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Elegant Universe _1_ - Greene.pdf

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Unformatted text preview: .« it #~‘:-« uflkLaeHfl m El tenant-weir Uxm WW1}! V EH om GYii nu Chapter 1 Tied Up with String C alling it a cover-up would be far too dramatic. But for more than half ‘ a century—even in the midst of some of the greatest scientific - achievements in history—physicists have been quietly aware of a dark cloud looming on a distant horizon. The problem is this: There are two foundational pillars upon which modern physics rests. Onelis Albert Ein- stein’s general relativity, which provides a theoretical framework for un- derstanding the universe on the largest of scales: stars, galaxies, clusters of galaxies, and beyond to the immense expanse of the universe itself. The other is quantum mechanics, which provides a theoretical framework for understanding the‘universe on the smallest of scales: molecules, atoms, and all the way down to subatomic particles like electrons and quarks. Through years of research, physicists have experimentally confirmed to al- most unimaginable accuracy virtually all predictions made by each of these theories. But these same theoretical tools inexorably lead to another disturbing conclusion: As they are currently formulated, general relativity and quantum mechanics cannot both be right. The two theories underly- ing the tremendous progress of physics during the last hundred years—— progress that has explained the expansion of the heavens and the fundamental structure of matter—are mutually incompatible. _ If you have not heard previously about this ferocious antagonism you may be wondering why. The answer is not hard to come by. In all but the most extreme situations, physicists study things that are either small and 3 fl 3‘} $1 The Elegant Universe light (like atoms and their constituents) or things that are huge and heavy {like stars and galaxies), but not both. This means that they need use only quantum mechanics or only general relativity and can, with a furtive glance. shrug off the barking admonition of the other. For fifty years this approach has not been quite as blissful as ignorance, but it has been pretty close. But the universe can be extreme. In the central depths of a black hole an enormous mass is crushed to a minuscule size. At the moment of the big bang the whole of the universe erupted from a microscopic nugget whose size makes a grain of sand look colossal. These are realms that are tiny and yet incredibly massive, therefore requiring that both quantum me- chanics and general relativity simultaneously be brought to bear. For rea- sons that will become increasingly clear as we proceed, the equations of general relativity and quantum mechanics, when combined, begin to shake, rattle, and gush with steam like a red—lined automobile. Put less fig- uratively, well-posed physical questions elicit nonsensical answers from the unhappy amalgam of these two theories. Even if you are willing to keep the deep interior of a black hole and the beginning of the universe shrouded in mystery, you can’t help feeling that the hostility between quantum mechanics and general relativity cries out for a deeper level of understanding. Can it really be that the universe at its most fundamental level is divided, requiring one set of laws when things are large and a dif— ferent, incompatible set when things are small? Superstring theory, a young upstart compared with the venerable edi- fices of quantum mechanics and general relativity, answers with a re- sounding no. Intense research over the past decade by physicists and mathematicians around the‘ world has revealed that this new approach to describing matter at its most fundamental level resolves the tension be— tween general relativity and quantum mechanics. In fact, superstring the— ory shows more: Within this new framework, general relativity and quantum mechanics require one another for the theory to make sense. Ac— cording to superstring theory, the marriage of the laws of the large and the small is not only happy but inevitable. That’s part of the good news. But superstring theory—string theory, for short—takes this union one giant step further. For three decades, Einstein sought a unified theory of physics, one that would interweave all of na- ture’s forces and material constituents within a single theoretical tapestry. 