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corgalius>> ideas>> the one

>>general relativity
>>across the universe
>>a theory of everyting?

"A human being is a part of the whole of what we call the "universe". A part limited in time and space. We experience thoughts; our feelings and ourselves as something separate from the rest, a kind of optical delusion of our consciousness. This delusion is really a prison for us, restricting us to our personal desires and to the affection of a few persons nearest to us. Our task must be to free ourselves from this prison by widening our circle of understanding and compassion to embrace all living creatures in the whole of nature and it's beauty."

-Albert Einstein

"The biggest cause of trouble in the world today is that the stupid people are so sure about things and the intelligent folks are so full of doubts. Fear is the main source of superstition, and one of the main sources of cruelty. To conquer fear is the beginning of wisdom."

- Bertrand Russell

>>general relativity

Introduced by Albert Einstein in 1915, this theory was developed to generalize the theory of special relativity . Contrary to the stories told by many high school physics teachers, special relativity does cover accelerating objects, but the math is difficult. Where general relativity is needed is in the presence of heavy objects or large amounts of energy.

The physics of general relativity is very simple:

MASS AND ENERGY CURVE SPACE AND TIME

and Einstein gave the equation which actually determines the relationship.

The rest of general relativity is based on the mathematics of curved surfaces. Tests of General Relativity The most exciting test of general relativity is the PERIHELION OF MERCURY. Since the middle ages, it has been observed that the planet mercury travels along an ellipse (a squashed circle), and that the orbit shifts a little each year. The observed movement was a rotation of about 42 degrees per year. According to general relativity, the shift is 43 degrees per year, which convinced the world the general relativity was a correct theory.
Another test is the GRAVITATIONAL REDSHIFT. According to general relativity, light will change color as it gets closer to the surface of the Earth. When researchers carefully measured light at the top and bottom of a tall building, they found exactly the same result! Unfortunately it was later shown that the same effect occurs in quantum mechanics without using general relativity.
The third classical test of general relativity is called the DEFLECTION OF LIGHT. According to general relativity, the sun should make other stars appear to move (this is only an illusion, the stars don't actually move). Of course you can't see stars when the sun is in the sky, so this is hard to measure. But a group of researchers waited for a total solar eclipse, and then measured the effect and found that it agrees with general relativity. However, in 1960 it was shown that the exact same effect can be predicted using special relativity, and so it wasn't a good test of general relativity.
The fourth test, performed in 1964, was the Shapiro radar bounce experiment. A beam of light was sent to a distant planet and then it returned to Earth. The light was deflected as predicted by general relativity, and so far no other explanation has been given for these results.
The fifth test is still being planned. According to general relativity, a spinning top which orbits Earth should wobble a very small amount. The group working on the project have developed extremely precise instruments to measure the tiny effect, but haven't tested it in orbit.

>>across the universe

At a particular instant roughly 12 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a dime, began to expand and cool at an incredibly rapid rate. By the time the temperature had dropped to 100 million times that of the sun's core, the forces of nature assumed their present properties, and the elementary particles known as quarks roamed freely in a sea of energy. When the universe had expanded an additional 1,000 times, all the matter we can measure filled a region the size of the solar system.

At that time, the free quarks became confined in neutrons and protons. After the universe had grown by another factor of 1,000, protons and neutrons combined to form atomic nuclei, including most of the helium and deuterium present today. All of this occurred within the first minute of the expansion. Conditions were still too hot, however, for atomic nuclei to capture electrons. Neutral atoms appeared in abundance only after the expansion had continued for 300,000 years and the universe was 1,000 times smaller than it is now. The neutral atoms then began to coalesce into gas clouds, which later evolved into stars. By the time the universe had expanded to one fifth its present size, the stars had formed groups recognizable as young galaxies. When the universe was half its present size, nuclear reactions in stars had produced most of the heavy elements from which terrestrial planets were made. Our solar system is relatively young: it formed five billion years ago, when the universe was two thirds its present size. Over time the formation of stars has consumed the supply of gas in galaxies, and hence the population of stars is waning. Fifteen billion years from now stars like our sun will be relatively rare, making the universe a far less hospitable place for observers like us.

