That is so common a misperception that it makes the many-worlds-faq
http://kuoi.asui.uidaho.edu/~kamikaze/documents/many-worlds-faq.html
Q21 Does many-worlds violate Ockham's Razor?
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William of Ockham, 1285-1349(?) English philosopher and one of the
founders of logic, proposed a maxim for judging theories which says that
hypotheses should not be multiplied beyond necessity. This is known as
Ockham's razor and is interpreted, today, as meaning that to account for
any set of facts the simplest theories are to be preferred over more
complex ones. Many-worlds is viewed as unnecessarily complex, by some,
by requiring the existence of a multiplicity of worlds to explain what
we see, at any time, in just one world.
This is to mistake what is meant by "complex". Here's an example.
Analysis of starlight reveals that starlight is very similar to faint
sunlight, both with spectroscopic absorption and emission lines.
Assuming the universality of physical law we are led to conclude that
other stars and worlds are scattered, in great numbers, across the
cosmos. The theory that "the stars are distant suns" is the simplest
theory and so to be preferred by Ockham's Razor to other geocentric
theories.
Similarly many-worlds is the simplest and most economical quantum theory
because it proposes that same laws of physics apply to animate observers
as has been observed for inanimate objects. The multiplicity of worlds
predicted by the theory is not a weakness of many-worlds, any more than
the multiplicity of stars are for astronomers, since the non-interacting
worlds emerge from a simpler theory.
(As an historical aside it is worth noting that Ockham's razor was also
falsely used to argue in favour of the older heliocentric theories
*against* Galileo's notion of the vastness of the cosmos. The notion
of vast empty interstellar spaces was too uneconomical to be believable
to the Medieval mind. Again they were confusing the notion of vastness
with complexity [15].)
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Sean Morgan (sean@lucifer.com)