Tests of The Weak Equivalence Principle
Tests of the weak equivalence principle are those that verify the equivalence of gravitational mass and inertial mass. An obvious test is dropping two contrasted objects in hard vacuum, e.g., inside Fallturm Bremen.
| Researcher | Year | Method | Result |
| John Philoponus | 6th century | Described correctly the effect of dropping balls of different masses | no detectable difference |
| Simon Stevin | ~1586 | Dropped lead balls of different masses off the Delft churchtower | no detectable difference |
| Galileo Galilei | ~1610 | Rolling balls down inclined planes | no detectable difference |
| Isaac Newton | ~1680 | measure the period of pendulums of different mass but identical length | no measurable difference |
| Friedrich Wilhelm Bessel | 1832 | measure the period of pendulums of different mass but identical length | no measurable difference |
| Loránd Eötvös | 1908 | measure the torsion on a wire, suspending a balance beam, between two nearly identical masses under the acceleration of gravity and the rotation of the Earth | difference is less than 1 part in 109 |
| Roll, Krotkov and Dicke | 1964 | Torsion balance experiment, dropping aluminum and gold test masses | |
| David Scott | 1971 | Dropped a falcon feather and a hammer at the same time on the Moon | no detectable difference (not a rigorous experiment, but very dramatic being the first lunar one) |
| Braginsky and Panov | 1971 | Torsion balance, aluminum and platinum test masses, measuring acceleration towards the sun | difference is less than 1 part in 1012 |
| Eöt-Wash group | 1987– | Torsion balance, measuring acceleration of different masses towards the earth, sun and galactic center, using several different kinds of masses |
See:
| Year | Investigator | Sensitivity | Method |
| 500? | Philoponus | "small" | Drop Tower |
| 1585 | Stevin | 5×10-2 | Drop Tower |
| 1590? | Galileo | 2×10-2 | Pendulum, Drop Tower |
| 1686 | Newton | 10-3 | Pendulum |
| 1832 | Bessel | 2×10-5 | Pendulum |
| 1910 | Southerns | 5×10-6 | Pendulum |
| 1918 | Zeeman | 3×10-8 | Torsion Balance |
| 1922 | Eötvös | 5×10-9 | Torsion Balance |
| 1923 | Potter | 3×10-6 | Pendulum |
| 1935 | Renner | 2×10-9 | Torsion Balance |
| 1964 | Dicke,Roll,Krotkov | 3x10-11 | Torsion Balance |
| 1972 | Braginsky,Panov | 10-12 | Torsion Balance |
| 1976 | Shapiro, et al. | 10-12 | Lunar Laser Ranging |
| 1981 | Keiser,Faller | 4×10-11 | Fluid Support |
| 1987 | Niebauer, et al. | 10-10 | Drop Tower |
| 1989 | Heckel, et al. | 10-11 | Torsion Balance |
| 1990 | Adelberger, et al. | 10-12 | Torsion Balance |
| 1999 | Baeßler, et al. | 5x10-14 | Torsion Balance |
| cancelled? | MiniSTEP | 10-17 | Earth Orbit |
| 2015? | MICROSCOPE | 10-16 | Earth Orbit |
| 2015? | Reasenberg/SR-POEM | 2×10-17 | vacuum free fall |
Experiments are still being performed at the University of Washington which have placed limits on the differential acceleration of objects towards the Earth, the sun and towards dark matter in the galactic center. Future satellite experiments – STEP (Satellite Test of the Equivalence Principle), Galileo Galilei, and MICROSCOPE (MICROSatellite pour l'Observation de Principe d'Equivalence) – will test the weak equivalence principle in space, to much higher accuracy.
Equivalence Principle (EP) tests include pulsar(neutron star)-solar star binaries versus General Relativity (GR) predictions for orbit, periastron precession, and gravitation radiation orbital decay. EP violation by contrasted measurable observables is limited by empirical observation to be less than one part in 20 trillion difference/average. No quantum theory of gravity reduces to GR or is predictive. Einstein-Hilbert action is incompatible with non-classical gravitation absent a Chern-Simons parity-violating contribution. String theory, supergravity and quintessence may violate the weak EP by containing light scalar fields with long Compton wavelengths. These fields source fifth forces and variation of fundamental constants. Mechanisms are invented to reduce EP violations below observable levels, or such theories are fundamentally flawed, or the EP has observable violations.
Einstein-Cartan-Kibble-Sciama (ECKS) gravitation does not postulate the EP. ECKS spacetime torsion transforms like Lorentz force in electromagnetism — parity-odd and chiral. GR is an EP = true parity-even achiral subset with otherwise equivalent spacetime curvature. ECKS offers EP parity-odd violations external to GR that do not contradict prior EP test results. Massless boson photons detect zero vacuum anisotropy, refraction, dispersion, dichroism, or gyrotropy. Modeling fermionic matter with mirror-symmetric theory unleashes furies of parity violations requiring hierarchies of manually inserted symmetry breakings. The vacuum is empirically observed to have parity-even isotropies f(x) = f(-x) overall plus trace parity-odd anisotropies f(x) = -f(-x) (e.g., chirality) acting only upon fermionic matter. GR has known issues with angular momentum.
Physical chirality can be observed (absence of improper rotation axes Sn) and calculated but it cannot be measured. Optical rotation does not measure atomic mass distribution in space. Opposite shoes fit with trace different energies into trace chiral vacuum. They locally vacuum free fall along trace non-identical minimum action trajectories, violating the EP. Eötvös experiments are 5×10−14 difference/average EP-violation sensitive. Crystallography's "opposite shoes" are chemically and macroscopically identical, single crystal test masses in enantiomorphic space groups. Examples of paired "opposite shoes" are P3121 versus P3221 α-quartz or P31 versus P32 γ-glycine. These contrasts compose two geometric Eötvös experiments.
The most intense EP composition contrast, titanium minus beryllium baryon number, is 0.2397% net active mass. Eötvös experiment geometric test masses are 99.97+% net active mass as nuclei relative positions in space (ignoring electrons). 99.97/0.2397 = 417 times more sensitive for the same Eötvös balance total mass loading. Understanding gravitation suffers the illusion of knowledge created by epicycles of curve-fittings added to postulated elegant theory. Instead, write theory fundamentally consistent with empirical observation however unintuitive reality is seen to be. A novel, theory-validated, 1.2×10−16 difference/average EP-violation sensitive, geometric Equivalence Principle test should be run in existing apparatus. The worst it can do is succeed.
Read more about this topic: Equivalence Principle, Modern Usage, The Weak Equivalence Principle
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