THE PHYSICS OF EVERYTHING: The merging of two opposing forces of nature and the universe that caused collision that expands the way of thinking of the humankind.

FUNDAMENTALS OF PHYSICS

(An In-Depth Discussion of the 11 Dimensions)

THE FIRST FOUR DIMENSIONS (SPACE-TIME)

SPACE (3 DIMENSIONS) + TIME (1 DIMENSION) = 4 DIMENSIONAL SPACE-TIME

The Difference between Mass and Weight

Weight is different from mass. To understand this difference, you must first understand gravity. Gravity is a force of attraction between objects that is due to their masses. This attraction causes objects to exert a pull on other objects. Because all matter has mass, all matter experiences gravity. The amount of attraction between objects depends on two things—the masses of the objects and the distance between them.

How mass and distance affect gravity between objects?

a.        Gravitational force is large between objects with large mass that are close together.

b.        Gravitational force is smaller between objects with smaller masses that are close together.

c.         An increase in distance reduces gravitational force between two objects with large masses is less if they are far apart.

Gravitational force is experienced by all objects in the universe all the time. But the ordinary objects you see every day have masses so small (relative to, say, planets) that their attraction toward each other is hard to detect. Therefore, the gravitational force experienced by objects with small masses is very slight. However, the Earth’s mass is so large that the gravitational force between objects, such as our atmosphere or the space shuttle, and the Earth is great. Gravitational force is what keeps you and everything else on Earth from floating into space.

A measure of gravitational force exerted on an object is called weight. Consider the brick. The brick has mass. The Earth also has mass. Therefore the brick and the Earth are attracted to each other. A force exerted on the brick because of its attraction to the Earth. The weight of the brick is measure of this gravitational force.

The attraction between objects decreases as the distance between them increases. As a result, the gravitational force exerted on objects also decreases as the distance increases.

On Earth, the gravitational force exerted on an object is about the same everywhere, so an object’s weight is also about same everywhere. Because mass and weight remain constant everywhere on Earth, the terms mass and weight are often used as though they  mean the same thing. But using the term interchangeably can lead to confusion. So remember, weight depends on mass, but weight is not the same thing as mass.

Gravity

For thousands of years, two very puzzling questions were “Why do objects fall towards Earth?” and “What keeps the planets in motion in the sky?” The two questions were treated as separate topics until a British scientist named Sir Isaac Newton (1642-1727) realized that they were two parts of the same question.

Law of Universal Gravitation

This law describes the relationships between gravitational force, mass, and distance. It is called universal because it applies to all objects in the universe.

The law of universal gravitation states the following: All objects in the universe attract each other through gravitational force. The size of force depends on the masses of the objects and the distance between them.

a.        Gravitational force is small between objects with small masses.

b.        Gravitational force is larger between objects with larger masses.

c.         If the distance between two objects is increased, the gravitational force pulling them together is reduced.

I.                      Gravitational force increases as mass increases

II.                     Gravitational force decreases as distance increases

Trivia:

Scientists think seeds can “sense” gravity. The ability to sense gravity is what causes seeds to always send roots down and the green shoot up. But scientists do not understand just how seeds do this. Astronauts have grown seedlings during space shuttle missions to see how seeds respond to changes in gravity. So far, there are no definite answers from the results of these experiments.

What Is the Fundamental Principle Underlying String Theory?

One overarching lesson we have learned during the past hundred years is that the known laws of physics are associated with principles of symmetry. Special relativity is based on the symmetry embodied in the principle of relativity—the symmetry between all constant-velocity vantage points. The gravitational force, as embodied in the general theory of relativity, is based on the equivalence principle—the extension of the principle of relativity to embrace all possible vantage points regardless of the complexity of their states of motion. And the strong, weak, and electromagnetic forces are based on the more abstract gauge

symmetry principles.

Physicists, as we have discussed, tend to elevate symmetry principles to a place of prominence by putting them squarely on the pedestal of explanation. Gravity, in this view, exists in order that all possible observational vantage points are on completely equal

footing—i.e., so that the equivalence principle holds. Similarly, the nongravitational forces exist in order that nature respect their

associated gauge symmetries. Of course, this approach shifts the question of why a certain force exists to why nature respects its

associated symmetry principle. But this certainly feels like progress, especially when the symmetry in question is one that seems

eminently natural. For example, why should one observer's frame of reference be treated differently from another's? It seems far

more natural for the laws of the universe to treat all observational vantage points equally; this is accomplished through the

equivalence principle and the introduction of gravity into the structure of the cosmos. Although it requires some mathematical

background to appreciate fully, as we indicated in Chapter 5, there is a similar rationale behind the gauge symmetries underlying

the three nongravitational forces.

String theory takes us down another notch on the scale of explanatory depth because all of these symmetry principles, as well as

another—supersymmetry—emerge from its structure. In fact, had history followed a different course—and had physicists come

upon string theory some hundred years earlier—we can imagine that these symmetry principles would have all been discovered by

studying its properties. But bear in mind that whereas the equivalence principle gives us some understanding of why gravity exists,

and the gauge symmetries give us some sense of why the nongravitational forces exist, in the context of string theory these

symmetries are consequences; although their importance is in no way diminished, they are part of the end product of a much larger

theoretical structure.

This discussion brings the following question into sharp relief: Is string theory itself an inevitable consequence of some broader

principle—possibly but not necessarily a symmetry principle—in much the same way that the equivalence principle inexorably

leads to general relativity or that gauge symmetries lead to the nongravitational forces? As of this writing, no one has any insight

into the answer to this question. To appreciate its importance, we need only imagine Einstein trying to formulate general relativity

without having had the happy thought he experienced in the Bern patent office in 1907 that led him to the principle of equivalence.

It would not have been impossible to formulate general relativity without first having this key insight, but it certainly would have

been extremely difficult. The equivalence principle provides a succinct, systematic, and powerful organizational framework for

analyzing the gravitational force. The description of general relativity we gave in Chapter 3, for example, relied centrally on the

equivalence principle, and its role in the full mathematical formalism of the theory is even more crucial.

Currently, string theorists are in a position analogous to an Einstein bereft of the equivalence principle. Since Veneziano's

insightful guess in 1968, the theory has been pieced together, discovery by discovery, revolution by revolution. But a central

organizing principle that embraces these discoveries and all other features of the theory within one overarching and systematic

framework—a framework that makes the existence of each individual ingredient absolutely inevitable—is still missing. The

discovery of this principle would mark a pivotal moment in the development of string theory, as it would likely expose the theory's

inner workings with unforeseen clarity There is, of course, no guarantee that such a fundamental principle exists, but the evolution

of physics during the last hundred years encourages string theorists to have high hopes that it does. As we look to the next stage in

the development of string theory, finding its "principle of inevitability"—that underlying idea from which the whole theory

necessarily springs forth—is of the highest priority.138

What Are Space and Time, Really, and Can We Do without Them?

In many of the preceding chapters, we have freely made use of the concepts of space and of spacetime. In Chapter 2 we described

Einstein's realization that space and time are inextricably interwoven by the unexpected fact that an object's motion through space

has an influence on its passage through time. In Chapter 3, we deepened our understanding of spacetime's role in the unfolding of

the cosmos through general relativity, which shows that the detailed shape of the spacetime fabric communicates the force of

gravity from one place to another. The violent quantum undulations in the microscopic structure of the fabric, as discussed in

Chapters 4 and 5, established the need for a new theory, leading us to string theory. And finally, in a number of the chapters that

followed, we have seen that string theory proclaims that the universe has many more dimensions than we are aware of, some of

which are curled up into tiny but complicated shapes that can undergo wondrous transformations in which their fabric punctures,

tears, and then repairs itself.

Through graphic representations such as Figures 3.4, 3.6, and 8.10, we have tried to illustrate these ideas by envisioning the fabric

of space and spacetime as if it were somewhat like a piece of material out of which the universe is tailored. These images have

considerable explanatory power; they are used regularly by physicists as a visual guide in their own technical work. Although

staring at figures such as the ones just mentioned gives a gradual impression of meaning, one can still be left asking, What do we

really mean by the fabric of the universe?

This is a profound question that has, in one form or another, been the subject of debate for hundreds of years. Newton declared

space and time to be eternal and immutable ingredients in the makeup of the cosmos, pristine structures lying beyond the bounds of

question and explanation. As he wrote in the Principia, "Absolute space, in its own nature, without relation to anything external,

remains always similar and immovable. Absolute, true, and mathematical time, of itself, and from its own nature, flows equably

without relation to anything external."139 Gottfried Leibniz and others vociferously disagreed, claiming that space and time are

merely bookkeeping devices for conveniently summarizing relationships between objects and events within the universe. The

location of an object in space and in time has meaning only in comparison with another. Space and time are the vocabulary of these

relations, but nothing more. Although Newton's view, supported by his experimentally successful three laws of motion, held sway

for more than two hundred years, Leibniz's conception, further developed by the Austrian physicist Ernst Mach, is much closer to

our current picture. As we have seen, Einstein's special and general theories of relativity firmly did away with the concept of an

absolute and universal notion of space and time. But we can still ask whether the geometrical model of spacetime that plays such a

pivotal role in general relativity and in string theory is solely a convenient shorthand for the spatial and temporal relations between

various locations, or whether we should view ourselves as truly being embedded in something when we refer to our immersion

within the spacetime fabric.

Although we are heading into speculative territory, string theory does suggest an answer to this question. The graviton, the smallest

bundle of gravitational force, is one particular pattern of string vibration. And just as an electromagnetic field such as visible light

is composed of an enormous number of photons, a gravitational field is composed of an enormous number of gravitons—that is, an

enormous number of strings executing the graviton vibrational pattern. Gravitational fields, in turn, are encoded in the warping of

the spacetime fabric, and hence we are led to identify the fabric of spacetime itself with a colossal number of strings all undergoing

the same, orderly, graviton pattern of vibration. In the language of the field, such an enormous, organized array of similarly

vibrating strings is known as a coherent state of strings. It's a rather poetic image—the strings of string theory as the threads of the

spacetime fabric—but we should note that its rigorous meaning has yet to be worked out completely.

Nevertheless, describing the spacetime fabric in this string-stitched form does lead us to contemplate the following question. An

ordinary piece of fabric is the end product of someone having carefully woven together individual threads, the raw material of

common textiles. Similarly, we can ask ourselves whether there is a raw precursor to the fabric of spacetime—a configuration of

the strings of the cosmic fabric in which they have not yet coalesced into the organized form that we recognize as spacetime.

