Over the weekend I happened upon a discussion online on whether the Universe and the laws of nature support free will or not. This is not a simple question, and the technical details are well beyond the scope of this blog. However I thought I would spur further debates with a simple model that suggests how the Universe could be deterministic and support free will at the same time. I must also add a disclaimer that this is a very simplistic overview of a much bigger debate, and I will be skipping over many interesting but superfluous details.

Allow me to begin with some background information and definitions. 

Determinism is simply the belief or theory that everything is predetermined, and we cannot change anything. In classical physics (ie. everything before the 1920s) the laws of physics were all deterministic. If you know the starting state of any system, then it will progress and evolve in a specific way following Newton's Laws/Maxwell's Laws/fluid dynamics or any number of other equations. Once the initial conditions are known, and the equations solved, then you will know everything about the system until the end of time. There can be no free will.

And perhaps of greatest importance, the theory of general relativity, which is our best model of gravity at present, not only is deterministic but posits that the entire history and future of the Universe already exists in some sense and we are only experiencing three-dimensional slices of a four-dimensional spacetime. There can be no free will in general relativity. (I must add a disclaimer that there are possible exclusions to this claim, and the idea that the entire four dimensional spacetime already exists and is unchanging is not universally accepted, but I will leave those for another article)

However quantum mechanics gives a very different view of nature. In quantum mechanics, the same state with the same initial conditions will simultaneously evolve in several different ways. The system effectively forms multiple 'virtual systems' which are each a little different from the others. Then something happens (exactly what happens depends on the still debated interpretation of quantum mechanics) and one of those virtual systems becomes real, while the others disappear. And while no one has developed an viable model of quantum consciousness, quantum mechanics at least opens the possibility that we can change our future as it is non-deterministic.  It is possible - though not proven - that  quantum mechanics allows for free will in some form.

But how can both of these arguments be true? How can the Universe on large scales be predetermined by general relativity and on the smallest scales consist of random events that are unpredictable?

The answer is we just do not know.

But here is one possibility. One interpretation of quantum mechanics is that every time a decision is made (or a measurement of a system is made) then the Universe itself divides into multiple copies, each of which has a slightly different evolution. If this is true, then each of the four dimensional parallel universes could be completely determined by general relativity, making it deterministic. But we are free to select which of the multiple predetermined Universes we are following. 

Consider the diagram above, where the red line represents a possible path that a traveler could follow. The traveler starts by moving along a straight line. Then the Universe splits into three copies of itself. In the top Universe, the traveler is destined to turn left. In the middle copy, the traveler is predestined to go straight through, and in the bottom copy the traveler is predestined to turn right. And so there are three predetermined paths, but the traveler believes they have chosen between the three and therefore would appear to possess free will.

Of course this is a great oversimplification. In reality, no one knows how general relativity and quantum mechanics will eventually be combined, or how quantum mechanics is to be interpreted correctly, or whether free will will eventually be revealed to be an illusion. And while I have presented the traveler in the example as choosing between three paths, in truth there would be three identical copies of the traveler, each of whom makes a different choice. Is that free will? That is a matter for the philosophers to debate.

However this debate is eventually resolved, it is certain to be a fascinating result and perhaps a troubling revelation for a lot of people if free will is ever disproven. Until then, all we can do is ponder the wonders of nature.