Could the speed of light be changing across time and space?
In short: While modern physics relies on a constant speed of light, a family of non-mainstream hypotheses suggests otherwise. These theories propose that light might vary with frequency, space, or time, offering radical alternatives to our understanding of gravity, the early universe, and the very stability of physical laws.
In the framework of Einstein's general relativity, the speed of light remains constant in any local, freely falling reference frame. However, relativity does allow for apparent variations when viewed by a distant observer, such as light appearing to slow to zero at an event horizon. Beyond this established effect, Variable Speed of Light (VSL) theories propose that the local speed of light itself could fluctuate, challenging the foundational equivalence principle.
One of the earliest significant attempts at this was by Robert Dicke in 1957. Dicke proposed a theory of gravity where the speed of light measured by a free-falling observer could vary. He even suggested a cosmology where the speed of light decreased over time, providing an alternative explanation to the cosmological redshift. Other researchers, such as Giere and Tan in 1986, and Sanejouand in 2009, have published similar hypotheses that appear to contradict general relativity.
The debate often centers on the fine-structure constant, a dimensionless value. Some researchers studying distant quasars claim to have detected variations in this constant at a level of one part in 10^5, though these findings are disputed by others who find no detectable variation at higher sensitivities. Similarly, studies of the Oklo natural nuclear reactor, looking at samarium isotopes from 2 billion years ago, suggest that the fine-structure constant has remained largely unchanged.
Critics like G. F. R. Ellis and Jean-Philippe Uzan argue that adopting a varying speed of light would necessitate a massive rewrite of modern physics. Such a shift would require redefining distance measurements, modifying Maxwell's equations, and potentially contradicting Lorentz invariance. Because of these profound implications, VSL cosmologies remain outside the mainstream of modern physics.
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