BRIAN GREENE
The Elegant Universe
Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory
Themes
The book is divided into three themes in the following parts:
- Part I: The Edge of Knowledge
- Part II: The Dilemma of Space, Time, and the Quanta
- Part III: The Cosmic Symphony
- Part IV: String Theory and the Fabric of Spacetime
- Part V: Unification in the Twenty-First Century
The
other is quantum mechanics, which provides a theoretical framework for
understanding the universe on the smallest of scales: molecules, atoms,
and all the way down to subatomic particles like electrons and quarks.
Through years of research, physicists have experimentally confirmed to
almost unimaginable accuracy virtually all predictions made by each of
these theories. But these same theoretical tools inexorably lead to
another disturbing conclusion: As they are currently formulated, general
relativity and quantum mechanics cannot both be right. The two
theories underlying the tremendous progress of physics during the last
hundred years — progress that has explained the expansion of the heavens
and the fundamental structure of matter — are mutually incompatible.
If
you have not heard previously about this ferocious antagonism you may
be wondering why. The answer is not hard to come by. In all but the most
extreme situations, physicists study things that are either small and
light (like atoms and their constituents) or things that are huge and
heavy (like stars and galaxies), but not both. This means that they need
use only quantum mechanics or only general relativity and can, with a
furtive glance, shrug off the barking admonition of the other. For fifty
years this approach has not been quite as blissful as ignorance, but it
has been pretty close.
But the universe can be
extreme. In the central depths of a black hole an enormous mass is
crushed to a minuscule size. At the moment of the big bang the whole of
the universe erupted from a microscopic nugget whose size makes a grain
of sand look colossal. These are realms that are tiny and yet incredibly
massive, therefore requiring that both quantum mechanics and general
relativity simultaneously be brought to bear.
For
reasons that will become increasingly clear as we proceed, the
equations of general relativity and quantum mechanics, when combined,
begin to shake, rattle, and gush with steam like a red-lined automobile.
Put less figuratively, well-posed physical questions elicit nonsensical
answers from the unhappy amalgam of these two theories.
Even
if you are willing to keep the deep interior of a black hole and the
beginning of the universe shrouded in mystery, you can't help feeling
that the hostility between quantum mechanics and general relativity
cries out for a deeper level of understanding. Can it really be that the
universe at its most fundamental level is divided, requiring one set of
laws when things are large and a different, incompatible set when
things are small?
Superstring
theory, a young upstart compared with the venerable edifices of quantum
mechanics and general relativity, answers with a resounding no. Intense
research over the past decade by physicists and mathematicians around
the world has revealed that this new approach to describing matter at
its most fundamental level resolves the tension between general
relativity and quantum mechanics. In fact, superstring theory shows
more: Within this new framework, general relativity and quantum
mechanics require one another for the theory to make sense.
According to superstring theory, the marriage of the laws of the large
and the small is not only happy but inevitable.
That's
part of the good news. But superstring theory — string theory, for
short — takes this union one giant step further. For three decades,
Einstein sought a unified theory of physics, one that would interweave
all of nature's forces and material constituents within a single
theoretical tapestry. He failed. Now, at the dawn of the new millennium,
proponents of string theory claim that the threads of this elusive
unified tapestry finally have been revealed. String theory has the
potential to show that all of the wondrous happenings in the universe —
from the frantic dance of subatomic quarks to the stately waltz of
orbiting binary stars, from the primordial fireball of the big bang to
the majestic swirl of heavenly galaxies — are reflections of one grand
physical principle, one master equation.
Because
these features of string theory require that we drastically change our
understanding of space, time, and matter, they will take some time to
get used to, to sink in at a comfortable level. But as shall become
clear, when seen in its proper context, string theory emerges as a
dramatic yet natural outgrowth of the revolutionary discoveries of
physics during the past hundred years. In fact, we shall see that the
conflict between general relativity and quantum mechanics is actually
not the first, but the third in a sequence of pivotal conflicts
encountered during the past century, each of whose resolution has
resulted in a stunning revision of our understanding of the universe.
The Three Conflicts
The
first conflict, recognized as far back as the late 1800s, concerns
puzzling properties of the motion of light. Briefly put, according to
Isaac Newton's laws of motion, if you run fast enough you can catch up
with a departing beam of light, whereas according to James Clerk
Maxwell's laws of electromagnetism, you can't. As we will discuss in
Chapter 2, Einstein resolved this conflict through his theory of special
relativity, and in so doing completely overturned our understanding of
space and time. According to special relativity, no longer can space and
time be thought of as universal concepts set in stone, experienced
identically by everyone. Rather, space and time emerged from Einstein's
reworking as malleable constructs whose form and appearance depend on
one's state of motion.
The
development of special relativity immediately set the stage for the
second conflict. One conclusion of Einstein's work is that no object —
in fact, no influence or disturbance of any sort — can travel faster
than the speed of light. But, as we shall discuss in Chapter 3, Newton's
experimentally successful and intuitively pleasing universal theory of
gravitation involves influences that are transmitted over vast distances
of space instantaneously. It was Einstein, again, who stepped in
and resolved the conflict by offering a new conception of gravity with
his 1915 general theory of relativity.
Just
as special relativity overturned previous conceptions of space and
time, so too did general relativity. Not only are space and time
influenced by one's state of motion, but they can warp and curve in
response to the presence of matter or energy. Such distortions to the
fabric of space and time, as we shall see, transmit the force of gravity
from one place to another. Space and time, therefore, can no longer to
be thought of as an inert backdrop on which the events of the universe
play themselves out; rather, through special and then general
relativity, they are intimate players in the events themselves.
Once
again the pattern repeated itself: The discovery of general relativity,
while resolving one conflict, led to another. Over the course of the
three decades beginning in 1900, physicists developed quantum mechanics
(discussed in Chapter 4) in response to a number of glaring problems
that arose when nineteenth-century conceptions of physics were applied
to the microscopic world.
And
as mentioned above, the third and deepest conflict arises from the
incompatibility between quantum mechanics and general relativity. As we
will see in Chapter 5, the gently curving geometrical form of space
emerging from general relativity is at loggerheads with the frantic,
roiling, microscopic behavior of the universe implied by quantum
mechanics. As it was not until the mid-1980s that string theory offered a
resolution, this conflict is rightly called the central problem of
modern physics.
Moreover,
building on special and general relativity, string theory requires its
own severe revamping of our conceptions of space and time. For example,
most of us take for granted that our universe has three spatial
dimensions. But this is not so according to string theory, which claims
that our universe has many more dimensions than meet the eye —
dimensions that are tightly curled into the folded fabric of the cosmos.
So central are these remarkable insights into the nature of space and
time that we shall use them as a guiding theme in all that follows.
String theory, in a real sense, is the story of space and time since
Einstein.
To
appreciate what string theory actually is, we need to take a step back
and briefly describe what we have learned during the last century about
the microscopic structure of the universe.
Sumber:
Prof. BRIAN GREENE
Sumber:
Prof. BRIAN GREENE
