Simulations are getting better and better over time. If technology keeps progressing like that, one day it might be possible to simulate a human brain. However, simulations work based on Newtonian laws, not quantum. I know that quantum mechanics do not influence something as big as a brain but since it is something so complex maybe the tiny difference that quantum mechanics does to each tiny interaction is added up throughout time, resulting in a totally different outcome. For example, suppose we are simulating a human inside an isolated room. In theory, we could predict in what direction he will walk after 1-hour using newtons laws (let's say... forward). But since the human brain has a lot of interactions (such as electrons in synapsis) and is a very complex structure with tons of variables, perhaps quantum effects could sum up during this one hour making it impossible to predict if he will walk forward or to the right because the result would just be probabilities of his "choice". This reasoning could be applied to many complex structures with a large number of interactions (or even any object given enough time). For instance, maybe fluid dynamics simulations could also serve as an example, like simulating a smoke explosion for a certain time span. According to my idea, after a while, all we could know are the different probabilities of the positions of each smoke particle.

Does this make any sense? Quantum mechanics makes it impossible to predict human behavior (and other complex things) with more than, let's say, 90% accuracy, during a long time period? Or the quantum effects are just so negligible that even considering millions or billions of years there wouldn't be a difference?

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    $\begingroup$ We don't even need to consider QM for there to be a problem. What do you know about chaos theory? $\endgroup$ – CDCM Jul 6 '17 at 9:36
  • $\begingroup$ Or actually physics.stackexchange.com/q/63811/25301 since the above is close as a dupe of this one $\endgroup$ – Kyle Kanos Jul 6 '17 at 14:05
  • $\begingroup$ @KyleKanos You shouldn't mark a question as a duplicate of a duplicate. Just mark it as a duplicate of the original question. $\endgroup$ – tparker Jul 7 '17 at 18:43
  • $\begingroup$ @tparker: I mostly agree and I had meant to use q/63811 but had copied both that one and q/239426 into a comment (that I never posted) about this being a dupe and didn't clear the cached copy from q/239426 when I went to close. $\endgroup$ – Kyle Kanos Jul 7 '17 at 18:51

Scott Aaronson wrote a long quasi-philosophical essay on exactly that question: https://arxiv.org/abs/1306.0159. The punch line is that he argues that it's logically possible that quantum mechanics really does inject a level of unpredictability (or equivalently, unsimulatability) which is qualitatively different from and stronger than what occurs in classical mechanics.

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