Edit: Broke out the book and checked, and yeah, it explicitly equates it to Schroedinger's cat and throws in this: "Then again, Cochrane knew, according to other views of quantum physics, whenever the universe reached a decision point in which two outcomes were equally probable, the universe itself split, so that both outcomes had equal existence. "
Which isn't how it actually works, by the way. The theory is that every outcome gets its own independent measurement history (i.e. "universe"), regardless of their relative probabilities.
And it happens on a quantum-particle level, not the level of macroscopic events like coin flips or human decisions. That's why Schroedinger's Cat required the poison capsule to be triggered by the decay of a radioactive particle rather than, say, a burning candle -- because the point of the thought experiment was to create a situation where a quantum-state change
could have a direct impact on a macroscopic outcome, something that usually would
not be the case.
In fact, the splitting of the universe would happen, not because of which way the coin landed or which way a person decided to act, but because of whether, say, a radioactive atom in the adjacent brick wall decayed or not, or whether an electron in a nearby leaf tunnelled or not. Most alternate quantum realities would probably have exactly the same events on a human scale, except for those very uncommon instances where a quantum-level event did somehow manage to have a macroscopic impact. (Although that would probably happen a lot more frequently once a civilization got to the point of using transistor-based electronics and quantum computers.)
I suppose you could argue, though, that if a quantum-state change did split the universe in two, and there were a macroscopic event going on that actually did have a perfectly balanced 50-50 probability, then maybe it would just happen to take place differently in one universe than in the other. It wouldn't cause the split on a quantum level, but it would cause the two divergent universes to be noticeably different rather than functionally identical. I think I had Ranjea explain something to that effect to the trainees in
Watching the Clock.
Although I'm not convinced such a truly balanced probability would ever really happen. A coin flip isn't an exactly 50-50 proposition to begin with,
for various reasons. And there could also be influences from air currents and other environmental factors. I suspect that usually when we see an outcome as random and unpredictable, it's only because we're unable to account for every environmental factor or circumstance that's affecting its outcome.
Of course, in fiction, it's not that interesting to say "There are countless different universes, but they're all pretty much the same and their splits have nothing whatsoever to do with human actions or choices." So we fudge quantum physics for dramatic effect. What bugs me is when actual scientists, trying to talk about the Many-Worlds Interpretation for lay audiences, claim that it really is about human choices and coin flips and the like.