Our understanding of the genesis and evolution of the universe is one of the great achievements of 20th-century science. This knowledge comes from decades of innovative experiments and theories. Modern telescopes on the ground and in space detect the light from galaxies billions of light-years away, showing us what the universe looked like when it was young. Particle accelerators probe the basic physics of the high-energy environment of the early universe. Satellites detect the cosmic background radiation left over from the early stages of expansion, providing an image of the universe on the largest scales we can observe.

Our best efforts to explain this wealth of data are embodied in a theory known as the standard cosmological model or the big bang cosmology. The major claim of the theory is that in the large-scale average, the universe is expanding in a nearly homogeneous way from a dense early state. At present, there are no fundamental challenges to the big bang theory, although there are certainly unresolved issues within the theory itself. Astronomers are not sure, for example, how the galaxies were formed, but there is no reason to think the process did not occur within the framework of the big bang. Indeed, the predictions of the theory have survived all tests to date.

Yet the big bang model goes only so far, and many fundamental mysteries remain. What was the universe like before it was expanding? (No observation we have made allows us to look back beyond the moment at which the expansion began.) What will happen in the distant future, when the last of the stars exhaust the supply of nuclear fuel? No one knows the answers yet.

Our universe may be viewed in many lights--by mystics, theologians, philosophers or scientists. In science we adopt the plodding route: we accept only what is tested by experiment or observation. Albert Einstein gave us the now well-tested and accepted general theory of relativity, which establishes the relations between mass, energy, space and time. Einstein showed that a homogeneous distribution of matter in space fits nicely with his theory. He assumed without discussion that the universe is static, unchanging in the large-scale average [see "How Cosmology Became a Science," by Stephen G. Brush; Scientific American, August 1992].

In 1922 the Russian theorist Alexander A. Friedmann realized that Einstein's universe is unstable; the slightest perturbation would cause it to expand or contract. At that time, Vesto M. Slipher of Lowell Observatory was collecting the first evidence that galaxies are actually moving apart. Then, in 1929, the eminent astronomer Edwin P. Hubble showed that the rate a galaxy is moving away from us is roughly proportional to its distance from us.

The existence of an expanding universe implies that the cosmos has evolved from a dense concentration of matter into the present broadly spread distribution of galaxies. Fred Hoyle, an English cosmologist, was the first to call this process the big bang. Hoyle intended to disparage the theory, but the name was so catchy it gained popularity. It is somewhat misleading, however, to describe the expansion as some type of explosion of matter away from some particular point in space.

That is not the picture at all: in Einstein's universe the concept of space and the distribution of matter are intimately linked; the observed expansion of the system of galaxies reveals the unfolding of space itself. An essential feature of the theory is that the average density in space declines as the universe expands; the distribution of matter forms no observable edge. In an explosion the fastest particles move out into empty space, but in the big bang cosmology, particles uniformly fill all space. The expansion of the universe has had little influence on the size of galaxies or even clusters of galaxies that are bound by gravity; space is simply opening up between them. In this sense, the expansion is similar to a rising loaf of raisin bread. The dough is analogous to space, and the raisins, to clusters of galaxies. As the dough expands, the raisins move apart. Moreover, the speed with which any two raisins move apart is directly and positively related to the amount of dough separating them.

The evidence for the expansion of the universe has been accumulating for some 60 years. The first important clue is the redshift. A galaxy emits or absorbs some wavelengths of light more strongly than others. If the galaxy is moving away from us, these emission and absorption features are shifted to longer wavelengths--that is, they become redder as the recession velocity increases.

Hubble's Law

Hubble's measurements indicated that the redshift of a distant galaxy is greater than that of one closer to Earth. This relation, now known as Hubble's law, is just what one would expect in a uniformly expanding universe. Hubble's law says the recession velocity of a galaxy is equal to its distance multiplied by a quantity called Hubble's constant. The redshift effect in nearby galaxies is relatively subtle, requiring good instrumentation to detect it. In contrast, the redshift of very distant objects--radio galaxies and quasars--is an awesome phenomenon; some appear to be moving away at greater than 90 percent of the speed of light.

