Does quantum theory imply the entire Universe is preordained?
Was there ever any choice in the Universe being as it is? Albert Einstein could have been wondering about this when he remarked to mathematician Ernst Strauss: “What I’m really interested in is whether God could have made the world in a different way; that is, whether the necessity of logical simplicity leaves any freedom at all.”
US physicist James Hartle, who died earlier this year aged 83, made seminal contributions to this continuing debate. Early in the twentieth century, the advent of quantum theory seemed to have blown out of the water ideas from classical physics that the evolution of the Universe is ‘deterministic’. Hartle contributed to a remarkable proposal that, if correct, completely reverses a conventional story about determinism’s rise with classical physics, and its subsequent fall with quantum theory. A quantum Universe might, in fact, be more deterministic than a classical one — and for all its apparent uncertainties, quantum theory might better explain why the Universe is the one it is, and not some other version.
In physics, determinism means that the state of the Universe at any given time and the basic laws of physics fully determine the Universe’s backward history and forward evolution. This idea reached its peak with the strict, precise laws about how the Universe behaves introduced by classical physics. Take Isaac Newton’s laws of motion. If someone knew the present positions and momenta of all particles, they could in theory use Newton’s laws to deduce all facts about the Universe, past and future. It’s only a lack of knowledge (or computational power) that prevents scientists from doing so.
Along with this distinctive predictive power, determinism underwrites scientific explanations that come close to the ‘principle of sufficient reason’ most famously articulated by German polymath Gottfried Leibniz: that everything has an explanation. Every state of the Universe (with one obvious exception, which we’ll come to) can be completely explained by an earlier one. If the Universe is a train, determinism says that it’s running on a track, with no option to switch to any other path because different tracks never cross. [Continue reading…]