Notice that it is somewhat inaccurate to picture this state as a jumbled mass of individual vibrating strings that have yet to stitch

themselves together into an ordered whole because, in our usual way of thinking, this presupposes a notion of both space and

time—the space in which a string vibrates and the progression of time that allows us to follow its changes in shape from one

moment to the next. But in the raw state, before the strings that make up the cosmic fabric engage in the orderly, coherent

vibrational dance we are discussing, there is no realization of space or time. Even our language is too coarse to handle these ideas,

for, in fact, there is even no notion of before. In a sense, it's as if individual strings are "shards" of space and time, and only when

they appropriately undergo sympathetic vibrations do the conventional notions of space and time emerge.

Imagining such a structureless, primal state of existence, one in which there is no notion of space or time as we know it, pushes

most people's powers of comprehension to their limit (it certainly pushes mine). Like the Stephen Wright one-liner about the

photographer who is obsessed with getting a close-up shot of the horizon, we run up against a clash of paradigms when we try to

envision a universe that is, but that somehow does not invoke the concepts of space or time. Nevertheless, it is likely that we will

need to come to terms with such ideas and understand their implementation before we can fully assess string theory. The reason is

that our present formulation of string theory presupposes the existence of space and time within which strings (and the other

ingredients found in M-theory) move about and vibrate. This allows us to deduce the physical properties of string theory in a

universe with one time dimension, a certain number of extended space dimensions (usually taken to be three), and additional

dimensions that are curled up into one of the shapes allowed by the equations of the theory. But this is somewhat like assessing an

artist's creative talent by requiring that she work from a paint-by-number kit. She will, undoubtedly, add a personal flair here or

there, but by so tightly constraining the format of her work, we are blinding ourselves to all but a slender view of her abilities.

Similarly, since the triumph of string theory is its natural incorporation of quantum mechanics and gravity, and since gravity is

bound up with the form of space and time, we should not constrain the theory by forcing it to operate within an already existing

spacetime framework. Rather, just as we should allow our artist to work from a blank canvas, we should allow string theory to

create its own spacetime arena by starting in a spaceless and timeless configuration.

The hope is that from this blank slate starting point—possibly in an era that existed before the big bang or the pre-big bang (if we

can use temporal terms, for lack of any other linguistic framework)—the theory will describe a universe that evolves to a form in

which a background of coherent string vibrations emerges, yielding the conventional notions of space and time. Such a framework,

if realized, would show that space, time, and, by association, dimension are not essential defining elements of the universe. Rather,

they are convenient notions that emerge from a more basic, atavistic, and primary state.

Already, cutting-edge research on aspects of M-theory, spearheaded by Stephen Shenker, Edward Witten, Tom Banks, Willy

Fischler, Leonard Susskind, and others too numerous to name, has shown that something known as a zero-brane—possibly the

most fundamental ingredient in M-theory, an object that behaves somewhat like a point particle at large distances but has

drastically different properties at short ones—may give us a glimpse of the spaceless and timeless realm. Their work has revealed

that whereas strings show us that conventional notions of space cease to have relevance below the Planck scale, the zero-branes

give essentially the same conclusion but also provide a tiny window on the new unconventional framework that takes over. Studies

with these zero-branes indicate that ordinary geometry is replaced by something known as noncommutative geometry, an area of

mathematics developed in large part by the French mathematician Alain Connes.140 In this geometrical framework, the

conventional notions of space and of distance between points melt away, leaving us in a vastly different conceptual landscape.

Nevertheless, as we focus our attention on scales larger than the Planck length, physicists have shown that our conventional notion

of space does re-emerge. It is likely that the framework of noncommutative geometry is still some significant steps away from the

blank-slate state anticipated above, but it does give us a hint of what the more complete framework for incorporating space and

time may involve.

Finding the correct mathematical apparatus for formulating string theory without recourse to a pre-existing notion of space and

time is one of the most important issues facing string theorists. An understanding of how space and time emerge would take us a

huge step closer to answering the crucial question of which geometrical form actually does emerge.

Will String Theory Lead to a Reformulation of Quantum Mechanics?

The universe is governed by the principles of quantum mechanics to fantastic accuracy. Even so, in formulating theories over the

past half century, physicists have followed a strategy that, structurally speaking, places quantum mechanics in a somewhat

secondary position. In devising theories, physicists often start by working in a purely classical language that ignores quantum

probabilities, wave functions, and so forth—a language that would be perfectly intelligible to physicists in the age of Maxwell and

even in the age of Newton—and then, subsequently, overlaying quantum concepts upon the classical framework. This approach is

not particularly surprising, since it directly mirrors our experiences. At first blush, the universe appears to be governed by laws

rooted in classical concepts such as a particle having a definite position and a definite velocity at any given moment in time. It is

only after detailed microscopic scrutiny that we realize that we must modify such familiar classical ideas. Our process of discovery

has gone from a classical framework to one that is modified by quantum revelations, and this progression is echoed in the way that

physicists, to this day, go about constructing their theories.

This is certainly the case with string theory. The mathematical formalism describing string theory begins with equations that

describe the motion of a tiny, infinitely thin piece of classical thread—equations that, to a large extent, Newton could have written

down some three hundred years ago. These equations are then quantized. That is, in a systematic manner developed by physicists

over the course of more than 50 years, the classical equations are converted into a quantum-mechanical framework in which

probabilities, uncertainty, quantum jitters, and so on are directly incorporated. In fact, in Chapter 12 we have seen this procedure in

action: The loop processes (see Figure 12.6) incorporate quantum concepts—in this case, the momentary quantum-mechanical

creation of virtual string pairs—with the number of loops determining the precision with which quantum-mechanical effects are

accounted for.

The strategy of beginning with a theoretical description that is classical and then subsequently including the features of quantum

mechanics has been extremely fruitful for many years. It underlies, for example, the standard model of particle physics. But it is

possible, and there is growing evidence that it is likely, that this method is too conservative for dealing with theories that are as farreaching

as string theory and M-theory. The reason is that once we realize that the universe is governed by quantum-mechanical

principles, our theories really should be quantum mechanical from the start. We have successfully gotten away with starting from a

classical perspective until now because we have not been probing the universe at a deep enough level for this coarse approach to

mislead us. But with the depth of string/M-theory, we may well have come to the end of the line for this battle-tested strategy.

We can find specific evidence for this by reconsidering some of the insights emerging from the second superstring revolution (as

summarized, for example, by Figure 12.11). As we discussed in Chapter 12, the dualities underlying the unity of the five string

theories show us that physical processes that occur in any one string formulation can be reinterpreted in the dual language of any of

the others. This rephrasing will at first appear to have little to do with the original description, but, in fact, this is simply the power

of duality at work: Through duality, one physical process can be described in a number of vastly different ways. These results are

both subtle and remarkable, but we have not yet mentioned what may well be their most important feature..

The duality translations often take a process, described in one of the five string theories, that is strongly dependent on quantum

mechanics (for example, a process involving string interactions that would not happen if the world were governed by classical, as

opposed to quantum, physics) and reformulate it as a process that is weakly dependent on quantum mechanics from the perspective

of one of the other string theories (for example, a process whose detailed numerical properties are influenced by quantum

considerations but whose qualitative form is similar to what it would be in a purely classical world). This means that quantum

mechanics is thoroughly intertwined within the duality symmetries underlying string/M-theory: They are inherently quantummechanical

symmetries, since one of the dual descriptions is strongly influenced by quantum considerations. This indicates

forcefully that the complete formulation of string/M-theory—a formulation that fundamentally incorporates the newfound duality

symmetries—cannot begin classically and then undergo quantization, in the traditional mold. A classical starting point will

necessarily omit the duality symmetries, since they hold true only when quantum mechanics is taken into account. Rather, it

appears that the complete formulation of string/M-theory must break the traditional mold and spring into existence as a full-fledged

quantum-mechanical theory.

Currently, no one knows how to do this. But many string theorists foresee a reformulation of how quantum principles are

incorporated into our theoretical description of the universe as the next major upheaval in our understanding. For example, as

Cumrun Vafa has said, "I think that a reformulation of quantum mechanics which will resolve many of its puzzles is just around the

corner. I think many share the view that the recently uncovered dualities point toward a new, more geometrical framework for

quantum mechanics, in which space, time, and quantum properties will be inseparably joined together."141 And according to

Edward Witten, "I believe the logical status of quantum mechanics is going to change in a manner that is similar to the way that the

logical status of gravity changed when Einstein discovered the equivalence principle. This process is far from complete with

quantum mechanics, but I think that people will one day look back on our epoch as the period when it began."142

With guarded optimism, we can envision that a reframing of the principles of quantum mechanics within string theory may yield a

more powerful formalism that is capable of giving us an answer to the question of how the universe began and why there are such

things as space and time—a formalism that will take us one step closer to answering Leibniz's question of why there is something

rather than nothing.

Can String Theory Be Experimentally Tested?

Among the many features of string theory that we have discussed in the preceding chapters, the following three are perhaps the

most important ones to keep firmly in mind. First, gravity and quantum mechanics are part and parcel of how the universe works

and therefore any purported unified theory must incorporate both. String theory accomplishes this. Second, studies by physicists

over the past century have revealed that there are other key ideas—many of which have been experimentally confirmed—that

appear central to our understanding of the universe. These include the concepts of spin, the family structure of matter particles,

messenger particles, gauge symmetry, the equivalence principle, symmetry breaking, and supersymmetry, to name a few. All of

these concepts emerge naturally from string theory. Third, unlike more conventional theories such as the standard model, which

has 19 free parameters that can be adjusted to ensure agreement with experimental measurements, string theory has no adjustable

parameters. In principle, its implications should be thoroughly definitive—they should provide an unambiguous test of whether the

theory is right or wrong.

The road from this "in principle" ratiocination to an "in practice" fact is encumbered by many hurdles. In Chapter 9 we described

some of the technical obstacles, such as determining the form of the extra dimensions, that currently stand in our way. In Chapters

12 and 13 we placed these and other obstacles in the broader context of our need for an exact understanding of string theory,

which, as we have seen, naturally leads us to the consideration of M-theory. No doubt, achieving a full understanding of string/Mtheory

will require a great deal of hard work and an equal amount of ingenuity.

At every step of the way, string theorists have sought and will continue to seek experimentally observable consequences of the

theory. We must not lose sight of the long-shot possibilities for finding evidence of string theory discussed in Chapter 9.

Furthermore, as our understanding deepens there will, no doubt, be other rare processes or features of string theory that will

suggest yet other indirect experimental signatures.

But most notably, the confirmation of supersymmetry, through the discovery of superpartner particles as discussed in Chapter 9,

would be a major milestone for string theory. We recall that supersymmetry was discovered in the course of theoretical

investigations of string theory, and that it is a central part of the theory. Its experimental confirmation would be a compelling,

albeit circumstantial, piece of evidence for strings. Moreover, finding the superpartner particles would provide a welcome

challenge, since the discovery of supersymmetry would do far more than merely answer the yes-no question of its relevance to our

world. The masses and charges of the superpartner particles would reveal the detailed way in which supersymmetry is incorporated

into the laws of nature. String theorists would then face the challenge of seeing whether this implementation can be fully realized

or explained by string theory. Of course, we can be even more optimistic and hope that within the next decade—before the Large

Hadron Collider in Geneva comes on-line—the understanding of string theory will have progressed sufficiently for detailed

predictions about the superpartners to be made prior to their hoped-for discovery. Confirmation of such predictions would be a

monumental moment in the history of science.

