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A maser is a device that produces coherent electromagnetic waves, through amplification by stimulated emission. The first maser was built by Charles H. Townes, James P. Gordon, and Herbert J. Zeiger at Columbia University in 1953. Townes, Nikolay Basov and Alexander Prokhorov were awarded the 1964 Nobel Prize in Physics for theoretical work leading to the maser. Masers are also used as the timekeeping device in atomic clocks, and as extremely low-noise microwave amplifiers in radio telescopes and deep-space spacecraft communication ground stations.
A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.
Columbus is a science laboratory that is part of the International Space Station (ISS) and is the largest single contribution to the ISS made by the European Space Agency (ESA).
The Primary Atomic Reference Clock in Space or PARCS was an atomic-clock mission scheduled to fly on the International Space Station (ISS) in 2008, but cancelled to make way for the Vision for Space Exploration. The mission, to have been funded by NASA, involved a laser-cooled caesium atomic clock, and a time-transfer system using Global Positioning System (GPS) satellites. PARCS was to fly concurrently with the Superconducting Microwave Oscillator (SUMO) a different type of clock that was to be compared against the PARCS clock to test certain theories. The objectives of the mission were to have been:
Gravity Probe A (GP-A) was a space-based experiment to test the equivalence principle, a feature of Einstein's theory of relativity. It was performed jointly by the Smithsonian Astrophysical Observatory and the National Aeronautics and Space Administration. The experiment sent a hydrogen maser—a highly accurate frequency standard—into space to measure with high precision the rate at which time passes in a weaker gravitational field. Masses cause distortions in spacetime, which leads to the effects of length contraction and time dilation, both predicted results of Albert Einstein's theory of general relativity. Because of the bending of spacetime, an observer on Earth should measure a slower rate at which time passes than an observer that is higher in altitude. This effect is known as gravitational time dilation.
The Pound–Rebka experiment monitored frequency shifts in gamma rays as they rose and fell in the gravitational field of the Earth. The experiment tested Einstein's 1907 and 1911 predictions, based on the equivalence principle, that photons would gain energy when descending a gravitational potential, and would lose energy when rising through a gravitational potential. It was proposed by Robert Pound and his graduate student Glen A. Rebka Jr. in 1959, and was the last of the classical tests of general relativity to be verified. The measurement of gravitational redshift and blueshift by this experiment validated the prediction of the equivalence principle that clocks should be measured as running at different rates in different places of a gravitational field. It is considered to be the experiment that ushered in an era of precision tests of general relativity.
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The Dick effect is an important limitation to frequency stability for modern atomic clocks such as atomic fountains and optical lattice clocks. It is an aliasing effect: High frequency noise in a required local oscillator (LO) is aliased (heterodyned) to near zero frequency by a periodic interrogation process that locks the frequency of the LO to that of the atoms. The noise mimics and adds to the clock's inherent statistical instability, which is determined by the number of atoms or photons available. In so doing, the effect degrades the stability of the atomic clock and places new and stringent demands on LO performance.
References
- ↑ "Swiss Space Atomic Clock Technologies and Applications in Space Science" (PDF). SpectraTime. Archived from the original (PDF) on 12 February 2017. Retrieved 11 February 2017.
- ↑ ESA. "Atomic clock ensemble in space (ACES)" (PDF). ERASMUS Centre - Directorate of Human Spaceflight and Operations. Retrieved 11 February 2017.
- ↑ "Swiss Space Atomic Clock Technologies and Applications in Space Science" (PDF). SpectraTime. Archived from the original (PDF) on 12 February 2017. Retrieved 11 February 2017.
- ↑ ESA (25 July 2014). "Timely Arrival of PHARAO Space Clock". ESA . Retrieved 11 February 2017.
- ↑ Cacciapuoti, L.; Salomon, Ch. (2009). "Space clocks and fundamental tests: The ACES experiment". The European Physical Journal Special Topics. THE EUROPEAN PHYSICAL JOURNAL. 172 (1): 57–68. Bibcode:2009EPJST.172...57C. doi:10.1140/epjst/e2009-01041-7. S2CID 119402539 . Retrieved 9 March 2023.
- ↑ "PHARAO". Centre national d’études spatiales(CNES). 24 April 2015. Retrieved 2 August 2020.
- ↑ "ACES Workshop 2022". CCSD. Retrieved 8 March 2023.
- ↑ "ISS Utilization: ACES (Atomic Clock Ensemble in Space) / PHARAO". eoPortal. ESA . Retrieved 31 January 2016.
- ↑ CNES (5 September 2016). "PHARAO". CNES . Retrieved 11 February 2017.
- ↑ "ACES Platform". ESA. 8 September 2014. Retrieved 31 January 2016.