4 .gmummw . . Tied Up with String He failed. Now, at the dawn of the new millennium, proponents of string theory claim that the threads of this elusive unified tapestry finally have been revealed. String theory has the potential to show that all of the won- drous happenings in the universe—from the frantic dance of subatomic quarks to the stately waltz of orbiting binary stars, from the primordial fire- ball of the big bang to the majestic swirl of heavenly galaxies—are reflec- tions of one grand physical principle, one master equation. Because these features of string theory require that we drastically change our understanding of space, time, and matter, they will take some time to get used to, to sink in at a comfortable level. But as shall become clear, when seen in its proper context, string theory emerges as a dramatic yet natural outgrth of the revolutionary discoveries of physics during the past hundred years. In fact, we shall see that the conflict between general relativity and quantum mechanics is actually not the first, but the third in a sequence of pivotal conflicts encountered during the past century, each of whose resolution has resulted in a stunning revision of our under- standing of the universe. The Three Conflicts The first conflict, recognized as far back as the late 1800s, concerns puz- zling properties of the motion of light. Briefly put, according to Isaac New- ton’s laws of motion, if you run fast enough you can catch up with a departing beam of light, whereas according to James Clerk Maxwell’s laws of electromagnetism, you can’t. As we will discuss in Chapter 2, Einstein resolved this conflict through his theory of special relativity, and in so doing completely overturned our understanding of space and time. Ac- cording to special relativity, no longer can space and time be thought of as universal concepts set in stone, experienced identically \by everyone. Rather, space and time emerged from Einstein’s reworking as malleable constructs whose form and appearance depend on one’s state of motion. The development of special relativity immediately set the stage for the second conflict. One conclusion of Einstein’s work is that no object—in fact, no influence or disturbance of any sort—-—can travel faster than the speed of light. But, as we shall discuss in Chapter 3, Newton’s experi- mentally successful and intuitively pleasing universal theory of gravita- 5 The Elegant Universe tion involves influences that are transmitted over vast distances of space instantaneously It was Einstein, again, who stepped in and resolved the conflict by offering a new conception of gravity with his 1915 general the- ory of relativity. Just as special relativity overturned previous conceptions of space and time, so too did general relativity. Not only are space and time influenced by one’s state of motion, but they can warp and curve in re- sponse to the presence of matter or energy. Such distortions to the fabric of space and time, as we shall see, transmit the force of gravity from one place to another. Space and time, therefore, can no longer to be thought of as an inert backdrop on which the events of the universe play them- selves out; rather, through special and then general relativity, they are in— timate players in the events themselves. Once again the pattern repeated itself: The discovery of general rela— tivity, while resolving one conflict, led to another. Over the course of the three decades beginning in 1900, physicists developed quantum me- chanics (discussed in Chapter 4) in response to a number of glaring prob- lems that arose when nineteenth-century conceptions of physics were applied to the microscopic world. And as mentioned above, the third and deepest conflict arises from the incompatibility between quantum me- chanics and general relativity. As we will see in Chapter 5, the gently curv- ing geometrical form of space emerging from general relativity is at loggerheads with the frantic, roiling, microscopic behavior of the universe implied by quantum mechanics. As it was not until the mid-19805 that string theory offered a resolution, this conflict is rightly called the central problem of modern physics. Moreover, building on special and general rel— ativity, string theory requires its own severe revamping of our conceptions of space and time. For example, most of us take for granted that our uni- verse has three spatial dimensions. But this is not so according to string theory, which claims that our universe has many more dimensions than meet the eye—dimensions that are tightly curled into the folded fabric of the cosmos. So central are these remarkable insights into the nature of space and time that we shall use them as a guiding theme in all that follows. String theory, in a real sense, is the story of space and time since Einstein. To appreciate what string theory actually is, we need to take a step back and briefly describe what we have learned during the last century about the microscopic structure of the universe. 