If the alternate universes implied by the mainstream MWI (as opposed to variants like consistent histories) are "actual" in some sense, with even the slightest chance of communication between universes, then why have we not seen solid evidence of such communication?
Amongst the universes, many ("many" is a huge number, obviously) of them will be way ahead of us. Some will have had galactic civilizations for a billion years. Some will be versions of Earth except that the Egyptians pioneered electronics and hence the world is a few thousand years "ahead" of our world...even assuming time is commensurate with ours. And so on. You can all imagine the rich possibilities. If these universes are even remotely able to affect each other, through perhaps enormously advanced technology, then the vast number of such possible worlds would suggest that at least some of them have figured out how to do so. And yet they aren't here. No visitors from alternate universes. No signals sent in, a la Benford's "Timescape." Perhaps we don't know how to listen. Perhaps there are so many possible universes to potentially visit that we just haven't been gotten to yet. Perhaps in a multiverse of so many possibilities, ours is just not an interesting destination. Maybe there's a kind of MWI censorship going on: since we are still debating the validity of MWI, we obviously are in a universe where MWI has not been proved through such a visit. (There are many divergent series here, making even crude estimates difficult and probably worthless.) Hmmmhhhh.... --Tim May Prime, resident of Earth Prime