Determinism (also called antiserendipity) is the philosophical proposition that every event, including human cognition and behavior, decision and action, is causally determined by an unbroken chain of prior occurrences. With numerous historical debates, many varieties and philosophical positions on the subject of determinism exist from traditions throughout the world.
Determinism, quantum mechanics and classical physics
Some people have argued that in addition to the conditions humans can observe and the rules they can deduce there are hidden factors or hidden variables that determine absolutely in which order electrons reach the screen. They argue that the course of the universe is absolutely determined, but that humans are screened from knowledge of the determinative factors. So, they say, it only appears that things proceed in a merely probabilistically determinative way. Actually, they proceed in an absolutely determinative way. Although matters are still subject to some measure of dispute, quantum mechanics makes statistical predictions that would be violated if some local hidden variables existed. There have been a number of experiments to verify those predictions, and so far they do not appear to be violated although many physicists believe better experiments are needed to conclusively settle the question. (See Bell test experiments.) It is, however, possible to augment quantum mechanics with non-local hidden variables to achieve a deterministic theory that is in agreement with experiment. An example is the Bohm interpretation of quantum mechanics.
So quantum mechanics is deterministic, provided that one accepts the wave function itself as reality (rather than as probability of classical coordinates). Since we have no practical way of knowing the exact magnitudes, and especially the phases, in a full quantum mechanical description of the causes of an observable event, this turns out to be philosophically similar to the "hidden variable" doctrine.
Libertarianism is a philosophical position in metaphysics with respect to free will and determinism. It entails the belief that human beings possess free will, that free will is incompatible with determinism, and that determinism is false.
Although not held by the majority of contemporary philosophers, libertarianism is still widely discussed and avidly defended by several leading philosophers in the field, such as Peter van Inwagen, Robert Kane, Timothy O'Connor and Laura Ekstrom.
Naturalistic libertarians believe that the universe contains an indeterminstic element, for instance as demonstrated by quantum mechanics, and that human beings exploit this to achieve freedom of choice. There is no separate, dualistic self in this theory: the self is the total activity of the brain as a system.
At one time, it was assumed in the physical sciences that if the behavior observed in a system cannot be predicted, the problem is due to lack of fine-grained information, so that a sufficiently detailed investigation would eventually result in a deterministic theory ("If you knew exactly all the forces acting on the dice, you would be able to predict which number comes up"). However, the advent of quantum mechanics removed the underpinning from that approach, with the claim that (at least according to the Copenhagen interpretation) the most basic constituents of matter behave indeterministically, in accordance with such properties as the uncertainty principle. Quantum indeterminism was controversial on its introduction, with Einstein among the opposition, but gradually gained ground. Experiments confirmed the correctness of quantum mechanics, with a test of the Bell's theorem by Alain Aspect being particularly important because it showed that determinism and locality cannot both be true. Bohmian quantum mechanics remains the main attempt to preserve determinism (albeit at the expense of locality).
The Bohm interpretation of quantum mechanics, sometimes called Bohmian mechanics, the ontological interpretation, or the causal interpretation, is an interpretation postulated by David Bohm in 1952 as an extension of Louis de Broglie's pilot-wave theory of 1927 . Consequently it is sometimes called the de Broglie-Bohm theory. Bohm's interpretation is an example of a hidden variables theory. It is hoped that the hidden variables would provide a local deterministic objective description that would resolve or eliminate many of the paradoxes of quantum mechanics, such as Schrödinger's cat, the measurement problem, the collapse of the wavefunction, and similar concerns. However, Bell's inequality complicates this hope, as it demonstrates that there is no local hidden variable theory that is compatible with quantum mechanics. Thus, one is left with choosing between the lesser of two evils: discarding locality, or discarding realism. The Bohmian interpretation opts for keeping realism and accepting nonlocality.
Others see the consequences of EPR and Bell's theorem in a different way. They regard the correct conclusion to be related not so much to quantum theory itself, but only to deterministic interpretations of the same (i.e., to hidden-variable theories such as Bohm's interpretation). According to the people who think this way, what has been shown is that all deterministic theories must be nonlocal. For example, Niels Bohr was a member of this group. This group would claim that retaining orthodox quantum mechanics — with its nondeterministic character — permits one to retain locality, or at least to avoid the EPR type of nonlocality, at the expense of having no way to picture particles as objective elements of reality that occupy definite regions of space at all times. Armed with such a viewpoint, these physicists tend to be less receptive to Bohm's interpretation.
Seen as isomorphic to many worlds
Explicitly non-local. Bohm accepts that all the branches of the universal wavefunction exist. Like Everett, Bohm held that the wavefunction is real complex-valued field which never collapses. In addition Bohm postulated that there were particles that move under the influence of a non-local "quantum- potential" derived from the wavefunction (in addition to the classical potentials which are already incorporated into the structure of the wavefunction). The action of the quantum- potential is such that the particles are affected by only one of the branches of the wavefunction. (Bohm derives what is essentially a decoherence argument to show this, see section 7,#I [B]).
The implicit, unstated assumption made by Bohm is that only the single branch of wavefunction associated with particles can contain self-aware observers, whereas Everett makes no such assumption. Most of Bohm's adherents do not seem to understand (or even be aware of) Everett's criticism, section VI , that the hidden- variable particles are not observable since the wavefunction alone is sufficient to account for all observations and hence a model of reality. The hidden variable particles can be discarded, along with the guiding quantum-potential, yielding a theory isomorphic to many-worlds, without affecting any experimental results.
—Michael Clive Price