Barbara Awes, age 12, of Newport News, Virginia, for her question:

What is meant by absolute zero?

The word zero gets around in our everyday world, We use three zeros, $0.00 to report a penniless condition. Zero degrees Centigrade, is the freezing point of water. Dry ice is about 80 degrees colder than this, or minus 80 degrees centigrade. But things can get a lot colder than dry ice, at least in the lab. Absolute zero is a temperature rating, invented to give the absolute limit of how cold things can get.

When temperature scales first were invented, their degrees were based on the freezing point and the boiling point of water. On the old Fahrenheit scale, water freezes to solid ice at 32 degrees. It boils to steam at 212 degrees. This is a difference of 180 Fahrenheit degrees. On the more streamlined Centigrade scale, the same difference is a neat 100 degrees. Water freezes at 0. and boils at 100. This scale made Lt easier for scientists to figure things on the metric scale. But it gave no clues to the real nature of heat and cold.

These clues are related to what really happens when temperatures rise and fall. Heat is a form of energy. Its energy is actually the speed of moving molecules and atoms. As water gets hotter, its molecules slide around faster. As it cools, the loss of heat energy slows down the molecules. At 0 degrees centigrade, they fit together in crystals of ice. But they still have enough heat to jog and vibrate. Some motion remains until a substance reaches the heatless limit of absolute zero.

There seems no limit as to how hot a substance can get. But it reaches a limit of coldness when its temperature drops to minus 273.15 degrees centigrade. This is about 273 degrees below the zero degree of freezing water on the Centigrade scale. It is the same temperature as minus 459.67 degrees Fahrenheit, which is about 427 degrees colder than the zero on the Fahrenheit scale.

Cryogenicists, who study the effects of super cold temperatures, obviously needed to simplify this zero confusion. It was decided to forget the old Fahrenheit scale altogether. The new absolute scale, alias the Kelvin scale, is based on the neat centigrade degrees. But its zero is absolute zero, and on the absolute scale, water freezes at 273 degrees above zero.

Cryogenics is rather a new branch of science. It is full of surprises and rewards. For all sorts of things happen when ordinary substances become colder than cold. However, removing the last fraction of heat from a substance is very very difficult. So far, the cryogenicists have not chilled anything right down to the bones of absolute zero. But they have come within a small fraction of one degree.

At super cold temperatures, many non magnets become magnetic. Lead and other stubborn metals conduct electricity. Cryogenic research has led to improvements in metallurgy, rocket fuels and new electrical devices. One experiment slowed down the bewildering spin of electrons and helped to solve an important nuclear problem. In this case, a temperature of one tenth of a degree above absolute zero was held for ten second periods.