Are There Limits to Explanation?

Explaining everything, even in the circumscribed sense of understanding all aspects of the forces and the elementary constituents

of the universe, is one of the greatest challenges science has ever faced. And for the first time, superstring theory gives us a

framework that appears to have sufficient depth to meet the challenge. But will we ever realize the promise of the theory fully and,

for example, calculate the masses of the quarks or the strength of the electromagnetic force, numbers whose precise values dictate

so much about the universe? As in the previous sections, we will have to surmount numerous theoretical hurdles on the way to

these goals—currently, the most prominent is achieving a full nonperturbative formulation of string/M-theory.

But is it possible that even if we had an exact understanding of string/M-theory, framed within a new and far more transparent

formulation of quantum mechanics, we could still fail in our quest to calculate particle masses and force strength? Is it possible that

we would still have to resort to experimental measurements, rather than theoretical calculations, for their values? And, moreover,

might it be that this failing does not mean that we need to look for an even deeper theory, but simply reflects that there is no

explanation for these observed properties of reality?

One immediate answer to all these questions is yes. As Einstein said some time ago, "The most incomprehensible thing about the

universe is that it is comprehensible."143 The astonishment at our ability to understand the universe at all is easily lost sight of in an

age of rapid and impressive progress. However, maybe there is a limit to comprehensibility. Maybe we have to accept that after

reaching the deepest possible level of understanding science can offer, there will nevertheless be aspects of the universe that remain

unexplained. Maybe we will have to accept that certain features of the universe are the way they are because of happenstance,

accident, or divine choice. The success of the scientific method in the past has encouraged us to think that with enough time and

effort we can unravel nature's mysteries. But hitting the absolute limit of scientific explanation—not a technological obstacle or the

current but progressing edge of human understanding—would be a singular event, one for which past experience could not prepare

us.

Although of great relevance to our quest for the ultimate theory, this is an issue we cannot yet resolve; indeed, the possibility that

there are limits to scientific explanation, in the broad way we have stated it, is an issue that may never be resolved. We have seen,

for instance, that even the speculative notion of the multiverse, which at first sight appears to present a definite limit to scientific

explanation, can be dealt with by dreaming up equally speculative theories that, at least in principle, can restore predictive power.

One highlight emerging from these considerations is the role of cosmology in determining the implications of an ultimate theory.

As we have discussed, superstring cosmology is a young field, even by the youthful standards set by string theory itself. It will,

undoubtedly, be an area of primary research focus for years to come, and it is likely to be one of the major growth areas of the

field. As we continue to gain new insight into the properties of string/M-theory, our ability to assess the cosmological implications

of this rich attempt at a unified theory will become ever sharper. It is possible, of course, that such studies may one day convince us

that, indeed, there is a limit to scientific explanation. But it is also possible, to the contrary, that they will usher in a new era—an

era in which we can declare that a fundamental explanation of the universe has finally been found.

Reaching for the Stars

Although we are technologically bound to the earth and its immediate neighbors in the solar system, through the power of thought

and experiment we have probed the far reaches of both inner and outer space. During the last hundred years in particular, the

collective effort of numerous physicists has revealed some of nature's best-kept secrets. And once revealed, these explanatory gems

have opened vistas on a world we thought we knew, but whose splendor we had not even come close to imagining. One measure of

the depth of a physical theory is the extent to which it poses serious challenges to aspects of our worldview that had previously

seemed immutable. By this measure, quantum mechanics and the theories of relativity are deep beyond anyone's wildest

albeit circumstantial, piece of evidence for strings. Moreover, finding the superpartner particles would provide a welcome

challenge, since the discovery of supersymmetry would do far more than merely answer the yes-no question of its relevance to our

world. The masses and charges of the superpartner particles would reveal the detailed way in which supersymmetry is incorporated

into the laws of nature. String theorists would then face the challenge of seeing whether this implementation can be fully realized

or explained by string theory. Of course, we can be even more optimistic and hope that within the next decade—before the Large

Hadron Collider in Geneva comes on-line—the understanding of string theory will have progressed sufficiently for detailed

predictions about the superpartners to be made prior to their hoped-for discovery. Confirmation of such predictions would be a

monumental moment in the history of science.

Are There Limits to Explanation?

Explaining everything, even in the circumscribed sense of understanding all aspects of the forces and the elementary constituents

of the universe, is one of the greatest challenges science has ever faced. And for the first time, superstring theory gives us a

framework that appears to have sufficient depth to meet the challenge. But will we ever realize the promise of the theory fully and,

for example, calculate the masses of the quarks or the strength of the electromagnetic force, numbers whose precise values dictate

so much about the universe? As in the previous sections, we will have to surmount numerous theoretical hurdles on the way to

these goals—currently, the most prominent is achieving a full nonperturbative formulation of string/M-theory.

But is it possible that even if we had an exact understanding of string/M-theory, framed within a new and far more transparent

formulation of quantum mechanics, we could still fail in our quest to calculate particle masses and force strength? Is it possible that

we would still have to resort to experimental measurements, rather than theoretical calculations, for their values? And, moreover,

might it be that this failing does not mean that we need to look for an even deeper theory, but simply reflects that there is no

explanation for these observed properties of reality?

One immediate answer to all these questions is yes. As Einstein said some time ago, "The most incomprehensible thing about the

universe is that it is comprehensible."143 The astonishment at our ability to understand the universe at all is easily lost sight of in an

age of rapid and impressive progress. However, maybe there is a limit to comprehensibility. Maybe we have to accept that after

reaching the deepest possible level of understanding science can offer, there will nevertheless be aspects of the universe that remain

unexplained. Maybe we will have to accept that certain features of the universe are the way they are because of happenstance,

accident, or divine choice. The success of the scientific method in the past has encouraged us to think that with enough time and

effort we can unravel nature's mysteries. But hitting the absolute limit of scientific explanation—not a technological obstacle or the

current but progressing edge of human understanding—would be a singular event, one for which past experience could not prepare

us.

Although of great relevance to our quest for the ultimate theory, this is an issue we cannot yet resolve; indeed, the possibility that

there are limits to scientific explanation, in the broad way we have stated it, is an issue that may never be resolved. We have seen,

for instance, that even the speculative notion of the multiverse, which at first sight appears to present a definite limit to scientific

explanation, can be dealt with by dreaming up equally speculative theories that, at least in principle, can restore predictive power.

One highlight emerging from these considerations is the role of cosmology in determining the implications of an ultimate theory.

As we have discussed, superstring cosmology is a young field, even by the youthful standards set by string theory itself. It will,

undoubtedly, be an area of primary research focus for years to come, and it is likely to be one of the major growth areas of the

field. As we continue to gain new insight into the properties of string/M-theory, our ability to assess the cosmological implications

of this rich attempt at a unified theory will become ever sharper. It is possible, of course, that such studies may one day convince us

that, indeed, there is a limit to scientific explanation. But it is also possible, to the contrary, that they will usher in a new era—an

era in which we can declare that a fundamental explanation of the universe has finally been found.

Reaching for the Stars

Although we are technologically bound to the earth and its immediate neighbors in the solar system, through the power of thought

and experiment we have probed the far reaches of both inner and outer space. During the last hundred years in particular, the

collective effort of numerous physicists has revealed some of nature's best-kept secrets. And once revealed, these explanatory gems

have opened vistas on a world we thought we knew, but whose splendor we had not even come close to imagining. One measure of

the depth of a physical theory is the extent to which it poses serious challenges to aspects of our worldview that had previously

seemed immutable. By this measure, quantum mechanics and the theories of relativity are deep beyond anyone's wildest

expectations: Wave functions, probabilities, quantum tunneling, the ceaseless roiling energy fluctuations of the vacuum, the

smearing together of space and time, the relative nature of simultaneity, the warping of the spacetime fabric, black holes, the big

bang. Who could have guessed that the intuitive, mechanical, clockwork Newtonian perspective would turn out to be so thoroughly

parochial—that there was a whole new mind-boggling world lying just beneath the surface of things as they are ordinarily

experienced?

But even these paradigm-shaking discoveries are only part of a larger, all-encompassing story. With solid faith that laws of the

large and the small should fit together into a coherent whole, physicists are relentlessly hunting down the elusive unified theory.

The search is not over, but through superstring theory and its evolution into M-theory, a cogent framework for merging quantum

mechanics, general relativity, and the strong, weak, and electromagnetic forces has finally emerged. And the challenges these

developments pose to our previous way of seeing the world are monumental: loops of strings and oscillating globules, uniting all of

creation into vibrational patterns that are meticulously executed in a universe with numerous hidden dimensions capable of

undergoing extreme contortions in which their spatial fabric tears apart and then repairs itself. Who could have guessed that the

merging of gravity and quantum mechanics into a unified theory of all matter and all forces would yield such a revolution in our

understanding of how the universe works?

No doubt, there are even grander surprises in store for us as we continue to seek a full and calculationally tractable understanding

of superstring theory. Already, through studies in M-theory, we have seen glimpses of a strange new domain of the universe

lurking beneath the Planck length, possibly one in which there is no notion of time or space. At the opposite extreme, we have also

seen that our universe may merely be one of the innumerable frothing bubbles on the surface of a vast and turbulent cosmic ocean

called the multiverse. These ideas are at the current edge of speculation, but they may presage the next leap in our understanding of

the universe.

As we fix our sight on the future and anticipate all the wonders yet in store for us, we should also reflect back and marvel at the

journey we have taken so far. The search for the fundamental laws of the universe is a distinctly human drama, one that has

stretched the mind and enriched the spirit. Einstein's vivid description of his own quest to understand gravity—"the years of

anxious searching in the dark, with their intense longing, their alternations of confidence and exhaustion, and final emergence into

the light"144—encompasses, surely, the whole human struggle. We are all, each in our own way, seekers of the truth and we each

long for an answer to why we are here. As we collectively scale the mountain of explanation, each generation stands firmly on the

shoulders of the previous, bravely reaching for the peak. Whether any of our descendants will ever take in the view from the

summit and gaze out on the vast and elegant universe with a perspective of infinite clarity, we cannot predict. But as each

generation climbs a little higher, we realize Jacob Bronowski's pronouncement that "in every age there is a turning point, a new

way of seeing and asserting the coherence of the world."145 And as our generation marvels at our new view of the universe—our

new way of asserting the world's coherence—we are fulfilling our part, contributing our rung to the human ladder reaching for the

stars.

