GRAVITATIONAL RED-SHIFT EXPLORER (GRESE) Philippe Jetzer 1 and Cai Zhiwu 2 1 Physics Department, University of Zürich 2 Time Laboratory, Beijing Navigation Center (BSNC) On behalf of : Chunhao Han, Stephan Schiller, Jianfeng Su, Philip Tuckey, Qinghua Wang, Yiqiu Wang, Zaocheng Zhai 2nd ESA – CAS Workshop, Copenhagen 23-24 September 2014 GRAVITATIONAL RED-SHIFT EXPLORER (GRESE) Theme: What are the fundamental physical laws? Primary Goal: To test Einstein's Equivalence Principle, in particular to test the Local Position Invariance via gravitational red-shift measurements using clocks. Spacecraft and Single spacecraft carrying Instruments: - Space Hydrogen Maser (SHM) - Microwave time and frequency link (MWL) - GNSS receiver - Laser link (optional) Orbit: Highly elliptical orbit around Earth Lifetime: 2-3 years Type: S-class mission Science Rationale . Unified description of Gravity and Quantum Field Theory not achieved . Nature of Dark Matter (DM): unknown . Dark Energy – Cosmological constant: what is its nature? . Models of unification and models of Dark Energy generally involve scalar fields that - couple to gravity - couple in different ways to different ordinary matter types and DM . Fundamental constants are expectation values of scalar fields . Such character can lead to time- and space-varying fundamental constants . Recent detection of first fundamental scalar field (Englert-Brout-Higgs field) Violation of EEP is a general consequence Motivation Unified theories string theory, quantum loop gravity ,... ? ? Standard Model Theory of Theory of Theory of electromagne- weak Theory of gravitation tic interaction interaction strong interaction Local Lorentz Invariance Lorentz Invariance Local Position Invariance CPT - Symmetry Weak Equivalence Principle exactly valid? . Study space-time structure with high accuracy . Search for hints of quantum effects in gravity The conceptual basis for tests of General Relativity Gravitational redshift Lense-Thirring effect General Relativity Shapiro delay Perihelion shift Schiff effect Earth & moon free fall Metric theory of gravity Binaries dynamics … Einstein Equivalence Principle Local Position Universality of Local Lorentz Invariance Free Fall Invariance (Universality of grav. Redshift (Weak equivalence princip.) (Special Relativity) constancy of constants) “The trajectory of a point mass in a “The result of any local non- gravitational field depends only on gravitational experiment is its initial position and velocity, and independent of its location is independent of its composition.” in space and time.” Testing Earth‘s gravitational time dilation U Clock Ground at apogee Clock Clock n at perigee s ng ns Comparison between ground clock and satellite clock at perigee and apogee - determine: ( ns (apogee) - ng ) - ( ns (perigee) - ng ) - in highly elliptic orbit, DU(perigee-apogee)/c2 6 x 10-10 Measurement of time dilation in terrestrial potential with 2 x 10-6 accuracy Gravity Probe – A (1976): 7 x 10-5 ACES (>2015): 2 x 10-6 Comparing terrestrial atomic clocks via GRESE A link compares the clock on-board of GRESE with two clocks on the ground, allowing a comparison of the received signal with the local clocks at both ends. Time dilation measurement in Sun field - Precise test of Sun gravitational time dilation . Ground-to-satellite links allow terrestrial clock comparisons in common-view . Solar clock redshift: daily amplitude of 4 x 10-13 . Compensated by Doppler shift due to Earth motion if U(r) = GMSun/r . Since Doppler shift effect is precisely known, one can extract the time dilation effect . Measurement sensitivity 2 x 10-6 after 3 years (B. Hoffmann, Phys. Rev. 121, 337 Unequal (1961)) clock integration time frequencies . Measurement does not require a satellite clock To sun Equal clock frequencies Moon gravitational frequency shift measurement - Precise test of Moon gravitational time dilation . Moon clock redshift: daily amplitude of 6 x 10-15 . Test of Moon gravitational redshift at level 4 x 10-4 Unequal clock frequencies Moon Equal clock frequencies The gravitational frequency shift n clock1()r U()() r12 U r 1 2 n clock 2 ()rc GM Ur() r ? Search for existence of additional scalar fields f emanating from constituents of Earth, Sun, or Moon (V. Flambaum et al., 2007) • Model fi(r) ~ Si/r where Si may depend on the particle species contained in the massive body, and may depend on the clock type Mass of Sun: protons • Comparison of identical clocks at different locations Mass of Earth: protons & neutrons r1 , r2 will show an additional frequency shift contribution, which depends on the source type • GRESE will set limits to SEARTH , SSUN , SMOON • Note: comparison of clocks of different type, co-located on Sun-orbiting Earth, will show a modulationHere I consider of the frequencythe Al+/Hg+ difference comparison if S ≠ 0 (for and alpha), if it depends or Al+/Cs on the comparison clock type (for the combination of alpha, me/mp, g-factors), done at few 10e-17 or 10e-16 level, and USun[Sundistanceperihelion]-USun[Sundistanceaphelion] 