Ask the Experts; April 2005; Scientific American Magazine; by Staff Editor; 1 Page(s)

The answer depends on how one interprets the word "measured." Both the accurate measurement of fleeting events and the recording or inference of such occurrences are of interest. So we suggest rephrasing the original query into two new ones: "What are the shortest time spans that can be measured with a particular accuracy?" and "What are the briefest happenings that can be recorded or inferred?"

Currently cesium atomic-fountain clocks are the best way to measure time with a certain accuracy. These "clocks" are actually frequency standards rather than timekeeping devices, and they achieve the defined cesium standard frequency with the exceptional precision of about one part in 1015. Put another way, in 30 million years of continuous operation, they would neither gain nor lose more than a second. Yet these frequency standards are rather "noisy," and to achieve such impressive results requires averaging many thousands of separate frequency measurements over a period of about one day. So fountain clocks are not generally useful for timing short-duration events.