Hubble contributed to another crucial part of the picture. He counted the number of visible galaxies in different directions in the sky and found that they appear to be rather uniformly distributed. The value of Hubble's constant seemed to be the same in all directions, a necessary consequence of uniform expansion. Modern surveys confirm the fundamental tenet that the universe is homogeneous on large scales. Although maps of the distribution of the nearby galaxies display clumpiness, deeper surveys reveal considerable uniformity.

The Milky Way, for instance, resides in a knot of two dozen galaxies; these in turn are part of a complex of galaxies that protrudes from the so-called local supercluster. The hierarchy of clustering has been traced up to dimensions of about 500 million light-years. The fluctuations in the average density of matter diminish as the scale of the structure being investigated increases. In maps that cover distances that reach close to the observable limit, the average density of matter changes by less than a tenth of a percent.

To test Hubble's law, astronomers need to measure distances to galaxies. One method for gauging distance is to observe the apparent brightness of a galaxy. If one galaxy is four times fainter than an otherwise comparable galaxy, then it can be estimated to be twice as far away. This expectation has now been tested over the whole of the visible range of distances.

Some critics of the theory have pointed out that a galaxy that appears to be smaller and fainter might not actually be more distant. Fortunately, there is a direct indication that objects whose redshifts are larger really are more distant. The evidence comes from observations of an effect known as gravitational lensing [see illustration on opposite page]. An object as massive and compact as a galaxy can act as a crude lens, producing a distorted, magnified image (or even many images) of any background radiation source that lies behind it. Such an object does so by bending the paths of light rays and other electromagnetic radiation. So if a galaxy sits in the line of sight between Earth and some distant object, it will bend the light rays from the object so that they are observable [see "Gravitational Lenses," by Edwin L. Turner; Scientific American, July 1988]. During the past decade, astronomers have discovered about two dozen gravitational lenses. The object behind the lens is always found to have a higher redshift than the lens itself, confirming the qualitative prediction of Hubble's law.

Hubble's law has great significance not only because it describes the expansion of the universe but also because it can be used to calculate the age of the cosmos. To be precise, the time elapsed since the big bang is a function of the present value of Hubble's constant and its rate of change. Astronomers have determined the approximate rate of the expansion, but no one has yet been able to measure the second value precisely.

Still, one can estimate this quantity from knowledge of the universe's average density. One expects that because gravity exerts a force that opposes expansion, galaxies would tend to move apart more slowly now than they did in the past. The rate of change in expansion is thus related to the gravitational pull of the universe set by its average density. If the density is that of just the visible material in and around galaxies, the age of the universe probably lies between 10 and 15 billion years. (The range allows for the uncertainty in the rate of expansion.)

Yet many researchers believe the density is greater than this minimum value. So-called dark matter would make up the difference. A strongly defended argument holds that the universe is just dense enough that in the remote future the expansion will slow almost to zero. Under this assumption, the age of the universe decreases to the range of seven to 13 billion years.

To improve these estimates, many astronomers are involved in intensive research to measure both the distances to galaxies and the density of the universe. Estimates of the expansion time provide an important test for the big bang model of the universe. If the theory is correct, everything in the visible universe should be younger than the expansion time computed from Hubble's law.

These two timescales do appear to be in at least rough concordance. For example, the oldest stars in the disk of the Milky Way galaxy are about nine billion years old--an estimate derived from the rate of cooling of white dwarf stars. The stars in the halo of the Milky Way are somewhat older, about 12 billion years--a value derived from the rate of nuclear fuel consumption in the cores of these stars. The ages of the oldest known chemical elements are also approximately 12 billion years--a number that comes from radioactive dating techniques. Workers in laboratories have derived these age estimates from atomic and nuclear physics. It is noteworthy that their results agree, at least approximately, with the age that astronomers have derived by measuring cosmic expansion.

Another theory, the steady-state theory, also succeeds in accounting for the expansion and homogeneity of the universe. In 1946 three physicists in England--Hoyle, Hermann Bondi and Thomas Gold--proposed such a cosmology. In their theory the universe is forever expanding, and matter is created spontaneously to fill the voids. As this material accumulates, they suggested, it forms new stars to replace the old. This steady-state hypothesis predicts that ensembles of galaxies close to us should look statistically the same as those far away. The big bang cosmology makes a different prediction: if galaxies were all formed long ago, distant galaxies should look younger than those nearby because light from them requires a longer time to reach us. Such galaxies should contain more short-lived stars and more gas out of which future generations of stars will form.

