What is (not) wrong with scalar gravity?

TL;DR

This paper re-examines Einstein's early arguments against scalar gravity, showing that such theories are theoretically viable despite contradicting later experimental results, which is relevant for teaching and history of GR.

Contribution

It challenges the traditional view that scalar gravity theories are inherently flawed, providing a detailed re-analysis of Einstein's original arguments.

Findings

Scalar gravity theories are theoretically consistent.

Einstein's arguments against scalar gravity are not conclusive.

Experimental contradictions do not invalidate the theory's formal viability.

Abstract

On his way to General Relativity (GR) Einstein gave several arguments as to why a special relativistic theory of gravity based on a massless scalar field could be ruled out merely on grounds of theoretical considerations. We re-investigate his two main arguments, which relate to energy conservation and some form of the principle of the universality of free fall. We find that such a theory-based a priori abandonment not to be justified. Rather, the theory seems formally perfectly viable, though in clear contradiction with (later) experiments. This may be of interest to those who teach GR and/or have an active interest in its history.