From what I read it was by the Heisenberg uncertainty principle but I don’t understand how, too technical for me when I read up on the topic
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To quote Wikipedia, Laplace’s “demon” idea suggests:
> if someone (the demon) knows the precise location and momentum of every atom in the universe, their past and future values for any given time are entailed; they can be calculated from the laws of classical mechanics.
i.e. if we know everything about *now* we can figure out everything in the past and everything in the future.
The uncertainty principle (again, quoting Wikipedia) means:
> the more precisely the position of some particle is determined, the less precisely its momentum can be predicted from initial conditions, and vice versa.
If the uncertainty principle is true, then Laplace’s “demon” cannot exist, as it cannot know both the position and momentum of *any* atom in the universe; the more it knows about the position, the less certainty it has over the momentum and vice versa.
In fact the Copenhagen interpretation of quantum mechanics takes us even further, and we get that there is always uncertainty in any (quantum) measurement; we cannot precisely predict the position *or* momentum of any (quantum) particle in the future even given perfect knowledge now, all we can do is come up with an estimate for where it could be, and what the probability of finding it there is.
The Copenhagen interpretation of quantum mechanics is *non-deterministic*; that is to say the future isn’t uniquely or necessarily determined by the present – there is true randomness.
Of course, there are other interpretations of quantum mechanics, which *are* deterministic (either via something like the Many Worlds Interpretation – although that is only deterministic on the multi-verse scale, or interpretations which don’t have localism).
There are also logical attempts to disprove Laplace’s demon, but they tend to be more technical counter-arguments (the “demon” couldn’t know all its future memory, or if you had two demons they wouldn’t be able to know everything about the other) and less important in thinking about determinism.
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