- Carr, Bernard, ed.
Universe or Multiverse?
Cambridge: Cambridge University Press, 2007.
ISBN 0-521-84841-5.
-
Before embarking upon his ultimately successful quest to discover
the
laws
of planetary motion,
Johannes Kepler
tried to explain the sizes of the orbits of the planets from
first principles: developing a mathematical model of the orbits
based upon
nested
Platonic solids. Since, at the time, the solar system was believed
by most to be the entire universe (with the fixed stars on a sphere
surrounding it), it seemed plausible that the dimensions of the
solar system would be fixed by fundamental principles of science and
mathematics. Even though he eventually rejected his model as
inaccurate, he never completely abandoned it—it was for
later generations of astronomers to conclude that there is nothing
fundamental whatsoever about the structure of the solar system: it
is simply a contingent product of the history of its
condensation from the solar nebula, and could have been entirely
different. With the discovery of planets around other stars in the
late twentieth century, we now know that not only do planetary systems
vary widely, many are substantially more weird than most astronomers
or even science fiction writers would have guessed.
Since the completion of the Standard Model of particle physics in
the 1970s, a major goal of theoretical physicists has been to
derive, from first principles, the values of the more than
twenty-five “free parameters” of the Standard Model
(such as the masses of particles, relative strengths of forces,
and mixing angles). At present, these values have to be measured
experimentally and put into the theory “by hand”, and
there is no accepted physical explanation for why they have the
values they do. Further, many of these values appear to be
“fine-tuned” to allow the existence of life in the
universe (or at least, life which resembles ourselves)—a
tiny change, for example, in the mass ratio of the up and down
quarks and the electron would result in a universe with no heavy
elements or chemistry; it's hard to imagine any form of life
which could be built out of just protons or neutrons. The
emergence of a Standard Model of cosmology has only deepened the
mystery, adding additional apparently fine-tunings to the
list. Most stunning is the cosmological constant, which appears
to have a nonzero value which is 124 orders of magnitude smaller
than predicted from a straightforward calculation from quantum
physics.
One might take these fine-tunings as evidence of a benevolent
Creator (which is, indeed, discussed in chapters 25 and 26 of this book),
or of our living in a simulation crafted by a clever programmer
intent on optimising its complexity and degree of interestingness
(chapter 27). But most physicists shy away from such
deus ex machina and
“we is in machina” explanations and seek purely
physical reasons for the values of the parameters we measure.
Now let's return for a moment to Kepler's attempt to derive the
orbits of the planets from pure geometry. The orbit of the
Earth appears, in fact, fine-tuned to permit the existence of
life. Were it more elliptical, or substantially closer to or
farther from the Sun, persistent liquid water on the surface would
not exist, as seems necessary for terrestrial life. The apparent
fine-tuning can be explained, however, by the high probability that
the galaxy contains a multitude of planetary systems of every
possible variety, and such a large ensemble is almost certain to
contain a subset (perhaps small, but not void) in which an earthlike
planet is in a stable orbit within the habitable zone of its star.
Since we can only have evolved and exist in such an environment, we
should not be surprised to find ourselves living on one of these
rare planets, even though such environments represent an
infinitesimal fraction of the volume of the galaxy and universe.
As efforts to explain the particle physics and cosmological
parameters have proved frustrating, and theoretical investigations
into cosmic inflation and string theory have suggested that the
values of the parameters may have simply been chosen at random by
some process, theorists have increasingly been tempted to retrace
the footsteps of Kepler and step back from trying to explain
the values we observe, and instead view them, like the masses and
the orbits of the planets, as the result of an historical process
which could have produced very different results. The apparent
fine-tuning for life is like the properties of the Earth's
orbit—we can only measure the parameters of a universe which
permits us to exist! If they didn't, we wouldn't be here to
do the measuring.
But note that like the parallel argument for the fine-tuning of
the orbit of the Earth, this only makes sense if there are a
multitude of actually existing universes with different random
settings of the parameters, just as only a large ensemble of
planetary systems can contain a few like the one in which we find
ourselves. This means that what we think of as our universe
(everything we can observe or potentially observe within the
Hubble volume) is just one domain in a vastly larger
“multiverse”, most or all of which may remain
forever beyond the scope of scientific investigation.
Now such a breathtaking concept provides plenty for physicists,
cosmologists, philosophers, and theologians to chew upon, and
macerate it they do in this thick (517 page), heavy (1.2 kg),
and expensive (USD 85) volume, which is drawn from papers
presented at conferences held between 2001 and 2005. Contributors
include two Nobel laureates (Steven Weinberg and Frank Wilczek),
and just about everybody else prominent in the multiverse
debate, including Martin Rees, Stephen Hawking, Max Tegmark, Andrei
Linde, Alexander Vilenkin, Renata Kallosh, Leonard Susskind, James
Hartle, Brandon Carter, Lee Smolin, George Ellis, Nick Bostrom, John
Barrow, Paul Davies, and many more. The editor's goal was that the
papers be written for the intelligent layman: like articles in the
pre-dumbed-down Scientific American or “front
of book” material in Nature or Science.
In fact, the chapters vary widely in technical detail and
difficulty; if you don't follow this stuff closely, your eyes
may glaze over in some of the more equation-rich chapters.
This book is far from a cheering section for multiverse
theories: both sides are presented and, in fact, the longest
chapter is that of Lee Smolin, which deems the anthropic
principle and anthropic arguments entirely nonscientific.
Many of these papers are available in preliminary form for free
on the arXiv preprint server; if
you can obtain a list of the chapter titles and authors from
the book, you can read most of the content for free. Renata
Kallosh's chapter contains an excellent example of why one
shouldn't blindly accept the recommendations of a spelling
checker. On p. 205, she writes “…the gaugino
condensate looks like a fractional instant on
effect…”—that's supposed to be “instanton”!
August 2007 