PTTI 2006 - Project GREAT

I was invited to present the evening special program at the annual PTTI (Precise Time and Time Interval) conference in 2006. The participants and presenters at the conference are, for the most part, professionals in the precise time & frequency field. The topic was Project GREČAT (General Relativity Einstein/Essen Anniversary Test) which describes the "Kids, Clocks, and Relativity" experiment I did in 2005.

Project GREAT Presentation at PTTI 2006 (PowerPoint slides/PDF, 10.3 MB)


The year 2005 was the 100th anniversary of Einstein's first paper on Relativity. It was also the 50th anniversary of Essen's first cesium atomic clock. Project GREČAT (General Relativity Einstein/Essen Anniversary Test) was conceived to celebrate both events in a single experiment.

This talk presents the historical background, implementation details, and surprisingly successful results of Project GREAT, a modern demonstration of relativistic time dilation by carrying multiple, portable, cesium clocks to high altitude and directly measuring the clock effects of gravitational blue shift as predicted by General Relativity. Although several similar, and now classic, traveling clock experiments have been performed since the early 1970s this one is unique for a number of reasons.

This is perhaps the first case where the General Theory of Relativity is confirmed with a family weekend road trip using surplus atomic timing equipment. The performance of a large collection of vintage and modern cesium standards was measured and the best three clocks pre-selected for the round-trip experiment. With an assortment of sub-nanosecond time interval counters, data logging equipment, AC/DC power distribution systems, and a large set of batteries the family mini-van was converted into a mobile atomic time lab.

The clocks were driven to the highest point accessible by road on Mt Rainier, the iconic volcano near Seattle, Washington, and kept at altitude for 40 hours. Continuous 3-way inter-clock phase measurements were collected during the trip and 5-way rate measurements were made against hydrogen maser references for days both before and after the trip. In addition to practical advice for constructing a mobile time lab, theoretical and actual results, the paper explores multiple algorithms to extract precise time dilation measurements from the raw data.

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