Copenhagen:
... You bounce a photon off a half-silvered mirror and look at the results: Bam! Split universe.
Copenhagen Interpretation never splits universes. Instead, you have a wave function collapse in the one and only universe.
You bounce a photon off a half-silvered mirror and don't look at the results. Since the physical state of your brain is not causally dependent on the destination of the photon, you don't branch into two mwenglers in any noticeable way.
In MWI, you NEVER branch in to two anythings in a "noticeable" way. All the myriads of branches have no interactions, there is nothing noticeable about any of the other branches from within the branch we are in. If there is something noticeable about other branches, then an experiment could be defined to check the hypothesis of branching, and we would start to gather evidence for or against branching. Until such time as an hypothesis is created and tested and shows evidence for branches, MWI is an interpretation, and not a theory.
So why does it even matter? I am thinking it through and realizing that an interpretation is in some way a pre-theory. As we sit with the idea of MWI, maybe one of us develops hypotheses about experiments whic might show evidence for the other branches, or not. Without the interpretation of MWI, that hypothetical progress might never be available.
All the myriads of branches have no interactions
They do interact. This is how quantum physics was discovered.
The problem is that the magnitude of interaction is getting very small very quickly, so after a few microseconds it becomes technically impossible to measure. This is what allows people to say: "Yeah, for a few microseconds there is something mathematically equivalent to branches, but then it disappears completely" and you can't experimentally prove them wrong.
One side believes that the interaction is getting smaller, but it never reach...
If it's worth saying, but not worth its own post, even in Discussion, it goes here.