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Is there a general consensus about whether the universe is deterministic? Is it still up in the air?

I have attempted to read other physics.stackexchange answers and do some independent research, but I'm not a physicist and most literature is beyond my comprehension. If you answer this question, I would most appreciate it if you could attempt to explain it as simply as possible. As soon as I have to start to look up theories or definitions that are used, the web pages that explain it generally go over my head.

The easiest way to phrase my question that I can think of:

Assume there is a being existing prior to the big bang, with infinite intelligence, physics knowledge, memory, and time. The only limitation is this being can not time travel to observe the future universe. However, this being knows everything about everything as it currently stands. Once the big bang happens, could this being map out every interaction in our universe throughout all time?

Martin
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temetvince
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1 Answers1

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No, since 1925 or 1926, we have known – thanks to Werner Heisenberg, Niels Bohr, Max Born, and colleagues who have discovered the so-called "quantum mechanics", the new foundation of modern physics – that the Universe is not deterministic in this sense. Even the maximal possible knowledge of the initial state is insufficient for the prediction of phenomena in the future.

In fact, the departure with the classical reasoning your question exhibits is much deeper than that. The sentence

However, this being knows everything about everything as it currently stands.

is impossible as well. It is not possible to know "everything" – in particular, it is not possible to know positions and velocities (non-commuting quantities) at the same time. One should go even further: the very words "everything as it currently stands" is forbidden by Nature because Nature doesn't objectively "stand" in a particular way. Or equivalently, the laws of Nature outlaw the "perfect agent" who knows everything.

Quantum mechanics uses well-defined differential equations for the operators (quantities such as $x,v$) or the wave functions ($\psi$) that determine these objects' future values. But these objects don't contain unambiguous answers to questions about the phenomena. Instead, they are only the building blocks of formulae to calculate the probabilities that something happens, or something else happens. So only probabilities of various answers to questions about the phenomena in Nature may be computed from the initial state.

Because these objects obey some "deterministic equations" but they don't actually determine the phenomena – because all these objects have a probabilistic interpretation – people sometimes use the term "quantum determinism" that holds in Nature. However, quantum determinism isn't a special case of the classical determinism as you defined it.

Luboš Motl
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