So earlier I was reading about Laplace and my brain literally froze for a second. Imagine this: there is a person in 1814, sitting in Paris, and calmly writing that if there was an intellect vast enough to know the position and momentum of all particles in the universe, that intellect could calculate everything that has ever happened and everything that will happen. Completely. Without remainder. What gives me the chills is not just the theory, but how confident Laplace was when he wrote that in his Essai Philosophique sur les Probabilités. Like someone who just finished proving something and casually said “yeah, that is exactly how it works.”
And the implications are really heavy if you think about it seriously. If Laplace’s demon is real, then there is no such thing as choice. All the decisions you have ever made, all the ones you will make tomorrow morning, everything has been “written” since the first particles of the universe started moving. Free will is not just questioned, but mathematically it cannot exist. If Laplace was right, even the act of contemplating determinism was already predetermined.
But then Heisenberg came along in 1927 and with one small equation killed that entire argument. Not because our tools are not sophisticated enough to measure the position and momentum of a particle simultaneously, but because the universe itself does not allow it. That uncertainty is not a matter of technological limitation, it is the most fundamental nature of reality. This is what sets it apart from all other criticisms of Laplace: it is not “we cannot know,” but “that information simply does not exist.” Laplace’s demon cannot exist not because it is not smart enough, but because the data it needs to work never existed in this universe in the first place.
Then there is chaos theory which is often called the “second killer” but we need to be careful here. Chaos theory shows that a deterministic system can become practically unpredictable due to extreme sensitivity to initial conditions. Weather, for example, technically follows the laws of physics perfectly but cannot be predicted more than about ten days in advance. The problem is, as noted on Wikipedia and several academic papers, chaos theory and Laplace’s demon are not actually entirely compatible with each other because chaos theory is relevant when our knowledge is imperfect, while Laplace precisely assumes perfect knowledge. So chaos theory undermines Laplace’s demon from the practicality side, not from the principle side. The one that truly kills it fundamentally is still Heisenberg.
What I find most interesting is the fact that Laplace himself actually knew he would never be that demon. At the end of his essay he wrote that the human mind will always remain infinitely separated from such an intellect. So he was not claiming to be able to predict everything, he was just saying that the universe is in principle deterministic. Ironically, it was precisely the science he loved that eventually proved that even that principle was wrong. Sometimes science is just like that, building something incredibly solid and then tearing it down itself even more elegantly.