Glossary of Scientific Terms

Absolute zero. The lowest possible temperature, about -273 degrees Celsius, or 0 on the Kelvin scale.

Acceleration. A change in an object's speed or direction. See also velocity.

Accelerator. See particle accelerator.

Amplitude. The maximum height of a wave peak or the maximum depth of a wave trough.

Anthropic principle. Doctrine that one explanation for why the universe has the properties we observe is that, were the properties

different, it is likely that life would not form and therefore we would not be here to observe the changes.

Antimatter. Matter that has the same gravitational properties as ordinary matter, but that has an opposite electric charge as well as

opposite nuclear force charges.

Antiparticle. A particle of antimatter.

ATB. Acronym for "after the bang"; usually used in reference to time elapsed since the big bang.

Atom. Fundamental building block of matter, consisting of a nucleus (comprising protons and neutrons) and an orbiting swarm of

electrons.

Big bang. Currently accepted theory that the expanding universe began some 15 billion years ago from a state of enormous energy,

density, and compression.

Big crunch. One hypothesized future for the universe in which the current expansion stops, reverses, and results in all space and

all matter collapsing together; a reversal of the big bang.

Black hole. An object whose immense gravitational field entraps anything, even light, that gets too close (closer than the black

hole's event horizon).

Black-hole entropy. The entropy embodied within a black hole.

Boson. A particle, or pattern of string vibration, with a whole number amount of spin; typically a messenger particle.

Bosonic string theory. First known string theory; contains vibrational patterns that are all bosons.

BPS states. Configurations in a supersymmetric theory whose properties can be determined exactly by arguments rooted in

symmetry.

Brane. Any of the extended objects that arise in string theory. A one-brane is a string, a two-brane is a membrane, a three-brane

has three extended dimensions, etc, More generally, a p-brane has p spatial dimensions.

Calabi-Yau space, Calabi-Yau shape. A space (shape) into which the extra spatial dimensions required by string theory can be

curled up, consistent with the equations of the theory

Charge. See force charge.

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Chiral, Chirality. Feature of fundamental particle physics that distinguishes left- from right-handed, showing that the universe is

not fully left-right symmetric.

Closed string. A type of string that is in the shape of a loop.

Conifold transition. Evolution of the Calabi-Yau portion of space in which its fabric rips and repairs itself, yet with mild and

acceptable physical consequences in the context of string theory. The tears involved are more severe than those in a flop transition.

Cosmic microwave background radiation. Microwave radiation suffusing the universe, produced during the big bang and

subsequently thinned and cooled as the universe expanded.

Cosmological constant. A modification of general relativity's original equations, allowing for a static universe; interpretable as a

constant energy density of the vacuum.

Coupling constant. See string coupling constant.

Curled-up dimension. A spatial dimension that does not have an observably large spatial extent; a spatial dimension that is

crumpled, wrapped, or curled up into a tiny size, thereby evading direct detection.

Curvature. The deviation of an object or of space or of spacetime from a flat form and therefore from the rules of geometry

codified by Euclid.

Dimension. An independent axis or direction in space or spacetime. The familiar space around us has three dimensions (left-right,

back-forth, up-down) and the familiar spacetime has four (the previous three axes plus the past-future axis). Superstring theory

requires the universe to have additional spatial dimensions.

Dual, Duality, Duality symmetries. Situation in which two or more theories appear to be completely different, yet actually give

rise to identical physical consequences.

Electromagnetic field. Force field of the electromagnetic force, consisting of electric and magnetic lines of force at each point in

space.

Electromagnetic force. One of the four fundamental forces, a union of the electric and magnetic forces.

Electromagnetic gauge symmetry. Gauge symmetry underlying quantum electrodynamics.

Electromagnetic radiation. The energy carried by an electromagnetic wave.

Electromagnetic wave. A wavelike disturbance in an electromagnetic field; all such waves travel at the speed of light. Visible

light, X rays, microwaves, and infrared radiation are examples.

Electron. Negatively charged particle, typically found orbiting the nucleus of an atom.

Electroweak theory. Relativistic quantum field theory describing the weak force and the electromagnetic force in one unified

framework.

Eleven-dimensional supergravity. Promising higher-dimensional supergravity theory developed in the 1970s, subsequently

ignored, and more recently shown to be an important part of string theory.

Entropy. A measure of the disorder of a physical system; the number of rearrangements of the ingredients of a system that leave

its overall appearance intact.

Equivalence principle. See principle of equivalence.

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Event horizon. The one-way surface of a black hole; once penetrated, the laws of gravity ensure that there is no turning back, no

escaping the powerful gravitational grip of the black hole.

Extended dimension. A space (and spacetime) dimension that is large and directly apparent; a dimension with which we are

ordinarily familiar, as opposed to a curled-up dimension.

Extremal black holes. Black holes endowed with the maximal amount of force charge possible for a given total mass.

Families. Organization of matter particles into three groups, with each group being known as a family The particles in each

successive family differ from those in the previous by being heavier, but carry the same electric and nuclear force charges.

Fermion. A particle, or pattern of string vibration, with half a whole odd number amount of spin; typically a matter particle.

Feynman sum-over-paths. See sum-over-paths.

Field, Force field. From a macroscopic perspective, the means by which a force communicates its influence; described by a

collection of numbers at each point in space that reflect the strength and direction of the force at that point.

Flat. Subject to the rules of geometry codified by Euclid; a shape, like the surface of a perfectly smooth tabletop, and its higherdimensional

generalizations.

Flop transition. Evolution of the Calabi-Yau portion of space in which its fabric rips and repairs itself, yet with mild and

acceptable physical consequences in the context of string theory.

Foam. See spacetime foam.

Force charge. A property of a particle that determines how it responds to a particular force. For instance, the electric charge of a

particle determines how it responds to the electromagnetic force.

Frequency. The number of complete wave cycles a wave completes each second.

Gauge symmetry. Symmetry principle underlying the quantum-mechanical description of the three nongravitational forces; the

symmetry involves the invariance of a physical system under various shifts in the values of force charges, shifts that can change

from place to place and from moment to moment.

General relativity. Einstein's formulation of gravity, which shows that space and time communicate the gravitational force

through their curvature.

Gluon. Smallest bundle of the strong force field; messenger particle of the strong force.

Grand unification. Class of theories that merge all three nongravitational forces into a single theoretical framework.

Gravitational force. The weakest of the four fundamental forces of nature. Described by Newton's universal theory of gravity, and

subsequently by Einstein's general relativity.

Graviton. Smallest bundle of the gravitational force field; messenger particle for the gravitational force.

Heterotic-E string theory (Heterotic E8 × E8 string theory). One of the five superstring theories; involves closed strings whose

right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string.

Differs in important but subtle ways from the Heterotic-O string theory.

Heterotic-O string theory (Heterotic O(32) string theory). One of the five superstring theories; involves closed strings whose

right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string.

Differs in important but subtle ways from the Heterotic-E string theory.

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Higher-dimensional supergravity. Class of supergravity theories in more than four spacetime dimensions.

Horizon problem. Cosmological puzzle associated with the fact that regions of the universe that are separated by vast distances

nevertheless have nearly identical properties such as temperature. Inflationary cosmology offers a solution.

Infinities. Typical nonsensical answer emerging from calculations that involve general relativity and quantum mechanics in a

point-particle framework.

Inflation, Inflationary cosmology. Modification to the earliest moments of the standard big bang cosmology in which universe

undergoes a brief burst of enormous expansion.

Initial conditions. Data describing the beginning state of a physical system.

Interference pattern. Wave pattern that emerges from the overlap and the intermingling of waves emitted from different

locations.

Kaluza-Klein theory. Class of theories incorporating extra curled-up dimensions, together with quantum mechanics.

Kelvin. A temperature scale in which temperatures are quoted relative to absolute zero.

Klein-Gordon equation. A fundamental equation of relativistic quantum field theory.

Laplacian determinism. Clockwork conception of the universe in which complete knowledge of the state of the universe at one

moment completely determines its state at all future and past moments.

Light clock. A hypothetical clock that measures elapsed time by counting the number of round-trip journeys completed by a single

photon between two mirrors.

Lorentz contraction. Feature emerging from special relativity, in which a moving object appears shortened along its direction of

motion.

Macroscopic. Refers to scales typically encountered in the everyday world and larger; roughly the opposite of microscopic.

Massless black hole. In string theory, a particular kind of black hole that may have large mass initially, but that becomes ever

lighter as a piece of the Calabi-Yau portion of space shrinks. When the portion of space has shrunk down to a point, the initially

massive black hole has no remaining mass—it is massless. In this state, it no longer manifests such usual black hole properties as

an event horizon.

Maxwell's theory, Maxwell's electromagnetic theory. Theory uniting electricity and magnetism, based on the concept of the

electromagnetic field, devised by Maxwell in the 1880s; shows that visible light is an example of an electromagnetic wave.

Messenger particle. Smallest bundle of a force field; microscopic conveyer of a force.

Mirror symmetry. In the context of string theory, a symmetry showing that two different Calabi-Yau shapes, known as a mirror

pair, give rise to identical physics when chosen for the curled-up dimensions of string theory.

M-theory. Theory emerging from the second superstring revolution that unites the previous five superstring theories within a

single overarching framework. M-theory appears to be a theory involving eleven spacetime dimensions, although many of its

detailed properties have yet to be understood.

Multidimensional hole. A generalization of the hole found in a doughnut to higher-dimensional versions.

Multi-doughnut, Multi-handled doughnut. A generalization of a doughnut shape (a torus) that has more than one hole.

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Multiverse. Hypothetical enlargement of the cosmos in which our universe is but one of an enormous number of separate and

distinct universes.

Neutrino. Chargeless species of particle, subject only to the weak force.

Neutron. Chargeless particle, typically found in the nucleus of an atom, consisting of three quarks (two down-quarks, one upquark).

Newton's laws of motion. Laws describing the motion of bodies based on the conception of an absolute and immutable space and

time; these laws held sway until Einstein's discovery of special relativity.

Newton's universal theory of gravity. Theory of gravity declaring that the force of attraction between two bodies is proportional

to the product of their masses and inversely proportional to the square of the distance between them. Subsequently supplanted by

Einstein's general relativity.

Nonperturbative. Feature of a theory whose validity is not dependent on approximate, perturbative calculations; an exact feature

of a theory.

Nucleus. The core of an atom, consisting of protons and neutrons.

Observer. Idealized person or piece of equipment, often hypothetical, that measures relevant properties of a physical system.

One-loop process. Contribution to a calculation in perturbation theory in which one virtual pair of strings (or particles in a pointparticle

theory) is involved.

Open string. A type of string with two free ends.

Oscillatory pattern. See vibrational pattern.

