Sometimes I encounter people whose impression is that what Einstein’s 1905 theory of special relativity (the one that said no object’s speed can exceed the speed of light in vacuum, etc.) did in “overthrowing” the ideas of the past was somehow like what the Bolsheviks did to the Czars twelve years later– out with the old order, in with the new, and let nothing remain behind. The widespread notion, inherited from philosopher and historian Thomas Kuhn, is that a new paradigm arose, and the old was swept away. The truth is far different. Important parts of the conceptual superstructure of 19th century physics had to be replaced, but the predictive mathematical core was not replaced, but rather was extended. It was like tearing the roof and facade off a building while keeping the interior beams and columns, then extending the structure to make it much larger than before, and finally giving it a very novel external appearance. That’s why the Einsteinian revolution was possible at all! Newton’s equations had already been used to design all sorts of real-world 18th and 19th century technology. If Einstein’s equations hadn’t contained those of Newton and his descendants as a special case, they would have been in conflict with the real world… a no-no for a scientific theory.
Sine this is so important to understand, I’ve written an article illustrating how Einstein’s equations relating energy, momentum, mass and speed were an extension, not a replacement, of the equations that were previously in use. It describes how Einstein unified two separate classes of equations, one set that could be used for massive objects moving slowly compared to the speed of light, and the other for light itself, into a single class of equations, one that not only included the two previous classes but made predictions for massive objects moving at speeds comparable to that of light.