TOP QUARK is the sole piece of the Standard Model yet to be unambiguously detected. This image shows a top-quark candidate spotted at the Fermi National Accelerator laboratory in 1989.
Particle Metaphysics John Horgan Scientific American Feb 94 (extract)
In a sense, physicists are victims of their own success. They have constructed a theory that accounts for particle interactions with extraordinary accuracy. Called the Standard Model (or; by Michael Dine of the University of California at Santa Cruz, the "theory of almost everything"), it rests on the sturdy foundation of quantum mechanics, a radical theory of matter and energy erected by such giants as Niels Bohr, "'emer Heisenberg and Erwin Schrödinger in the 1920s and 1930s. In the 195Os Richard Feynman and others invented a theory of electromagnetism, called quantum electrodynamics, that accounts for virtually all chemical and electronic phenomena. During the following decade, physicists developed a theory for the strong nuclear force: quantum chromodynamics, or QCD. It holds that protons and neutrons are manifestations of more elementary particles named quarks. Each proton and neutron is composed of three quarks, which are themselves bound together by force-carrying particles described as gluons. (The prefix "chromo" refers to the fact that quarks are categorized according to their "color," a quantum mechanical property unrelated to color in its usual sense.) The field took a huge step toward unification in the 1960s, when Weinberg, Sheldon I.. Glashow, Abdus Salam and others proposed that electromagnetism and the weak nuclear force are actually two manifestations of the same electroweak force, [the weak force being carried by heavy photons W+, W- and Zo]. Both quantum chromodynamics and the electroweak theory have been validated by increasingly stringent tests at the world's major accelerator laboratories.
No GUTs, No Glory
Meanwhile theorists had alreadl, forged far beyond the Standard Model in search of a deeper theory. They were encouraged by the fact that quantum chromodynamics and the electroweak theory are both gauge theories, which posit that all the elements of a system can undergo transformations-such as rotation or reflection in a mirror-without being ftmdamentally altered. This feature, called symmetry, has become for many particle physicists the epitome of truth and beauty. Early in the 1970s Glashow and Georgi, his younger colleague at Harvard, invented a gauge theory, called SU(5), that could yield both electroweak interactions and strong interactions. (The term "SU(5)" refers to the number of symmetries displayed by the theory.) The grand unified theory-a term that Glashow and Georgi did not invent and insist they do not like-made what was then a startling prediction. Quarks, it seemed, could change into neutrinos, electrons and their antimatter counterparts; that meant protons (which are composed of quarks) were unstable and would eventually decay. Although the estimated half-life of any particular proton could be longer than the age of the sun (according to calculations done later by Weinberg, Georgi and Helen Quinn), physicists could test the prediction by watching a sufficiently large number of protons.
Proton-decay detectors have now been built at more than half a dozen sites around the ivorld. Most are placed deep underground to minimize signals from cosmic rays-high-energy particles from outer space. One of the largest experirnents began operating in a salt niine near Cleveland, Ohio, some 10 years ago. It consists of a gigantic vat of water surrounded by photodetectors that watch for the minute flashes of light that should be released by the decay of a proton in the water. So far neither this nor any other detector has observed proton decay. The lack of experimental support for the SU(5) theory merely opened the door to alternatives, notably a more general approach known as supersymmetry. Sunpesymmetry holds that fermions, the particles that constitute matter, and bosons, which transmit forces, share deep symmetries. The scheme requires that each known particle have a relatively massive, supersymmetrical partner, or "sparticle." One striking feature of supersymmetry is that its power can be increased if it is extended into extra dimensions. Just as an astronaut rising above the two-dimensional plane of the earth can apprehend its global symmetry so can theorists discern the more subtle symmetries underlying particle interactions by viewing them from a higher-dimensional standpoint.
