Most people have heard absolute zero described as the lowest possible temperature, but what does that mean? Is it really the coldest cold, or just the lowest temperature that we can measure? Is there a corresponding highest temperature? According to Moses Chan, Evan Pugh professor of physics at Penn State, answering these questions requires understanding the meaning of temperature.

"Temperature is a measure of the degree of 'disorder' or 'messiness' of a system," said Chan. "When a system is cooled down to absolute zero, then that system is perfectly ordered and all its constituents -- molecules and atoms -- are in their proper place. That is the lowest possible temperature." Absolute zero, or 0 K (kelvins) corresponds to -273.16 C, or -459.688 F.

Before quantum mechanics was developed as a model to explain the behavior of atomic and subatomic particles, scientists thought that all atoms would stop moving at absolute zero. However, even at this temperature, atoms and molecules retain what is known as zero-point energy, the lowest possible energy a system can have. As Chan explained it, the energy in the vacuum of empty space is considered a form of zero-point energy. Also described as the "ground" or "stationary" state, absolute zero is considered a stable state from which no energy can be removed.

"At low temperatures," Chan continued, "quantum mechanical effects dominate the properties of all matter." In some materials, the effect is truly spectacular. At sufficiently low temperatures, for instance, some types of matter become superconducting, carrying electric current with absolutely no resistance. Practical applications of this phenomena include high magnetic field MRI machines and very efficient electric motors and transformers. Read More

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