The many-worlds interpretation of quantum mechanics seems to have been developed with extreme carelessness, as far as I can tell.
Suppose the universe is a one-dimensional harmonic oscillator in an energy eigenstate. That's an extremely simple quantum state for the universe to be in, it should be easy to 'interpret'. So: if that's the global quantum state of the universe, where are the many worlds? What are their states, their histories? Frank Tipler (in _Physics of Immortality_) advances himself as a many-worlds advocate. When he tries to describe what the many worlds *are*, at one point he says they are *all* the trajectories through the classical state space. At another point he refers just to the Bohmian-mechanical trajectories through that state space, those corresponding to a particular choice of universal wavefunction. You can see more of my complaints about the MWI at http://threads.hotwired.com/cgi-bin/interact/replies_index?msg.53983, under "Challenge to many-worlds advocates". According to John Bell, at one time physicists would say that Niels Bohr solved all the problems of interpretation. I think that Everett is starting to play that role - people who have doubts or puzzles about what QM "means" are referred to Everett. -mitch http://www.thehub.com.au/~mitch