We have shown in Schrödinger’s Cat that the wave function often contains many different versions of reality, with a different version of the observer in each version of reality. And we know from experience that our perceptions will correspond to those of one and only one version of the observer. We also know that the versions of the observer are the only ‘entities’ that perceive in QMA.

The important point in this section is that each version of the observer can only be aware of what is happening within their own version of reality. The version of the observer that sees a dead cat can know nothing about what is happening on the live cat branch. She cannot know whether the live version of the cat is purring, or whether the live-cat version of the observer is relieved, or jumping up and down or whatever. That is, each separate version of reality constitutes, in effect, a separate, isolated universe that evolves in its own quantum mechanical way totally without regard to what happens in the other versions. This implies in particular that there can be no communication between different versions—by light, sound, touch, email or any other means.

[P5] Different versions of reality in the wave function are in separate, isolated universes that cannot communicate with or affect each other in any way. Thus no version of the observer can perceive anything other than that which happens within its own, single universe.

This result shows there can be no perception of more than one version of reality in QMA (see Quantum Physics and Classical Perception). It is also used to disqualify several potential interpretations of quantum physics. Because if its importance, its technical derivation is given in Details of Separate Universes.