Laplace's Demon: Is Destiny Already Written?

Laplace's demon is a philosophical argument that claims that all events in the universe are determined by the unbreakable chain of cause and effect. Let's see what the hypothesis consists of.

Written and verified by the philosopher Maria Alejandra Morgado Cusati.

Last update: 01 June, 2022

Laplace’s demon is an imaginary character that has the ability to know the precise location and momentum of each particle of the universe. From there, he deduces their past and future.

In other words, he’s an entity outside the universe that, thanks to the information he has of the present moment, can’t only predict what will happen in the future, but is also capable of knowing the beginning of everything that exists.

This approach is based on the deterministic perspective of the universe, which holds that all physical events – even human thought and actions – are causally determined by the unbreakable chain of cause and effect. Therefore, their present state in some sense “determines” the future.

Let’s take a look at this interesting theory in more detail.

Causal determinism: Laplace’s demon

Causal determinism is a philosophical current that defends that everything that happens has a preceding cause. Therefore, the events of the present are the result of a series of past events and, in turn, are the cause of future events.

To understand this better, imagine that the events of the universe are an infinite series of dominoes placed in a vertical position to fall one after the other and can’t be interrupted. In this sense, the fall of each domino will be due to the fall of the one that precedes it, thus forming the unbreakable chain of cause and effect.

From there, it follows that everything has been predestined since the beginning of time. Therefore, destiny is already written.

Likewise, accepting determinism would imply rejecting free will. After all, according to this theory, everything we do is already determined by the cause-effect chain.

Laplace’s demon

Now, Pierre-Simon Laplace (1749 – 1826) was a French astronomer, physicist, and mathematician. He’s often remembered as one of the greatest scientists of all time. In fact, he’s sometimes referred to as the Newton of France.

Laplace dedicated himself to the analysis of celestial mechanics, perfecting the model proposed by Newton. He also participated in the study of some gas molecules and atomic particles. Thanks to his knowledge in classical mechanics, he imagined the possibility of predicting all events through science.

From there, he created the imaginary character known as Laplace’s demon. This being has the ability to know everything about the universe (including every detail of the subatomic particles that compose it). From this, he applies the laws of physics to predict the future and describe the past accurately.

This argument is valid if determinism is accepted as a premise. As we’ve already analyzed, it claims that the present state of events are the effect of their previous state and, in turn, are the cause of the one that follows.

Arguments against Laplace’s demon

The deterministic perspective was particularly successful while the universe was conceived solely in terms of classical mechanics and linear cause-effect systems. However, the later emergence of quantum mechanics, entropy, and chaos theory challenged the tendency to apply the logic of prediction to everything we know.

Quantum mechanics and Laplace’s demon

Quantum mechanics presents an indeterministic model of the universe. This is because it’s discovered that subatomic particles don’t really obey the laws of classical mechanics.

Therefore, one cannot predict their behavior, but only establish probabilities. Thus, Laplace’s demon really can’t accurately establish future events.

Chaos theory

One of the most popular components of chaos theory is the butterfly effect, which is mostly cited to represent the large impacts of small actions.

However, this effect deals with the unpredictability of events. So the flapping of a butterfly may cause a tornado, but it may also produce a gentle wind somewhere else. We can’t know the real outcome.

In fact, even if all the variables of a system were known, with enough iterations, it can behave unpredictably.

Entropy

Entropy is disorder, and the entropy of the universe is always increasing. Therefore, according to this theory, it will become increasingly disordered over time and it will be more difficult to predict what will happen accurately.

Furthermore, Laplace’s demon is supposed to decipher the past by reverse-engineering the present. However, the concept of thermodynamic irreversibility complicates this. After all, some things cannot be restored to the initial state in which they were, so it becomes impossible to know what was the initial state of what is being examined now.

It’s not an obsolete theory

As we have seen, quantum mechanics, thermodynamics, and chaos theory provide the basis for refuting the existence of Laplace’s demon.

However, we must bear in mind that determinism has had and continues to have an undeniable value in the development of the sciences. In fact, these have based a large part of their discoveries on the principle of cause and effect.

So, the presence of determinism and indeterminism in the universe reminds us of its vastness and how little we know about it.