The fundamental particles of the universe that physicists have
identified—electrons, neutrinos, quarks, and so on—are the
“letters” of all
matter. Just like their linguistic counterparts, they appear to have no
further
internal substructure. String theory proclaims otherwise. According to
string
theory, if we could examine these particles with even greater
precision—a
precision many orders of magnitude beyond our present technological
capacity—we would find that each is not pointlike but instead
consists of a tiny,
one-dimensional loop. Like an infinitely thin rubber band, each particle
contains a vibrating, oscillating, dancing filament that physicists have named a
string.
In the figure at right, we illustrate this essential idea of string theory by
starting with an ordinary piece of matter, an apple, and repeatedly magnifying
its structure to reveal its ingredients on ever smaller scales. String theory
adds the new microscopic layer of a vibrating loop to the previously known
progression from atoms through protons, neutrons, electrons, and quarks.
Although it is by no means obvious, this simple replacement of point-particle
material constituents with strings resolves the incompatibility between quantum
mechanics and general relativity (which, as currently formulated, cannot
both be right). String theory thereby unravels the central Gordian knot of
contemporary theoretical physics. This is a tremendous achievement, but it is
only part of the reason string theory has generated such excitement.
Field of dreams
In Einstein’s day, the strong and weak forces had not yet been discovered, but
he found the existence of even two distinct forces—gravity and
electromagnetism—deeply troubling. Einstein did not accept that nature is
founded on such an extravagant design. This launched his 30-year voyage in
search of the so-called unified field theory that he hoped would show
that these two forces are really manifestations of one grand underlying
principle. This quixotic quest isolated Einstein from the mainstream of
physics, which, understandably, was far more excited about delving into the
newly emerging framework of quantum mechanics. He wrote to a friend in the
early 1940s, “I have become a lonely old chap who is mainly known because he
doesn’t wear socks and who is exhibited as a curiosity on special
occasions.”
Einstein was simply ahead of his time. More than half a century later, his
dream of a unified theory has become the Holy Grail of modern physics. And a
sizeable part of the physics and mathematics community is becoming increasingly
convinced that string theory may provide the answer. From one principle—that
everything at its most microscopic level consists of combinations of vibrating
strands—string theory provides a single explanatory framework capable of
encompassing all forces and all matter.
String theory proclaims, for instance, that the observed particle
properties—that is, the different masses and other properties of
both the
fundamental particles and the force particles associated with the four
forces
of nature (the strong and weak nuclear forces, electromagnetism, and
gravity)—are a reflection of the various ways in which a string
can vibrate.
Just as the strings on a violin or on a piano have resonant frequencies
at
which they prefer to vibrate—patterns that our ears sense as
various musical
notes and their higher harmonics—the same holds true for the
loops of string
theory. But rather than producing musical notes, each of the preferred
mass and
force charges are determined by the string’s oscillatory pattern. The
electron
is a string vibrating one way, the up-quark is a string vibrating
another way,
and so on.
Far from being a collection of chaotic experimental facts, particle properties
in string theory are the manifestation of one and the same physical feature:
the resonant patterns of vibration—the music, so to speak—of fundamental
loops of string. The same idea applies to the forces of nature as well. Force
particles are also associated with particular patterns of string vibration and
hence everything, all matter and all forces, is unified under the same rubric
of microscopic string oscillations—the “notes” that strings can play.
A theory to end theories
For the first time in the history of physics we therefore have a framework with
the capacity to explain every fundamental feature upon which the universe is
constructed. For this reason string theory is sometimes described as possibly
being the “theory of everything” (T.O.E.) or the “ultimate” or “final” theory.
These grandiose descriptive terms are meant to signify the deepest possible
theory of physics—a theory that underlies all others, one that does not
require or even allow for a deeper explanatory base.
In practice, many string theorists take a more down-to-earth approach and think
of a T.O.E. in the more limited sense of a theory that can explain the
properties of the fundamental particles and the properties of the forces by
which they interact and influence one another. A staunch reductionist would
claim that this is no limitation at all, and that in principle absolutely
everything, from the big bang to daydreams, can be described in terms of
underlying microscopic physical processes involving the fundamental
constituents of matter. If you understand everything about the ingredients, the
reductionist argues, you understand everything.
The reductionist philosophy easily ignites heated debate. Many find it
fatuous
and downright repugnant to claim that the wonders of life and the
universe are
mere reflections of microscopic particles engaged in a pointless dance
fully
choreographed by the laws of physics. Is it really the case that
feelings of
joy, sorrow, or boredom are nothing but chemical reactions in the
brain—reactions between molecules and atoms that, even more
microscopically, are
reactions between some of the fundamental particles, which are really
just
vibrating strings?
In response to this line of criticism, Nobel laureate Steven Weinberg
cautions in Dreams of a Final Theory:
At the other end of the spectrum are the opponents of reductionism who are
appalled by what they feel to be the bleakness of modern science. To whatever
extent they and their world can be reduced to a matter of particles or fields
and their interactions, they feel diminished by that knowledge….I would not
try to answer these critics with a pep talk about the beauties of modern
science. The reductionist worldview is chilling and impersonal. It has to be
accepted as it is, not because we like it, but because that is the way the
world works.
Some agree with this stark view, some don’t.
Others have tried to argue that developments such as chaos theory tell us that
new kinds of laws come into play when the level of complexity of a system
increases. Understanding the behavior of an electron or quark is one thing;
using this knowledge to understand the behavior of a tornado is quite another.
On this point, most agree. But opinions diverge on whether the diverse and
often unexpected phenomena that can occur in systems more complex than
individual particles truly represent new physical principles at work, or
whether the principles involved are derivative, relying, albeit in a terribly
complicated way, on the physical principles governing the enormously large
number of elementary constituents.
My own feeling is that they do not represent new and independent laws of
physics. Although it would be hard to explain the properties of a tornado in
terms of the physics of electrons and quarks, I see this as a matter of
calculational impasse, not an indicator of the need for new physical laws. But
again, there are some who disagree with this view.