General relativity predicts that standard clocks placed at differing
gravitational potentials will run at different rates. Although experiments
confirming the gravitational redshift have been done, they involve
frequency and not time, and need not appeal to general relativity
for explanation. Therefore, considerable interest exists as to the result
of an accurate experiment in which real macroscopic clocks are
brought together for comparison before and after separation to differing
potentials.
This experiment consists of flying an ensemble of atomic clocks in
a military aircraft and comparing them before and after flight to another
clock ensemble remaining on the ground. The ground ensemble included
several Hewlett-Packard Cesium Beam clocks, three Efratom optically
pumped Rubidium clocks, and two hydrogen masers. The flying ensemble
included at least three Hewlett-Packard Cesium clocks and three Efratom
Rubidium clocks. Five of the Cesium clocks were new models delivered
with a high beam current option resulting in higher stability than standard
models. The clocks were maintained under stringent environmental controls to protect against vibration, magnetic fields, and changes in
temperature, pressure, and power supply voltage.
Five main flights were ma de, each at approximately 30,000 feet
altitude for fifteen hours. The aircraft was continuously tracked by
a theodolite calibrated radar which obtained position and velocity
measurements for every second of flight. This allowed an accurate
calculation of a theoretical prediction to compare to experiment.
The flying clocks gained approximately 45 nanoseconds (45 x 10-9 s)
with respect to the ground clocks. The normalized results (measured
effect divided by predicted effect) and the experimental standard
deviations of the mean for each of the five flights were as follows:
.999 + .016 .977 + .026 .963 + .013 1.002 + .026 .991 + .037
The result for the entire experiment, with standard deviation of the
mean, was .987 ±. .011. The statistically expected standard deviation
of the mean based on knowledge of clock quality was approximately .015.
Considering this result as well as systematic errors, a final result
is established of Measured value/
Predicted value =
0.987 ± .01