Testing the Steady-State Hypothesis

The test is simple conceptually, but it took decades for astronomers to develop detectors sensitive enough to study distant galaxies in detail. When astronomers examine nearby galaxies that are powerful emitters of radio wavelengths, they see, at optical wavelengths, relatively round systems of stars. Distant radio galaxies, on the other hand, appear to have elongated and sometimes irregular structures. Moreover, in most distant radio galaxies, unlike the ones nearby, the distribution of light tends to be aligned with the pattern of the radio emission.

Likewise, when astronomers study the population of massive, dense clusters of galaxies, they find differences between those that are close and those far away. Distant clusters contain bluish galaxies that show evidence of ongoing star formation. Similar clusters that are nearby contain reddish galaxies in which active star formation ceased long ago. Observations made with the Hubble Space Telescope confirm that at least some of the enhanced star formation in these younger clusters may be the result of collisions between their member galaxies, a process that is much rarer in the present epoch.

So if galaxies are all moving away from one another and are evolving from earlier forms, it seems logical that they were once crowded together in some dense sea of matter and energy. Indeed, in 1927, before much was known about distant galaxies, a Belgian cosmologist and priest, Georges Lema”tre, proposed that the expansion of the universe might be traced to an exceedingly dense state he called the primeval "super-atom." It might even be possible, he thought, to detect remnant radiation from the primeval atom. But what would this radiation signature look like?

When the universe was very young and hot, radiation could not travel very far without being absorbed and emitted by some particle. This continuous exchange of energy maintained a state of thermal equilibrium; any particular region was unlikely to be much hotter or cooler than the average. When matter and energy settle to such a state, the result is a so-called thermal spectrum, where the intensity of radiation at each wavelength is a definite function of the temperature. Hence, radiation originating in the hot big bang is recognizable by its spectrum.

In fact, this thermal cosmic background radiation has been detected. While working on the development of radar in the 1940s, Robert H. Dicke, then at the Massachusetts Institute of Technology, invented the microwave radiometer--a device capable of detecting low levels of radiation. In the 1960s Bell Laboratories used a radiometer in a telescope that would track the early communications satellites Echo-1 and Telstar. The engineer who built this instrument found that it was detecting unexpected radiation. Arno A. Penzias and Robert W. Wilson identified the signal as the cosmic background radiation. It is interesting that Penzias and Wilson were led to this idea by the news that Dicke had suggested that one ought to use a radiometer to search for the cosmic background.

Astronomers have studied this radiation in great detail using the Cosmic Background Explorer (COBE) satellite and a number of rocket-launched, balloon-borne and ground-based experiments. The cosmic background radiation has two distinctive properties. First, it is nearly the same in all directions. (As the COBE team, led by John Mather of the National Aeronautics and Space Administration Goddard Space Flight Center, showed in 1992, the variation is just one part per 100,000.) The interpretation is that the radiation uniformly fills space, as predicted in the big bang cosmology. Second, the spectrum is very close to that of an object in thermal equilibrium at 2.726 kelvins above absolute zero. To be sure, the cosmic background radiation was produced when the universe was far hotter than 2.726 kelvins, yet researchers anticipated correctly that the apparent temperature of the radiation would be low. In the 1930s Richard C. Tolman of the California Institute of Technology showed that the temperature of the cosmic background would diminish because of the universe's expansion.

The cosmic background radiation provides direct evidence that the universe did expand from a dense, hot state, for this is the condition needed to produce the radiation. In the dense, hot early universe thermonuclear reactions produced elements heavier than hydrogen, including deuterium, helium and lithium. It is striking that the computed mix of the light elements agrees with the observed abundances. That is, all evidence indicates that the light elements were produced in the hot young universe, whereas the heavier elements appeared later, as products of the thermonuclear reactions that power stars.

