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Does Heisenberg’s Uncertainty Principle mean that the universe cannot deterministically be predicted by observers, or does it mean that the universe is inherently indeterministic, meaning that the exact same initial conditions could give different results over time (or is this a wrong interpretation)?

If the second case is true, what should be understood by this indeterminism? Is it that some processes have random or stochastic components? I do not understand how true randomness can exist. Where would randomness come from, apart from imperfect knowledge about the system?

I am not a physicist and am potentially completely misunderstanding many concepts in this question. Any explanation or references are welcome!

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  • $\begingroup$ Note that parts of this post contains questions about metaphysics and/or philosophy. E.g. "where would randomness come from?". That aside, the question is important and could generate some good answers. $\endgroup$ May 16 at 11:34
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    $\begingroup$ There is no consensus. $\endgroup$
    – Jbag1212
    May 16 at 20:45

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Randomness and probability distributions of random events that depend on the deterministic mathematical equations of classical physics are well understood. The data though at very small dimensions cannot be modeled with the deterministic classical mechanics and quantum mechanics developed in order to be able to fit the data and be predictive.

In quantum mechanics theories , the universe is inherently indeterministic, meaning that the exact same initial conditions give different results for individual interactions. The theory can only model and predict probability distributions for the measurements, the individual particle interactions are indeterminant, the value of velocities angles etc to be measured for as single event has a probability of having that value given by the solutions of the quantum mechanical equations for the problem.

This answer of mine may help.

As for consensus among physicists, the mainstream model of physics accepts that the underlying level of all physical phenomena depends on quantum mechanics, which is probabilistic, not deterministic.

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When discussing the status of randomness in quantum physics, more than the Heisenberg inequality, I'd have a look at Bell's inequalities.

In a nutshell, Bell's inequalities are a criterium to determine whether a quantum process has an underlying deterministic mechanism (called "hidden variables"), or is just random. They're an experimentally verifiable criterium, and a lot of efforts were made in the past 40 years to check them in various ways.

Without getting into too much detail, when Bell's inequalities are violated in an experiment, it excludes the possibility of a "traditional" deterministic mechanism, but a few other, more exotic possibilities remain. But as time passes, those remaining possibilities are more and more constrained, so that any theory describing them would be increasingly awkward (non-local, violating relativity... take your pick).

While the debate is still going, the physicists actively working to build a deterministic theory compatible with Bell's inequalities are few and far between. For the most part, their objections are useful to force quantum theory's users to sharpen their arguments and get a deeper understanding of their tool.

Time will tell!

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I think the question misses a subtle and important point. Even outside the realm of quantum mechanics, it is now known that some macroscopic processes are inherently chaotic. The evolution of these systems over time has been found to be EXTREMELY sensitive to the system's starting conditions, to the point that it is probably impossible to measure such system's starting parameters to a high enough precision to predict their complete evolution over time. That "warm blanket of predictability" that is implied by Newtonian mechanics has proven to be an illusion, both for quantum mechanical systems and for macroscopic systems.

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Heisenberg’s Uncertainty Principle is the exact point after which physics departs from a theory resting on the causally deterministic to a theory resting on the causally probabilistic. There is a fundamental limit to how accurately measurements can be made. Yes, it is possible that the exact same conditions could give different answers (measurements), but the probability distribution associated will be the same. As a result of our universe, or rather, the best model of the universe we have as of now, randomness exists because we do not know in a definite manner what happens next in a quantum system. Keep in mind that this is just my interpretation of quantum mechanics. There are various others out there that could give you insight.

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The probabilistic issues with QM occur at the level of individual subatomic particles. But when you average together the individual behavior of a very, very large number of those same particles, the QM properties of those particles is mostly averaged out (except for exotic things like lasers and liquid helium)- which means the uncertainty in position of, for example, a baseball or planet due to QM effects is so absurdly tiny that it doesn't enter into their dynamics at all.

Now, what about an object the size of the whole universe? During the big bang, matter and energy were so densely mashed together into such a tiny volume that QM effects were correspondingly huge and therefore must be taken into account in any realistic description of what was going on at that time. But as the universe got cooler and less dense, QM effects faded out and no longer dominated the dynamics.

So to answer your question- the only occasion where QM effects like indeterminancy and probabilities would affect the sort of universe we inhabit (if the evolution of that universe could even have us in it!) is during the first few minutes of its existence. And the conditions present then and how the dynamics worked is a very active field of research in astrophysics and cosmology.

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