Gravitational redshift revisited: inertia, geometry, and charge

TL;DR

This paper clarifies the interpretation of gravitational redshift experiments, emphasizing that curvature isn't always necessary for explanation and that charge effects can influence redshift, challenging assumptions about spacetime flatness.

Contribution

It demonstrates that gravitational redshift can be explained without spacetime curvature and explores how charge can mask redshift effects, revising common interpretations.

Findings

Experiments can often be explained with special relativity alone.

Spacetime curvature isn't always required to explain redshift results.

Charge can cancel gravitational redshift effects, affecting spacetime interpretation.

Abstract

Gravitational redshift effects undoubtedly exist; moreover, the experimental setups which confirm the existence of these effects-the most famous of which being the Pound-Rebka experiment-are well-known. Nonetheless-and perhaps surprisingly-there remains a great deal of confusion in the literature regarding what these experiments establish. Our goal in the present article is to clarify these issues, in three concrete ways. First, although (i) Brown and Read (2016) are correct to point out that, given their sensitivity, the outcomes of experimental setups such as the original Pound-Rebka configuration can be accounted for using solely the machinery of accelerating frames in special relativity (barring some subtleties due to the Rindler spacetime necessary to model the effects rigorously), nevertheless (ii) an explanation of the results of more sensitive gravitational redshift outcomes does in fact require more. Second, although typically this 'more' is understood as the invocation of spacetime curvature within the framework of general relativity, in light of the so-called 'geometric trinity' of gravitational theories, in fact curvature is not necessary to explain even these results. Thus (a) one can often explain the results of these experiments using only the resources of special relativity, and (b) even when one cannot, one need not invoke spacetime curvature. And third: while one might think that the absence of gravitational redshift effects would imply that spacetime is flat, this can be called into question given the possibility of the cancelling of gravitational redshift effects by charge in the context of the Reissner-Nordstr\"om metric. This argument is shown to be valid and both attractive forces as well as redshift effects can be effectively shielded in the charged setting. Thus, it is not the case that the absence of gravitational effects implies a Minkowskian spacetime setting.