The Variable Speed of Light (VSL) theory of gravity, proposed by physicist Robert Dicke in 1957, presents a provocative alternative to Einstein’s general theory of relativity. Departing from the notion of a constant speed of light, the VSL theory posits that the speed of light can vary in gravitational contexts, offering a novel perspective on cosmological phenomena such as redshift. In this critical review, we examine the foundational tenets, impacts, and limitations of the VSL theory, shedding light on its theoretical underpinnings, experimental status, and interpretational challenges.

The VSL theory emerged as a response to the desire for alternative explanations to observed cosmological phenomena, challenging the framework of general relativity. By allowing for the variability of the speed of light, the VSL theory presents a radical departure from established principles of modern physics.

**Basic tenets:**

**Variability of the Speed of Light: **In the VSL theory, the speed of light is not held constant but can vary depending on the gravitational environment, offering a departure from the fundamental postulate of Einstein’s special theory of relativity.

**Incorporation of Mach’s Principle:** Dicke’s theory incorporates Mach’s principle, suggesting a connection between the distribution of matter in the universe and the properties of spacetime, providing a theoretical framework for understanding the variability of the speed of light.

**Redshift Interpretation:** The VSL theory offers an alternative interpretation of cosmological redshift, attributing it to changes in the speed of light over time rather than the expansion of the universe, challenging the prevailing cosmological paradigm.

**Strengths:**

**Exploration of Alternative Cosmological Models:** The VSL theory has spurred exploration into alternative cosmological models that depart from the framework of general relativity, fostering new avenues for theoretical investigation and cosmological inquiry.

**Linkage to Fundamental Constants:** Some models linking VSL theories to Dirac’s Large Numbers Hypothesis suggest a potential connection between fundamental constants and the size and age of the universe, stimulating further research into the underlying principles of cosmology and fundamental physics.

**Weaknesses:**

**Incompatibility with General Relativity:** The VSL theory represents a departure from general relativity, raising questions about its consistency with established experimental evidence and observational constraints.