[donaq]

Correct me if I'm wrong, but chaotic systems are unpredictable because they are complex and infinitesimally small variations in starting conditions result in hugely different outcomes. They are not, however, non-deterministic. If the universe is merely chaotic and not actually stochastic, then the existence of you and your children are not only not unlikely, it was inevitable.

[dreamcompiler]

Exactly right. Unless you believe quantum mechanics throws a monkey wrench into determinism. Which it certainly does in the realm of the very small.

Chaos theory and QM are completely orthogonal (by which I mean they say different things about the universe, not that they are in conflict with one another). And they both screw up the idea of a predictable universe.

[colordrops]

Quantum mechanics demonstrating non-determinism in the realm of the very small necessarily means that the macroscopic realm is also non-deterministic. A non-zero percentage of events occur in which that one particle appearing in one place rather than an is just enough to cause the macroscopic object to take a materially different path. In fact a simple thought experiment shows that a system could be engineered to demonstrate this. Add a sensor that reads which way a particle goes in a double slit experiment and based on the direction, flip a switch that makes a train go east or west based on the sensor read out.

[eridius]

Adding a sensor to the double slit experiment to measure which slit the particle goes through removes the quantum effects from the experiment.

[colordrops]

Not sure what you mean. Quantum effects don't "disappear". They are physical laws. Which slit the particle enters after the wave decoheres is precisely a non-deterministic element of quantum mechanics.

[bouncycastle]

But wouldn't measuring something also alter the outcome? Ie. Observer Effect https://en.m.wikipedia.org/wiki/Observer_effect_(physics)

[colordrops]

It doesn't alter the outcome. It causes it to decohere into a particular outcome. There is no way to force a particular outcome, hence it being non-deterministic.

[theprotocol]

How do we know that what we perceive as randomness in quantum mechanics is "true" randomness, and not just something we say is "random" because we fail to model it?

[donaq]

Aren't there deterministic interpretations of QM? Or have those been disproven?

[TomasSedovic]

(disclaimer: not a physicist, never looked into the maths, but I'm intrigued by QF)

There are deterministic interpretations that are very much on the table.

The Everett's Many Worlds[1] is fully deterministic (and depending on your view, the simplest one too): the universe is a quantum wave function evolving according to the Shrödinger's equation. That's it.

A lot (if not all) of the hidden variables theories are also deterministic. The apparent non-determinism stems from the aforementioned variables that we don't/can't see.

I keep hearing more and more about the Pilot wave theory[2] recently. And that's a hidden-variables deterministic interpretation.

[1]: https://en.wikipedia.org/wiki/Many-worlds_interpretation

[2]: https://en.wikipedia.org/wiki/Pilot_wave_theory