3.31895×10-10 I also use formula (4) in EGE Proposal 0-3 Final-commented by SS Red arrows : “null red-shift measurements” (differences ai - aj for two different types of clocks i and j). Green arrow : best direct LPI tests in the Sun field. Blue arrow : best direct LPI tests in Earth field. GRESE: Earth field ~ 10-6, Sun field ~ 10-6, Moon field ~ 10-4 OpticalTime clocksand Reference and Earth‘s Frames gravitational potential A new generation of atomic clocks: optical clocks Optical Clock (NIST) Observed frequency offset lifted up by 30 cm 3×10-17 30 cm Chou CW, Hume DB, Rosenband T, Wineland DJ (2010) Science 329 (5999),1630. DOI: 10.1126/science.1192720 PROBLEM: No system to Compare Measurements of Gravitational Potential: Clocks in Timing Labs (TAI/UTC) Unified Reference Frame for: Time (TAI) Positioning (Terrestrial/Celestial Reference Frame) Gravity (static/temporal) Differential measurements of the geopotential . With clocks of accuracy at 1x10-18, it is possible to measure geopotential differences with GRESE equivalent height resolution of 1 cm U . Extremely high spatial resolution . Good time resolution ( 1 day) . Transportable clocks can be positioned anywhere U(r ) 1 U(r2) n clock1()r UU()()rr12 * 1 2 n clock 2 ()r c Frequency comparison by: . The onboard atomic clock is not required - free-space link (~ 10 km) (only the link) - optical fiber (~ 100 km) - transponder (any distance, intercontinental) Link can be at optical or microwave frequency (*U includes the velocity term) Two-way link permits Doppler cancellation Altitude of clocks must be independently determined with < 1 cm error Terrestrial and Celestial Reference Frame ICRF2 Extragalactic Sources Coordinate System of the Earth: Rotation Axis •Reference FRAME for all EARTH OBSERVATION and SPACE SCIENCE Polar Motion missions •1 mm accuracy and 0.1 mm/yr stability Geocenter •Combination of VLBI, GNSS, SLR and DORIS UT1-UTC •Milli-Astrometry (using onboard/ground clocks) GRESE Measurements of Neutrino Speed in Sun Gravitational Field CERN Daya Bay WHAT is the SPEED of Neutrinos? CERN Neutrinos possess mass, the speed of Daya Bay neutrinos should be slightly smaller than the speed of light! First measurements carried out between CERN and Rome (limited by GPS) SHM under development for ACES - 1 SHM is an Active H-Maser for space applications developed by Spectratime under ESA contract with funding provided by the Swiss Space Office. To provide excellent medium-term stability to ACES Launch: 2016 International Space Station Mission duration: 18 months 1.E-11 SHM under development for ACES - 2 1.E-12 ACES SHM spec. Weight: < 44 kg 1.E-13 TRL 6 (09.2014) 1.E-14 Allan Deviation Allan Expected frequency stability: 1.E-15 < 1e-15@1day 1 10 100 1000 10000 Average Time [sec] Current status and plan: . EM2 testing phase on-going SHM EM fully assembled . FM manufacturing started . FM delivery 2015 SHM: relatively small, low-price, reliable, high-performance, high TRL space atomic clock available today for the fast, small mission in 2021 Radiation design improvement of SHM • More severe radiation Total Ionising Dose 1.E+09 GALILEO, MEO environment in the 23'615 km - 56° - 12 years 1.E+08 STE-QUEST - 5 years highly elliptical orbit ~51'000 km apogee, ~1'000 km perigee ACES, ISS LEO 1.E+07 than ACES ~400 km - 3 years GALILEO • Radiation design 1.E+06 1.E+05 improvements: STE-QUEST EEE components selection: 1.E+04 Ionizing radiation tolerant, Total Dose in Si (Rads) 1.E+03 radiation hardened ACES 1.E+02 Shielding optimization 0 5 10 15 20 Spherical Aluminium Shielding Thickness (mm) • Galileo H-maser and rubidium clocks (RAFS) have been developed and are operating in the harsh radiation environment of MEO. SHM radiation design improvement: NOT critical for GRESE orbit Microwave link Bi-directional, triple frequency link Performance: Link stability (ADEV): 1.6E-13/τ, 1 s ≤ τ ≤106 s Ranging uncertainty: ≤ 1cm (30 ps), Code ≤ 0.1mm (0.3 ps), Carrier Resources: Mass: ≤ 20 kg Power: ≤ 80 W Operation Modes: Continuous operation TRL: need upgrade from ACES/Beidou MWL; TRL 6 by 2016 via European & Chinese Heritage: MWL link for Beidou GNSS (double frequency) developments MWL link for ACES (2016): 16.5 kg, 51 W, TRL 5-6 (currently) Similar system to be developed for Tiangong space station Laser link (optional) Bi-directional laser link: Independent means to verify and calibrate MWL Satellite playloads Clock Performance Data transfer; telemetering 1pps and remote Meas data(ts) TimeCounter Link stability (ADEV): control system （t meas） s 5 Electronic pulse of laser 1 E-13/τ, 1 s ≤ τ ≤10 s Laser Laser Data transmitting reflector explorer Ranging uncertainty: ≤ 1 cm Refelcted laser Resources: Radio signal Uploaded Laser signal signal Mass: ≤10 kg, Power: ≤30 W Receiving tick Ground staion telescope Operational mode: Data aquisition Discontinuous operation emission tick (cloud-free conditions) Meas data(ts) Laser device TRL: 9 for 100 ps uncertainty; Meas data Trigger signal (tg ,τ) TimeCounter 1pps Timing signal computer Clock To be improved to 30 ps.