Particle accelerator. Machine for boosting particles to nearly light speed and slamming them together in order to probe the

structure of matter.

Perturbation theory. Framework for simplifying a difficult problem by finding an approximate solution that is subsequently

refined as more details, initially ignored, are systematically included.

Perturbative approach, Perturbative method. See perturbation theory.

Phase. When used in reference to matter, describes its possible states: solid phase, liquid phase, gas phase. More generally, refers

to the possible descriptions of a physical system as features on which it depends (temperature, string coupling constant values,

form of spacetime, etc.) are varied.

Phase transition. Evolution of a physical system from one phase to another.

Photoelectric effect. Phenomenon in which electrons are ejected from a metallic surface when light is shone upon it.

Photon. Smallest packet of the electromagnetic force field; messenger particle of the electromagnetic force; smallest bundle of

light.

Planck energy. About 1,000 kilowatt hours. The energy necessary to probe to distances as small as the Planck length. The typical

energy of a vibrating string in string theory.

Planck length. About 10-33 centimeters. The scale below which quantum fluctuations in the fabric of spacetime would become

enormous. The size of a typical string in string theory.

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Planck mass. About ten billion billion times the mass of a proton; about one-hundredth of a thousandth of a gram; about the mass

of a small grain of dust. The typical mass equivalent of a vibrating string in string theory.

Planck's constant. Denoted by the symbol h, Planck's constant is a fundamental parameter in quantum mechanics. It determines

the size of the discrete units of energy, mass, spin, etc. into which the microscopic world is partitioned. Its value is 1.05 × 10-17

grams-cm/sec.

Planck tension. About 1039 tons. The tension on a typical string in string theory.

Planck time. About 10-43 seconds. Time at which the size of the universe was roughly the Planck length; more precisely, time it

takes light to travel the Planck length.

Primordial nucleosynthesis. Production of atomic nuclei occurring during the first three minutes after the big bang.

Principle of equivalence. Core principle of general relativity declaring the indistinguishability of accelerated motion and

immersion in a gravitational field (over small enough regions of observation). Generalizes the principle of relativity by showing

that all observers, regardless of their state of motion, can claim to be at rest, so long as they acknowledge the presence of a suitable

gravitational field.

Principle of relativity. Core principle of special relativity declaring that all constant-velocity observers are subject to an identical

set of physical laws and that, therefore, every constant-velocity observer is justified in claiming that he or she is at rest. This

principle is generalized by the principle of equivalence.

Product. The result of multiplying two numbers.

Proton. Positively charged particle, typically found in the nucleus of an atom, consisting of three quarks (two up-quarks and one

down-quark).

Quanta. The smallest physical units into which something can be partitioned, according to the laws of quantum mechanics. For

instance, photons are the quanta of the electromagnetic field.

Quantum chromodynamics (QCD). Relativistic quantum field theory of the strong force and quarks, incorporating special

relativity.

Quantum claustrophobia. See quantum fluctuations.

Quantum determinism. Property of quantum mechanics that knowledge of the quantum state of a system at one moment

completely determines its quantum state at future and past moments. Knowledge of the quantum state, however, determines only

the probability that one or another future will actually ensue.

Quantum electrodynamics (QED). Relativistic quantum field theory of the electromagnetic force and electrons, incorporating

special relativity.

Quantum electroweak theory. See electroweak theory.

Quantum field theory. See relativistic quantum field theory.

Quantum fluctuation. Turbulent behavior of a system on microscopic scales due to the uncertainty principle.

Quantum foam. See spacetime foam.

Quantum geometry. Modification of Riemannian geometry required to describe accurately the physics of space on

ultramicroscopic scales, where quantum effects become important.

-- 183 --

Quantum gravity. A theory that successfully mergers quantum mechanics and general relativity, possibly involving modifications

of one or both. String theory is an example of a theory of quantum gravity.

Quantum mechanics. Framework of laws governing the universe whose unfamiliar features such as uncertainty, quantum

fluctuations, and wave-particle duality become most apparent on the microscopic scales of atoms and subnuclear particles.

Quantum tunneling. Feature of quantum mechanics showing that objects can pass through barriers that should be impenetrable

according to Newton's classical laws of physics.

Quark. A particle that is acted upon by the strong force. Quarks exist in six varieties (up, down, charm, strange, top, bottom) and

three "colors" (red, green, blue).

Radiation. The energy carried by waves or particles.

Reciprocal. The inverse of a number; for example, the reciprocal of 3 is 1/3, the reciprocal of ½ is 2.

Relativistic quantum field theory. Quantum-mechanical theory of fields, such as the electromagnetic field, that incorporates

special relativity.

Resonance. One of the natural states of oscillation of a physical system.

Riemannian geometry. Mathematical framework for describing curved shapes of any dimension. Plays a central role in Einstein's

description of spacetime in general relativity.

Schrödinger equation. Equation governing the evolution of probability waves in quantum mechanics.

Schwarzschild solution. Solution to the equations of general relativity for a spherical distribution of matter; one implication of

this solution is the possible existence of black holes.

Second law of thermodynamics. Law stating that total entropy always increases.

Second superstring revolution. Period in the development of string theory beginning around 1995 in which some nonperturbative

aspects of the theory began to be understood.

Singularity. Location where the fabric of space or spacetime suffers a devastating rupture.

Smooth, Smooth space. A spatial region in which the fabric of space is flat or gently curved, with no pinches, ruptures, or creases

of any kind.

Space-tearing flop transition. See flop transition.

Spacetime. A union of space and time originally emerging from special relativity. Can be viewed as the "fabric" out of which the

universe is fashioned; it constitutes the dynamical arena within which the events of the universe take place.

Spacetime foam. Frothy, writhing, tumultuous character of the spacetime fabric on ultramicroscopic scales, according to a

conventional point-particle perspective. An essential reason for the incompatibility of quantum mechanics and general relativity

prior to string theory.

Special relativity. Einstein's laws of space and time in the absence of gravity (see also general relativity).

Sphere. The outer surface of a ball. The surface of a familiar three-dimensional ball has two dimensions (which can be labeled by

two numbers such as "latitude" and "longitude," as on the surface of the earth). The concept of a sphere, though, applies more

generally to balls and hence their surfaces, in any number of dimensions. A one-dimensional sphere is a fancy name for a circle; a

-- 184 --

zero-dimensional sphere is two points (as explained in the text). A three-dimensional sphere is harder to picture; it is the surface of

a four-dimensional ball.

Spin. A quantum-mechanical version of the familiar notion of the same name; particles have an intrinsic amount of spin that is

either a whole number or half a whole number (in multiples of Planck's constant), and which never changes.

Standard model of cosmology. Big bang theory together with an understanding of the three nongravitational forces as

summarized by the standard model of particle physics.

Standard model of particle physics, Standard model, Standard theory. An enormously successful theory of the three

nongravitational forces and their action on matter. Effectively the union of quantum chromodynamics and the electroweak theory.

String. Fundamental one-dimensional object that is the essential ingredient in string theory

String coupling constant. A (positive) number that governs how likely it is for a given string to split apart into two strings or for

two strings to join together into one—the basic processes in string theory. Each string theory has its own string coupling constant,

the value of which should be determined by an equation; currently such equations are not understood well enough to yield any

useful information. Coupling constants less than 1 imply that perturbative methods are valid.

String mode. A possible configuration (vibrational pattern, winding configuration) that a string can assume.

String theory. Unified theory of the universe postulating that fundamental ingredients of nature are not zero-dimensional point

particles but tiny one-dimensional filaments called strings. String theory harmoniously unites quantum mechanics and general

relativity, the previously known laws of the small and the large, that are otherwise incompatible. Often short for superstring theory.

Strong force, Strong nuclear force. Strongest of the four fundamental forces, responsible for keeping quarks locked inside

protons and neutrons and for keeping protons and neutrons crammed inside of atomic nuclei.

Strong force symmetry. Gauge symmetry underlying the strong force, associated with invariance of a physical system under shifts

in the color charges of quarks.

Strongly coupled. Theory whose string coupling constant is larger than 1.

Strong-weak duality. Situation in which a strongly coupled theory is dual—physically identical—to a different, weakly coupled

theory.

Sum-over-paths. Formulation of quantum mechanics in which particles are envisioned to travel from one point to another along

all possible paths between them.

Supergravity. Class of point-particle theories combining general relativity and supersymmetry.

Superpartners. Particles whose spins differ by ½ unit and that are paired by supersymmetry.

Superstring theory. String theory that incorporates supersymmetry.

Supersymmetric quantum field theory. Quantum field theory incorporating supersymmetry.

Supersymmetric standard model. Generalization of the standard model of particle physics to incorporate supersymmetry. Entails

a doubling of the known elementary particle species.

Supersymmetry. A symmetry principle that relates the properties of particles with a whole number amount of spin (bosons) to

those with half a whole (odd) number amount of spin (fermions).

-- 185 --

Symmetry. A property of a physical system that does not change when the system is transformed in some manner. For instance, a

sphere is rotationally symmetrical since its appearance does not change if it is rotated.

Symmetry breaking. A reduction in the amount of symmetry a system appears to have, usually associated with a phase transition.

Tachyon. Particle whose mass (squared) is negative; its presence in a theory generally yields inconsistencies.

Thermodynamics. Laws developed in the nineteenth century to describe aspects of heat, work, energy, entropy, and their mutual

evolution in a physical system.

Three-brane. See brane.

Three-dimensional sphere. See sphere.

Time dilation. Feature emerging from special relativity, in which the flow of time slows down for an observer in motion.

T.O.E. (Theory of Everything). A quantum-mechanical theory that encompasses all forces and all matter.

Topologically distinct. Two shapes that cannot be deformed into one another without tearing their structure in some manner.

Topology. Classification of shapes into groups that can be deformed into one another without ripping or tearing their structure in

any way

Topology-changing transition. Evolution of spatial fabric that involves rips or tears, thereby changing the topology of space.

Torus. The two-dimensional surface of a doughnut.

Two-brane. See brane.

Two-dimensional sphere. See sphere.

Type I string theory. One of the five superstring theories; involves both open and closed strings.

Type IIA string theory. One of the five superstring theories; involves closed strings with left-right symmetric vibrational

patterns.

Type IIB string theory. One of the five superstring theories; involves closed strings with left-right asymmetric vibrational

patterns.

Ultramicroscopic. Length scales shorter than the Planck length (and also time scales shorter than the Planck time).

Uncertainty principle. Principle of quantum mechanics, discovered by Heisenberg, that there are features of the universe, like the

position and velocity of a particle, that cannot be known with complete precision. Such uncertain aspects of the microscopic world

become ever more severe as the distance and time scales on which they are considered become ever smaller. Particles and fields

undulate and jump between all possible values consistent with the quantum uncertainty. This implies that the microscopic realm is

a roiling frenzy, awash in a violent sea of quantum fluctuations.

