In effect what we’re doing when we make a reference frame is to treat as equivalent events that are happening “on different branches of history”. At first, that may seem like a very odd thing to do. But the thing to understand is that as entities embedded in the same universe that’s generating all these different branches of history, we too are branching. So it’s really a question of how a “branching brain” will perceive a “branching universe”. And that depends on what reference frame (or “quantum observation frame”) we pick. But as soon as we insist that we maintain a single thread of experience, or, equivalently, that we sequentialize time, then—together with computational boundedness—this puts all sorts of constraints on the reference frames we pick.