Theorists have constructed various supersymmetrical grand unified theories and even quantum gravity theories. An example of the latter is supergravity, which assumes that gravitons, the particles that transmit gravity, have supersymmetrical partners called gravitinos. In 1980 supergravity seemed so promising that Stephen Hawking announced that it rrdght represent the long-sought "complete and unified theory of physics." But supergravity soon bogged down in mathematical problems related to the definition of gravitons as points. just as division by zero yields an infinite and hence mearingless result, so do calculations involving pointlike particles. Gauge theories had helped physicists constructing models of electromagnetism and the nuclear forces overcome this problem. But gravity with its distortions of space and time, seemed to demand an even more radical approach. Many physicists think superstring theory represents that approach. Superstring theory began rather modestly. In the early1970s theorists proposed that the strong nuclear force could stem from interactions of string-like particles. In the same way that vibrations of violin strings give rise to different notes, so could the vibrations of these strings yield the disparate particles ininvolved in the strong force. That route was abandoned in favor of the far more successful method using quarks and gluons. But string theory was resurrected in supersymmetrical form in the late 1970s by Michael B. Green and John H. Schwarz. Somewhat to their amazement, Green and Schwarz found that supersynunetrical strings generated all the forces of nature, including gravity. Best of all, the substitution of strings for points eliminated many of the mathematical problems arising in other quantum gravity theories. The theory demands acceptance of some far-fetched assumptions about physical reality. The strings are thought to inhabit as many as 26 dimensions, and they are as small in comparison to a proton as a proton is in comparison to the solar system. This micro-realm, named the Planck scale, is inaccessible to any conceivable experiment. Physicists and increasingly mathematicians have nonetheless become entranced by the theory's rich structure. Indeed, Edward Witten, a premier superstring theorist, has become as potent an influence in mathematics as in physics. Yet even Witten, whose analytical skills are legendary has struggled to link superstrings to known physical phenomena. Recently he has forged a bridge between superstrings and black holes, which have traditionally been the playground of theorists specializing in general relativity rather than particle physics. In 1991 Witten showed how superstring theory could yield black holes-albeit only highly-simplified, two-dimensional ones. Witten's paper triggered a burst of theoretical activity that continues to this day. Superstrings may also help wrap up a conundrum related to black holes pointed out by Hawking two decades ago. Hawking showed that quantum effects might cause black holes to radiate away energy-and therefore mass-until they eventually evaporate. He summed up his finding with the phrase "Black holes ain't so black." Because a black hole represents, at least in principle, a record of the processes that created it, its evaporation results in a permanent loss of information. The past, in a sense, is eradicated. Hawking proclaimed, and many theorists agreed, that he had uncovered a paradox that could be resolved only by modifying either quantum mechanics or general relativity.
In a paper published in Physical Review Letters last October , Leonard Susskind of Stanford University shows how superstrings might solve the puzzle. The paradox identified by Hawking, Susskind explains, arises out of the assumption, embedded in general relativity that different observers have the same picture of how information is stored in a given region of space and time. But according to superstring theory different observers can have different pictures. For any single observer, the past is preserved.
Critics charge that such work is not even physics, because it is so divorced from any experimentally accessible phenomena. Susskind retorts that progress in physics can no longer be achieved in traditional ways. "It's quite apparent to me that if the questions raised oner the past 15 to 20 years are going to be resoIveed, it is not goin.to be through experiments proceeding in incremental steps," Susskind declares. "it would be hopeless to get to the Planck scale this way. People who ignore this are simply going to become irrelevant.' Witten expresses the same vew, though somewhat more mildly: "I think, we could do much better with experiments, but I have faith in human perseverance."
Some workers claim CERN has already provided what may be tentative evidence of supersymmetry. Researchers there have been performing higldy precise measurements of the so-called coupling constants of electromagnetism, the weak force and the strong force. (The coupling constant of a force is a measure of its strengtIL) Grand unified theories predict that the couphng constants of the three shortrange forces, which have different values at low energies, should converge at high energies. CERN's data disagree with the predictions of the old SU(5) GUF, but when supersymmetry is added to the theory the predictions match .. exactly," according to Ellis of CERN. Others find these results unconvincing.
is modeled by a sculpture that is made of key rings.
Does Particle Physics Need a New Paradigm?
