>I'd like to know what it means for one universe to "interfere with"
Ah. I believe Deutsch's ontological answer to this invokes what he calls
`shadow light', a kind of explicit leakage between adjacent universes (not
to be confused, I gather, with Bohmian pilot waves or Cramer offer waves).
I say `ontological' because obviously to date everyone uses the same
equations and calculations in solving QT problems, irrespective of how they
suppose these formalisms cash out in ontic terms.