 In the 1860s, people like Michael Faraday, Andre Ampere and James Maxwell and others were studying electric and magnetic fields. Maxwell proposed that the existence of an electric charge filled empty space with an electric field. Accelerating the charge causes the electric field to change. Furthermore, he showed that a changing electric field created a magnetic field, and a changing magnetic field created an electric field. So the accelerated electron creates a disturbance in the electric field that propagates itself through space as an electromagnetic wave. Earlier, Faraday had measured the resistance of empty space to the forming of an electric field called permittivity, and Ampere had measured the resistance of empty space to the forming of a magnetic field called permittability. In 1864, using their numbers, Maxwell calculated the speed of his waves at around 311,000 kilometers per second, or 193,000 miles per second. This was in good agreement with Fizzou's for light. Maxwell had demonstrated that light indeed is an electromagnetic wave. This was a remarkable achievement, but it created a problem. The Galilean transformations would have the speed of light adjusted for the relative speed between reference frames. But Maxwell's value is a constant. It does not change for the observer on the train and the observer on the ground. Given that the speed of the wave is equal to its wavelength times its frequency, to resolve this conflict we need an exact measurement for these two quantities. For visible light frequencies, like the color red, we see that we would need to measure wavelengths on the order of 750 nanometers. That's three one hundred thousandths of an inch. In the 1800s there was no way to measure lengths this small. That changed when Albert Michelson invented the interferometer.