6 Tied Up with String The Universe at Its Smallest: What We Know about Matter . e ancient Greeks surmised that the stuff of the universe was made up tiny “uncuttable” ingredients that they called atoms. Just as the enor- ious number of words in an alphabetic language is built up from the 'ealth of combinations of a small number of letters, they guessed that the 'st range of material objects might also result from combinations of a mall number of distinct, elementary building blocks. It was a prescient ; ess. More than 2,000 years later we still believe it to be true, although i e identity of the most fundamental units has gone through numerous re— "sions. In the nineteenth century scientists showed that many familiar ubstances such as oxygen and carbon had a smallest recognizable con- tituent; following in the tradition laid down by the Greeks, they called em atoms. The name stuck, but history has shown it to be a misnomer, ince atoms surely are “cuttable.” By the early 19305 the collective works ‘ J. J. Thomson, Ernest Rutherford, Niels Bohr, and James Chadwick had * tablished the solar system—like atomic model with which most of us are familiar. Far from being the most elementary material constituent, atoms consist of a nucleus, containing protons and neutrons, that is surrounded by a swarm of orbiting electrons. For a while many physicists thought that protons, neutrons, and elec- trons were the Greeks’ “atoms." But in 1968 experimenters at the Stanford Linear Accelerator Center, making use of the increased capacity of tech— nology to probe the microscopic depths of matter, found that protons and neutrons are not fundamental, either. Instead they showed that each con- sists of three smaller particles, called quarks—a whimsical name taken from a passage in James Joyce’s Finnegan's Wizke by the theoretical physi- cist Murray Cell-Mann, who previously had surmised their existence. The experimenters confirmed that quarks themselves come in two varieties, which were named, a bit less creatively, up and down. A proton consists of two up-quarks and a down-quark; a neutron consists of two down-quarks and an up-quark. Everything you see in the terrestrial world and the heavens above ap- pears to be made from combinations of electrons, up-quarks, and down— quarks. No experimental evidence indicates that any of these three 7 The Bhgiaflr ”diverse miles is built up from something smaller. But a great deal of evidence staircases that the universe itself has additional particulate ingredients. In mt- rrmi~i9505, Frederick Reines and Clyde Cowan found conclusive ex- perimental evidence for a fourth kind of fundamental particle called a neutrino—a particle whose existence was predicted in the early 19305 by Wolfgang Pauli. Neutrinos proved very difficult to find because they are ghostly particles that only rarely interact with other matter: an average— energy neutrino can easily pass right through many trillion miles of lead without the slightest effect on its motion. This should give you significant relief. because right now as you read this, billions of neutrinos ejected into space by the sun are passing through your body and the earth as well, as part of their lonely journey through the cosmos. In the late 19305, another particle called a muon—identical to an electron except that a muon is about 200 times heavier—was discovered by physicists studying cosmic rays (showers of particles that bombard earth from outer space). Because there was nothing in the cosmic order, no unsolved puzzle, no tailor—made niche, that necessitated the muon’s existence, the Nobel Prize—winning particle physicist Isidor Isaac Rabi greeted the discovery of the muon with a less than enthusiastic ‘Who ordered that?" Nevertheless, there it was. And more was to follow. Using ever more powerful technology, physicists have continued to slam bits of matter together with ever increasing energy, momentarily recreating conditions unseen since the big bang. In the debris they have searched for new fundamental ingredients to add to the growing list of par— ticles. Here is what they have found: four more quarks—charm, strange, bottom, and top—and another even heavier cousin of the electron, called a tau, as well as two other particles with properties similar to the neutrino (called the muon—neutrino and tail—neutrino to distinguish them from the original neutrino, now called the electron-neutrino). These particles are produced through high-energy collisions and exist only ephemerally; they are not constituents of anything we typically encounter. But even this is not quite the end of the story. Each of these particles has an antiparticle partner—a particle of identical mass but opposite in certain other respects such as its electric charge (as well as its charges with respect to other forces discussed below). For instance, the antiparticle of an electron is called a positron—it has exactly the same mass as an electron, but its elec- tric charge is +1 whereas the electric charge of the electron is —1. When 8 Tied Up with String ‘Jcontact, matter and antimatter can annihilate one another to produce e energy—that’s why there is extremely little naturally occurring anti- , ‘ en called families. Each family contains two of the quarks, an electron 1". one of its cousins, and one of the neutrino species. The corresponding farticle types across the three families have identical properties except for their mass, which grows larger in each successive family. The upshot is that physicists have now probed the structure ,of matter to scales of about 1‘ a billionth of a billionth of a meter and shown that everything