You have to be very careful about which multiverse theory you refer to. There is a Many-worlds interpretation of quantum mechanics which don't lead to any diffe

You have to be very careful about which multiverse theory you refer to. There is a Many-worlds interpretation of quantum mechanics which don't lead to any different physics. It's fundamentally untestable and doesn't matter. So far you can't derive new measurable consequences (to my knowledge) of this, and there's no physics that really hinges on it. If you don't like this interpretation, you can pick some other one until you're happy and shut up and calculate with the schroedinger or dirac equation which has been verified stupidly well with experiment. Multiverse theories in general may say that the universes interact. They may have differing coupling constants. You know you might try to come up with some new physics but your new physics hinges on these things existing... and that's fine... that's how new things are discovered, but we're at a point where SUSY is taken as a given and multiverses are taken as a given and theorists have this really much more convenient model of the universe that they're starting from and making unmeasurable conclusions and it just seems so removed from reality! That's the issue.