Unified theory, Unified field theory. Any theory that describes all four forces and all of matter within a single, all-encompassing

framework.

Uniform vibration. The overall motion of a string in which it moves without changes in shape.

Velocity. The speed and the direction of an object's motion.

-- 186 --

Vibrational mode. See vibrational pattern.

Vibrational pattern. The precise number of peaks and troughs as well as their amplitude as a string oscillates.

Vibration number. Whole number describing the energy in the uniform vibrational motion of a string; the energy in its overall

motion as opposed to that associated with changes in its shape.

Virtual particles. Particles that erupt from the vacuum momentarily; they exist on borrowed energy, consistent with the

uncertainty principle, and rapidly annihilate, thereby repaying the energy loan.

Wave function. Probability waves upon which quantum mechanics is founded.

Wavelength. The distance between successive peaks or troughs of a wave.

Wave-particle duality. Basic feature of quantum mechanics that objects manifest both wavelike and particle-like properties.

W bosons. See weak gauge boson.

Weak force, Weak nuclear force. One of the four fundamental forces, best known for mediating radioactive decay.

Weak gauge boson. Smallest bundle of the weak force field; messenger particle of the weak force; called W or Z boson.

Weak gauge symmetry. Gauge symmetry underlying the weak force.

Weakly coupled. Theory whose string coupling constant is less than 1.

Winding energy. The energy embodied by a string wound around a circular dimension of space.

Winding mode. A string configuration that wraps around a circular spatial dimension.

Winding number. The number of times a string is wound around a circular spatial dimension.

World-sheet. Two-dimensional surface swept out by a string as it moves.

Wormhole. A tube-like region of space connecting one region of the universe to another.

Z boson. See weak gauge boson.

Zero-dimensional sphere. See sphere.

-- 187 --

References and Suggestions for Further Reading

Abbot, Edwin A. Flatland: A Romance of Many Dimensions. Princeton: Princeton University Press, 1991.

Barrow, John D. Theories of Everything. New York: Fawcett-Columbine, 1992.

Bronowski, Jacob. The Ascent of Man. Boston: Little, Brown, 1973.

Clark, Ronald W. Einstein, The Life and Times. New York: Avon, 1984.

Crease, Robert P., and Charles C. Mann. The Second Creation. New Brunswick, N.J.: Rutgers University Press, 1996.

Davies, P. C. W. Superforce. New York: Simon & Schuster, 1984.

Davies, P. C. W., and J. Brown, eds. Superstrings: A Theory of Everything? Cambridge, Eng.: Cambridge University Press, 1988.

Deutsch, David. The Fabric of Reality. New York: Allen Lane, 199 7.

Einstein, Albert. The Meaning of Relativity. Princeton: Princeton University Press, 1988.

——. Relativity. New York: Crown, 1961.

Ferris, Timothy Coming of Age in the Milky Way New York: Anchor, 1989.

——. The Whole Shebang. New York: Simon & Schuster, 1997.

Fölsing, Albrecht. Albert Einstein. New York: Viking, 1997.

Feynman, Richard, The Character of Physical Law. Cambridge, Mass.: MIT Press, 1995.

Gamow, George. Mr. Tompkins in Paperback. Cambridge, Eng.: Cambridge University Press, 1993.

Gell-Mann, Murray. The Quark and the Jaguar. New York: Freeman, 1994.

Glashow, Sheldon. Interactions. New York: Time-Warner Books, 1988.

Guth, Alan H. The Inflationary Universe. Reading, Mass.: Addison-Wesley, 1997.

Hawking, Stephen. A Brief History of Time. New York: Bantam Books, 1988.

Hawking, Stephen, and Roger Penrose. The Nature of Space and Time. Princeton: Princeton University Press, 1996.

Hey, Tony, and Patrick Walters. Einstein's Mirror. Cambridge, Eng.: Cambridge University Press, 1997.

Kaku, Michio. Beyond Einstein. New York: Anchor, 1987.

——. Hyperspace. New York: Oxford University Press, 1994.

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Lederman, Leon, with Dick Teresi. The God Particle. Boston: Houghton Mifflin, 1993.

Lindley, David. The End of Physics. New York: Basic Books, 1993.

——. Where Does the Weirdness Go? New York: Basic Books, 1996.

Overbye, Dennis, Lonely Hearts of the Cosmos. New York: HarperCollins, 199 1.

Pais, Abraham. Subtle Is the Lord: The Science and the Life of Albert Einstein. New York: Oxford University Press, 1982.

Penrose, Roger. The Emperor's New Mind. Oxford, Eng.: Oxford University Press, 1989.

Rees, Martin J. Before the Beginning. Reading, Mass.: Addison-Wesley, 1997.

Smolin, Lee. The Life of the Cosmos. New York: Oxford University Press, 1997.

Thorne, Kip. Black Holes and Time Warps. New York: Norton, 1994.

Weinberg, Steven. The First Three Minutes. New York: Basic Books, 1993.

——. Dreams of a Final Theory. New York: Pantheon, 1992.

Wheeler, John A. A Journey into Gravity and Spacetime. New York: Scientific American Library, 1990.

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SPACE, TIME, AND RELATIVITY

Space, time, and gravity are interlinked. Gravity affects everything in space. The gravity of a black hole is so strong it can even change the pace of time.

Today, as in the past, scientists work to understand how the Universe works, as we find out more, our new ideas improve, or even replace, old ones. This happened in the early years of the 20^th century. Isaac Newton’s theory of gravity was improved and in part replace, by a new theory. In 1915, the German-born scientist Albert Einstein developed a theory called General Relativity which related space and gravity.

In Einstein’s theory, the gravity of an object is a distortion in time and space. This is difficult to imagine but start by thinking of a rubber sheet. An object like a marble placed on the sheet will dent it. Now imagine the marble is a massive star and the rubber sheet is a space. The star dents the space near it. Other similar massive objects will make a dent or gravitational well, in space near them.

Starlight passing close to the Sun is deflected slightly by its gravitational well, but these stars can only be seen during eclipses. Measurements made during the 1919 eclipse were used to test Einstein’s theory. It is now known that the results were not accurate enough to prove Einstein right, though later tests did so.

The gravity of the Sun bends space near it. As the light from a distant star passes the Sun on its way to Earth, it too is bent. So astronomers on Earth observe the star away from its true position.

The scientist Albert Einstein (1879-1955) published his theory of General Relativity, concerning gravity, in 1915. His work led to ideas such as black holes and the Big Bang.

A black hole is the remains of a giant star that has blown up. The core that is left after the explosion creates such a powerful gravitational field that any object passing close enough will be pulled into it.

BLACK HOLES

An object such as a black hole makes more than a dent in the space around it. A black hole is what Is left over from a massive star that has collapsed at the end of its life. The original star has been super-compressed and now has gravity so strong that it has formed a deep, steep-sided, gravitational well. Light cannot escape so it is ‘black’. Nothing at all can escape the well so we think of it as a ‘hole’ cut off from the rest of space. A hole’s gravity not only distorts space around it but also time. The flow of time is disturbed so that it runs slower and slower near the hole.

BIG BANG

The universe may not always existed. Scientists think it began about 13 to 15 billion years ago with a ‘Big Bang’. It is believed that one moment there was just a tiny hot ball containing everything in the universe and then a moment later, the universe burst into existence with the biggest explosion of all time, separating into basic forces such as electricity and gravity—so big that everything is still hurtling out from it—even today.

Scientists questioned the origin of the Universe for the first time in the 20^th century.

Could it really have existed in one form, and forever—or was there a starting point, and a process of continual change?

The evidence pointed more and more forward the Big Bang, and the Universe only 13.7 billion years old.

Until around 100 years ago, the Universe was thought not to change over time. but during the 20^th century, scientists collected evidence proving that the Universe is not static, but is in a constant state of expansion and change. Its expansive state is linked to an explosive event known as the Big Bang.

Origin of the Big Bang Theory

The seeds for the new view of the Universe were planted in 1916 when Albert Einstein published his theory of general relativity, from which the Dutch astronomer Willem de Sitter developed an imaginary expanding Universe. The idea became reality in 1929, when Edwin Hubble showed the Universe is expanding, which means that it was once smaller and denser. It has, however, Belgian Georges Lemaitre’s suggestion, in 1931, that the Universe’s origin was a primeval “cosmic egg” that exploded, creating an expanding Universe, which served as the first Big Bang model.

1. At first, the entire universe was a hot ball tinier than an atom and much hotter than any star. This swelled much, much faster than the speed of light, growing to the size of a galaxy in just a tiny fraction of a second.

2. As the universe expanded, it began to cool and tiny particles of energy and matter—each of them much smaller than atoms—began to form a thick, soup-like material.

3. After about 3 minutes, gravity started to pull the particles together. Atoms joined together to make gases such as hydrogen and helium, and the thick ‘soup’ began to clear and thin out. By the ends of the third minute, the matter that surrounds us today had been created.

4. Over time, as the young universe grew larger, the gases clumped into clouds. After several hundred million years, the clouds began to form stars and galaxies.

Astronomers developed the Big Bang Theory, and worked out how long ago it occurred, by looking at how galaxies move in space. They also discovered that every galaxy in the universe is zooming away from Earth. If this is so, the universe must be expanding. If the universe is getting bigger, there must have been a time when it was much smaller—this is known as the ‘Expanding Universe’ theory.

The Evidence mounts

In the 1940s, Fred Hoyle together with Americans Hermann Bondi and Thomas Gold, put forward an alternative view of the Universe. The “Steady State” theory they proposed that the Universe appears the same in every location and at all times. It has no beginning or end, and matter is commonly created.

In 1948, George Gamow outlined how the relative proportions of the hydrogen and helium in today’s Universe could be produced in a Big Bang Universe. The case for the Big Bang strengthened after 1955, when Martin Ryle showed that the distant, older radio galaxies were more numerous and densely packed than those nearby. This disproved the vital characteristic of the hypothetical Steady State Universe, in which density would always stay the same.

Direct evidence for the Big Bang came a decade later. George Gamow had predicted that a remnant of the Universe’s initial radiation would remain: the microwave background that would have permeated all space, before starting to cool. This was detected in 1946 by Arno Penzias and Robert Woodrow Wilson, confirming that the Universe was intensely hot in the past.

Astronomers are able to tell that a galaxy is moving by its color. If a galaxy is moving away from Earth, its light waves stretched and the redder it appears. This is called RED SHIFT.

Some astronomers think the universe will expand forever. Others think the expansion will stop then collapse back to a ‘big crunch.’

In March 2002, astronomers spotted the oldest galaxy yet seen—13 billion light-years away and 15 billion years old. Since this is older than the estimated age of the universe, scientists may have to work out the universe’s age again.

