The debacle of determinism

Isaac Newton

By the end of the eighteenth century, Isaac Newton’s theory of universal gravitation was well established. As this theory makes it possible to predict very accurately the orbits of the bodies in the solar system, the French astronomer Pierre Simon de Laplace believed he had sufficient reasons to say the following:

An intelligence that knew all the forces that animate nature, as well as the respective situation of the beings that make it... could cover in a single formula the movements of the largest bodies of the universe and those of the lighter atom. Nothing would be uncertain and both the future and the past would be present before his eyes.

This assertion became the dogma of deterministic materialism, a philosophical (not scientific) doctrine asserting that only matter exists (taking the term broadly) and that the whole history of the universe is determined. Therefore there is no human freedom, nor intentionality, nor final causes in nature. There are just efficient causes.

Laplace’s statement can be expressed in more modern terms:

If we knew the position and the momentum of all the particles of the universe at a given instant, we could predict all their past and future development.

During the nineteenth century, determinism came to dominate the philosophical mentality (by the way, not the scientific), but in the twentieth century it suffered three devastating attacks that made it unsustainable. I will cite here these three attacks in logical order, not in chronological order:

1. Heisenberg’s uncertainty principle, which in its best known form says the following:

Werner Heisenberg

It is not possible to know with absolute accuracy the position and the momentum of a particle at a given instant of time.

Note that this principle applies not just to all particles of the universe, as the modernized form of Laplace’s statement says, but to each of these particles.

Physicists and philosophers disagree on the transcendence of the uncertainty principle. Some think that each particle has a well-defined position and momentum, although we cannot know both properties at the same time with arbitrary accuracy, which would mean there is an epistemological uncertainty. Others believe that the uncertainty is physical, that is, that the particles, in fact, do not have clearly determined position and momentum.

Faced with the challenge of the uncertainty principle, determinists can answer the following:

Okay, we’ll never be able to know exactly the properties of all the particles of the universe at any given time. But could we not know them with sufficient approximation to be able to predict, within a reasonable margin, all the past and future development of the cosmos?

The second devastating attack on determinism answered this question:

2. The theory of chaos, pre-formulated by Poincaré and formulated by Edward Lorenz during the 1950s, which says this:

In chaotic systems, very small differences in the initial conditions give rise to enormous differences in the final results.

Since Newton’s equations describe a chaotic system, and since the universe has existed for a very long time (about 13.8 billion years) it follows that we cannot make correct predictions about its long-term evolution.

As in the previous case, physicists and philosophers do not agree on the kind of uncertainty that stems from chaos theory. Although most think it is epistemological (which only limits our ability to know), some like Ilya Prigogine think that it is a physical uncertainty, i.e., intrinsic.

Given the double challenge of the uncertainty principle and chaos theory, determinists, relying on the hypothesis that these uncertainties can be epistemological, could answer the following:

Okay, we will never be able to calculate the trajectories of all the particles in the universe. But that does not imply that these trajectories do not exist, even though they are unknowable to us, to the Laplace demon and to any other conscious entity within the universe.

The answer to this is given by the third attack:

Niels Bohr

3. Copenhagen interpretation of quantum mechanics: According to Niels Bohr and his school, the physics of elementary particles is not deterministic, but random, with physical indeterminism, which means that:

Not only is it impossible to predict the future development of the universe in the long term, but also in each of the instantaneous steps of its existence (in every quantum collapse).

It is true that the Schrödinger equation, which describes the behavior of elementary particles before quantum collapse, is deterministic. But the quantum collapse, which according to this interpretation is the interface between quantum microscopic phenomena and classical macroscopic phenomena, is indeterministic. The only way to avoid it is by rejecting the Copenhagen interpretation.

The Copenhagen interpretation has been predominant in quantum physics throughout the 20th century. Many attempts to knock it down have failed. To save determinism, the only option left to its supporters is to deny this interpretation, to replace it by a different one that does not imply physical indeterminism. So far, all these proposals have failed, so the only escape from determinists has been to resort to the quantum multiverse (see another article in this blog), according to which, in each quantum collapse (i.e. a huge number of times every second) the universe splits into two parallel universes where the two possible outcomes of the collapse will have taken place (one in a universe, the other in the other). The quantum multiverse would imply that there is not a single universe, but a huge number, greater than any other known number, and anything that can happen will have happened in at least one of these universes. This is a way to recover determinism. Our universe would not be deterministic, but quantum multiverse would be.

This insane and non-scientific theory (it claims something whose falsity cannot be proved) does not explain why we are in this particular universe and not in another of the many that would exist simultaneously.

Meanwhile, the intrinsic indeterminism of the physical basis of our universe makes several things possible, such as human intentionality, free will, and God’s interaction with the universe, not because indeterminism is the same as freedom, but because, by stating that the set of efficient causes is open, other causes (final causes) are made possible, whose existence determinism denies.

The same post in Spanish
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Manuel Alfonseca