Project GREAT: General Relativity Einstein/Essen Anniversary Test

*Created 24-Sep-2005, updated 22-March-2007*

In September 2005 the kids and I took several very accurate cesium atomic clocks from home and parked 5400 feet up Mt Rainier (the volcano near Seattle) for a full two days. The goal was to see if the clocks actually gained time, even if billionths of a second, as predicted by Einstein's general theory of relativity. Does gravity really alter time and can this weird phenomenon be detected with a family road trip experiment?

See photos of Project GREAT (the clocks, the kids, the van, the mountain, etc.)

- Project GREAT: Photo Tour (a family relativity road trip in pictures)

Project GREAT in Physics Today Letters:

- An Adventure in Relative Time-Keeping (as PDF local) in response to
- Daniel Kleppner's article: Time Too Good To Be True (as PDF local)

WIRED magazine coverage:

- Amateur Time Hackers Play With Atomic Clocks at Home (article)
- Time Hackers Tinker With Their Atomic Toys (photos)

Atomic clock humor pages related to Project GREAT:

- Most Accurate Wristwatch (the first true atomic wristwatch)
- Climbing With Atomic Clocks (a relativity experiment that proved too difficult)

Below is an informal description of Project GREAT. References to more technical papers can be found at the end of this page.

If you don't want to read the details about the experiment,
at least look at this one graph.
Superimposed on a photo of the kids at Glacier Vista on Mt Rainier is
a graph showing the timekeeping accuracy of three cesium clocks (**red**,
**green**, **blue**
lines).
We, and the clocks, spent three days at home, then two days up on the
mountain, and three more days back at home.
The plot covers 8 days; measurements while at home before and after the trip were made against atomic clocks in my lab and are accurate better than a nanosecond.

According to Einstein, fast-moving clocks run slow (*special relativity*), and high-elevation
clocks run fast (*general
relativity*). Clocks that run fast gain time, so given our high elevation and how long we stayed,
the prediction was that
these clocks would gain about 22 nanoseconds. This, not because the clocks were
moving (they were in a parked minivan),
but simply because the clocks experienced a lower gravitational field by being 5400 feet above sea level for two days.

Of course, the predicted effect is incredibly small,
but with clocks *accurate* enough,
elevations *high* enough,
the stay *long* enough,
and time interval counters *precise* enough,
the effect becomes measurable.
The goal of this fun experiment was to measure, or at least to demonstrate,
relativistic time dilation using equipment I had at home.

The plot show the clocks aren't perfect
(even these excellent atomic clocks vary randomly at the nanosecond level) but they each
undergo positive time dilation and the
average (**black** line) time jump in the three clocks while up on the mountain
for two days is around 23 nanoseconds. Not bad for a first attempt.

// next //

My letter about Project GREAT appeared in the March 2007 issue of Physics Today.

- An Adventure in Relative Time-Keeping (as PDF) in response to
- Daniel Kleppner's article: Time Too Good To Be True (as PDF)

Title:
Letter: 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 GREAT (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 GPS; the net altitude gain was +1340 meters.
Given the terrestrial blueshift of 1.1 × 10 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. |

Project GREČAT was presented at the PTTI conference in December 2006. Below is the abstract of the talk.

Title:
Abstract: 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 GREAT (General Relativity Einstein/Essen Anniversary Test) was conceived to celebrate both events in a single experiment. This paper 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 blueshift 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 minivan 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. |

PTTI technical paper PDF, available Spring 2007

PTTI PowerPoint presentation PDF, available by request.

Here's the email about Project GREAT to the Time-Nuts mailing list.

From: "Tom Van Baak" <tvb*leapsecond.com> |

- [1] A short introduction on Einstein, clocks, and time can be found here:

Beginners Guides to Measurement - Einstein

http://www.npl.co.uk/publications/einstein/relativity_time.html - GREČAT or GREAT, doesn't matter.

Send comments or questions to: tvb

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Project GREAT: General Relativity Einstein/Essen Anniversary Test