Atom Time

Time, the fourth dimension of the world man lives in, is measured by the motion of the earth. The basic unit of time is a day: one revolution of the earth. But recently the battered old earth has fallen into disrepute as a timekeeper. It is not accurate enough for the needs of modern science. Its spin is gradually slowing down-because of the drag of the tides. It also varies slightly, from day to day, for reasons that science does not understand.

Last week the National Bureau of Standards announced a better timekeeper than the earth: the constant vibrations of the atoms in ammonia molecules.

Because of the characteristics of its molecules, gaseous ammonia absorbs radio microwaves of certain sharp frequencies. When such waves are shot through ammonia gas, the radio waves are absorbed as long as they are vibrating at the right speed. If they vibrate too fast or too slow, an electronic device retunes the transmitter and makes the waves vibrate at the exact frequency that is absorbed most strongly. Thus the waves, regulated by the ammonia molecules as the escapement of a clock is regulated by its pendulum, keep to a steady beat. Hitched to a "frequency divider," they measure time accurately in ordinary minutes and seconds.

On this principle the Bureau of Standards has constructed an "atomic clock" that has nothing to do with the erratic earth. The present model is accurate to one part in 10 million. An improved model, the Bureau thinks, should be accurate to one part in 100 million; it would vary one second in about three years. This is better than the earth, whose revolutions vary, from one day to another, by one part in 20 to 30 million.

Some sentimentalists may be sorry to see the earth discarded as a timekeeper. But there are consolations. If the world's official time is kept by the atom clock, the length of the day, in atomic hours, will increase as the motion of the earth slows down. Eventually man will enjoy a 25-hour day. This will happen, according to some calculations, in about 1,800,000 centuries.

*At an estimated rate of two one-thousandths of a second per century.

This file is automatically generated by a robot program, so reader's discretion is required.