Historic Speed of Light measurements in Southern California

If you’ve heard of Dr. Albert Michelson at all, then you’re probably a minor science dweeb and you know him from the Michelson-Morely interferometer.  Every science student knows of it, a device, created in 1887 to look for the presence of aether.  Much to the surprise of a large number of people at the time, no aether was found, nor was it when Michelson repeated the experiment in later years to a higher level of precision.  But not even  most hardcore science types are aware that between 1922 and 1931, Michelson did substantial work in Southern California trying to obtain an accurate value for the velocity of light.

This was a very big deal at the time and his experiments were followed with much interest, even from the general public.  Why?  Well, light was still somewhat mysterious at the time.  Einstein had published his Special Theory of Relativity only a few years earlier in 1905, and people were still wound up over it.  According to Einstein, the velocity of light was a fundamental constant, yet our measurement of it at the time still held significant error.  Que the good Dr. Michelson.

Michelson was not only an outstanding physicist, he was also one of the best experimenters who ever lived.  It’s one thing to be able to understand and wield esoteric theory, but it’s another thing altogether to make theories work in the real world, subject to equipment limitations.  Because of Michelson’s experimental genius in 1907  he was the first American to be awarded a Nobel Prize in Physics “for his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid”. So a precision measurement of the velocity of light was right up Michelson’s alley.

Throughout the latter part of his career, Michelson was associated with the University of Chicago.  However most of his velocity of light measurements were performed using the facilities at the Mount Wilson Observatory as a base.  Michelson typically would spend summers in California, setting up equipment and taking measurements, then return to Chicago and continuing to direct Mount Wilson staff by correspondence.

Most of Michelson’s work in Southern California involved, put simply,  sending flashes of light from Mount Wilson to distant mountain peaks.  These distant peaks had mirrors on them, which returned the light flashes back to Mount Wilson where their travel time (and thus velocity) could be measured by a clever experimental apparatus.  While it sounds simple, to do it with the precision Michelson did was rather extraordinary.

In an attempt to obtain even better results, Michelson’s final experiment was to measure the velocity of light in a mile long, 36″ diameter evacuated pipe, pushing the technology of the day to its limit.  The pipe was located on the Irvine Ranch in Orange County, about a mile northeast of where the John Wayne Airport is today.  Michelson died before that experiment was final, but his associates completed Michelson’s work and published the results.

This is a somewhat brief story of Michelson’s remarkable velocity of light measurements in Southern California.  I’ll share the history I dug up and things I found unexpected and surprising.  I was also able to unearth some remarkable old photos of Michelson’s installations.  Finally, I’ll show what Michelson’s sites look like today, should anyone wish to visit these historic science locations.

A note to picky readers:  The proper terminology is of course “velocity of light”, but most people conventionally use “speed of light”, which I shall do so from this point forward.  It flows better.  Deal with it.

I’ve divided this story into five roughly consecutive sections, listed below.  In the case of any of the images, clicking on them should bring up a much larger version.  Some of the images are really extraordinary and have never been on the Internet before.

The Mount Wilson station (1922 – 1928)

The Mount Wilson-Mount San Antonio measurements (1922 – 1926)

The Mount Wilson-San Jacinto Mountain measurements (1926)

The Mount Wilson-Santiago Peak measurements (1928)

The Irvine Ranch measurements (1929 – 1933)

The data for this report comes primarily from the following sources:

A special thank you to Dr. Dan Lewis and Catherine Wehrey of the Huntington Library for access to the Library’s historic papers and their assistance in the hunt and to Dave Jurasevich of the Mount Wilson Observatory for allowing access to Michelson’s Mount Wilson site.

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