Re: Everett

Nicholas Bostrom (
Thu, 14 Aug 1997 01:09:16 +0000

Hal Finney wrote:

> Sean Morgan, <>, writes:
> > [Excerpt from the many-worlds FAQ at
> >]
> 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.

In the case of the Fermi paradox, if there are other civilizations
well within our past light cone, and large scale space
colonization is possible, then either they destroy themselves before
they do large scale space colonization, or they take pain to avoid
detection. One of these occurances would have to happen to 100% of
all civilizations in our past light cone. It seems not be absurd to
assume that this is what happens in our universe, but if all possible
series of quantum events actually occur in some world, then there
should be some civilization that miraculously managed to escape
extinction and then happened to adopt a policy of not hiding. So in
the case there are MW then these two answers (self destruction and
hiding) to the inter-worlds version of the Fermi paradox aren't

I don't know exactly how the possible non-linearity of the
wave function could be exploited to travel between worlds. It is
imaginable that there would be some limitation analogous to the light
speed. Since there would be extremely many worlds, and the
number would continually explode, perhaps a worlds colonizing
civilization would simply not be able to keep up with the number of
new worlds that are born every second. This could then explain why
our world hasn't been colonized. I think this is a better
objection to the cited argument that MW implies quantum linearity.
Nicholas Bostrom

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