In the late 1800s, when James Clerk Maxwell realised that light was an electromagnetic wave, his equations showed that light's speed should be about 300,000 kilometres per second. This was close to the value experimenters had measured, but Maxwell's equations left a nagging loose end: 300,000 kilometres per second relative to what? At first, scientists pursued the makeshift resolution that an invisible substance permeating space, the "aether", provided this unseen standard of rest.
It was Einstein who in the early 20th century argued that scientists needed to take Maxwell's equations more seriously. If Maxwell's equations did not refer to a standard of rest, then there was no need for a standard of rest. Light's speed, Einstein forcefully declared, is 300,000 kilometres per second relative to anything. The details are of historical interest, but I'm describing this episode for a larger point: everyone had access to Maxwell's mathematics, but it took the genius of Einstein to embrace it fully. His assumption of light's absolute speed allowed him to break through first to the special theory of relativity – overturning centuries of thought regarding space, time, matter and energy – and eventually to the general theory of relativity, the theory of gravity that is still the basis for our working model of the cosmos.
The story is a prime example of what the Nobel laureate Steven Weinberg meant when he wrote: "Our mistake is not that we take our theories too seriously, but that we do not take them seriously enough." Weinberg was referring to another great breakthrough in cosmology, the prediction by Ralph Alpher, Robert Herman and George Gamow of the existence of the cosmic microwave background radiation, the afterglow of the big bang. This prediction is a direct consequence of general relativity combined with basic thermodynamics. But it rose to prominence only after being discovered theoretically twice, a dozen years apart, and then being observed through a benevolent act of serendipity.
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