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2. Atomic clock - Wikipedia

   en.wikipedia.org/wiki/Atomic_clock

   An atomic clock is based on a system of atoms which may be in one of two possible energy states. A group of atoms in one state is prepared, then subjected to microwave radiation. If the radiation is of the correct frequency, a number of atoms will transition to the other energy state.

3. Caesium standard - Wikipedia

   en.wikipedia.org/wiki/Caesium_standard

   The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK [1] and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory. Caesium atomic clocks are one of the most accurate time and frequency standards, and serve as the primary standard for the definition of the second in ...

4. NIST-F1 - Wikipedia

   en.wikipedia.org/wiki/NIST-F1

   NIST-F1. NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard. The clock took less than four years to test and build, and was developed by Steve Jefferts and Dawn Meekhof of the Time and ...

5. History of timekeeping devices - Wikipedia

   en.wikipedia.org/wiki/History_of_timekeeping_devices

   The most accurate pendulum clocks were controlled electrically. [166] The Shortt–Synchronome clock, an electrical driven pendulum clock designed in 1921, was the first clock to be a more accurate timekeeper than the Earth itself. [167] A succession of innovations and discoveries led to the invention of the modern quartz timer.

6. List of atomic clocks - Wikipedia

   en.wikipedia.org/wiki/List_of_atomic_clocks

   Department of Defense master clock. United States Naval Observatory. Washington, D.C. United States. 18 cesium atomic clocks and 4 hydrogen maser clocks. Cs, H. National Institute of Information and Communications Technology. Koganei, Japan [ 13 ] Optical lattice clock.

7. Hafele–Keating experiment - Wikipedia

   en.wikipedia.org/wiki/Hafele–Keating_experiment

   The Hafele–Keating experiment was a test of the theory of relativity. In 1971, [1] Joseph C. Hafele, a physicist, and Richard E. Keating, an astronomer, took four caesium -beam atomic clocks aboard commercial airliners. They flew twice around the world, first eastward, then westward, and compared the clocks in motion to stationary clocks at ...