Although Einstein is best known for his theories of relativity, he was also the main driving force behind the advent of quantum mechanics (QM). His early work in photo-voltaic effect paved way for future developments in QM. And he won the Nobel prize, not for the theories of relativity, but for this early work.
It then should come as a surprise to us that Einstein didn’t quite believe in QM. He spent the latter part of his career trying to device thought experiments that would prove that QM is inconsistent with what he believed to be the laws of nature. Why is it that Einstein could not accept QM? We will never know for sure, and my guess is probably as good as anybody else’s.
Einstein’s trouble with QM is summarized in this famous quote.
It is indeed difficult to reconcile the notions (or at least some interpretations) of QM with a word view in which a God has control over everything. In QM, observations are probabilistic in nature. That is to say, if we somehow manage to send two electrons (in the same state) down the same beam and observe them after a while, we may get two different observed properties.
We can interpret this imperfection in observation as our inability to set up identical initial states, or the lack of precision in our measurements. This interpretation gives rise to the so-called hidden variable theories — considered invalid for a variety of reasons. The interpretation currently popular is that uncertainty is an inherent property of nature — the so-called Copenhagen interpretation.
In the Copenhagen picture, particles have positions only when observed. At other times, they should be thought of as kind of spread out in space. In a double-slit interference experiment using electrons, for instance, we should not ask whether a particular electron takes on slit or the other. As long as there is interference, it kind of takes both.
The troubling thing for Einstein in this interpretation would be that even God would not be able to make the electron take one slit or the other (without disturbing the interference pattern, that is). And if God cannot place one tiny electron where He wants, how is he going to control the whole universe?