Michael Clive Price wrote and widely distributed his "Many-Worlds FAQ"
back in the 90s, and he has a couple of questions that touch on this
Is physics linear?
Could we ever communicate with the other worlds?
Is linearity exact?
In the first answer above, Price argues that communication between the
Everett worlds is impossible if the quantum mechanics formulation is
correct and the Schrodinger equation evolves in a linear fashion as is
presently believed. I'm not sure he is right about this, because I was
under the impression that the worlds could exert slight influences on each
other. Worlds never completely split, as I understand it; but rather,
as decoherence develops between two branches, the degree of influence
between them declines exponentially and soon they are effectively split.
In his second answer above, Price build on this to argue that physics
really is linear, because if it were not, inter-world communication and
possibly transportation would be possible. He then argues along similar
lines as Tim May has done here, that in some world an early civilization
would have arisen and colonized through world-space as well as physical
As I said, I'm not sure that Price is 100% correct in his reasoning here,
but still there is a good argument to be made that inter-world travel
cannot be too easy, or else they would certainly be here by now.