There are two items on my life's bucket list that I fear I will never attain--a general understanding of quantum mechanics and relativity. There's nothing more humbling to me than my repeated attempts to start up these mountains.
I came across a book a few weeks ago, The Perfect Theory, that I'm giving a little time to this week on vacation. It's about Einstein's theory of general relativity. I doubt I'll get too far into it but I thought I might blog a little to jog the memory when I retire at 70 and return to that bucket list with all that free time. :-)
The first chapter starts interestingly enough in 1907. That's further than I've ever gotten before. Einstein came up with his theory of special relativity in 1905. That was the year he published his paper arguing that clocks move more slowly and objects appear to shrink as they approach the speed of light.
He was working as a patent clerk from 8-6 every day at the time and talking through the problems of modern physics with his old friend Marcel Grossman on the side. Einstein didn't play the game of academics very well. He did what he wanted and didn't accept the principle that professors assign grades which have something to do with getting jobs. So patent office it was, a mercy job even at that.
Einstein set out to resolve the consequent contradictions from two claims of the physics of his day: 1) the laws of physics work the same in any inertial frame and 2) the speed of light always has the same value. These two principles contradicted each other because if you shine a light from the front of a moving train, you would think that light would move faster than some light you shined from a flashlight on the ground in the same direction.
If I have it right, Einstein's famous solution was to suggest that, from your perspective standing on the ground, the time for the light shining from the front of the train moves more slowly than it does for you with the light shining from you the ground. But on the train, time moves the same as always.
Thus the grandfather paradox. If I am on a train moving close to the speed of light, time proceeds normally for me. But if my son does not get on that train. He may grow up and have children who, by the time I return to the speed he is moving, are older than I am.
In 1907, Einstein was asked to summarize his theory and give implications, which he did in the Yearbook of Electronics and Radioactivity. His series was called, "On the Relativity Principle and the Conclusions Drawn from It." There was one very significant addition.
The Special Theory of Relativity only works for frames of reference that are moving at a constant speed in relation to each other. It does not apply to frames of reference that are speeding up in relation to other frames of reference. In other words, it only works if the train is moving at a constant speed in relation to the ground.
Meanwhile, gravity involves acceleration. Einstein didn't have it all figured out (and he would need help), but he had an insight in 1907 that would later bear the appropriate fruit. "If a person falls freely he will not feel his own weight." What I take this insight to be is that, in the frame of reference of the person falling, there is a constancy that is different from that perceived by a person looking on, much as that experienced by a person on a train who does not feel the speed an observer on the ground observes.
With a little help, this seed would lead to a general theory in 1915 that could accommodate gravity and accelerating frames of reference.