Einstein by Walter Isaacson

is no designated “rest” frame of reference that has preference over any other.
    One of Einstein’s clearest explanations of what he had wrought was in a letter to his Olympia Academy colleague Solovine:
The theory of relativity can be outlined in a few words. In contrast to the fact, known since ancient times, that movement is perceivable only as
relative
movement, physics was based on the notion of
absolute
movement. The study of light waves had assumed that one state of movement, that of the light-carrying ether, is distinct from all others. All movements of bodies were supposed to be relative to the light-carrying ether, which was the incarnation of absolute rest. But after efforts to discover the privileged state of movement of this hypothetical ether through experiments had failed, it seemed that the problem should be restated. That is what the theory of relativity did. It assumed that there are no privileged physical states of movement and asked what consequences could be drawn from this.
     
    Einstein’s insight, as he explained it to Solovine, was that we must discard concepts that “have no link with experience,” such as “absolute simultaneity” and “absolute speed.” 61
    It is very important to note, however, that the theory of relativity does not mean that “everything is relative.” It does not mean that everything is subjective.
    Instead, it means that measurements of time, including duration and simultaneity, can be relative, depending on the motion of the observer. So can the measurements of space, such as distance and length.But there is a union of the two, which we call spacetime, and that remains invariant in all inertial frames. Likewise, there are things such as the speed of light that remain invariant.
    In fact, Einstein briefly considered calling his creation Invariance Theory, but the name never took hold. Max Planck used the term
Relativtheorie
in 1906, and by 1907 Einstein, in an exchange with his friend Paul Ehrenfest, was calling it
Relativitätstheorie.
    One way to understand that Einstein was talking about invariance, rather than declaring everything to be relative, is to think about how far a light beam would travel in a given period of time. That distance would be the speed of light multiplied by the amount of time it traveled. If we were on a platform observing this happening on a train speeding by, the elapsed time would appear shorter (time seems to move more slowly on the moving train), and the distance would appear shorter (rulers seem to be contracted on the moving train). But there is a relationship between the two quantities—a relationship between the measurements of space and of time—that remains invariant, whatever your frame of reference. 62
    A more complex way to understand this is the method used by Hermann Minkowski, Einstein’s former math teacher at the Zurich Polytechnic. Reflecting on Einstein’s work, Minkowski uttered the expression of amazement that every beleaguered student wants to elicit someday from condescending professors. “It came as a tremendous surprise, for in his student days Einstein had been a lazy dog,” Minkowski told physicist Max Born. “He never bothered about mathematics at all.” 63
    Minkowski decided to give a formal mathematical structure to the theory. His approach was the same one suggested by the time traveler on the first page of H. G. Wells’s great novel
The Time Machine,
published in 1895: “There are really four dimensions, three which we call the three planes of Space, and a fourth, Time.” Minkowski turned all events into mathematical coordinates in four dimensions, with time as the fourth dimension. This permitted transformations to occur, but the mathematical relationships between the events remained invariant.
    Minkowski dramatically announced his new mathematical approach in a lecture in 1908. “The views of space and time which I wishto lay before you have sprung from the soil of experimental physics, and