Author:
Roux, Jean-Claude
Category:
Research Papers
Sub-Category:
Relativity Theory
Date Published:
March 27, 2025
Keywords:
General relativity; geometric optics; refraction index; light beam; Schwarzschild radius; flat space-time; Newtonian gravity; weak gravitational field; Shapiro’s time delay
Abstract:
We explore the propagation of light in the vicinity of a massive body by introducing a variable speed of light under the assumption of a weak gravitational field. Starting from general relativity, we reformulate the equation of motion of a light beam into an equivalent description in flat spacetime, governed by classical physics—namely geometric optics and Newtonian gravity. Crucially, we show that this transformation is reversible: the curved spacetime formulation and the flat spacetime with a gravitational index are mathematically equivalent in this regime. This duality allows for a simpler treatment of light propagation without loss of generality. As an application, we derive the Shapiro’s time delay within this flat-space framework and recover an expression that closely matches the result obtained in general relativity. This finding offers a new perspective on a phenomenon historically regarded as a key confirmation of Einstein’s theory.
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