The theory for the origin of the light elements emerged from the burst of research that followed the end of World War II. George Gamow and graduate student Ralph A. Alpher of George Washington University and Robert Herman of the Johns Hopkins University Applied Physics Laboratory and others used nuclear physics data from the war effort to predict what kind of nuclear processes might have occurred in the early universe and what elements might have been produced. Alpher and Herman also realized that a remnant of the original expansion would still be detectable in the existing universe.

Despite the fact that significant details of this pioneering work were in error, it forged a link between nuclear physics and cosmology. The workers demonstrated that the early universe could be viewed as a type of thermonuclear reactor. As a result, physicists have now precisely calculated the abundances of light elements produced in the big bang and how those quantities have changed because of subsequent events in the interstellar medium and nuclear processes in stars.

Putting the Puzzle Together

Our grasp of the conditions that prevailed in the early universe does not translate into a full understanding of how galaxies formed. Nevertheless, we do have quite a few pieces of the puzzle. Gravity causes the growth of density fluctuations in the distribution of matter, because it more strongly slows the expansion of denser regions, making them grow still denser. This process is observed in the growth of nearby clusters of galaxies, and the galaxies themselves were probably assembled by the same process on a smaller scale.

The growth of structure in the early universe was prevented by radiation pressure, but that changed when the universe had expanded to about 0.1 percent of its present size. At that point, the temperature was about 3,000 kelvins, cool enough to allow the ions and electrons to combine to form neutral hydrogen and helium. The neutral matter was able to slip through the radiation and to form gas clouds that could collapse into star clusters. Observations show that by the time the universe was one fifth its present size, matter had gathered into gas clouds large enough to be called young galaxies.

A pressing challenge now is to reconcile the apparent uniformity of the early universe with the lumpy distribution of galaxies in the present universe. Astronomers know that the density of the early universe did not vary by much, because they observe only slight irregularities in the cosmic background radiation. So far it has been easy to develop theories that are consistent with the available measurements, but more critical tests are in progress. In particular, different theories for galaxy formation predict quite different fluctuations in the cosmic background radiation on angular scales less than about one degree. Measurements of such tiny fluctuations have not yet been done, but they might be accomplished in the generation of experiments now under way. It will be exciting to learn whether any of the theories of galaxy formation now under consideration survive these tests.

The present-day universe has provided ample opportunity for the development of life as we know it--there are some 100 billion billion stars similar to the sun in the part of the universe we can observe. The big bang cosmology implies, however, that life is possible only for a bounded span of time: the universe was too hot in the distant past, and it has limited resources for the future. Most galaxies are still producing new stars, but many others have already exhausted their supply of gas. Thirty billion years from now, galaxies will be much darker and filled with dead or dying stars, so there will be far fewer planets capable of supporting life as it now exists.

The universe may expand forever, in which case all the galaxies and stars will eventually grow dark and cold. The alternative to this big chill is a big crunch. If the mass of the universe is large enough, gravity will eventually reverse the expansion, and all matter and energy will be reunited. During the next decade, as researchers improve techniques for measuring the mass of the universe, we may learn whether the present expansion is headed toward a big chill or a big crunch.

In the near future, we expect new experiments to provide a better understanding of the big bang. New measurements of the expansion rate and the ages of stars are beginning to confirm that the stars are indeed younger than the expanding universe. New telescopes such as the twin 10-meter Keck telescopes in Hawaii and the 2.5-meter Hubble Space Telescope, other new telescopes at the South Pole and new satellites looking at background radiation as well as new physics experiments searching for "dark matter" may allow us to see how the mass of the universe affects the curvature of space-time, which in turn influences our observations of distant galaxies.

We will also continue to study issues that the big bang cosmology does not address. We do not know why there was a big bang or what may have existed before. We do not know whether our universe has siblings--other expanding regions well removed from what we can observe. We do not understand why the fundamental constants of nature have the values they do. Advances in particle physics suggest some interesting ways these questions might be answered; the challenge is to find experimental tests of the ideas.

In following the debate on such matters of cosmology, one should bear in mind that all physical theories are approximations of reality that can fail if pushed too far. Physical science advances by incorporating earlier theories that are experimentally supported into larger, more encompassing frameworks. The big bang theory is supported by a wealth of evidence: it explains the cosmic background radiation, the abundances of light elements and the Hubble expansion. Thus, any new cosmology surely will include the big bang picture. Whatever developments the coming decades may bring, cosmology has moved from a branch of philosophy to a physical science where hypotheses meet the test of observation and experiment.

