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absolute zero

unattainability of absolute zero
The unattainability of absolute zero is graphically demonstrated by this illustration, in which the absolute temperature is plotted in degrees kelvin on a logarithmic scale rather than a normal linear scale. The logarithmic scale better represents the task confronted by low-temperature physicists, since in nature the ratio of two absolute temperature T1/T2, is more important than their difference T1 - T2. Thus the distance from 10K to 1K is just as great as the distance from 0.001K to ).0001K. On the logarithmic scale, the point at which T = 0K is infinitely far to the left.

apparatus used to reach very low temperatures
McGill University student Cory Dean works on a dilution refrigerator capable of reaching 8 mK above absolute zero
In theory, the lowest possible temperature, and therefore the lowest possible total energy of a system. Although it might be expected that all particle motion would stop at absolute zero, this is not in fact the case. The Heisenberg uncertainty principle asserts that even at the minimum conceivable temperature, subatomic particles would still possess a residual kinetic energy known as zero point energy. A strange outcome of this fact is that the closely packed electrons in a metal at absolute zero would have the same energy as an ordinary gas at 50,000°C. Thus, although at absolute zero a system's entropy is zero, the total energy of a system is not zero.

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.

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