"God doesn't play dice."
I'm sure the reader has heard this famous saying from Einstein in a 1926 letter to fellow physicist Max Born. Perhaps not so clear to most people is what God and what dice Einstein was referring to. His worries reflect a deep concern about how far our explanations of Nature can go. They speak to the heart of what science is, an issue that remains contentious to this day.
Einstein was referring to quantum physics, the physics that describes the behavior of molecules, atoms and subatomic particles — like electrons and the Higgs boson. The "dice" relate to probabilities, the fact that in the quantum world the cozy determinism of our classical worldview goes down the drain.
In our everyday life objects follow well-behaved histories from point A to point B. In the realm of the very small this determinism fails completely. We can, at most, compute probabilities that a particle will be at this or that point in space (within the accuracy of the measuring device). Even more bizarre, before we detect a particle we can't even tell if it exists. All we have is potentiality.
In an extreme interpretation, we can say that the act of detection "creates" the particle. But if that's the case, what about bigger objects? Aren't they made of atoms, which are quantum objects? Does a mountain only exist when we look at it? Surely, that's kind of ridiculous. Mount Everest is there whether we look at it or not. But how can you tell? Do we know that Mount Everest is out there when we are not looking, or do we infer that from common sense?
To Einstein, this loss of predictive determinism couldn't be the last word in our description of Nature. Another theory, deeper and broader, should be able to explain the paradoxes of the quantum world. Was he right?
A lot has happened in eight decades. Experiments have tried again and again to find flaws in traditional quantum mechanics, perhaps opening a window into an alternative theory. All to no avail: it really looks as if quantum mechanics is here to stay. Nature is inherently uncertain and we have to come to terms with it.
Heisenberg's uncertainty principle, stating that we can't know both position and velocity of a particle with arbitrary accuracy, is more than an obstacle to knowledge; it's the way Nature operates. God does seem to play dice, and the tremendous successes of quantum physics are a testament to our ability to make sense of a very bizarre state of affairs.
Einstein's sentence in his letter to Born is actually different from the snippet above:
Quantum mechanics demands serious attention. But an inner voice tells me that this is not the true Jacob. The theory accomplishes a lot, but it does not bring us closer to the secrets of the Old One. In any case, I am convinced that He does not play dice.