Temperatures within a few billionths of a degree of absolute zero have been achieved in the laboratory. At such low temperatures, substances have been seen to enter a peculiar state, known as the Bose-Einstein condensate, in which their quantum wavefunctions merge and particles lose their individual identities.
Denoted by zero degrees on the kelvin temperature scale (0 K = -273.16°C = -459.69°F), absolute zero is physically unattainable according to the third law of thermodynamics. At first sight, this might seem unreasonable. There is no upper temperature limit, so why should there be a lower one? In trying to understand this, it is helpful to think in terms of temperature ratios rather than temperature differences – the ratio from 10,000 K to 1,000 K, say, being the same as that from 0.001 K to 0.0001 K. Just as by supplying more and more energy to a system we can add as many zeros before the decimal point of the kelvin reading as we choose, so by continuing to take energy out of a system we can add an arbitrary number of zeros after the decimal point. Yet just as we can never reach an infinitely high temperature, so we can never attain an infinitely low one – absolute zero itself.
In a deep sense, absolute zero lies at the asymptotic limit of low energy just as the speed of light lies, for particles with mass, at the asymptotic limit of high energy. In both cases, energy of motion – kinetic energy – is the key quantity involved. At the high energy end, as the average speed of the particles of a substance approaches the speed of light, the temperature rises without limit.
Related category• HEAT AND THERMODYNAMICS
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