The farthest galaxies from Earth are hurtling away at almost the speed of light.

The afterglow of the Big Bang can be detected in microwave radiation which emanates from all over space.

The Start of the Universe

We now think that the Big Bang occurred 13.7 billion years ago, starting our Universe. It produced all energy, matter, and space, and marked the recognized start of time. At the outset the Universe was dense, hot, and contained pure energy. It rapidly ballooned in size, before settling to steadier state of expansion. It consisted of tiny particles of energy, which became tiny particles of matter within seconds. Within minutes, the Universe had turned almost entirely into the nuclei of hydrogen and helium atoms.

TIME

Before the invention of clocks, people measured time by natural rhythm of the Earth—by watching the movements of the Sun, Moon, and stars in the sky. Now we can see time passing as hands move or figures change on a clock. Modern atomic clocks can measure time with astonishing accuracy. Yet scientists and philosophers still find it hard to agree on what time is. Scientists say time is dimension (like length and width) and what we move through time just as we can move sideways or backwards and forwards, or up and down. Time is said to be the fourth dimension: the other three are length, width, and depth. But time only runs one way: a candle cannot be unburned, or your life lived backwards.

ATOMIC TIME

Just as a guitar string vibrates at certain pitch or frequency, so do atoms and molecules. Atomic vibrations are so regular that they can be used to make the world’s most accurate clocks—atomic clocks. These special clocks, all housed in special laboratory, mostly use the cesium-133 atom. Since 1967, 1 second has been defined as 9,192,631,770 vibrations of a cesium-133 atom. Atomic clocks are also used to set the world’s standard time, called Co-ordinated Universal Time (UTC), set by the United States National Institute of Standards and Technology (NIST) agency.

NATURAL CLOCK

Sundials work well in the sunshine, but they do not work at night or in cloudy weather. Other ways of measuring time that did not depend on sunshine were invented in the ancient world. A candle burns down steadily, so time can be measured by the changing length of a burning candle. Water or sand running steadily from one container to another can also mark the passing of time. then in the 17^th century, the great Italian scientist Galileo Galilei noticed that a pendulum of a certain length (a weight on the end of a wire or rod) always swings at the same rate. This discovery made it possible to build accurate clocks by linking the swinging pendulum to pointers (hands) that showed the time on a dial (clock face).

TIME TRAVEL

The brilliant scientist Albert Einstein suggested that time does not exist on its own but is a part of something called space-time, which includes the whole universe. When we move about, we are not moving through space while time passes separately. We move through space-time. if time is linked to space, some scientists wonder if we might be able to move through the time part of space-time into the past or future. Objects in space called black holes have such a strong force of gravity that they might be able to bend space-time enough to create pathways called wormholes that link to other places or times. If wormholes do exist then they are understood to be even smaller than an atom. Some scientists believe it might be possible to enlarge them using an extremely powerful electric field, holding them open long enough to make a tunnel through space-time and thus travel through time.

ADDITIONAL INFO

All living things have their own ‘biological’ clocks to control their lives and stimulate certain behavior at specific times.

The shortest time that can be measured is pico-second—one trillionth of a second.

The length of a day on Earth has not always been the same. In the past, days were shorter and in the future, days will be longer because the Earth is spinning more slowly as time goes on. The average day is 1.6 milliseconds (thousandths of a second) longer every 100 years.

Einstein showed that the faster things travel, the slower time runs. When Apollo 11 went to the Moon, an accurate clock on board lost a few seconds, not because of any fault but because time ran slowly in the speeding space ship.

TIME AND SPACE

A century ago, our idea of the way the world worked was straightforward. It seemed obvious that things happen, one after another. But two ordinary scientific ideas—quantum science and relativity—have shown that it is not as simple as that. Quantum science shows that effect does not inevitably follow cause. Relativity overturns our common view sense view of time, with time running identically anywhere in the universe. While these ideas have only a limited impact on our everyday lives, they have revolutionized science—from the study of our vast universe to the study of minute atoms.

EVERYTHING IS RELATIVE

It would seem to be the case that time is the same everywhere and moves in only one direction, from past to future. German physicist Albert Einstein (1879-1955) showed that this is not so. In his theory of relativity, Einstein completely overturned this idea of time and was the first to show that time is relative. Time is not fixed but depends entirely on how you measure it—and you can only measure it relative to something else. Einstein showed that time does not run one-way, but is a dimension, just like length, breadth, and depth, and that it can run backwards as well as forwards.

QUANTUM WORLD

Most scientists once though light and other kinds of radiation travel in waves. But in the 1980s, German scientist Max Planck (1858-1947) observed the ranged of radiation sent out by a hot object—and it did not support the theory that radiation travels in waves. Planck realized that the range of radiation made sense if it was emitted in chunks, or what he called ‘quanta’. Quanta are very, very small. When lots are emitted together, they appear like smooth waves; when they are emitted separately, they are like particles. Soon scientists realized the quantum concept applied to all kinds of particles smaller than atom, and quantum mechanics became a whole new science.

FOUR-DIMENSIONAL

Hermann Minkowski (1864-1909) developed Einstein’s theory of relativity to suggest that space and time are not separate things. Space has three-dimensions—up, down, and sideways. Minkowski suggested that time was just another dimension. So it made sense to talk of time as the fourth dimension, and join space and time together to create four-dimensional spacetime. Minkowski used geometry to solve problems in number theory, mathematical physics, and number theory.

ADDITIONAL INFO

Relativity explains why astronauts who come back from the Moon are a few seconds younger than they would have been had they stayed on Earth—because time ran slower in space.

Einstein thought light was the fastest thing in the universe; recent experiments with particles have surpassed the speed of light.

BLACK HOLES

Einstein showed that gravity pulls things closer by shrinking the space and time between them. If gravity is incredibly powerful it will shrink space and time to the point where it disappears. In his book, A Brief History of Time, English Physicist Stephen Hawking (born 1942) suggests that this is what happens in the center of a black hole—a place in space where gravity is so strong that it sucks everything in, including light.

Black holes may be linked by tunnels through space and time called wormholes, to exactly opposite ‘white’ holes. Wormholes may make it possible through time in the future.

Teleporting may seem to have come straight from Star Trek, but scientists have succeeded in teleporting particles. Using a phenomenon called quantum entanglement, particles are destroyed in one place only to reappear instantly some distance away.

TIME

From the earliest times, humans have used the seasons, the movement of the Earth, and the passage of the Moon and the Sun across the sky to measure time.

The time system we use is based on the movement of the Earth. One spin of the planet on its axis is a day, and one orbit round the Sun is a year. As Earth spins, the Sun is seen to rise in the sky, climb to its highest point and then descend until it disappears from view.

The Sun is at its highest point in the sky when a place on Earth faces the Sun directly. This happens day after day, as Earth continually spins. Part of the day it is lit by the Sun is out of view and it is nighttime. The starting- and finishing-point for the day is midnight, and halfway through the day is noon, the hour closest to the time when the Sun is at its highest in the sky.

EINSTEIN’S THEORY OF RELATIVITY: A SMALL REVOLUTION

Is everything relative?

No, and that may sound surprising. Many quantities behave relative to each other, for example speed and place, but there is one absolute quantity: the speed of light in a vacuum.

What does relative mean?

If I see an object approaching me, there are three basic possibilities: first, I am stationary and the object is moving towards me; second, the object is stationary and I am moving towards it; third, we are both moving towards each other. Who is still and who is moving can only be decided with regard to a fixed reference frame, for example a tree at the side of the road. If the tree is stationary relative to me, then I know I am stationary and the object is approaching me. It could also be argued that the tree is moving with the Earth’s rotation and therefore it is not a suitable reference point as I am also moving together with the Earth. The ultimate consequence means that it cannot be decided who or what is moving, but this statement can apply only with reference to arbitrarily definable frames of reference—thus relative to them.

What does time have to do with speed?

The time of an object passes all the more slowly the faster it is moving; and also its mass increases with the speed. Both effects are only observable at very high speeds way beyond our level of experience, but they are measurable with accurate atomic clocks placed in rockets. At close to the speed of light, time would pass very slowly and it would even stop if the speed of light were able to be reached. If an astronaut flies away from the Earth at close to the speed of light and returns—after what for him would be a short period—it could be that hundreds of years had already passed on Earth.

How high is the greatest possible speed?

The speed of light in vacuum is unbelievably 186,000 miles/s (299,792,458 km/s). nothing travels faster than this, and matter cannot quite reach that speed.

Why can’t time go backwards?

At speeds less than that of light, time travels forwards. At the speed of light, it stops. To move backwards it would have to move at more than the speed of light. However, as matter can only ever be slower than light because the energy to get closer and closer to the speed of light becomes enormous, time also moves for it in only one direction—forward. Logic also precludes time travel into the past. Otherwise the sequence of events could be changed retroactively. For instance, say you travel back in time and prevent your parents from getting to know one another by going with your father to play bowls; you would never have been born, so how can you go bowling with your father? Retrospectively, if you thus ceased to exist, then you would not have gone bowling and your parents would have got to know one another. You would still have been born and would then once again prevent your conception. The resulting endless circle is an insoluble paradox that would throw the world out of a kilter.

 The Theory of Everything

String theory. M-theory. Loop quantum gravity. The holographic universe. Several theories are competing for the solution to physics’ ultimate problem: finding a single theory to unify all of the forces of nature—a theory of everything.

The idea seemed logical enough when Einstein first set out on it in the 1920s. if the general relativity explains the universe from afar—why gravity pulls the earth around the sun—and quantum mechanics explains the world up close—how atoms, protons, and neutrons react to electromagnetism and the strong and weak forces—surely there must be a way to put the two theories together. After all, whether cosmic in size or miniscule, the particles and forces that govern our universe were all born at the same primordial moment. Yet Einstein failed. And in interim, armies of physicists, equipped with similarly well-intentioned yet ultimately faulty or unprovable ideas, have followed him to the same well-trod dead end.

Since the mid-1980s, the leading contender for a grand unifying theory has been string theory. The idea is deceptively simple: at the core of every particle in the universe is a tiny thread of energy. Each of these filaments vibrates like a violin string, and its rate of vibration determines its vital characteristics, or tone. There are neutrino strings and electron strings, photon strings, and graviton strings. When played together, they compose the symphony of the universe. Or at least, that’s the theory.

IS THERE A THEORY OF EVERYTHING?

Perhaps, anyway the scientific world is looking for it. But is not nature too complex and rich in structure to allow itself to be squeezed into a single formula? Take matter, for example. It is made of the most diverse molecules and atoms, substances and compounds, but the bottom line is that it is fundamentally formed from quarks and electrons. Moreover, matter can be converted in energy. So if photons (as carriers of light) and quarks and electrons are transferrable into one another, they can be combined in a unified theory. Meanwhile, of the four known fundamental forces of nature, three can be combined into one theory: electromagnetic force, the strong nuclear force, and the weak nuclear force. Up until now only gravity cannot be included. If it were, we would have a theory for all the forces of nature and could express a theory for everything. Nevertheless, nature would not allow itself to be “calculated” since, as a result of the Heisenberg Uncertainty Principle, the exact starting point for future calculations is not arbitrarily measurable.