When I was a child of 8, I heard a story that will stay with me for the rest of my life. I remember my school teachers telling us about a great scientist who had just died. They talked about him with great reverence, calling him one of the greatest scientists in all history. They said that very few people could understand his ideas, but that his discoveries changed the entire world and everything around us.

But what most intrigued me about this man was that he died before he could complete his greatest discovery. They said he spent years on this theory, but he died with his unfinished papers still sitting on his desk. I was fascinated by the story. To a child, this was a great mystery. What was his unfinished work? What problem could possibly be that difficult and that important that such a great scientist would dedicate years of his life in its pursuit? Curious, I decided to learn all I could about Albert Einstein and his unfinished theory. Some of the happiest moments of my childhood were spent quietly reading every book I could find about this great man and his theories. When I exhausted the books in our local library, I began to scour libraries and bookstores across the city and state eagerly searching for more clues. I soon learned that this story was far more exciting than any murder mystery and more important than anything I could ever imagine. I decided that I would try to get t o the root of this mystery, even if I had to become a theoretical physicist to do it.

Gradually, I began to appreciate the magnitude of his unfinished quest. I learned that Einstein had three great theories. His first two theories, the special and the general theory of relativity, led to the development of the atomic bomb and the present-day theory of black holes and the Big Bang. These two theories by themselves earned him the reputation as the greatest scientist since Isaac Newton. However, Einstein was not satisfied. The third theory, which he called the Unified Field Theory, was to have been his crowning achievement. It was to be the Theory of the Universe, the Holy Grail of physics, the theory which finally unified all physical laws into one simple framework. It was to be the ultimate goal of all physics, the theory to end all theories.

Sadly, it consumed Einstein for the last 30 years of his life; he spent many lonely years in a frustrating pursuit of the greatest theory of all time. But he wasn't alone; I also learned that some of the greatest minds of the twentieth century, such Werner Heisenberg and Wolfgang Pauli, also struggled with this problem and ultimately gave up.

Given the fruitless search that has stumped the world's Nobel Prize winners for half a century, most physicists agree that the Theory of Everything must be a radical departure from everything that has been tried before. For example, Niels Bohr, founder of the modern atomic theory, once listened to Pauli's explanation of his version of the unified field theory. Bohr finally stood up and said, "We are all agreed that your theory is absolutely crazy. But what divides us is whether your theory is crazy enough."

Today, however, after decades of false starts and frustrating dead ends, many of the world's leading physicists think that they have finally found the theory "crazy enough" to be the Unified Field Theory. Scores of physicists in the world's major research laboratories now believe we have at last found the Theory of Everything.

The theory which has generated so much excitement is called the superstring theory. Nearly every science publication in the world has featured major stories on the superstring theory, interviewing some of its pioneers, such as John Schwarz, Michael Green, and Yoichiro Nambu. (Discover magazine even featured it twice on its cover.) My book, Beyond Einstein: the Cosmic Search for the Theory of the Universe, was the first attempt to explain this fabulous theory to the lay audience.

Naturally, any theory which claims to have solved the most intimate secrets of the universe will be the center of intense controversy. Even Nobel Prize winners have engaged in heated discussions about the validity of the superstring theory. In fact, we are witnessing the liveliest debate in theoretical physics in decades over this theory.

To understand the power of the superstring theory and why it is heralded as the theory of the universe (and to understand the delicious controversy that it has stirred up), it is necessary to understand that there are four forces which control everything in the known universe, and that the superstring theory gives us the first (and only) description which can unite all four forces into a single framework.

>>The Four Fundamental Forces

Over 2,000 years ago, the ancient Greeks thought that all matter in the universe could be reduced down to four elements: air, water, earth, and fire. Today, after centuries of research, we know that these substances are actually composites; they, in turn, are made of smaller atoms and sub-atomic particles, held together by just four and only four fundamental forces.

