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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

   A caesium atomic fountain used as part of an atomic clock. The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical ...

4. Chip-scale atomic clock - Wikipedia

   en.wikipedia.org/wiki/Chip-scale_atomic_clock

   A chip scale atomic clock (CSAC) is a compact, low-power atomic clock fabricated using techniques of microelectromechanical systems (MEMS) and incorporating a low-power semiconductor laser as the light source. The first CSAC physics package was demonstrated at the National Institute of Standards and Technology (NIST) in 2003, [1] based on an ...

5. International Atomic Time - Wikipedia

   en.wikipedia.org/wiki/International_Atomic_Time

   International Atomic Time. International Atomic Time (abbreviated TAI, from its French name temps atomique international[1]) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. [2] TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories ...

6. 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 ...

7. Caesium - Wikipedia

   en.wikipedia.org/wiki/Caesium

   Caesium (IUPAC spelling; [ 9 ] also spelled cesium in American English) is a chemical element; it has symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F; 301.6 K), which makes it one of only five elemental metals that are liquid at or near room temperature.