 All these effects were known before Albert Einstein's theory of relativity was published, and he was familiar with them. The thinking by 1905 was that there is still an ether that represented nature's absolute reference frame, but the time dilation and space contraction made it impossible to detect. The ether was held to exist for two main reasons. One, it gave light a medium to propagate through, and two, it provided for an absolute frame of reference like the one we used to have with the Earth. With it, we could claim that there is such a thing as simultaneous events in two places in at least one preferred by nature reference frame. Einstein parted with this theory for two reasons. One was that he studied James Maxwell's equations that showed the speed of light was the result of empty space's resistance to the formation of an electric field, permittivity, and a magnetic field, permeability, as we covered in the microscopic segment of our How Small Is It video book. So there was no need for a propagating medium like the ether. Empty space will do just fine. And relative motion would not change these basic properties of empty space, thus making the speed of light the same for all observers. The other reason was more philosophical. What does it mean to postulate the existence of a thing and then explain that it cannot be detected? In the end, he accepted the idea that there is no universal frame of reference and all time, space, and simultaneity were relative for all inertial frames. In Einstein's paper, he articulated two postulates. One, the laws of physics are the same in all inertial reference frames. This means that formulas like force equals mass times acceleration will hold in all inertial reference frames. And all experiments run the same in all inertial reference frames. For example, if you drop a ball standing at home, it falls straight down to your feet. The same thing happens if you are standing on a moving train. The ball falls straight down to your feet. Postulate number two is that the speed of light in a vacuum has the same value in all inertial reference frames. With these two postulates, Einstein generated all relativity equations of Lorentz and others. It was his study of relativistic mass, momentum, and energy that led Einstein to the famous equation E equals mc squared. No one else had made this relationship between mass and energy. Today's nuclear power industry is a testament to the accuracy of this special relativity formula. Thank you. Thank you.