These four forces are:
Gravity is the force which keeps our feet anchored to the spinning earth and binds the solar system and the galaxies together. If the force of gravity could somehow be turned off, we would be immediately flung into outer space at l,000 miles per hour. Furthermore, without gravity holding the sun together, it would explode in a catastrophic burst of energy. Without gravity, the earth and the planets would spin out into freezing deep space, and the galaxies would fly apart into hundreds of billions of stars.

Electro-magnetism is the force which lights up our cities and energizes our household appliances. The electronic revolution, which has given us the light bulb, TV, the telephone, computers, radio, radar, microwaves, light bulbs, and dishwashers, is a byproduct of the electro-magnetic force. Without this force, our civilization would be wrenched several hundred years into the past, into a primitive world lit by candlelight and campfires.

The strong nuclear force is the force which powers the sun. Without the nuclear force, the stars would flicker out and the heavens would go dark. Without the sun, all life on earth would perish as the oceans turned to solid ice. The nuclear force not only makes life on earth possible, it is also the devastating force unleashed by a hydrogen bomb, which can be compared to a piece of the sun brought down to earth.

The weak force is the force responsible for radioactive decay. The weak force is harnessed in modern hospitals in the form of radioactive tracers used in nuclear medicine. For example, the dramatic color pictures of the living brain as it thinks and experiences emotions are made possible by the decay of radioactive sugar in the brain.

It is no exaggeration to say that the mastery of each of these four fundamental forces has changed every aspect of human civilization. For example, when Newton tried to solve his theory of gravitation, he was forced to develop a new mathematics and formulate his celebrated laws of motion. These laws of mechanics, in turn, helped to usher in the Industrial Revolution, which has lifted humanity from uncounted millennia of backbreaking labor and misery.

Furthermore, the mastery of the electromagnetic force by James Maxwell in the 1860s has revolutionized our way of life. Whenever there is a power blackout, we are forced to live our lives much like our forebears in the last century. Today, over half of the world's industrial wealth is now connected, in some way or other, to the electromagnetic force. Modern civilization without the electromagnetic force is unthinkable.

Similarly, when the nuclear force was unleashed with the atomic bomb, human history, for the first time, faced a new and frightening set of choices, including the total annihilation of all life on earth. With the nuclear force, we could finally understand the enormous engine that lies within the sun and the stars, but we could also glimpse for the first time the end of humanity itself.

Thus, whenever scientists unraveled the secrets of one of the four fundamental forces, it irrevocably altered the course of modern civilization. In some sense, some of the greatest breakthroughs in the history of the sciences can be traced back to the gradual understanding of these four fundamental forces. Some have said that the progress of the last 2,000 years of science can be summarized by the mastery of these four fundamental forces.

Given the importance of these four fundamental forces, the next question is: can they be united into one super force? Are they but the manifestations of a deeper reality?

>>Two Great Theories

At present there are two physical frameworks which have partially explained the mysterious features of these four fundamental forces. Remarkably, these two formalisms, the quantum theory and general relativity, allow us to explain the sum total of all physical knowledge at the fundamental level. Without exception.

The laws of physics and chemistry, which can fill entire libraries with technical journals and books, can in principle be derived from these two fundamental theories, making them the most successful physical theories of all time, withstanding the test of thousands of experiments and challenges.

Ironically, these two fundamental frameworks are diametrically opposite to each other. The quantum theory, for example, is the theory of the microcosm, with unparalleled success at describing the sub-atomic world. The theory of relativity, by contrast, is a theory of the macrocosmic world, the world of galaxies, super clusters, black holes, and Creation itself.

The quantum theory explains three of the four forces (the weak, strong, and electro-magnetic forces) by postulating the exchange of tiny packets of energy, called "quanta." When a flashlight is turned on, for example, it emits trillions upon trillion of photons, or the quanta of light. Everything from lasers to radar waves can be described by postulating that they are caused by the movement of these tiny photons of energy. Likewise, the weak force is governed by the exchange of subatomic particles called W-bosons. The strong nuclear force, in turn, binds the proton together by the exchange of "gluons."

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.

>>Superstrings

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 number 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.

>>What Happened Before the Big Bang?

One of the nagging problems of Einstein's old theory of gravity was that it did not explain the origin of the Big Bang. It did not give us a clue as to what happened before the Big Bang.

