What do such measurements offer to the working physicist? Their great precision offers the prospect of new time and frequency standards that have considerable improvements in accuracy. The present time standard is an atomic clock that relies on cesium atoms. It keeps time with an error of one part in 1013, corresponding to a few ten-thousandths of a second over a human lifetime. That certainly is far better accuracy than anyone needs for ordinary purposes, but there are situations where even this is barely adequate. Tests of Einstein's relativity, for instance, demand all the precision a physicist can achieve. Now the atomic fountain offers the prospect of a thousandfold improvement, reducing the error to one part in 1016.

Chu notes that this would make it possible to determine whether the constants of physics may be changing slowly with time. These constants include the speed of light, the charge on the electron, and Planck's constant in quantum mechanics. Standard physical theories hold that they all should indeed remain invariant, but it would be useful to check. As with Dehmelt's study of the radius of the electron, such a check might show that some of our basic ideas are wrong. Chu notes that such an experiment could feature atomic clocks of very great precision that would rely on distinctly different physical principles. One clock, based on optical transitions, would have its frequency determined primarily by quantum electrodynamics (QED), while another, based on the hyperfine splitting of certain spectral lines of atoms, would be determined by a combination of QED and nuclear forces. If the strength of the nuclear forces changed relative to the electromagnetic forces, the two very precise clocks would begin to "tick" at different rates.

Other types of observation would involve novel aspects of quantum mechanics. This branch of physics predicts that an atom has wave properties similar to those of photons of light. This is different from saying that the atom absorbs photons having the right wavelength; it means that the atom itself behaves like a wave rather than as a particle. Further, its wavelength increases as the atom cools to ultralow temperatures. This opens the prospect of carrying out experiments with atoms that are done ordinarily with light.

Among these is interferometry, which demonstrates that light consists of waves. Such an experiment conventionally features two thin slits set close together, with a screen behind them. A laser shines through the slits. It does not produce shadows but yields a pattern of alternating light and dark bands on the screen, as a result of interference

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