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This nOde last updated July 31st, 2007 and is
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A hypothetical particle consisting of a very short one-dimensional string existing in ten dimensions. It is the elementary particle in a theory of space-time incorporating supersymmetry.
[super(symmetry) + string.]
Kaku - _Visions_
However, the quantum theory stands in sharp contrast to Einstein's general relativity, which postulates an entirely different physical picture to explain the force of gravity. Imagine, for the moment, dropping a heavy shot put on a large bed spread. The shot put will, of course, sink deeply into the bed spread. Now imagine shooting a small marble across the bed. Since the bed is warped, the marble will execute a curved path. However, for a person viewing the marble from a great distance, it will appear that the shot put is exerting an invisible "force" on the marble, forcing it to move in a curved path. In other words, we can now replace the clumsy concept of a "force" with the more elegant bending of space itself. We now have an entirely new definition of a "force." It is nothing but the byproduct of the warping of space.
In the same way that a marble moves on a curved bed sheet, the earth moves around the sun in a curved path because space-time itself is curved. In this new picture, gravity is not a "force" but a byproduct of the warping of space-time. In some sense, gravity does not exist; what moves the planets and stars is the distortion of space and time.
However, the problem which has stubbornly resisted
solution for 50 years is that these two frameworks do not resemble each
other in any way. The quantum theory reduces "forces" to the exchange of
discrete packet of energy or quanta, while Einstein's theory of gravity,
by contrast, explains the cosmic forces holding the galaxies together by
postulating the smooth deformation of the fabric of space-time. This
is the root of the problem, that the quantum theory and general relativity
have two different physical pictures (packets of energy versus smooth space-time
continuums) and different mathematics to describe them. All attempts by
the greatest minds of the twentieth century at merging the quantum theory
with the theory of gravity have failed. Unquestionably, the greatest problem
of the century facing physicists today is the unification of these two
physical frameworks into one theory.
This sad state of affairs can be compared to Mother Nature having two hands, neither of which communicate with the other. Nothing could be more awkward or pathetic than to see someone whose left hand acted in total ignorance of the right hand.
Today, however, many physicists think that we have finally solved this long-standing problem. This theory, which is certainly "crazy enough" to be correct, has astounded the world's physics community. But it has also raised a storm of controversy, with Nobel Prize winners adamantly sitting on opposite sides of the fence.
This is the superstring theory, which postulates that all matter and energy can be reduced to tiny strings of energy vibrating in a 10 dimensional universe. Edward Witten of the Institute for Advanced Study at Princeton, who some claim is the successor to Einstein, has said that superstring theory will dominate the world of physics for the next 50 years, in the same way that the quantum theory has dominated physics for the last 50 years.
As Einstein once said, all great physical theories can be represented by simple pictures. Similarly, superstring theory can be explained visually. Imagine a violin string, for example. Everyone knows that the notes A,B,C, etc. played on a violin string are not fundamental.
The note A is no more fundamental than the note B. What is fundamental, of course, is the violin string itself. By studying the vibrations or harmonics that can exist on a violin string, one can calculate the infinite number of possible frequencies that can exist.
Similarly, the superstring can also vibrate in different frequencies.
Each frequency, in turn,
corresponds to a sub-atomic particle, or a "quanta." This explains why
there appear to be an infinite
number of particles. According to this theory, our bodies, which are made
of sub-atomic particles, can be described by the resonances of trillions
upon trillions of tiny strings.
In summary, the "notes" of the superstring are the subatomic particles, the "harmonies" of the superstring are the laws of physics, and the "universe" can be compared to a symphony of vibrating superstrings.
As the string vibrates, however, it causes the surrounding space-time continuum to warp around it. Miraculously enough, a detailed calculation shows that the superstring forces the space-time continuum to be distorted exactly as Einstein originally predicted.
Thus, we now have a harmonious description which merges the theory of quanta with the theory of space-time continuum. 10 Dimensional Hyperspace. The superstring theory represents perhaps the most radical departure from ordinary physics in decades. But its most controversial prediction is that the universe originally began in 10 dimensions. To its supporters, the prediction of a 10 dimensional universe has been a conceptual tour de force, introducing a startling, breath-taking mathematics into the world of physics.
To the critics, however, the introduction of 10 dimensional hyperspace borders on science fiction. To understand these higher dimensions, we remember that it takes three numbers to locate every object in the universe, from the tip of your nose to the ends of the universe.
For example, if you want to meet some friends for lunch in Manhattan, you say that you will meet them at the building at the corner of 42nd and 5th Ave, on the 37th floor. It takes two numbers to locate your position on a map, and one number to specify the distance above the map. It thus takes three numbers to specify the location of your lunch. However, the existence of the fourth spatial dimension has been a lively area of debate since the time of the Greeks, who dismissed the possibility of a fourth dimension. Ptolemy, in fact, even gave a "proof" that higher dimensions could not exist. Ptolemy reasoned that only three straight lines can be drawn which are mutually perpendicular to each other (for example, the three perpendicular lines making up a corner of a room.) Since a fourth straight line cannot be drawn which is mutually perpendicular to the other three axes, Ergo!, the fourth dimension cannot exist.
