Dear Colleagues,Science & Ultimate Reality
Einstein once remarked that the thing that most interested him was whether
God had any choice in the nature of his creation. What he was referring to
is whether the physical universe necessarily has the properties it does, or
whether it might have been otherwise. Obviously some of the fine details
could be different, such as the location of this or that atom. But what
about the underlying laws? For example, could there be a universe in which
gravity is a bit stronger, or the electron a bit heavier?
Physicists remain divided on the issue. Some have flirted with the idea that
if we knew enough about the laws of physics, we would find that they are
logically and mathematically unique. Then to use Einstein's terminology, God
would have had no choice: there is only one possible universe. However, most
scientists expect that there are many - probably and infinite number - of
possible alternative universes in which the laws and conditions are
self-consistent. For example, the universe could have been Newtonian,
consisting of flat space populated by hard spheres that fly about and
sometimes collide. It would probably be a boring world, but it's hard to say
why it is impossible.
If the universe could have been otherwise, then two questions follow. First,
is there anything special about the actual universe that we inhabit as
opposed to the alternatives? Second, why has this particular universe been
selected for actuality from among a limitless number of different
possibilities?
One feature that might be special about our universe is that there are
conscious beings inhabiting it, beings that can look in awe at their world
and reflect on the nature of existence. Mathematical studies suggest that
some key features of our universe are rather sensitive to the precise form
of the laws of physics, so that if we could play God and tinker with, say,
the masses of the elementary particles, or the relative strengths of the
fundamental forces, those key features would be compromised. Probably life
would not arise in a universe with even slightly different laws.
The discovery that we live in a remarkably bio-friendly universe has
prompted some scientists and philosophers to conclude that there is
something contrived about the way our physical world is put together. Others
have opted instead for the "multiverse" explanation. According to this
theory, our universe is but one among an infinity of actually existing
universes. In other words, the alternative possibilities are not failed
worlds, but real worlds that exist alongside ours. It is, of course, no
surprise that we find ourselves inhabiting one of those relatively rare
life-permitting universes; we would not be able to exist in a bio-hostile
alternative.
The multiverse theory is an old one, and often goes under the name "the
anthropic principle." What is new is that physicists now have some halfway
reasonable conjectures about how all these other universes might come to be.
John Wheeler, together with Hugh Everett, suggested an early possibility, in
which the alternative worlds are different branches of the wave function in
a quantum superposition. (In later years Wheeler moved on from this
interpretation of quantum mechanics.) Another idea is that, what we normally
call "the universe," is but a small component in a much larger system, in
which other spatial regions have different laws and properties. Perhaps the
most extreme version of a multiverse has been suggested by Max Tegmark, a
theoretical physicist at the University of Pennsylvania. Tegmark proposes
that all mathematically self-consistent world descriptions enjoy real
existence. There is thus a sliding scale of extravagance, ranging from
multiplying worlds with the mathematical laws fixed, to multiplying laws
within an overall mathematical scheme, to multiplying the mathematical
possibilities too.
Some people conclude that invoking an infinity of unseen worlds merely to
explain some oddities about the one we do see is the antithesis of Occam's
Razor. Others believe the multiverse is a natural extension of modern
theoretical cosmology. (It's worth noting that Max's website lists his
multiverse theory under the "Crazy Stuff" category!) Whatever you think,
Tegmark is the ideal guide to tell you about the various multiverses on
offer. A brief summary of his paper follows.
Paul Davies
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Title:: Parallel Universes
Author: Max Tegmark
Summary:
The notion that our observable Universe is merely a small part of a larger
"multiverse" is attracting increasing attention among physicists,
particularly those working on cosmology (motivated by inflation theory and
apparent fine-tuning) and string theory (motivated by multiple "vacuum"
states). The goal of the article is to survey the various types of parallel
Universes proposed, focusing on - the evidence and physical motivation for
each type - the extent to which the ideas are experimentally testable - what
each type would be like to inhabit - philosophical implications and the
structure of the multiverse It is not widely appreciated that there are in
fact numerous different types of multiverses being proposed. The key goal of
this article is to describe the different kinds and the relationship between
them. People have speculated about such things since the dawn of time, but
now it's gradually becoming experimentally testable.
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