Also in the March Physics Today is an essay (which PT calls "Reference Frame") entitled "On the Matter of the Meter" by David Kleppner. This two page article is, in my opinon, quite well written. One phrase that brought a grin to my face was "Most physicists are deeply unconcerned with, if not totally unaware of, legal definitions [of units of measurement], ....." Kleppner's point is that the CGPM has a knack, due to the slowness of the political process involved, for defining the second and the meter in terms of a technology just as that technology becomes obsolete. The current definition of the meter must rely on a technique called optical frequency chains (of lasers locked to one frequency and offset precisely to provide another). The precision of this technique is already enough to make it essential for the two main atomic clock standards for time (in the U.S. and Europe) and for GPS clocks to require corrections for the effect of gravitational fields on time -- bone up on General Relativity for details. (To use a figure from the article, "time varies with height at the surface of the Earth by one part in 10^16 per meter.") But the technology of optical chains is being overtaken by a device called a frequency comb generator, which also uses lasers. Kleppner envisions a new definition coming out of CGPM someday (year? decade?) as a result of these issues. Jim -- James R. Frysinger University/College of Charleston 10 Captiva Row Dept. of Physics and Astronomy Charleston, SC 29407 66 George Street 843.225.0805 Charleston, SC 29424 http://www.cofc.edu/~frysingj [EMAIL PROTECTED] Cert. Adv. Metrication Specialist 843.953.7644
