My main issue with Many Worlds is that it is always superfluous.
We know that the exponential complexity of the quantum state cannot be explained by saying every outcome simply occurs in another branch. That would make it mathematically equivalent to an ensemble, and ensembles can be decomposed into large collections of simple deterministic systems with only linear complexity. If that were how reality worked, quantum mechanics would be unnecessary. The theory could be reduced to classical statistical mechanics.
A quantum superposition, such as an electron being spin up and spin down, is not an electron doing both in some proportions. If it were, it would again be equivalent to an ensemble and fully describable using classical probability theory. If the quantum state has any ontology at all, it cannot merely represent particles doing multiple things at once. It must be something else, a distinct beable that either influences particles, as in pilot wave theories, or gives rise to them, as in collapse models.
Some Many Worlds advocates eventually concede this, but then argue that particles never really existed and are only subjective illusions, while the quantum state alone is real. Calling something a subjective illusion does not remove the need for explanation. Hallucinations are still physical processes with physical causes. You can explain them by analyzing the brain and its interactions.
Likewise, you still need a physical explanation for how the illusion of particles arises. Any such explanation ends up equivalent to explaining how real particles arise, and once you do that, Many Worlds becomes unnecessary. You can always replace the multiverse with a single universe by making the process stochastic instead of deterministic.
The crucial point is that we know a particle in a superposition of states cannot be a particle in multiple states at the same time. That is mathematically impossible and if that is what it was then it could be reduced to a classical description! Any interpretation which relies on thinking the quantum state represents an ensemble, i.e. it represents things “taking all possible paths” or “in multiple states at once,” is just confused as to the mathematics as this is not what the mathematics says.
My main issue with Many Worlds is that it is always superfluous.
We know that the exponential complexity of the quantum state cannot be explained by saying every outcome simply occurs in another branch. That would make it mathematically equivalent to an ensemble, and ensembles can be decomposed into large collections of simple deterministic systems with only linear complexity. If that were how reality worked, quantum mechanics would be unnecessary. The theory could be reduced to classical statistical mechanics.
A quantum superposition, such as an electron being spin up and spin down, is not an electron doing both in some proportions. If it were, it would again be equivalent to an ensemble and fully describable using classical probability theory. If the quantum state has any ontology at all, it cannot merely represent particles doing multiple things at once. It must be something else, a distinct beable that either influences particles, as in pilot wave theories, or gives rise to them, as in collapse models.
Some Many Worlds advocates eventually concede this, but then argue that particles never really existed and are only subjective illusions, while the quantum state alone is real. Calling something a subjective illusion does not remove the need for explanation. Hallucinations are still physical processes with physical causes. You can explain them by analyzing the brain and its interactions.
Likewise, you still need a physical explanation for how the illusion of particles arises. Any such explanation ends up equivalent to explaining how real particles arise, and once you do that, Many Worlds becomes unnecessary. You can always replace the multiverse with a single universe by making the process stochastic instead of deterministic.
The crucial point is that we know a particle in a superposition of states cannot be a particle in multiple states at the same time. That is mathematically impossible and if that is what it was then it could be reduced to a classical description! Any interpretation which relies on thinking the quantum state represents an ensemble, i.e. it represents things “taking all possible paths” or “in multiple states at once,” is just confused as to the mathematics as this is not what the mathematics says.
I go into this in more detail here: https://medium.com/p/f67aacb622d5