What Ptolemy actually proved was that it is impossible for us humans to visualize the fourth dimension. Although computers routinely manipulate equations in N-dimensional space, we humans are incapable of visualizing spatial dimensions beyond three. The reason for this unfortunate accident has to do with biology, rather than physics. Human evolution put a premium on being able to visualize objects moving in three dimensions. There was a selection pressure placed on humans who could dodge lunging saber tooth tigers or hurl a spear at a charging mammoth. Since tigers do not attack us in the fourth dimension, there simply was no advantage in developing a brain with the ability to visualize objects moving in four dimensions.
From a mathematical point of view, however, adding higher dimensions is a distinct advantage: it allows us to describe more and more forces. There is more "room" in higher dimensions to insert the electromagnetic force into the gravitational force. (In this picture, light becomes a vibration in the fourth dimension.) In other words, adding more dimensions to a theory always allows us to unify more laws of physics.
A simple analogy may help. The ancients were once puzzled by the weather. Why does it get colder as we go north? Why do the winds blow to the West? What is the origin of the seasons? To the ancients, these were mysteries that could not be solved. From their limited perspective, the ancients could never find the solution to these mysteries. The key to these puzzles, of course, is to leap into the third dimension, to go up into outer space, to see that the earth is actually a sphere rotating around a tilted axis. In one stroke, these mysteries of the weather become transparent. The seasons, the winds, the temperature patterns, etc. all become obvious once we leap into the third dimension.
Likewise, the superstring is able to accommodate a large number of forces because it has more "room" in its equations to do so.
To understand the intense controversy surrounding superstring theory, think of the following parable.
Imagine that, at the beginning of time, there was once a beautiful, glittering gemstone. Its perfect symmetries and harmonies were a sight to behold. However, it possessed a tiny flaw and became unstable, eventually exploding into thousands of tiny pieces.
Imagine that the fragments of the gemstone rained
down on a flat, two-dimensional world, called Flatland,
where there lived a mythical race of beings called Flatlanders.
These Flatlanders were intrigued by the beauty of the fragments, which
could be found scattered all over Flatland. The scientists of Flatland
postulated that these fragments must have come from a crystal of unimaginable
beauty that shattered in a titanic Big Bang. They then decided to embark
upon a noble quest, to reassemble all these pieces of the gemstone.
After 2,000 years of labor by the finest minds of Flatland, they were finally
able to fit many, but certainly not all, of the fragments together into
two chunks. The first chunk was called the "quantum,"
and the second chunk was called "relativity."
Although they Flatlanders were rightfully proud of their progress, they were dismayed to find that these two chunks did not fit together. For half a century, the Flatlanders maneuvered these two chunks in all possible ways, and they still did not fit. Finally, some of the younger, more rebellious scientists suggested a heretical solution: perhaps these two chunks could fit together if they were moved in the third dimension.
This immediately set off the greatest scientific controversy in years. The older scientists scoffed at this idea, because they didn't believe in the unseen third dimension. "What you can't measure doesn't exist," they declared.
Furthermore, even if the third dimension existed, one could calculate that the energy necessary to move the pieces up off Flatland would exceed all the energy available in Flatland. Thus, it was an untestable theory, the critics shouted. However, the younger scientists were undaunted. Using pure mathematics, they could show that these two chunks fit together if they were rotated and moved in the third dimension. The younger scientists claimed that the problem was therefore theoretical, rather than experimental. If one could completely solve the equations of the third dimension, then one could, in principle, fit these two chunks completely together and resolve the problem once and for all.
We Are Not Smart Enough
That is also the conclusion of today's superstring enthusiasts, that the fundamental problem is theoretical, not practical. The true problem is to solve the theory completely, and then compare it with present-day experimental data. The problem, therefore, is not in building gigantic atom smashers; the problem is being clever enough to solve the theory.
Edward Witten, impressed by the vast new areas of mathematics opened up by the superstring theory, has said that the superstring theory represents "21th century physics that fell accidentally into the 20th century." This is because the superstring theory was discovered almost by accident. By the normal progression of science, we theoretical physicists might not have discovered the theory for another century.
The superstring theory may very well be 21st century physics, but the bottleneck has been that 21st century mathematics has not yet been discovered. In other words, although the string equations are perfectly well-defined, no one is smart enough to solve them. This situation is not entirely new to the history of physics.
When Newton first discovered the universal law of gravitation at the age of 23, he was unable to solve his equation because the mathematics of the 17th century was too primitive. He then labored over the next 20 years to develop a new mathematical formalism (calculus) which was powerful enough to solve his universal law of gravitation. Similarly, the fundamental problem facing the superstring theory is theoretical. If we could only sharpen our analytical skills and develop more powerful mathematical tools, like Newton before us, perhaps we could solve the theory and end the controversy. Ironically, the superstring equations stand before us in perfectly well-defined form, yet we are too primitive to understand why they work so well and too dim witted to solve them. The search for the theory of the universe is perhaps finally entering its last phase, awaiting the birth of a new mathematics powerful enough to solve it.
Imagine a child gazing at a TV set. The images and stories conveyed on the screen are easily understood by the child, yet the electronic wizardry inside the TV set is beyond the child's ken. We physicists are like this child, gazing in wonder at the mathematical sophistication and elegance of the superstring equations and awed by its power. However, like this child, we do not understand why the superstring theory works.