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Edit: Based on @WillO's very relevant comment, I want to clarify what I mean by "fully governed by QM" - it means governed only by the laws of quantum mechanics, as opposed to classical physics. It effectively means "if classical systems don't exist in the universe".

If those logical steps below are correct, it would seem to me that saying "The universe is fully governed by QM, and only by QM" and saying "MWI is correct" both have to have the same answer - either both are true, or both are false.

Is that statement correct?

From what I understood, MWI says the following:

  1. The universe is fully governed by QM, and only by QM.
  2. This means it is described by a universe-sized quantum wavefunction which evolves according to the Schroedinger equation.
  3. The universal wavefunction contains the universe, and the universe contains everything that exists.
  4. This means that nothing exists outside of the quantum wavefunction to observe it.
  5. This means the quantum system can never collapse, and remains in a pure state forever.
  6. The universe is an ordinary quantum system capable of superposition.
  7. Superposition is where the so called "many worlds" would come from.

Now, as far as I understand, the Coppenhaggen Interpretation says the following:

  1. Universe is partitioned between classical systems governed by classical physical laws, and quantum systems governed by QM.
  2. At the boundary between microscopic and macroscopic scales, systems "switch" which set of physical laws governs them.
  3. Collapse happens when classical and quantum systems interact.

So, to repeat the question - if the universe is fully governed by QM, and only by QM, does that necessitate that MWI is correct? If MWI is incorrect, does that imply that the universe cannot be governed only by QM?

Qmechanic
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sashoalm
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1 Answers1

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I think that the conclusion drawn in the post is correct. If we take the entire universe as our system and apply quantum mechanics to it, it follows that the collapse postulate never gets applied as there is nothing external to the universe to measure the universe.

So we are led to something resembling the Many Worlds Interpretation. But we should be careful here. The first question to ask is :

Does quantum mechanics, when applied to the whole universe, even fit our empirical observations?

The immediate answer to this question is 'no', as the Born rule is part of our observations. We observe certain outcomes with more often than others, in a way given by the Born rule. The Schrodinger equation does not reproduce the Born rule.

One way this can be answered in MWI is by postulating that certain worlds have more copies or duplicates than others. This is the approach Everett took in his original paper. But some relatively recent approaches have tried to derive the Born rule from some other principles. I am linking a critique of these approaches.

Apart from additional postulates to derive the Born rule, MWI also uses few subtle assumptions that cannot be called "conventional Quantum Mechanics". Like several commentators have stated, conventional Quantum mechanics is identified with "Schrodinger equation + The collapse postulate", and there is good reason for that.

Conventional Quantum mechanics takes the wavefunction as a tool to predict what happens in reality upon measurements. If we carry over this idea to MWI, then MWI cannot be related to empirical observations as there can be no measurements of the universal wavefunction. You are just left with a meaningless wavefunction.

To evade this conclusion, MWI needs an additional assumption that the wavefunction itself is mapped to reality by identifying parts of the wavefunction with "worlds". Now, the notion of a world needs to be well-defined which is a challenge to MWI. See this answer for a critique.

In summary, applying QM to the whole universe does lead to something resembling MWI, as we are left with a wavefunction obeying the Schrodinger equation. But MWI needs additional assumptions to connect this wavefunction to empirical observations. There are problems for the Many Worlders to tackle.

In my view, the lesson to be learned is that quantum mechanics isn't supposed to be applied to the whole universe. Approaches more nuanced than MWI need to be taken to solve the measurement problem.

Ryder Rude
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