A while back I wrote three posts in a row about why Determinism in the physical sense is not actually a real thing in the real universe and is not a consequence of physical law. The initial conditions of the universe do not fully determine all future states of the universe, and in particular do not determine all future actions of organic multi-cellular agents. Such actions may be and often are determined in a sense, or perhaps more accurately predisposed, but not by the universe's initial conditions. The actions of a dog may be constrained by instinct, conditioning, interactions with other dogs, reactions to random (stochastic) events such as lightning or earthquakes, or done just because the dog felt like it. The universe's initial conditions certainly had nothing to do with it. Nor was the appearance of dogs on a tiny planet orbiting a tiny star in one of billions of ordinary galaxies predetermined. The exact sequence of events that lead to that particular dog existing was in no way inevitable, although one may claim inevitability that some form of life evolved somewhere at some time, given enough of it.
My refutation of physical Determinism requires a certain proficiency in Physics to understand. That may be inevitable as well, but I would like to add some Explainy Words to the body of work that may help fellow physicists as well as, like, musicians and such, to grasp the idea of our Non-Deterministic Universe. How can a Universe entirely operating on natural physical laws be non-deterministic?
This must be framed by an understanding of what people mean when they say Determinism. The physical meaning of physical Determinism is that all future states are "locked in" by the combination of the physical laws which 100% control the evolution of the universe from one state to the next, and the initial conditions from which the universe starts off. Many will focus only on the fact that the evolution of states is 100% determined by the physical laws (which we call physics) and set aside the important role of the initial conditions or the starting point, or even the precise nature of the physics that controls the operation of the universe. While the fact of the universe evolving or functioning entirely according to physics is not called into question except by Deists,Theists, or other proponents of willful ignorance, this sense of Determinism does not do what its claimants suppose: it does not fix as inevitable all future states of the universe or all future events. I show that future states are not necessarily fully determined by previous states even when the evolution of states is entirely governed by physics. I also show that initial states can and often are sufficiently ambiguous to allow future states to be not inevitable even when arrived at by entirely natural physical processes.
At the most basic level that we are yet to understand, the universe possess a Quantum Mechanical physics. Quantum Mechanics is an explicitly non-deterministic kind of physics. Hell, even ordinary cause-and-effect struggles to assert itself in the Quantum regime. That is the universe at the lowest, most basic level we know so far. The nature of Quantum Physics is that future states while determined by physics are not fully determined by initial conditions, and any given evolution of states is not foreordained by initial conditions even though the laws of Quantum Physics are always fully complied with. But this is not even remotely the end of the story.
When a system of Quantum particles increases sufficiently in number, Classical or Newtonian Physics emerges as a result. Let me repeat that: Classical Physics emerges from Quantum Mechanics when you add enough interacting parts. A Classical body emerges from putting together a sufficiently large (maybe 10,000) quantum objects in a semi-bound state, such as a baseball or a tiny pebble. Classical bodies obey classical physics including all of Newton's Laws, Hooke's Law, Electromagnetism, and Gravity. All of this emerges from or arises from Quantum Mechanics, a fully non-deterministic physics.
Classical Physics applied to a small number of Classical Bodies in a simple system over a Reasonable time period exhibits some properties of Determinism. Determinism is an emergent and temporary property of Classical Physics, which is emergent from non-deterministic Quantum Physics. What is a simple system and a reasonable time period? By simple I mean that the number of interactions is about the same order of magnitude as the number of bodies. In other words we do not see every body interacting with every other body and hence an exponentially larger number of interactions than bodies. Reasonable time is the time it takes for there to be about the same order of magnitude of interactions as there are bodies. Beyond that, this newly emerged Determinism starts to break down because of its intrinsic sensitivity to initial conditions and the number of interactions which tend to magnify ambiguity as the number of interactions increases.
When the system is not so simple and/or the time becomes unreasonably long, a new physics begins to emerge: Chaos. Chaotic physics emerges or arises from Classical Physics and Classical bodies when the sensitivity to initial conditions becomes so great that only infinitely determined initial conditions could constrain all future states of the system. Without infinitely determined initial states, Chaotic Physics becomes Non-Deterministic: the Non-Determinism has emerged from the increasing complexity of the deterministic Classical system. Notice we do not have infinity precision of initial conditions at any time anywhere. Remember, Classical bodies and Classical Physics emerged from Quantum Physics, which is non-deterministic and ambiguous with respect to the exact state of every particle. It is not the case that the states exist but are unknown; it is more the case that an infinitely unambiguous initial state did not exist in reality.
But we're not done yet. As Classical bodies get smaller and greater in number, yet another new Physics emerges: Thermodynamics or Statistical Physics. This physics is again inherently stochastic and non-deterministic. Specific internal states of Thermodynamic Systems are not determined by initial conditions for the reasons I explained in the post on that topic. Due to thermodynamic irreversibility (aka entropy increase) information about previous states is irretrievably lost and those exact states no longer play any role in determining future states or the evolution of the system. Again, from conditionally deterministic Classical Physics emerges a new non-deterministic physics, and the universe is back to being Non-Deterministic again.
To summarize, the Determinism that people are familiar with is only an emergent property of a limited range of physical phenomenon which itself emerges from more fundamental and fundamentally non-deterministic physics. This limited Determinism dissolves back into non-determinism as soon as the scope of physical phenomena is further expanded. Only certain Goldilocks conditions can give rise to a temporary illusion of physical Determinism in our universe.