Everything depends on causality, so a ‘partly causal’ universe is impossible.

Causal is as causal does, Harry

If you're religious, you have a bit of a problem. You need the universe to be causal, so free will remains possible, but you also need your 'god' to be able to reach in and force exceptions to the laws of physics whenever divinely necessary. In short, you need a universe that is 'partly causal'. Such a state of affairs is actually impossible, and here's why.

Physical laws are our models of reality that we use to describe events and equivalences. They have a particularly impressive power:- to predict what will happen, given specific starting conditions. While it's debatable whether any law could describe an aspect of reality perfectly (even if reality has any absolute characteristics), our current set of physical laws are robustly abstract, and the marvellous technological society you witness around you is a direct result of that robustness. Observations that appear to be inexplicable using a current law cause a problem, and the law has to be modified or replaced to account for the new observation, because the model is obviously incomplete or wrong; it can no longer describe the observation.

Whether a modification or new law, the replacement must also still account for the observations covered by the previous version, which is where the problem with supernatural events arises. An uncaused event is an event that happens spontaneously. It's literally not causal because there was no 'cause' for it. It therefore can't be covered by a physical law, because the laws of physics are descriptions of causality, or equivalences inferred by observing causal activity. No new version of a physical law can predict such an event either:- it was spontaneous; its occurrence wasn't dependent upon any starting conditions. In a real universe, reducing the comfortably causal 'laws of physics' to the 'suggestions of physics' would be disastrous from a human perspective, because everything relies on causality. From the reliable repetition of particle interactions that allows stellar bodies to form, to the chemical processes that allow our cells to be 'alive', right up to the mechanisms that provide human consciousness, all of it requires causality. While there's still philosophical debate about the topic, and even gentle disagreements within the scientific community, the principle of causality is empirically unassailable. But what actually makes an event 'causal'?

A bit of groundwork:- consider a single particle, 'Bob' (in honour of the name in the previous article). Bob's current state (his location, momentum etc) is the product of all the events experienced by Bob and his antecedents (if he has any) since the Big Bang. Those events are interactions with the universe. For Bob's current state to be described as 'causal', all those interactions must have been causal. That means Bob's current causal state is actually just the end of an immense chain of causal interactions stretching back to the Big Bang, each link of which can be described by the laws of physics. So what's a 'causal interaction'? It's a process, in which the particles involved start in a causal state, and transition causally into another causal state.

For the overall interaction to be causal, all of its components must be causal too, which is the only case in which the laws of physics can fully describe it, including its end state. If the starting state was a-causal, the laws of physics wouldn't be able to describe the transition, and if the transition was a-causal, the laws of physics couldn't describe the end state. A-causal states or transitions can never be described by the laws of physics, because by definition, such spontaneous events have no cause, so there would be no way to rejig the physical laws being violated in order to account for the new a-causal observation. An a-causal event wouldn't be predictable either because its occurrence isn't dependent upon starting conditions. The particles that constitute the end of the a-causal event would therefore be in an a-causal state:- there's no way to get to describe how to get to that end state without invoking the 'magic, Harry!' clause.

Any interactions between those a-causal particles and the currently causal Bob would put Bob into an a-causal state too, because there's now no longer any way to describe why Bob is in that state using the laws of physics, and no way to 'correct' the laws to account for the current situation. From that point onwards, all Bob's future states are a-causal. Even worse, any particles Bob interacts with also enter an a-causal state, because Bob is now in their causal history too, so there's no way to describe their state either. The a-causality blooms outwards in a chain reaction, converting causality to a-causality as it goes. That does many terrible things (from a human perspective), such as eradicating any possibility of free will. This is because you can no longer decide:- your 'decisions', as events, are now nothing but spontaneous uncaused selections, not internally rational choices; to all intents and purposes, they'd be simply random, and whether or not free will exists, there's no definition of it that is based on random selections.

No one has ever witnessed a macroscopic a-causal event, so whether they are even possible remains debatable. You don't need to worry about this sort of speculative scenario though; if an a-causal (a.k.a 'supernatural') event happened, our local volume of spacetime would flip from causal to a-causal at light speed, and your intelligence would cease to function, so 'you' would no longer exist to experience the ensuing chaotic nightmare. You can have causal, or you can have a-causal, you just can't have partly causal.

Cheerie-bye, Harry.

You're a Wizard Harry! Except you can't do magic. Not in THIS universe, at least. Click To Tweet