encountered to date——whether it occurs naturally or is produced artificially with giant atom-smashers—consists of some combination of particles from these ‘ three families and their antimatter partners. A glance at Table 1.1 will no doubt leave you with an even stronger 1 sense of Rabi’s bewilderment at the discovery of the muon. The arrange ment into families at least gives some semblance of order, but innumerable "whys” leap to the fore. Why are there so many fundamental particles, es- pecially when it seems that the great majority of things in the world around us need only electrons, up-quarks, and down-quarks? Why are there three families? Why not one family or four families or any other number? Why do the particles have a seemingly random spread of masses—why, for in- Family 1 Family 2 Family 3 Particle Mass Particle Mass Particle Mass Electron .00054 Muon .1 1 Tau 1.9 Electron- Muon- Tau- neutrino < 10‘8 neutrino < .0003 neutrino < .033 Up-quark .0047 ‘Charm Quark 1.6 Top Quark 189 Down-quark .0074 Strange Quark .16 Bottom Quark 5.2 Table 1.1 The three families of fundamental particles and their masses (in multiples of the proton mass). The values of the neutrino masses have so far eluded experimental determination. The Elegant Universe stance. does the tau weigh about 3,520 times as much as an electron? Why does the top quark weigh about 40,200 times as much an up—quark? These are such strange, seemingly random numbers. Did they occur by chance, by some divine choice, or is there a comprehensible scientific explanation for these fundamental features of our universe? The Forces, or, Where’s the Photon? Things only become more complicated when we consider the forces of na- ture. The world around us is replete with means of exerting influence: balls can be hit with bats, bungee enthusiasts can throw themselves earthward from high platforms, magnets can keep superfast trains suspended just above metallic tracks, Geiger counters can tick in response to radioactive material, nuclear bombs can explode. We can influence objects by vigor- ously pushing, pulling, or shaking them; by hurling or firing other objects into them; by stretching, twisting, or crushing them; or by freezing, heat- ing, or burning them. During the past hundred years physicists have ac- cumulated mounting evidence that all of these interactions between various objects and materials, as well as any of the millions upon millions of others encountered daily, can be reduced to combinations of four fun— damental forces. One of these is the gravitational force. The other three are the electromagnetic force, the weak force, and the strong force. Gravity is the most familiar of the forces, being responsible for keep- ing us in orbit around the sun as well as for keeping our feet firmly planted on earth. The mass of an object measures how much gravitational force it can exert as well as feel. The electromagnetic force is the next most fa- miliar of the four. It is the force driving all of the conveniences of modem life—lights, computers, TVs, telephones—and underlies the awesome might of lightning storms and the gentle touch of a human hand. Micro- scopically, the electric charge of a particle plays the same role for the elec- tromagnetic force as mass does for gravity: it determines how strongly the particle can exert as well as respond electromagnetically. The strong and the weak forces are less familiar because their strength rapidly diminishes over all but subatomic distance scales; they are the nuclear forces. This is why these two forces were discovered only much more recently. The strong force is responsible for keeping quarks “glued” 10 "i Tied Up with String together inside of protons and neutrons and keeping protons and neu- trons tightly crammed together inside atomic nuclei. The weak force is best known as the force responsible for the radioactive decay of substances such as uranium and cobalt. During the past century, physicists have found two features common to all these forces. First, as we will discuss in Chapter 5, at a microscopic level all the forces have an associated particle that you can think of as being the smallest packet or bundle of the force. If you fire a laser beam— an “electromagnetic ray gun”—you are firing a stream of photons, the smallest bundles of the electromagnetic force. Similarly, the smallest con- stituents of weak and strong force fields are particles called weak gauge bosons and gluons. (The name gluon is particularly descriptive: You can think of gluons as the microscopic ingredient in the strong glue holding atomic nuclei together.) By 1984 experimenters had definitively estab- lished the existence and the detailed properties of these three kinds of force particles, recorded in Table 1.2. Physicists believe that the gravita- tional force also has an associated particle—the graviton—but its exis- tence has yet to be confirmed experimentally. The second common feature of the forces is that just as mas...
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