MICROSCOPE mission: final results of the test of the Equivalence Principle
Pierre Touboul · Gilles Métris · Manuel Rodrigues · Joel Bergé · Alain Robert · Quentin Baghi · Thibault Damour · Pierre Fayet · Claus Lämmerzahl · Serge Reynaud · Timothy Sumner · and the MICROSCOPE collaboration
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MICROSCOPE is the most precise test ever made of the oldest observation in gravity: that everything falls the same way. Galileo did it with inclined planes, Eötvös with a torsion balance, and this collaboration did it by putting two hollow cylinders of different metals — one a platinum-rhodium alloy, one titanium-aluminium-vanadium — inside each other aboard a drag-free French satellite and letting them orbit the Earth for two and a half years. Both cylinders are held in place by electrostatic forces, and the experiment simply asks how much those two restoring forces differ. If the two metals fell even fractionally differently, the difference would appear as a signal at the orbital frequency. The satellite carries a second pair made of the same metal as a reference, so the instrument can be checked against itself. The collaboration reports no violation. The Eötvös ratio for the titanium and platinum pair comes out at minus 1.5 parts in a thousand million million, with an uncertainty of about the same size, improving the previous limit by a factor of 4.6.
Why it matters hereChapter 3 needs a yardstick, and this is the finest one that exists. Any mechanism that proposes to change how a mass responds to a push — inertia from the zero-point field included — has to explain why an orbiting titanium cylinder and an orbiting platinum cylinder respond identically to fifteen decimal places. That is not an obstacle to the programme; it is the precision the programme gets to work against, and the same instrument has already been turned into constraints on Lorentz invariance, on long-range forces and on dark matter.
What it claims
01The experiment compares two concentric hollow cylindrical test masses of different composition — a platinum-rhodium alloy and a titanium-aluminium-vanadium alloy — held in relative equilibrium by electrostatic forces in a differential accelerometer aboard a drag-free satellite. Any difference in the forces needed to hold them would be evidence of a violation.Introduction, description of sensor units SUREF and SUEP, and Table I (arXiv:2209.15487v1)
Settled physics02The final result is a null one: the Eötvös ratio for the titanium and platinum pair is measured as minus 1.5, plus or minus 2.3 statistical and plus or minus 1.5 systematic, in units of ten to the minus fifteen — close to the ten to the minus fifteen precision the mission was designed for, and an improvement on the collaboration's own earlier constraint by a factor of 4.6.Abstract and equation 5 (arXiv:2209.15487v1)
Settled physics03The reference instrument, whose two test masses are the same alloy, also returns a null result — zero plus or minus 1.1 statistical and plus or minus 2.3 systematic, in the same units — which the authors read as showing no sign of unaccounted systematic error in the main measurement.Paragraph following equation 5 (arXiv:2209.15487v1)
Settled physics04Temperature variation is the dominant systematic. The authors report an overall systematic upper bound of 1.5 parts in ten to the fifteen for the test instrument and 2.3 for the reference, against a design specification of 0.2, and describe the dedicated heater sessions used to characterise the thermal sensitivity.Section on systematic errors, following Figure 1 (arXiv:2209.15487v1)
Settled physics05The authors note that the same dataset yields constraints on Lorentz invariance, on long-range interactions and on dark matter, beyond the equivalence-principle limit itself.Closing section, list of derived constraints (arXiv:2209.15487v1)
Published and peer-reviewed06A next-generation mission should reach ten to the minus seventeen, the authors argue, by replacing the gold charge-management wire with a contactless device of the kind flown on LISA Pathfinder, reducing the glitches traced to crackling in the satellite coating, and improving thermal stability.Closing section, upgrades for a next-generation MICROSCOPE (arXiv:2209.15487v1)
What to watch
The way in
https://doi.org/10.1103/PhysRevLett.129.121102LICENCE CHECKED. Published as Physical Review Letters volume 129, article 121102, 14 September 2022. Unpaywall records the article as open access of the bronze kind — free to read at the publisher, under no open licence — and Crossref lists only the APS default licence, with no Creative Commons statement anywhere on the record. Bronze access permits reading, not redistribution, so no text of the published article is reproduced here. SOURCE READ. The authors' own preprint, arXiv:2209.15487v1 dated 30 September 2022 and carrying the same title, was downloaded and read in full for this page. It is distributed under the arXiv non-exclusive distribution licence, which permits arXiv to distribute it but is not an open licence, so it too is summarised rather than quoted. The locators below cite that preprint. The collaboration author list runs to forty-four names across fifteen institutions, led by ONERA and the French space agency CNES; the creators field above names the first authors and the senior theorists and marks the rest, rather than reproducing the full list twice. RELATED PAGES. Chapter 3's precision-measurement family on this site includes the Casimir force measurements at /library/stm-911b036fc2 and /library/stm-7e33aa610f, and the Casimir-force test of non-Newtonian gravitation at /library/stm-8a6a8e0653.
How to cite it
Pierre Touboul, Gilles Métris, Manuel Rodrigues, Joel Bergé, Alain Robert, Quentin Baghi, Thibault Damour, Pierre Fayet, Claus Lämmerzahl, Serge Reynaud, Timothy Sumner, and the MICROSCOPE collaboration (2022) MICROSCOPE mission: final results of the test of the Equivalence Principle. doi:10.1103/PhysRevLett.129.121102
Where it sits in the curriculum