One aspect which I disagree with is that collapse is the important thing to look at.
Decoherence is sufficient to get classical behaviour on the branches of the wave function. There is no need to consider collapse if we care about ‘weird’ vs. classical behaviour. This is still the case even if the whole universe is collapse-resistant (as is the case in the many worlds interpretation).
The point of this is that true cat states ( = superposed universe branches) do not look weird.
The whole ‘macroscopic quantum effects’ are interferences between whole universes branches from the view of this small quantum object in they brain.
Superposition of universe—We can certainly regard the possibility that the macroscopic world is in a superposition as seen from our brain. This is what we should expect (absent collapse) just from the sizes of universe and brain:
The size of our brain corresponds to a limited number for the dimensionality of all possible brain states (we can include all sub-atomic particles for this)
If the number of branches of the universe is larger than the number of possible brain states, there is no possible wave function in which there aren’t some contributions in which the universe is in a superposition with regards to the brain. Some brain states must be associated with multiple branches.
the universe is a lot larger than the brain and dimensionality scales exponentially with particle number
further, it seems highly likely that many physical brain-states correspond to identical mind states (some unnoticeable vibration propagating through my body does not seem to scramble my thinking very much)
Because of this, anyone following the many worlds interpretation should agree that from our perspective, the universe is always in a superposition—no unknown brain properties required. But due to decoherence (and assuming that branches will not meet), this makes no difference and we can replace the superposition with a probability distribution.
Perhaps this is captured by your “why Everett called his theory relative interpretation of QM”—I did not read his original works.
The question now becomes the interference between whole universe branches:
A deep assumption in quantum theory is locality which implies that two branches must be equal in all properties[1] in order to interfere[2].
Because of this, interference of branches can only look like “things evolving in a weird direction” (double slit experiment) and not like “we encounter a wholly different branch of reality” (fictional stories where people meet their alternate-reality versions).
Because of this, I do not see how quantum mechanics could create the weird effects that it is supposed to explain.
If we do assume that human minds have an extra ability to facilitate interaction between otherwise distant branches if they are in a superposition compared to us, this of course could create a lot of weirdness.
But this seems like a huge claim to me that would depart massively from much of what current physics believes. Without a much more specific model, this feels closer to a non-explanation than to an explanation.
This is not a necessary property of quantum theories, but it is one of the core assumptions used in e.g. the standard model. People who explore quantum gravity do consider theories which soften this assumption
One aspect which I disagree with is that collapse is the important thing to look at. Decoherence is sufficient to get classical behaviour on the branches of the wave function. There is no need to consider collapse if we care about ‘weird’ vs. classical behaviour. This is still the case even if the whole universe is collapse-resistant (as is the case in the many worlds interpretation). The point of this is that true cat states ( = superposed universe branches) do not look weird.
Superposition of universe—We can certainly regard the possibility that the macroscopic world is in a superposition as seen from our brain. This is what we should expect (absent collapse) just from the sizes of universe and brain:
The size of our brain corresponds to a limited number for the dimensionality of all possible brain states (we can include all sub-atomic particles for this)
If the number of branches of the universe is larger than the number of possible brain states, there is no possible wave function in which there aren’t some contributions in which the universe is in a superposition with regards to the brain. Some brain states must be associated with multiple branches.
the universe is a lot larger than the brain and dimensionality scales exponentially with particle number
further, it seems highly likely that many physical brain-states correspond to identical mind states (some unnoticeable vibration propagating through my body does not seem to scramble my thinking very much)
Because of this, anyone following the many worlds interpretation should agree that from our perspective, the universe is always in a superposition—no unknown brain properties required. But due to decoherence (and assuming that branches will not meet), this makes no difference and we can replace the superposition with a probability distribution.
Perhaps this is captured by your “why Everett called his theory relative interpretation of QM”—I did not read his original works.
The question now becomes the interference between whole universe branches: A deep assumption in quantum theory is locality which implies that two branches must be equal in all properties[1] in order to interfere[2]. Because of this, interference of branches can only look like “things evolving in a weird direction” (double slit experiment) and not like “we encounter a wholly different branch of reality” (fictional stories where people meet their alternate-reality versions).
Because of this, I do not see how quantum mechanics could create the weird effects that it is supposed to explain.
If we do assume that human minds have an extra ability to facilitate interaction between otherwise distant branches if they are in a superposition compared to us, this of course could create a lot of weirdness. But this seems like a huge claim to me that would depart massively from much of what current physics believes. Without a much more specific model, this feels closer to a non-explanation than to an explanation.
more strictly: must have mutual support in phase-space. For non-physicists: a point in phase-space is how classical mechanics describes a world.
This is not a necessary property of quantum theories, but it is one of the core assumptions used in e.g. the standard model. People who explore quantum gravity do consider theories which soften this assumption