Gravitational Waves and the Limits of Testing General Relativity

A recent paper titled "Unitarity, Causality, and Solar System Bounds, Significantly Limit Using Gravitational Waves to Test General Relativity" by Alexander Cassem and Mark P. Hertzberg explores the limitations of using gravitational waves (GWs) to test general relativity (GR). The authors discuss how the detection of deviations from GR, particularly through the observation of GWs from merging black holes, has been a significant motivation for projects like LIGO and Virgo.

The study highlights that deviations in GR can arise from higher-order derivative corrections in the effective action, specifically through higher powers of the Riemann curvature tensor. However, the authors argue that while observational bounds suggest these coefficients could be large enough to detect, fundamental principles such as causality and unitarity complicate this assumption.

Cassem and Hertzberg review existing bounds on these coefficients, concluding that they imply a low cutoff on the effective theory. They also propose a mechanism for generating these higher-order terms through minimally coupled light scalars. A notable finding is that this mechanism could lead to quantum corrections to Newton's potential, which have already been ruled out by solar system tests.

The paper emphasizes that to avoid these solar system constraints, a significant improvement—over seven orders of magnitude—in interferometer sensitivity would be necessary. This finding has implications for future gravitational wave research and the ongoing efforts to test the foundations of general relativity.