According to Einstein’s general theory of relativity, a clock runs more slowly if it is close to a large gravitating object. This principle was highlighted in the movie Interstellar, in which the main character spends several hours on a planet orbiting a massive black hole, and returns to find that his young daughter has become an elderly woman. In the movie, one hour on the planet equates to seven years on Earth. The same effect can be measured on a much smaller scale by changing locations within Earth’s gravitational field. Using atomic clocks, we have compared the ticking of time at Colorado College (elevation 1845 m above sea level), at the United States Air Force Academy (elevation 2165 m), and near the summit of Pikes Peak (elevation 4288 m). Our results confirm that clocks do indeed tick more quickly at higher altitudes, in accordance with Einstein’s theory. This work was carried out as a collaboration of physics majors and faculty at Colorado College (CC) and the United States Air Force Academy (USAFA). The experimental technique is straightforward and our results can be reproduced by other students and teachers, given access to an atomic clock and reasonably large changes in elevation. Indeed, a similar endeavor has been reported by amateur clock enthusiasts.

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The HP 5071A frequency standards used in this work were obtained at no cost as surplus from another laboratory. New 5071A standards are available from Microsemi Corp. for approximately $65,000. As these frequency standards have a long useful life (ours were manufactured in 1999 and 2000), many used units are available over a wide range of prices. We found some used units online for about $2000.
3.
In principle, the measurement described here could be done in a very tall building, given sufficient data accumulation times. For reference, the height of the highest occupied floor of One World Trade Center in New York City, at 386 m above ground level, is greater than the elevation difference between USAFA and CC.
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