In 1939 Walter A Shewhart published a chart of the history of measurement of the speed of light (1). His attention had turned from his earlier work on industrial statistics and business improvement (2) to issues of scientific measurement and operational definitions. The chart shows a gradual decrease in the measured values and substantial inconsistency between studies, even when measurement uncertainty is considered. Much of Shewhart's intention in publishing the chart was to expose some of the issues around measurement uncertainty and how it is addressed by scientists. I was always intrigued as to the subsequent history of the chart so decided to research the data and bring the chart up to date. Of course, at the 1983 Conference Generale des Poids et Mesures the following SI (Systeme International) definition of the metre was adopted:
"The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second."
This brought an end to speed of light measurement so the whole history can be told, at least until the current definition of the metre becomes sufficiently imprecise for current needs and the definition needs to be refined.
The up-to-date chart is shown in Figure 1. The vertical lines illustrate the experimenters' estimate of uncertainty though it is not always clear what these are meant to suggest. Click on the chart for more detail.
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This page last updated 24th May 2001
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