The 10 dimensional superstring theory, however, gives us a compelling explanation of the origin of the Big Bang. According to the superstring theory, the universe originally started as a perfect 10 dimensional universe with nothing in it.

However, this 10 dimensional universe was not stable. The original 10 dimensional space-time finally "cracked" into two pieces, a four and a six dimensional universe. The universe made the "quantum leap" to another universe in which six of the 10 dimensions curled up into a tiny ball, allowing the remaining four dimensional universe to inflate at enormous rates.

The four dimensional universe (our world) expanded rapidly, eventually creating the Big Bang, while the six dimensional universe wrapped itself into a ball and collapsed down to infinitesimal size.

This explains the origin of the Big Bang, which is now viewed as a rather minor aftershock of a more cataclysmic collapse: the breaking of a 10 dimensional universe into a four and six dimensional universe.

In principle, it also explains why we cannot measure the six dimensional universe, because it has shrunk down to a size smaller than an atom. Thus, no earth-bound experiment can measure the six dimensional universe.

>>Recreating Creation

Although the superstring theory has been called the most sensational discovery in theoretical physics in the past decades, its critics have focused on its weakest point, that it is almost impossible to test. The energy at which the four fundamental forces merge into a single, unified force occurs at the fabulous "Planck energy," which is a billion billion times greater than the energy found in a proton.

Even if all the nations of the earth were to band together and single-mindedly build the biggest atom smasher in all history, it would still not be enough to test the theory. Because of this, some physicists have scoffed at the idea that superstring theory can even be considered a legitimate "theory." Nobel laureate Sheldon Glashow, for example, has compared the superstring theory to the former Pres. Reagan's Star Wars program (because it is untestable and drains the best scientific talent).

The reason why the theory cannot be tested is rather simple. The Theory of Everything is necessarily a theory of Creation, that is, it must necessarily explain everything from the origin of the Big Bang down to the lilies of the field. Its full power is manifested at the instant of the Big Bang, where all its symmetries were intact. To test this theory on the earth, therefore, means to recreate Creation on the earth, which is impossible with present-day technology.

Although this is discouraging, a piece of the puzzle may be supplied by the Superconducting Supercollider (SSC), which, if built, will be the world's largest atom smasher.

>>The SSC - Biggest Experiment of All Time

These questions about unifying the fundamental forces are not academic, because the largest scientific machine ever built, the SSC, may be built to test some of these ideas about the instant of Creation. (Although the SSC was originally approved by the Reagan administration, the project, because of its enormous cost, is still touch-and-go, depending every year on Congressional funding.)

The SSC is projected to accelerate protons to a staggering energy of tens of trillions of electron volts. When these subatomic particles slam into each other at these fantastic energies, the SSC will create temperatures which have not been seen since the instant of Creation (although it is still too weak to fully test the superstring theory). That is why it is sometimes called a "window on Creation."

The SSC is projected to cost over $8 billion (which is large compared to the science budget, but insignificant compared to the Pentagon budget). By every measure, it will be a colossal machine. It will consist of a ring of powerful magnets stretched out in a tube over 50 miles in diameter. In fact, one could easily fit the Washington Beltway, which surrounds Washington D.C., inside the SSC. Inside this gigantic tube, protons will be accelerated to unimaginable energies.

At present, it is scheduled to be finished near the turn of the century in Texas, near the city of Austin. When completed, it will employ thousands of physicists and engineers and cost millions of dollars to operate.

At the very least, physicists hope that the SSC will find some exotic sub-atomic particles, such as the "Higgs boson" and the "top quark," in order to complete our present-day understanding of the quantum theory. However, there is also the small chance that physicists might discover "supersymmetric" particles, which may be remnants of the original superstring theory. In other words, although the superstring theory cannot be tested directly by the SSC, one hopes to find resonances from the superstring theory among the debris created by smashing protons together.

>>Parable of the Gemstone

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.

In conclusion, perhaps some of the readers will be inspired by this story to read every book in their libraries about the superstring theory. Perhaps some of the young readers of this article will be the ones to complete this quest for the Theory of the Universe, begun so many years ago by Einstein.