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Story Name: Speed of Light
Story Topics: Physics
Datafile Name: Michelson
Methods: Boxplot , T-test , ANOVA
Abstract: In 1879, A. A. Michelson made 100 determinations of the velocity of light in air using a modification of a method proposed by the French physicist Foucault. These measurements were grouped into five trials of 20 measurements each. The numbers are in km/sec, and have had 299,000 subtracted from them. The currently accepted "true" velocity of light in vacuum is 299,792.5 km/sec.

The data are given here as reported by Stigler. Stigler has applied the corrections used by Michelson and reports that the "true" value appropriate for comparison to these measurements is 734.5. Each trial may be a summary of several experimental observations.

Because the speed of light is a physical constant, we know (to a close approximation) the "true" value that Michelson was trying to measure. It is therefore possible to test the null hypothesis that the true mean = 734.5 for each of the trials or for all 100 determinations taken together.

There is evidence of trouble in the data. Boxplots of the trials side-by-side indicate that not all were equally variable nor even centered on the same value. A one-way ANOVA confirms this. One might consider startup effects and underlying bias in the instrument.

Image: Boxplot of the five trials to help see where the differences lie.