universe supports life because of some rather remarkable coincidences.
If the values of the physical constants that govern the fundamental
forces and interactions in our universe were just a bit different, then life (or
at least, life as we know it) would be impossible.
I devoted a previous column (“The ‘Real World’ and The Standard
May-1996) to a discussion of some of the consequences of tinkering with some of
the physical constants, but let me give some further examples here.
gravity were a bit stronger, the universe would have long since collapsed to a
black hole. If gravity were a bit
weaker, galaxies would never have formed. If
either the strong or electromagnetic forces were a bit different in strength,
the neutron would be less massive than the proton, and the universe would be
filled with neutrons and neutron stars, with few atoms or nuclei.
If the 7.654 MeV energy level in carbon-12 was not precisely where it is,
the nuclei of carbon and the heavier elements could not have formed from helium
in burned-out stars and supernovas, and there would be no heavy elements to make
planets and people. And so on …
most recent realization that our universe is “special” comes from the
observation (see “Our Runaway Universe and Einstein's Cosmological
May-1999) that the rate of expansion of the universe is itself accelerating.
This implies that Einstein’s cosmological constant is not zero, in
other words, that there is a small but non-zero density of “dark” energy in
the vacuum itself, which creates a negative pressure driving the accelerating
expansion. Cosmologists are coming
to realize that the remarkably small but non-zero size of this vacuum energy is
another “accident” that makes life in our universe possible.
fair to ask (while difficult to answer) the question of whether the values of
the fundamental constants, including the cosmological constant, are just lucky
accidents, or whether there is some mechanism that arranged them to make life in
the universe possible. One
way providing an answer to this question is through the Anthropic Principle.
The Anthropic Principle asserts that since we, as living beings,
are present to measure the fundamental constants and ask where they come from,
they must be arranged to make living beings possible.
Otherwise, there would be nobody around to ask the question.
This is an answer of sorts, but
it is not a very satisfying one.
situation in cosmology is a bit like the theology of the middle ages, which
insisted that the Earth was the center of the universe because God made it that
way. Galileo got in a lot of trouble
for discovering the moons of Jupiter, a planetary system in miniature visible
with his newly invented telescope, and suggesting that by analogy the Earth
might be just a satellite orbiting the Sun, as Copernicus had previously
claimed, rather than the center of the universe.
As long as there was only one Planet Earth and we were living on it, it appeared that divine intervention was required to make things come out the way they are. However, we now know that there are a huge number of galaxies in our universe, a huge number of stars in each galaxy, and that most of those star probably have planets orbiting them. From such a cosmic perspective, the Earth is a much less special place, perhaps just one of a very large number of planets in the universe that can support life, perhaps including intelligent life. Earth has been demoted from the center of the universe to the sidelines.
Leonard Susskind of
deals with this problem by asserting that there must be “islands” of reduced
symmetry lying “off the coast” of the super-symmetric main body of string
theory. And he further asserts that
the number of such islands must be extremely large, numbering far more than the
number of electrons in our universe.
distinguishes these islands (which can be thought of as universes) from one
another is the value of the cosmological constant (which has an indirect effect
on some of the other fundamental constants) within them.
Susskind calls the ensemble of such string theory universes “the
landscape of string theory”. This
landscape is populated by a vast number of universes, each with a different
“ground state vacuum”, in other words, with a different cosmological
constant and amount of dark energy in a given volume of vacuum.
observed value of the cosmological constant constitutes a difficult puzzle.
The difficulty for both orthodox quantum chromodynamics (QCD) and the
more speculative string theory with the density of vacuum energy in our universe
is that is that it is very small, but not zero.
These theories were well prepared to explain either a zero vacuum energy
(e.g., with supersymmetry) or a very large vacuum energy (e.g., with standard
QCD). However, a small-but-not-zero
vacuum energy density is far more difficult, because it requires “fine
tuning” to get the vacuum energy “just right” to make a universe like
ours. This is sometimes called
“the Goldilocks Problem”. Up to
now, cosmological theorist has been unable to suggest a mechanism that could
solve the Goldilocks Problem. Susskind
is attempting to supply such a mechanism by invoking the Anthropic Principle, as
applied to universes.
he argues that of all the universes populating the string-theory landscape, at
least one must have a ground state vacuum in which has a “just-right” amount
of dark energy in the vacuum, small but not zero.
In that universe, life would be possible.
The vast majority of other universes would be lifeless.
Therefore, since we exist as living beings, we must reside in that
just-right universe. In other words,
the dark energy in the vacuum is small, not because some fine tuning set it that
way, but because our universe in one of the few populating the string theory
landscape in which in which it is low enough to make life possible.
the Susskind scenario, the Big Bang created a vast quantity of “bubble”
universes. The vacuum of each of
these universe was initially filled with dark energy, driving the superluminal
expansion we call inflation. At the
end of the inflation period, the vacuum energy density dropped until it reached
a bottom level, a vacuum energy ground state.
And because of the vast number of islands in the landscape of string
theory, there was enormous variation from universe to universe in the value of
the ground state energy. This, if
our universe has a low ground state energy, it should be no surprise, because some
universe should have such a situation.
Susskind’s scenario tells us, therefore, that most of these universes will be lifeless and inhospitable to visitors, perhaps empty of stars, perhaps empty of matter, perhaps the site of a black-hole collapse or a Big Rip super expansion.
are the science-fiction applications of Susskind’s ideas?
Obviously, the idea of a multitude of universes, parallel or otherwise,
has been written about many times. Steven
Baxter, in his novel Raft hypothesized
a universe in which the gravitational force was much stronger than in our
universe and in which castaway humans struggled for survival in an alien and
hostile universe, into which they has somehow been deposited.
own hard-SF novel Einstein’s Bridge is built on the assumption that there a
multiplicity of separated “bubble universes”, and that wormholes (in the
physics literature originally called “Einstein-Rosen Bridges”) could be use
to communicate and travel from one such universe to another.
In Susskind’s scenario, universes containing life as we know it would be few and far between. The average universe would have a large vacuum energy, which would cause it to have accelerated expansion to a state where it was essentially empty. Universe-hopping would be a lonely business, with a very low probability of finding anything interesting. Explorers would need some advance signal of the presence of intelligent life (see, for example, my novel Einstein’s Bridge) to make it worth expending the resources to make contact with another universe.
Susskind’s scenario viable? Time
will tell. It requires some advances
in string theory to verify his speculation of “islands” with varying
cosmological constants. From another
perspective, it may have a problem with Occam’s Razor, since it justifies the
state of our present observable universe by hypothesizing that it must have a
very large number of unobservable siblings.
It therefore suffers from the same malady as the rest of string theory,
in that untestable hypotheses, particularly those making spectacular claims, are
not subject to testing and improvement through the use of the scientific method.
in any case, it’s an interesting idea that may be worth considering by writers
of science fiction.
Anthropic Landscape of String Theory”, Leonard Susskind, (2003), preprint
Breaking in the Anthropic Landscape”, Leonard Susskind, (2004), preprint
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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