An adventure in relative time-keeping

I enjoyed Daniel Kleppner’s Reference Frame about the relativistic effects of elevation on precise clocks (Physics Today, March 2006, page 10 ). He would be amused with an experiment I did with my kids last year.

The year 2005 was the widely publicized 100th anniversary of Einstein’s first paper on relativity and the lesser-known 50th anniversary of Louis Essen’s first cesium clock. To celebrate, I created Project GRE²AT (General Relativity Einstein/Essen Anniversary Test), perhaps the first “kitchen science” relativity experiment.

As a collector of vintage and modern atomic clocks, I discovered it was possible, using gear found at home, to convert our family minivan into a mobile high-precision time laboratory, complete with batteries, power converters, time interval counters, three children, and three cesium clocks (see photograph). We drove as high as we could up Mount Rainier, the volcano near Seattle, Washington, and parked there for two days. The trip was continuously logged with the global positioning system; the net altitude gain was +1340 meters.

Given the terrestrial blueshift of 1.1 × 10⁻¹⁶ per meter mentioned by Kleppner and integrating our altitude profile, we predicted the round-trip time dilation to be +22 nanoseconds. This is remarkably close to what we experimentally observed when, after we returned, the ensemble of portable cesium clocks was again compared with atomic clocks left at home (see graph).

Instead of fanciful stories of rocket ships and twins, the kids got a hands-on introduction to general relativity with real clocks and a family road trip. Furthermore, by being at high altitude for the weekend, we experienced more time together, relatively speaking. It was the best extra 22 nanoseconds I’ve ever spent with the kids.

So, yes, not only do we live in a time when atomic clocks are altimeters, but when relativity is child’s play.