WHAT IS THE PRINCIPLE OF LEAST ACTION?

The principle of least action is a basic principle of mechanics. In school we learn that a ball’s flight curve is a parabola, which can be calculated using the law of the force of gravity. It can also be expressed thus: the chosen path of an object is the one for which the sum of the kinetic and potential energy of each point along the path gives a minimum value.

THE SCIENCE OF STRING THEORY

String theory is surprisingly easy to explain. It goes like this. The fundamental particle is not a quark, but a string whose dimensions are calculated to be 10^-33 centimeters. The important feature of this string is that, in the same manner as the strings of a violin, it vibrates to produce different notes and harmonics.

In biology, theories generally arise to explain data encountered in the real world. For instance, the theory that the structure of DNA was a double helix was developed after Rosalind Franklin took hundreds of X-ray crystallography images of DNA. The idea of a double helix explained her images as well as data obtained in other experiments.

But unlike biological theories, string theory did not arise from data. Instead, it was born of the belief that the universe is elegant and simple, and that the four known fundamental forces of the universe would be unified, as Einstein believed. In the 19th century, James Clerk Maxwell rocked the world by showing that electricity and magnetism were part of one force – electromagnetism. Ever since then, a mathematical ideal that allowed for the ‘unification’ of all the major forces in nature has been a primary goal for theoretical physicists. Besides electromagnetism, these include gravity and the two forces that operate at the subatomic level: the weak and the strong force. The strong force holds quarks together inside protons and neutrons and stops atoms from disintegrating. The weak force explains why atoms sometimes do disintegrate – it underlies radioactive decay.

The blithely named ‘standard theory’, has done a masterful job at explaining the operations of the strong and weak forces. And it has also managed to unify the strong and weak forces with the electromagnetic force. But it hasn’t been able to accommodate the existence of gravity.

What Einstein struggled to do – and what has defeated everyone until the 1980s – was to unify all the forces with a single type of law. This law should be as simple and elegant at describing the interactions and manifestations of the four forces as Einstein’s equation of E=mc².

String theory achieves this goal. Just as the mathematics of a vibrating violin string can explain all of music, so the mathematics of tiny vibrating strings that vibrate in 11 dimensions, are capable of explaining the four forces, plus matter and energy; and hence the universe itself.

The gestation of string theory has so far taken the past four decades and is still far from being fully developed. Its beginnings trace to Gabriel Veneziano who, in 1968, while studying at the European Centre for Theoretical Physics (CERN) in Switzerland, stumbled across a mathematical formula that explained the interactions of the strong force. The formula turned out to describe not ‘point particles’ but vibrating strings. However string theory as an explanation for the strong force was abandoned because it predicted something absurd: a ‘massless spin 2 particle’. Later physicists realized this was just the characteristic of a graviton – the hypothetical particle proposed to carry the force of gravity. With that, a number of enthusiastic physicists proposed that string theory could be used to explain gravity, and in 1984 – after some more mathematical alchemy – John Schwarz at California Institute of Technology and Michael Green at Queen Mary’s College in London, found that string theory could also encompass all the other forces. String field theory is to string theory what the electromagnetic field is to photons: a way of describing how strings act coherently in the real world. In 1974, Michio Kaku and Keiji Kikkawa demonstrated that it could work in 10 dimensions.

Nearly 20 years later, in 1993, Ed Witten at Princeton University in the U.S. sparked a mini-revolution when he showed that string theory worked best in 11 dimensions. His seminal demonstration has taken string field theory physicists, Kaku among them, back to the drawing board. String theory predicts alternate universes because it fits with the physics of the subatomic scale (quantum dynamics) as well as with cosmology (it is the only theory to achieve this unification, and that’s why it remains compelling).

At the quantum level, particles of matter such as electrons can be in two places at once. In fact, this seemingly absurd statement is validated by modern electronics. Extrapolated, this means that the building blocks of our world might actually be leading an alternative existence somewhere else... as you sit reading this, there may be another you reading in another parallel world.

String theory as applied to cosmology suggests that strings were the first thing to be created, and they were originally created in 11 dimensions. Since the world as humans experience it has only four dimensions (three of space plus one of time), these other dimensions are probably out there and our universe is perhaps just one bubble in an infinite sea of bubble universes.

BIG BANG

The universe may not always existed. Scientists think it began about 13 to 15 billion years ago with a ‘Big Bang’. It is believed that one moment there was just a tiny hot ball containing everything in the universe and then a moment later, the universe burst into existence with the biggest explosion of all time, separating into basic forces such as electricity and gravity—so big that everything is still hurtling out from it—even today.

The Big Bang Theory

1. At first, the entire universe was a hot ball tinier than an atom and much hotter than any star. This swelled much, much faster than the speed of light, growing to the size of a galaxy in just a tiny fraction of a second.

2. As the universe expanded, it began to cool and tiny particles of energy and matter—each of them much smaller than atoms—began to form a thick, soup-like material.

3. After about 3 minutes, gravity started to pull the particles together. Atoms joined together to make gases such as hydrogen and helium, and the thick ‘soup’ began to clear and thin out. By the ends of the third minute, the matter that surrounds us today had been created.

4. Over time, as the young universe grew larger, the gases clumped into clouds. After several hundred million years, the clouds began to form stars and galaxies.

HOW DO WE KNOW?

Astronomers developed the Big Bang Theory, and worked out how long ago it occurred, by looking at how galaxies move in space. They also discovered that every galaxy in the universe is zooming away from Earth. If this is so, the universe must be expanding. If the universe is getting bigger, there must have been a time when it was much smaller—this is known as the ‘Expanding Universe’ theory.

RED SHIFT

Astronomers are able to tell that a galaxy is moving by its color. If a galaxy is moving away from Earth, its light waves stretched and the redder it appears. This is called RED SHIFT.

ABOVE and BEYOND

Some astronomers think the universe will expand forever. Others think the expansion will stop then collapse back to a ‘big crunch’

In March 2002, astronomers spotted the oldest galaxy yet seen—13 billion light-years away and 15 billion years old. Since this is older than the estimated age of the universe, scientists may have to work out the universe’s age again.

IT’S A FACT

The farthest galaxies from Earth are hurtling away at almost the speed of light.

The afterglow of the Big Bang can be detected in microwave radiation which emanates from all over space.

Scientists questioned the origin of the Universe for the first time in the 20^th century.

Could it really have existed in one form, and forever—or was there a starting point, and a process of continual change?

The evidence pointed more and more forward the Big Bang, and the Universe only 13.7 billion years old.

Until around 100 years ago, the Universe was thought not to change over time. but during the 20^th century, scientists collected evidence proving that the Universe is not static, but is in a constant state of expansion and change. Its expansive state is linked to an explosive event known as the Big Bang.

Origin of the Big Bang Theory

The seeds for the new view of the Universe were planted in 1916 when Albert Einstein published his theory of general relativity, from which the Dutch astronomer Willem de Sitter developed an imaginary expanding Universe. The idea became reality in 1929, when Edwin Hubble showed the Universe is expanding, which means that it was once smaller and denser. It has, however, Belgian Georges Lemaitre’s suggestion, in 1931, that the Universe’s origin was a primeval “cosmic egg” that exploded, creating an expanding Universe, which served as the first Big Bang model.

Scientists questioned the origin of the Universe for the first time in the 20^th century.

Could it really have existed in one form, and forever—or was there a starting point, and a process of continual change?

The evidence pointed more and more forward the Big Bang, and the Universe only 13.7 billion years old.

Until around 100 years ago, the Universe was thought not to change over time. but during the 20^th century, scientists collected evidence proving that the Universe is not static, but is in a constant state of expansion and change. Its expansive state is linked to an explosive event known as the Big Bang.

Origin of the Big Bang Theory

The seeds for the new view of the Universe were planted in 1916 when Albert Einstein published his theory of general relativity, from which the Dutch astronomer Willem de Sitter developed an imaginary expanding Universe. The idea became reality in 1929, when Edwin Hubble showed the Universe is expanding, which means that it was once smaller and denser. It has, however, Belgian Georges Lemaitre’s suggestion, in 1931, that the Universe’s origin was a primeval “cosmic egg” that exploded, creating an expanding Universe, which served as the first Big Bang model.

The Evidence mounts

In the 1940s, Fred Hoyle together with Americans Hermann Bondi and Thomas Gold, put forward an alternative view of the Universe. The “Steady State” theory they proposed that the Universe appears the same in every location and at all times. It has no beginning or end, and matter is commonly created.

In 1948, George Gamow outlined how the relative proportions of the hydrogen and helium in today’s Universe could be produced in a Big Bang Universe. The case for the Big Bang strengthened after 1955, when Martin Ryle showed that the distant, older radio galaxies were more numerous and densely packed than those nearby. This disproved the vital characteristic of the hypothetical Steady State Universe, in which density would always stay the same.

Direct evidence for the Big Bang came a decade later. George Gamow had predicted that a remnant of the Universe’s initial radiation would remain: the microwave background that would have permeated all space, before starting to cool. This was detected in 1946 by Arno Penzias and Robert Woodrow Wilson, confirming that the Universe was intensely hot in the past.

The Start of the Universe

We now think that the Big Bang occurred 13.7 billion years ago, starting our Universe. It produced all energy, matter, and space, and marked the recognized start of time. At the outset the Universe was dense, hot, and contained pure energy. It rapidly ballooned in size, before settling to steadier state of expansion. It consisted of tiny particles of energy, which became tiny particles of matter within seconds. Within minutes, the Universe had turned almost entirely into the nuclei of hydrogen and helium atoms.

The thousands of galaxies, stars, and planets that are visible from Earth are some of the billions of objects that make up the Universe, astronomy is the study of the Universe and how it changes with time.

Stars and planets have been studied since the first humans looked up the night sky. They made sense of the bright dots they saw by grouping them into constellations. They followed the movements of the Moon and planets, and developed a simple model of the Universe. More recently, scientists have studied what stars are made of and how they form, evolve, and die. Most scientists believe the Universe was born in a ‘big bang’—an instant when all matter was created and time began.

Almost all the objects in the Universe are too far away for missions from Earth and in orbit to gather information  from the light, X-rays, radio, and infrared radiation given out by such distant objects, space probes have visited some of the planets, comets, and asteroids, and twelve men have walked on the Moon. Each year, astronomers and space scientists discover more objects in space and learn new details about those objects in space and learn new details about those objects that have already been known for some time.