In his monumental work, Lectures on Physics, Richard Feynman made this comment regarding J. C. Maxwell’s work—which surprised many because Einstein was then considered a very big deal:
“From a long view of the history of mankind—seen from, say, ten thousand years from now—there can be little doubt that the most significant event of the 19th century will be judged as Maxwell's discovery of the laws of electrodynamics. The American Civil War will pale into provincial insignificance in comparison with this important scientific event of the same decade.”
Now don’t be scared: These famous T-shirt equations define the properties of Electromagnetism. They read like a foreign language, but learning about them is not all that difficult once you know what the symbols mean. However, they are clearly meant for people who fill blackboards full of this stuff.
There are many ways of writing these equations and regardless of how they are written they say something absolutely remarkable. For example, the second equation (Gauss’ Law of Magnetism) says there can’t be magnetic monopoles, even though the equation above it defines the characteristics of electric monopoles. Maxwell's third equation is Faraday's Law. It says that an electric field (E) is produced by a varying magnetic field (B).
But remarkably, the fourth equation can be solved for the speed of electromagnetic phenomena in a vacuum, “c” which works out to be 2.99792458 x 10^8 meters per second (present day, agreed-upon number).
Now Maxwell’s calculation of “c” was actually a simple wave equation that was a function of only two physical constants, the Permittivity (ε) and Permeability (μ) of a vacuum; values that were not hard to measure. So Maxwell’s “c” depends on the accuracy of two constants, which describe only the nature of the medium…nothing else! To believe anything can go faster than light is to believe that there is a medium with a Permittivity and Permeability less than vacuum. So in a very fundamental way, you can’t get a more accurate measurement for “c” for any electromagnetic phenomenon in a vacuum. Remember, Maxwell was only talking about the characteristic of the electromagnetic field in a vacuum. All the relativity stuff came much later with Einstein.
James Clerk Maxwell published his equations in 1861-1862. Experimenters were excited! It immediately became apparent that light had to be an electromagnetic phenomenon, because many careful experimenters, who were at the time independently trying to measure the speed of light by strictly opto-mechanical methods, were coming up with velocities not one part in a thousand different!
It is hard to overestimate the importance of Maxwell’s accomplishment. He put together the basic ideas of Faraday, Gauss, Ampere and other 19th century scientists who were then experimenting with the newly discovered principles of electricity and magnetism. The telegraph had just been invented. The American Civil War would soon begin. It was the era of wood-fired steam engines and horses and muskets. People used whale oil lamps and tallow candles to drive away the darkness. But the ideas of Maxwell in the early 1860s were almost miraculous. Interest in his theories among scientists was phenomenal.