Tim May writes:
> IF the MWI universe branchings are at all communicatable-with, that is, 
> at least _some_ of those universes would have very, very large amounts 
> of power, computer power, numbers of people, etc. And some of them, if 
> it were possible, would have communicated with us, colonized us, 
> visited us, etc.
> This is a variant of the Fermi Paradox raised to a very high power.

A similar idea has been advanced about time travel.  If it will someday
be possible, why aren't we being visited from the future?

However both proposals could be patched up against this objection by
postulating that we have to build the "other end" of the communication
link.  Time travel will be possible but you won't be able to travel
back to before you built the time machine.  Inter-world travel will
be possible but the people in both worlds have to cooperate to build a
common quantum system, perhaps implying that they have to begin building
the system before their worlds split.

In this formulation (which I believe arises quite naturally in the general
relativity based approaches to time travel), the only way that visitors
could be here now would be if there were some natural phenomenon which
happened to provide the necessary conditions, or if there wre some other
civilization in our local universe who had built the required machinery.

Hal Finney

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