Colder than Coldest

One of the firmest hitching posts in science is absolute zero, the temperature ( -273DEG C. or 0DEG Kelvin) where vibratory motions of the atoms cease. Generations of physics students have been told that nothing can get colder than 0DEG K. It gave them a comfortable feeling that here at least was a final point beyond which they need not worry. Recently in Manhattan, Professor Norman F. Ramsey of Harvard University told a meeting of the American Ordnance Association about a new set of laws that must be worked out to deal with a temperature range that reaches below absolute zero.

Ordinary heat is motions of atoms or molecules, but when the motion has died away at 0DEG K., the nuclei of the atoms still have a property called "spin." Some spins have more energy than others, and the spinning nuclei can affect the spin of other nuclei near them. So high-energy spin can spread through a substance in much the same way that heat does. Low-energy spin can spread, too, so a substance whose atoms are motionless in the ordinary sense can still lose energy and cool below absolute zero.

The laws that govern spin temperatures at --K. are not for physics beginners, and ordinary rules of thermodynamics do not work. They lead to the incorrect conclusion that a heat engine operating below absolute zero can do work, e.g., produce mechanical energy, without affecting the temperature of the material that it is ' using as an energy source. Professor Ramsey proposes that one of the thermodynamics laws (among the most sacred in physics) be changed to preclude the possibility of a --K. perpetual-motion machine.

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