In March, Stephen Hawking, the Cambridge University scientist and author of "A Brief History of Time," gave 50-50 odds that researchers would develop in the next 20 years a successful "theory of everything." In his recent book "The Elegant Universe," a Columbia physicist in his 30s named Brian Greene offers us one of the first intelligible glimpses of this ultimate-theory-in-progress.
Many physicists charge that TOEs, as they are sometimes abbreviated, are mathematical abstractions without any basis in physical reality. But Greene demonstrates that the TOE represents one of the most promising pursuits in science today.
As it stands now, each major physics theory describes only part of the universe. For example, Einstein's General Theory of Relativity describes gravity, the force that holds galaxies together and makes apples fall. Quantum theory predicts the behavior of very small particles such as electrons. Electrons, in turn, are mainly governed by electromagnetism the force that holds magnets to your refrigerator and causes lightning bolts to strike the earth.
As Greene points out in "The Elegant Universe," these theories clash when applied to the same phenomena. For instance, when equations from quantum theory and general relativity are combined to describe the behavior of a black hole, they provide answers that turn out to be meaningless. Greene thinks this means one or both theories must have something wrong.
So Greene and other physicists are devoting their careers to building a single theory that can accommodate all forces and particles and anticipate their behavior together. They want a single theory that not only predicts all the possible particles but correctly describes the four different forces -- gravity, electromagnetism, and their two lesser-known siblings: the weak force, best known for causing radioactive atoms to decay, and the strong force, which binds the cores of atoms.
Although several ideas for the TOE exist, few have aroused the passion of physicists the way superstring theory has. Greene explains the theory in remarkably simple language. Granted, it's sometimes difficult to endure his brimming confidence that physics may soon give us the deepest possible description of nature. Yet to his credit, Greene persuasively argues that superstring theory is the most revolutionary idea in physics since Einstein's time.
First, superstring theory refutes the notion that electrons and nature's other fundamental particles are pointlike objects. Instead, it says they are tiny stringlike loops or line segments. Just as a guitar string produces notes of a musical scale, the vibration patterns in a superstring produce particles such as electrons and quarks. One vibration pattern in strings leads to the graviton -- the long-sought but as-yet experimentally undetected particle believed to transmit the force of gravity.
Second, superstring theory says that the universe cannot be squeezed to smaller than the size of a string. This notion resolves certain theoretical troubles with the Big Bang theory. Previously, physicists believed that the universe began with a size of zero while still containing all the matter and energy it has today. Greene points out that setting a minimum size solves the physical, mathematical and logical problems posed by an infinite density of matter and allows the Big Bang to make a little more sense.
But string theory introduces some very bizarre ideas of its own. It says that the universe must have 10 dimensions of space and time, instead of the three dimensions of space and the one dimension of time that we know. It's nearly impossible to visualize, but Greene offers an analogy: It's like walking on a cosmic garden hose that has infinite length but is so thin that it looks like it has no width. On such a hose, you would be able to tell easily when you moved forward or backward, but it would be harder to determine if you moved clockwise or counterclockwise because the dimension is so small.
Superstringers also contend that not only does our universe have the three "extended" spatial dimensions we are familiar with, but other "compactified" dimensions too small for us to see or notice. We can't visit them, since we're already there; nothing from them can bother us, because only certain subatomic objects are small enough to fit in them. But physicists have recently proposed tests for searching for evidence of these extra dimensions, using next-generation particle smashers. So by 2010 or thereabouts, don't be surprised if you pick up a newspaper and read that extra dimensions have been discovered.
Strings are so tiny -- about 10 billionths of a trillionth of a quadrillionth of a meter -- that no equipment can discern them with any detail. According to Greene, some physicists believe that "we are ready to tackle questions that are beyond our present technological ability to test directly." Indeed, perhaps the most revolutionary idea of string theory is that it defies the scientific tradition of developing a hypothesis to be tested by experiment.
Lacking experimental verification, physicists seeking the TOE seem to be guided by aesthetic beauty. Just as the title of Greene's book suggests, the core of this epic search is about scientific "elegance," that poetic notion that our physical world is ruled by an underlying and fundamentally simple order. Physicists feel they're on the right track when they find a mathematical description that is both simple and universal, like Isaac Newton discovering a single law to explain both the falling apple and the Earth's orbit.
Other physicists question whether string theory has a basis in the physical world or is just mathematical abstraction. As science writer John Gribbin shows in "The Search for Superstrings, Symmetry, and the Theory of Everything," many great physics theories seemed crazy and untestable at first. For example, the idea that each type of particle has an antimatter counterpart arose out of mathematical equations as an apparently meaningless result. Antimatter particles first seemed to have no basis in reality, but they were subsequently detected in cosmic rays.
Ironically, the search for elegance reveals how messy the scientific process is. Even with mathematical beauty as a guide, the road to a TOE will contain some surprises. One of those surprises comes in the last 100 pages of Greene's book: Superstring theory doesn't really exist anymore.
In the mid-1990s, there were five different versions of superstring theory, each following an acceptable mathematical logic. The fact that they were all plausible but mutually incompatible ruled them all out in the same way that five equally plausible explanations to Amelia Earhart's disappearance would cast doubt upon each one.
But Edward Witten, a physicist at the Institute of Advanced Study in Princeton, showed that all five versions of the theory were different aspects of yet a larger theory termed M-theory. M-theory adds new building blocks in the form of thin sheets called "membranes." It calls for an extra dimension in which these membranes can roll up and look like strings. It's the "Theory Formerly Known as Strings," and it looks significantly more powerful.
What will happen when physicists announce a theory of everything? As Greene says, "Almost everyone agrees that finding the TOE would in no way mean that psychology, biology, geology, chemistry or even physics had been solved or in some sense subsumed." What it will mean is that physicists will have discovered the basic grammatical rules for the language of forces along with the full alphabet of particles. Just as complete knowledge of the English language does not prevent another great novel from being written, a theory of everything will only open us to more of the universe's possibilities.