An Improved Torsion Balance Test of the Equivalence Principle Towards the Sun

Abstract

We search for violations of the Equivalence Principle towards the Sun using a rotating torsion balance apparatus. We set 95\%-confidence limits on violations with beryllium and aluminum test bodies of $η_{\odot, Be-Al} \leq 2.1 \times 10^{-13}$. These results are a factor of four improvement of previously reported results towards the Sun and a $\sim20\%$ improvement on previous torsion balance tests regardless of source.

Figures (3)

• Figure 1: Schematic of the rotating torsion balance apparatus. The torsion pendulum, vacuum system, and angular readout are mounted on a rotating air-bearing turntable. The tilt of the apparatus is measured by a pair of co-rotating tilt sensors and controlled by thermal expansion feet that the turntable rests on. Thermal gradients and changes of the temperature, magnetic field, and gravity gradients are controlled, shielded, and compensated for, respectively. Reprinted from Ref. ross2025probing.
• Figure 2: Once per turntable revolution differential acceleration amplitudes ($\Delta a_{TT}$) along with the corresponding fits to the solar basis functions. Each two-day long cut was independently fit. Gaps in the data are due to local construction activity and hardware failures. The right pane shows the histogram of the acceleration amplitudes.
• Figure 3: Measured differential accelerations from each two-day long segments towards the Sun (in-phase) and the orthogonal direction (out-of-phase).