Changing constants: measuring progress.

Changing constants: Measuring progress

The values of fundamental physicalconstants, such as the elementary charge and the mass of an electron, are only as good as the techniques used to measure them. When measurement accuracy improves, then the numerical values of these constants change. The publication this month of new values for a range of fundamental constants represents the first major revision of the numbers since 1973. At least one decimal place has been added to many of the constants. Several values had to be adjusted by larger-than-expected margins.

"The advances [in measurement]across the board were amazing,' says physicist Barry N. Taylor of the National Bureau of Standards in Gaithersburg, Md. "There were a lot of new results that had to be taken into account.'

Taylor and E. Richard Cohen of theRockwell International Science Center in Thousand Oaks, Calif., spent almost six years gathering information, reviewing measurements and analyzing data to come up with recommended values. Their report was adopted last summer by the Committee on Data for Science and Technology (CODATA) of the International Council of Scientific Unions, and the final numbers appear in CODATA "Bulletin 63,' recently published by Pergamon Press, Inc., in Elmsford, N.Y.

"Nothing else needs to be done exceptto get the information out to the troops,' says Taylor, "and for everyone to start using the numbers.' This also means changes in tables found in textbooks and reference works. "It takes some time for [the adjustment] to diffuse through the system,' he says.

Changes in the values of fundamentalconstants won't affect the definitions of the seven basic units of measurement-- the meter, kilogram, second, ampere, kelvin, mole and candela--in the International System of Units (SI). They may, however, alter the practical, laboratory methods used to represent certain units.

Although the volt, for example, is derivedfrom the meter, kilogram, second and ampere, it's more convenient to define a practical voltage standard in terms of an electrical measurement on a Josephson junction. That measurement depends on how accurately the Josephson frequency-voltage ratio is known. The recent change in its value means that current practical representations of the volt are about eight parts per million too small.

Researchers measuring wavelengths inspectroscopy, computing energy levels in atoms or working with high-precision instruments may also find the higher degree of accuracy helpful. But in general, the changes in the fundamental constants will have little impact on everyday life or on normal laboratory work.

"It's the testing of physical theorywhere it's most critical,' says Taylor. If two measurements, done in different ways, produce slightly different results for some constant, then there may be a problem in either or both measurements, or the theory may be incorrect.

"The overall consistency of the wholestructure tests how well we can measure,' Taylor says. This type of analysis brings inconsistencies to the attention of the scientific community, "and all the people involved then have a renewed motivation to go back and look harder and to try and understand,' he says. "That's how science progresses.'

Table: Recommended values for selected fundamental constants