A deterministic universe is logically impossible

determinism

determinism, philosophical theory, according to which all events within the universe, including human actions, are uniquely predetermined by unchangeable (natural) laws. This determinateness need not necessarily be of a causal nature. The only decisive factor is the existence of a law-like connection. This is important insofar as the majority of the physical laws cannot be interpreted causally, but are nonetheless deterministic. A careful distinction must therefore be made between determinism and the assumption of an unrestricted validity of the causal principle (causality).

As a philosophical principle, determinism has its roots in Greek philosophy; for physics, however, it only became relevant in the 18th century with the expansion of Newtonian mechanics, which provides a deterministic description of motion processes or, more generally, of changes in state: are all state variables of a mechanical system at a point in time t0 known, the state can in principle be determined for any future point in time t calculate with the help of the laws of mechanics. With this unambiguous determination of the future by the past, mediated by the mechanical laws, Newtonian mechanics first provided a more precise definition of the general idea of ​​determinism, which found its concrete expression in the successes, for example, in the description of the planetary movements. Above all, however, with its technical applications, which for the first time established a scientifically determined working practice with the possibility of reliable predictions with the construction of instruments and machines based on mechanical knowledge, it provided the basis for a "mechanization of the worldview", which nature as Uhrwerk (Holbach) and humans as machines (de la Mettrie). In its purest form, the idea of ​​a complete determination of the world can finally be found in Laplace's demon, a being with superhuman intelligence, who should be able to use the laws of nature to know the coordinates and impulses of all particles in the universe for a certain point in time determine past and future constellations of the universe. This mechanical determinism has decisively shaped the worldview of physics into the 20th century, regardless of the fact that even in classical mechanics the initial conditions can never be determined with any precision. However, this was seen only as a practical and not a fundamental problem that would have to be overcome by advances in measurement technology. The development of statistical mechanics, which began in the middle of the 19th century, was not viewed as an objection to the universal validity of determinism, because statistics were only understood as the result of ignorance of the initial conditions.

A rethink only began with the development of quantum mechanics, when it became clear that, according to Heisenberg's uncertainty principle, the simultaneous definition of position and momentum, which is required for a deterministic description of the movement of a particle in terms of mechanics, is in principle not feasible. From this fact, however, the conclusion cannot be drawn that the assumption of a continuous determination of natural events is refuted. If one interprets the uncertainty relation in such a way that only the measurement-technical realization of these initial conditions is impossible, one can still claim that the motion of quantum mechanical particles is deterministic as long as no measurement takes place. Such a claim can of course no longer be refuted using empirical methods. On this basis, it could then be argued that quantum mechanics is incomplete, i.e. that a set of physical quantities existed, the so-called hidden parameters (Bohm's theory), which restore a consistent determinacy of events. After all, quantum mechanics is a deterministic theory insofar as the temporal development of a quantum mechanical state between two measurements is thoroughly deterministic. The decisive arguments against an unrestrictedly valid determinism are provided less by quantum mechanics than by nonlinear dynamics. It shows that the determinism of classical mechanics is essentially due to the fact that its equations of motion are linear differential equations with respect to time t are (chaos, deterministic chaos). Nevertheless, it can be said that quantum mechanics initiated the developments that led to the change in basic attitudes characteristic of modern physics: determinism is viewed neither as a necessary property of nature nor as a methodological or heuristic principle. Law-like description and explainability have nothing to do with determinism and are also possible where processes are not deterministic down to the last detail. Overall, however, there is currently no consensus within physics about the ontological status and methodological relevance of determinism, but the following attitudes to the problem of determinism can be found:

1. Metaphysical determinism: Nature, i.e. all real processes that take place in the outside world, are strictly deterministic. Everything happens with absolute necessity and there are no random events. All statistical laws must consequently be reducible to deterministic ones, at least in principle. Physical theories that do not guarantee this must be viewed as incomplete and should be expanded accordingly.

2. Scientific determinism: Physics does not provide any evidence for a metaphysical determinism; However, determinism is indispensable as a methodological principle, i.e. without the heuristic assumption of determinism, the law-like description characteristic of physics is impossible. Processes that do not proceed deterministically are not accessible to a law-like description.

3. Any kind of assumption about determinism is irrelevant to physics. What is meant by determinism can only be meaningfully defined in relation to a theory. In an absolute sense, there are both deterministic and statistical laws in physics that cannot be reduced to deterministic ones. In relation to a certain theory, however, the statistical laws can certainly be interpreted as deterministic with regard to the description of the behavior of groups. In particular, it makes no sense to supplement quantum mechanics with hidden parameters, because this imposes a concept of determinism on quantum mechanics that only makes sense relative to classical mechanics.