File: 6 October 2026 · Source date: February 25, 2026 · NIST
Why do clocks work differently?
According to general relativity, a clock in a stronger gravitational field moves slightly slower than a clock in a weaker field. In special relativity, motion also affects clock speed. These are not metaphors, but physical differences comparable to precision atomic clocks.
What will the 2026 experiments test?
NIST describes a comparison of high-precision clocks on the International Space Station with those on the ground, and a comparison of the clock on Mount Blue Sky in Colorado with the clock approximately 2,700 meters below. The aim is to test more rigorously whether the clocks tick as predicted by the theory in lower gravity conditions.
Why is GPS a good example?
In satellite clocks, both motion and the weaker gravitational effect are taken into account. Relativity corrections are part of the engineering calculation for geolocation in phones to work correctly. This success does not mean that the theory is complete on every scale; It shows that it is extremely strong under the conditions it is tested.
discussion question
What is “real time” if two clocks start at the same time and measure different times at different altitudes? Answer by considering physical measurement and everyday time experience separately.
Resources and further reading
NIST — Putting Einstein to the Test With the World’s Most Accurate Clocks ↗Share your answer
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