The light speed vs the observer: the Kennedy-Thorndike test from GRAAL-ESRF

TL;DR

This study uses high-precision Compton Edge measurements at ESRF to significantly improve constraints on the invariance of light speed with respect to observer velocity, supporting Special Relativity.

Contribution

It introduces a novel application of the Compton Edge method to the Kennedy-Thorndike experiment, achieving three orders of magnitude better limits on Lorentz invariance violation.

Findings

Limit of 7×10^-12 on light speed invariance with respect to observer velocity

Improved constraints by three orders of magnitude over previous tests

Utilized dual energy scales for enhanced calibration accuracy

Abstract

High precision tests of the light speed constancy for all observers as of empirical basis of the Special Relativity have continuously been among the goals of advanced experimental studies. Based on the Compton Edge method proposed by us [1], a constraint on the one-way light speed isotropy and the Lorentz invariance violation has been obtained at the dedicated GRAAL experiment at European Synchrotron Radiation Facility (ESRF, Grenoble) [2-5]. Using the GRAAL's data we now get a new constraint on one of key tests of Special Relativity - the Kennedy-Thorndike experiment [6] - in probing the light speed invariance with respect to the velocity of the observer (apparatus). Our analysis takes advantage of GRAAL's setup where two separate energy scales are involved: first, via the position of the Compton Edge determining the light speed in the reference frame of incident 6 GeV electrons within the tagging system, second, in the calorimeter via the 1.27 MeV photons of the ^22 Na source. The two energy scales are engaged to each other through production of $\eta$ mesons by tagged laser Compton backscattered $\gamma$-rays. Accuracy of the calibration and stability of energies reached in each section enable us to obtain the limit 7 10^-12 for the Kennedy-Thorndike test, which improves the currently existing limits by three orders of magnitude.