While Einstein is best remembered by the general public for his theory of relativity, this was largely complete by 1916. Quantum mechanics, by that time, was only just beginning to be developed into a full, general theory. Einstein made important contributions to quantum mechanics early on, in 1905 using the hypothesis that light is quantized to explain the photoelectric effect (Nobel prize!), and spent much of the later part of his career thinking and writing about quantum mechanics. He was a vocal critic of the way quantum mechanics describes the universe—notably the fact that quantum mechanics is inherently non-deterministic in the sense that it can, in general, only predict the probability that a certain outcome will happen, regardless of how much is known about the system and its initial state. This is the origin of the now famous (and often paraphrased) remark
I, at any rate, am convinced that He [God] does not throw dice.
Einstein argued that quantum mechanics must be an incomplete description of reality, and that there must be additional information, termed “hidden variables,” which determine the course of events, and which are as of yet, inaccessible to our scrutiny. It’s hard to argue with this view. How can one make the case that in calculating a quantum mechanical probability we then know all that we ever can know about the future of our system? How can we say that there isn’t just a little bit more beneath the surface which is not yet within reach of our instruments?
Quantum mechanics introduces an unsettling moment of truth: a nanoscale Seldon Crisis in which the system goes from having many possible outcomes to just one. This moment of actually throwing the proverbial dice occurs (according to the standard interpretation of quantum mechanics during Einstein’s lifetime) at the instant in which the system is observed. At that instant the system collapses from many possibilities, with one being chosen at random to be the outcome actually observed. The illusion of determinism comes from things which are more or less constantly observed. They are simply not left unperturbed long enough for the hallmarks of quantum mechanical behavior to disturb that illusion.
With this observation effect, it seems as if the results of measurements do not exist independently—they don’t come into being, they are not “real” until the results are actually observed. This is a hard thing to get one’s mind around. Quantum mechanics seems to say that there is no independent, objective reality without observation (there is a great deal of philosophical work these days to try to resolve this without invoking Einstein’s “hidden variables,” but most of it is beyond my current understanding).
This is exactly the old “if a tree falls in the forest” question, but with an actual physical theory at the center of the (ostensibly) philosophical debate. Einstein said he believed that the moon continued to exist even when he wasn’t looking at it. If quantum mechanics is God throwing dice, then hidden variables are a bit like playing cards. You don’t know what card will come up when you say, “hit me,” but you can be reasonably assured that the outcome does actually exist before the card is turned over.