Relevance of the weak equivalence principle and experiments to test it: lessons from the past and improvements expected in space

Abstract

Tests of the Weak Equivalence Principle (WEP) probe the foundations of physics. Ever since Galileo in the early 1600s, WEP tests have attracted some of the best experimentalists of any time. Progress has come in bursts, each stimulated by the introduction of a new technique: the torsion balance, signal modulation by Earth rotation, the rotating torsion balance. Tests for various materials in the field of the Earth and the Sun have found no violation to the level of about 1 part in 1e13. A different technique, Lunar Laser Ranging (LLR), has reached comparable precision. Today, both laboratory tests and LLR have reached a point when improving by a factor of 10 is extremely hard. The promise of another quantum leap in precision rests on experiments performed in low Earth orbit. The Microscope satellite, launched in April 2016 and currently taking data, aims to test WEP in the field of Earth to 1e-15, a 100-fold improvement possible thanks to a driving signal in orbit almost 500 times stronger than for torsion balances on ground. The `Galileo Galilei' (GG) experiment, by combining the advantages of space with those of the rotating torsion balance, aims at a WEP test 100 times more precise than Microscope, to 1e-17. A quantitative comparison of the key issues in the two experiments is presented, along with recent experimental measurements relevant for GG. Early results from Microscope, reported at a conference in March 2017, show measurement performance close to the expectations and confirm the key role of rotation with the advantage (unique to space) of rotating the whole spacecraft. Any non-null result from Microscope would be a major discovery and call for urgent confirmation; with 100 times better precision GG could settle the matter and provide a deeper probe of the foundations of physics.

To appear: Physics Letters A, special issue in memory of Professor Vladimir Braginsky, 2017. Available online: http://dx.doi.org/10.1016/j.physleta.2017.09.027

References in corpus (7)

• Testing General Relativity with Atom Interferometry
• Quantum Test of the Universality of Free Fall
• Test of Equivalence Principle at $10^{-8}$ Level by a Dual-species Double-diffraction Raman Atom Interferometer
• Lunar Laser Ranging Tests of the Equivalence Principle
• Testing the strong equivalence principle with the triple pulsar PSR J0337+1715
• Gravity gradient suppression in spaceborne atomic tests of the equivalence principle
• Do general relativistic effects limit experiments to test the universality of free fall and the weak equivalence principle?

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• Quantum free fall motion and quantum violation of weak equivalence principle
• Scattering Times of Quantum Particles from the Gravitational Potential, and Equivalence Principle Violation
• Free fall in KvN mechanics and Einstein's principle of equivalence
• Systematic errors in high-precision gravity measurements by light-pulse atom interferometry on the ground and in space
• Violation of the equivalence principle in curvature based extended gravity at finite temperature
• Explanation of the EPF experiment in terms of gravity gradients
• On the first test of the Weak Equivalence Principle in low Earth orbit