By Walter Isaacson
ACCORDING to the special theory of relativity, all inertial reference frames are equally valid. It is not a question of whether rods actually shrink (1) or time really slows down; all we know is that the observers in different states of motion will measure things differently. And now that we have dispensed with the ether as “superfluous,” there 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 has assumes that one state of movement, that of 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 though experiments had failed, it seemed that the problem should be restated. This 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.”
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 which remain invariant.
In fact, Einstein briefly considered calling his creation Invariance Theory, but that name never really 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 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.
— Einstein: His Life and Universe, Ch. 6. 'Special Relativity.'
1. Isaacson is making a reference to Einstein's thought experiment of imagining a rod that has a certain length when it is measured while it is stationary relative to the observer. Now imagine the rod in motion. An attempt to measure the rod while it is in motion will produce a different result. I’ll omit here any explanation for the discrepancy in measurements.
