For the last 20 years of his life, Albert Einstein was something
of an oddity
in the physics community, like a beloved eccentric uncle whose favorite
subject of conversation draws embarrassed looks around the table.
While quantum theory, the theory of the infinitesimally small, was being tested
with accuracy never attained before, he refused to accept that it
was the ultimate theory. For the last years of his life, he worked on a
way to reconcile his own theory of gravitation and the quantum description
of the world. He didn't succeed and died without seeing his dearest dream realized.
More than 40 years later, Einstein is almost vindicated: The long lasting
problem of incompatibility between general relativity and quantum mechanics
seems to be on its way to a resolution.
The solution may be difficult to grasp. If the handful of
physicists involved in what are called "superstring theories"
(or string, for short) are correct, we live in a world weirder than you
can probably imagine.
It's a world of 10 dimensions, with some curled up at a microscopic level
and some "big" dimensions that we perceive as "real."
A world where the distinction between space and time is spurious (as taught
by general relativity). A world where, in fact, the very notion of space
and time is bound to disappear. In the words of Brian Greene, a professor
at Columbia University and author of a book on the subject,"if string
theory is correct, the fabric of our universe has properties that would
have dazzled even Einstein."
In string theory, there are no elementary particles (like electrons
or quarks), but pieces of vibrating strings. Each vibration mode
corresponds to a different particle and determines its charge and its
mass. In the current understanding of the theory, those strings are
not "made of" anything: they are the fundamental constituent of
matter. The consequences of replacing point-like particles by
vibrating microscopic strings are enormous. The only consistent
framework to describe those strings implies a 10- or even conceivably an
11-dimension world in which 6 or 7 dimensions are curled up. Those extra
dimensions are the ones which determine the properties of the world we
live in. The larger dimensions are what we perceive as the ordinary
space and time.
Above is a closed string mode that is
characteristic of a spin-2 massless graviton (the particle
that mediates the force of gravity). This is one of the most attractive
features of string theory. It naturally and inevitably includes
gravity as one of the fundamental interactions. Image courtesy of
In superstring theory's 10-dimensional spacetime,
we still observe only a 4-dimensional spacetime. Somehow we need to link
the two if superstrings are to describe our universe. To do this we
curl up the extra 6 dimensions into a small compact space.
If the size of the compact space is of order the string
scale (10-33 cm) we wouldn't be able to detect the presence
extra dimensions directly - they're just too small. The end result is that
we get back to our familiar (3+1)-dimensional world, but there is a tiny
"ball" of 6-dimensional space associated with every point in our 4-dimensional
universe. This is shown in an extremely schematic way in the
illustration to the right.
Image courtesy of John Pierre
As a "unified" theory, string theory attempts to explain all
four forces observed in nature. And indeed, one of the solutions of
the string equations is a force that looks like gravity.
It is a testimony to the power and the beauty of string theory that
physicists would rather give up the very notion of space and time--
and admit a 10-dimension world--than question the path on which the
quest for a unified theory has led them.
String theory could successfully account for gravity and predict
super-symmetric particles. But until a couple of years ago it had
little connection with puzzles in physics. There were no results or
concrete predictions to show off. It could have been nothing more than
a beautiful mathematical construction.
Things changed in 1996. Andrew Strominger, then at the Institute for
Theoretical Physics in Santa Barbara, and Cumrun Vafa from
Harvard University, used string theory to "construct" a certain type of black
hole, much the same way one can "construct" a hydrogen atom by jotting down the equations, derived from quantum mechanics, that describe an electron bound to a proton.
Strominger and Vafa confirmed a result derived by Jacob Bekenstein
and Stephen Hawking back in the late 1970's. Bekenstein and Hawking
found that the amount of disorder (or "entropy") in a special kind
of black hole was very large. This was a surprising result, since no
one could understand (and nor did the computations give any insight)
how an object as simple as a black hole (which can be
characterized simply by its mass and its spin) could have such a large
amount of disorder within it.
As a result of building this special black hole using string
theory, Strominger and Vafa were able to obtain the correct value for
the disorder predicted by Bekenstein and Hawking. This result
electrified the physics community! For the first time, a result
derived with "classical physics" could be obtained from
string theory. Even though the black holes for which the result was
derived have very little in common with the black holes which are
believed to sit in the middle of galaxies, this new computation
illustrated the connection between strings and gravity. In addition,
the computation provides insight into the physical reasons for the answer.
No one knows yet if string theory is the ultimate theory--the
theory of everything, if there is such thing. But the theory's
incredible elegance and potential make it a strong front-runner to
further explain the inner workings of the universe well into the next
century. In the words of Edward Witten, a pioneer and one of its
leaders: "String theory is a part of twenty-first century physics
that fell by chance into the twentieth century."
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