Quantum mechanics was not the only theory that bothered Einstein. Few people have appreciated how dissatisfied he was with his own theories of relativity. Special relativity grew out of Einstein’s insight that the laws of electromagnetism cannot depend on relative motion and that the speed of light therefore must always be the same, no matter how the source or the observer moves. Among the consequences of that theory are that energy and mass are equivalent (the now-legendary relationship E = mc2) and that time and distance are relative, not absolute. Special relativity was the result of 10 years of intellectual struggle, yet Einstein had convinced himself it was wrong within two years of publishing it. He rejected his own theory, even before most physicists had come to accept it, for reasons that only he cared about. For another 10 years, as others in the world of physics slowly absorbed special relativity, Einstein pursued a lonely path away from it.
Why? The main reason was that he wanted to extend relativity to include all observers, whereas his special theory postulates only an equivalence among a limited class of observers—those who aren’t accelerating. A second reason was his concern with incorporating gravity, making use of what he called the equivalence principle, which postulates that observers can never distinguish the effects of gravity from those of acceleration as long as they observe phenomena only in their neighborhood. By this principle, he linked the problem of gravity with the problem of extending relativity to all observers.
Einstein was the only one who worried about these two problems. Meanwhile, other physicists came up with ways to incorporate gravitational phenomena directly into special relativity. This was the reasonable thing to do, for they were building directly on the success of the new theory Einstein had invented. And they succeeded in making the theory consistent. Moreover, their extensions of special relativity agreed with all the experiments that had been done. So why did Einstein reject it? His reason was that his colleagues’ approach—folding descriptions of gravity into special relativity rather than crafting a whole new theory—disagreed with his equivalence principle. He understood quickly that there was a key experiment that could distinguish between the incremental approach of the other physicists and his own radical approach. This was to measure the bending of light by the sun’s gravity, an effect predicted by the equivalence principle. A reasonable person might have waited to see how the experiment came out, and indeed, an opportunity to test the theory came in 1919. By that time, Einstein had invented his second theory of relativity, which he called general relativity. The experiment appeared to confirm the new theory’s predictions. The result was announced on the front pages of the world’s newspapers, making Einstein the first scientist to be a media star.
General relativity is the most radical and challenging of Einstein’s discoveries—so much so that I believe the majority of physicists, even theoretical physicists, have yet to fully incorporate it into their thinking. The flashy stuff, like black holes, gravitational waves, the expanding universe, and the Big Bang are, it turns out, the easy parts of general relativity. The theory goes much deeper: It demands a radical change in how we think of space and time.
All previous theories said that space and time have a fixed structure and that it is this structure that gives rise to the properties of things in the world, by giving every object a place and every event a time. In the transition from Aristotle to Newton to Einstein and special relativity, that structure changed, but in each case the structure is absolute. We and everything we observe live in a set space-time, with fixed and unchanging properties. That is the stage on which we play, but nothing we do or could do affects the structure of space and time themselves.
General relativity is not about adding to those structures. It is not even about substituting those structures for a list of possible new structures. It rejects the whole idea that space and time are fixed at all. Instead, in general relativity the properties of space and time evolve dynamically, in interaction with everything they contain. Furthermore, the essence of space and time now is just a set of relationships between events that take place in the history of the world. It is sufficient, it turns out, to speak only of two kinds of relationships: how events are related to each other causally (the order in which they unfold) and how many events are contained within a given interval of time, measured by a standard clock (how quickly they unfold relative to each other).
Thus, in general relativity there is no fixed framework, no stage on which the world plays itself out. There is only an evolving network of relationships, making up the history of space, time, and matter. All the previous theories described space and time as fixed backgrounds on which things happen. The implication of general relativity is that there is no background.
This point is subtle and elusive. I was very fortunate to know the great astrophysicist Subrahmanyan Chandrasekhar during his last years. Chandra, as we called him, demonstrated in 1930 that relativity implied that stars above a certain mass would collapse into what we now call a black hole. Much later, he wrote a beautiful book describing the different solutions of the equations of general relativity that describe black holes. As I got to know him, Chandra shocked me by speaking of a deep anger toward Einstein. Chandra was upset that Einstein, after inventing general relativity, had abandoned this masterpiece, leaving it to others to struggle through it.
I now believe that Chandra partly missed the point, and he is certainly not alone. The deepest implication of general relativity is not that the universe may expand or that there are black holes. To think this way is to believe that general relativity is just another step in the progression from Aristotle to Newton to special relativity. Chandra, in his interest in the solutions of the theory, was, I fear, acting like so many others—reaching for a beautiful flower but missing the beauty of how it is that flowers come to be.
Chandra was right that in spite of the great triumph general relativity represented, Einstein did not linger long over it. For Einstein, quantum physics was the essential mystery, and nothing could be really fundamental that was not part of the solution to that problem. Because general relativity didn’t explain quantum theory, it had to be provisional as well. It could only be a step toward Einstein’s goal, which was to find a theory of quantum phenomena that would agree with all the experiments and satisfy his demand for clarity and completeness.
Einstein imagined for a time that such a theory could come from an extension of general relativity. Thus he entered into the final period of his scientific life, his search for a unified field theory. He sought an extension of general relativity that would incorporate electromagnetism, thereby wedding the large-scale world, where gravity dominates, with the small-scale world of quantum physics. He tried a variety of means, such as adding new dimensions of space-time or loosening somewhat the mathematical structure of general relativity. The irony is that some of these gambits worked, but they still led nowhere. For it turns out that unified theories are a dime a dozen. There are many ways to generalize general relativity so as to incorporate the laws of electromagnetism. Nor is it much harder, as has been done recently, to extend the theory a bit further to incorporate the nuclear forces, and so have a unified theory of all the forces.