A Review of One-Way and Two-Way Experiments to Test the Isotropy of the Speed of Light

TL;DR

This paper reviews historical experiments testing the isotropy of the speed of light, categorizing methods and summarizing experimental verification levels, to guide future research in fundamental physics.

Contribution

It provides a comprehensive categorization and analysis of one-way and two-way experiments testing light speed isotropy since 1881, highlighting gaps for future studies.

Findings

Experiments have progressively verified light speed isotropy since 1881.

Two-way experiments generally show higher precision than one-way tests.

The review suggests directions for future experimental investigations.

Abstract

As we approach the 125th anniversary of the Michelson-Morley experiment in 2012, we review experiments that test the isotropy of the speed of light. Previous measurements are categorized into one-way (single-trip) and two-way (round-trip averaged or over closed paths) approaches and the level of experimental verification that these experiments provide is discussed. The isotropy of the speed of light is one of the postulates of the Special Theory of Relativity (STR) and, consequently, this phenomenon has been subject to considerable experimental scrutiny. Here, we tabulate significant experiments performed since 1881 and attempt to indicate a direction for future investigation.