Our 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 Model”, Analog, 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.
If 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 . . .
The most recent realization that our universe is “special” comes from the observation (see “Our Runaway Universe and Einstein’s Cosmological Constant”, Analog, 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.
It’s 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.
This 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.
Prof. Leonard Susskind of Stanford University has recently proposed a view that resembles this cosmic perspective, but as applied to universes rather than planets. This perspective comes from string theory, an area of theoretical physics that has for the past decade been the intellectual focus of major theoretical activity in the physics community. String theorists have been exploring a mathematics that describes fundamental particles (quarks, leptons . . .) as vibrational standing waves on hyperdimensional strings. The string theory variants that describe such a worldview, as they have developed, were found to be divisible into five different string theory classes. Later, new insights regrouped them into “M-theory,” within which the five separate string theories were special cases, special solutions of a master theory. However, there is one problem in comparing our universe with all of those “universes” described by string theory variants. All of the string-theory universes have a zero cosmological constant. The problem is that the master theory is too symmetricthere is a boson for every fermionand this symmetry sets the cosmological constant to zero.
Susskind 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.
What 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.
The 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 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 theorists have 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.
Basically, he argues that of all the universes populating the string-theory landscape, at least one must have a ground state vacuum that has a “just-right” amount of dark energy in the vacuumsmall, 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.
In the Susskind scenario, the Big Bang created a vast quantity of “bubble” universes. The vacuum of each of these universes 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. Thus, if our universe has 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.
What 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 have somehow been deposited.
My own novel Einstein’s Bridge is built on the assumption that there is 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, Einstein’s Bridge) to make it worth expending the resources to make contact with another universe.
Is 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.
But in any case, it’s an interesting idea that may be worth consideration by writers of science fiction.
AV Columns Online: Electronic reprints of over 120 “The Alternate View” columns by John G. Cramer, previously published in Analog, are available online at: http://www.npl.washington.edu/av. The preprints referenced below can be obtained at: http://www.arxiv.org.
“The Anthropic Landscape of String Theory”, Leonard Susskind, (2003), preprint hep-th/0302219;
“Supersymmetry Breaking in the Anthropic Landscape”, Leonard Susskind, (2004), preprint hep-th/0405189.