89 degrees Celsius or 184 kelvins (temperature taken at Vostok in Antarctica in 1983). Even in deepest space the coldest atoms have a temperature a few degrees above absolute zero. This slight temperature is due to background radiation left over from the Big Bang itself and so it is almost inconceivable that anyway in space actually lies at absolute zero.
In a laboratory situation temperatures even colder than outer space have been reached but even they have not been as low as absolute zero. Liquid helium has a temperature of just 4 kelvins but is still above absolute zero. In 2003 scientists at the Massachusetts Institute of Technology managed using lasers to cool caesium atoms to within 0.5 billionths of a kelvin and this is the coldest temperature ever artificially produced.
Absolute zero is really an abstract idea. Although scientists have been able to get tantalisingly close, they have had to accept that absolute zero may never actually be reached with certainty.
Sources:
Baker, Joanne (2007) 50 Physics Ideas You Really Need to Know (Quercus Publishing)
Holzner, Steve (2005) Physics for Dummies (John Wiley & Sons)
Kuhn, Karl (1996) Basic Physics: A Self-Teaching Guide (John Wiley & Sons)
Lord Kelvin
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