[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