Exploring Gravity with Optical Clocks

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The Einstein Gravity Explorer Mission Proposal

This proposal was submitted in June 2007 to ESA in the framework of ESA′s Cosmic Vision program call for proposals, by a consortium of researchers for Europe, US, Australia, Japan and China.

Figure: General concept of the EGE mission proposal. Atomic clocks on the satellite and on ground are intercompared as the satellite orbits Earth on a highly elliptic orbit.

Fundamental physics science objectives:

Primary
- High-precision measurement of the earth gravitational time dilation (specification: 25 ppb, a factor 3000 improvement; goal: 8 ppb)
- First precise measurement of the sun gravitational time dilation (specification: 1 ppm, factor 2 x 104 improvement)
- Test of space and time variability of fundamental constants with accuracy 30 ppb (specification)
- First search for neutron scalar charge, with accuracy 30 ppb (specification)
- Test of anomalous coupling of matter to the Standard Model quantum fields

Secondary
- Tests of Lorentz Invariance in the matter and photon sector (factor 20 improvement)
- Contribution towards establishing a new definition of the unit of time

Spin-off to other fields (outside fundamental physics):
- Comparison of distant terrestrial clocks at the 10-18 level
- Establishment of a new approach to the determination of the geopotential, with few cm geoid height resolution
- Demonstration of clock and link technology for future applications, e.g. in precision spacecraft navigation
- Demonstration on high performance real-time range determination with precision 25 times (based on code phase) to 1000 times (based on carrier phase) higher compared to systems onboard ERS-2 or TOPEX POSEIDON
- Comparison of 3 different orbit determination systems: Laser-Ranging, μm-precision Microwave Ranging, GPS-based orbit determination
- Atmospheric signal propagation study, precise determination of 3rd order ionospheric term
- Monitoring stability of the Galileo/GPS/Glonass time scales

Payload:
- The instruments on board of EGE are: the cold atom microwave clock PHARAO, a single-ion optical clock based on Ytterbium, a frequency comb, and an upgraded version of the microwave link developed for the ISS mission ACES. As a second option, a lattice optical clock based on Strontium was suggested.

The proposal was evaluated but not selected because its technological readiness was regarded to be insufficient.