Here is another excerpt which I thought was an interesting take on a subject
we have often discussed:
> Q39 Is linearity exact?
> -------------------
> Linearity (of the wavefunction) has been verified to hold true to better
> than 1 part in 10^27 [W]. If slight non-linear effects were ever
> discovered then the possibility of communication with, or travel to, the
> other worlds would be opened up. The existence of parallel Everett-
> worlds can be used to argue that physics must be *exactly* linear, that
> non-linear effects will never be detected. (See "Is physics linear" for
> more about linearity.)
>
> The argument for exactness uses a version of the weak anthropic
> principle and proceeds thus: the exploitation of slight non-linear
> quantum effects could permit communication with and travel to the other
> Everett-worlds. A sufficiently advanced "early" civilisation [F] might
> colonise uninhabited other worlds, presumably in an exponentially
> spreading fashion. Since the course of evolution is dictated by random
> quantum events (mutations, genetic recombination) and environmental
> effects (asteroidal induced mass extinctions, etc) it seems inevitable
> that in a minority, although still a great many, of these parallel
> worlds life on Earth has already evolved sapient-level intelligence and
> developed an advanced technology millions or even billions of years ago.
> Such early arrivals, under the usual Darwinian pressure to expand, would
> spread across the parallel time tracks, if they had the ability,
> displacing their less-evolved quantum neighbours.
>
> The fossil record indicates that evolution, in our ancestral lineage,
> has proceeded at varying rates at different times. Periods of rapid
> development in complexity (eg the Cambrian explosion of 530 millions
> years ago or the quadrupling of brain size during the recent Ice Ages)
> are interspersed with long periods of much slower development. This
> indicates that we are not in the fast lane of evolution, where all the
> lucky breaks turned out just right for the early development of
> intelligence and technology. Ergo none of the more advanced
> civilisations that exist in other worlds have ever been able to cross
> from one quantum world to another and interrupt our long, slow
> biological evolution.
>
> The simplest explanation is that physics is sufficiently linear to
> prevent travel between Everett worlds. If technology is only bounded
> by physical law (the Feinberg principle [F]) then linearity would have
> to be exact.
Here he is essentially using the Fermi paradox to prove that travel to
other parallel Everett worlds is impossible. Of course, the same argument
proves either that travel to other stars is impossible, or that there are
no other civilizations within our past light cone.
Many people believe that neither of these is the case, and that the Fermi
paradox must have some other explanation. The same belief would invalidate
the claim that MW implies quantum linearity.
Hal