Although it is difficult to understand and to manipulate, superstring theory is by far the leading candidate for a quantum gravity theory. One reason may be that it represents an extension of ideas, such as symmetry, that are deeply ingrained in the culture of particle physics. Some theorists think it is time to consider fresh approaches. One innovative quantum gravity scheme has emerged over the past half a dozen years from a group of specialists in general relativity. It is called loopspace theory, or the Ashtekar theory, after one of its originators, Abhay Ashtekar. Ashtekar and his colleagues have found a way to rewrite the equations of general relativity so that they resemble the equations of quantum electrodynamics. This method allows them to treat gravity as a quantum mechanical phenomenon without encountering the mathematical problems that have blocked other attempts. One of the implications of the theory is that space is not a seamless entity but is composed, like a sheet of chain mail, of discrete, infinitesimal loops. An even more radical concept has been put forward by Gerard 't Hooft of the University of Utrecht in the Netherlands. 't Hooft played a major role in the development of gauge theories, which are the language in which the Standard Model is written. Yet he has become increasingly dissatisfied with current approaches to quantum gravity, which he feels gives short shrift to such essential concepts as causality. He suggests that physicists try constructing physical models based on cellular automata, creations of computer science that have causality as a basic feature. A cellular automaton consists of a grid of discrete units, or cells, that evolve according to specific rules. The state of each cell is determined by the states of its immediate neighbors. Another prominent physicist advocating new modes of thinking is Roger Penrose of the University of Oxford. He questions the assumption of most particle physicists that symmetry is a fundamental feature of nature rather than of their theories. This belief, Penrose contends, rests ultimately on an aesthetic preference, one that he and other physicists do not necessarily share. "In my view," Penrose writes in a recent essay, "if there is to be a final theory, it could only be a scheme of a very different nature. Rather than being a physical theory in the ordinary sense, it would have to be a principle-a mathematical principle whose implementation might itself involve non-mechanical subtlety."
Loops Spaces/2 Scientifc American Sept 92
Shortly after Ashtekar reported on his work in 1985, Theodore A. Jacobson, Carlo Rover and Lee Smolin determined that Ashtekar's new formulation could indeed generate exact solutions for certain quantum giravitational states. Borrowing from the mathematics of knot theory, Rover and Smolin then showed that each of these quantum states could be considered as a different knot, of which the simplest form was a loop. These loops are akin to the lines of force generated by an electric field. Ashtekar, Smolin and Rover have been trying to demonstrate how these knots and loops can be woven together like the links in a chain-mail coat, to form the apparently seamless fabric of space. The loops are so tightly knitted that be distinguished except at a distance of 10^-35 meter, a length as small in comparison to an atom as an atom is in comparison to the solar system. General relativity predicts that at precisely this scale, known as the Planck scale, quantum gravitational effects should become significant. This is also the scale at wlach superstrings are thought to operate. One crucial difference between loop-space theory and superstring theory is that the latter purports to explain not just gravity but all the forces and particles of nature. Such a theory is sometimes called a theory of everything. The loop-space theory is currently less ambitious, generally describing only the passage of gravitons through otherwise empty space. The gravitons appear as 'embroidery' knitted into the loops. Ashtekar and his colleagues have just begtun inserting particles corresponding to other forces and to mass, such as photons or protons, into the loop-space model. Although the model correctly describes the interactions of gravity and matter, Smolin concedes that it may never "explain' all the forces of nature from first causes in the way superstring theory is intended to do. 'We don't have some overreaching ambition to have a complete theory of nature,' Smolin says. "Our idea is to have a quantum mechanical theory of gravity.' That in itself would be a tremendous achievement. Gravity is by far the weakest of all the forces, and under most circumstances its effects on small-scale, quantum interactions can be ignored. But a quantum gravity theory is crucial for understanding what happens in the first instant of the big bang or toward the center of a black hole, where energies and densities surge toward infinity and Planck-scale effects predominate.
Some physicists [notably super-string theorists have suggested that the loop-space theory is based on flawed assumptions and will not lead to any significant breakthrough. Smolin counters that the approaches are likely to be complementary. A consistent theory of quantum gravity, Smolin adds, may also help resolve some of the nagging philosophical questions that quantum mechanics injected into physics early in this century. Is uncertainty intrinsic to reality? Does reality exist independent of observation? 'I don't think you can solve these problems with philosophy or just by talkingg about them it will require some new physics' -John Horgan
Gathering String Scientific American June 94
Standing a safe distance outside a black hole, toss in a coin. As it nears the black hole's horizon-the point of no return-the coin will seem to fall ever more slowly until it hardly moves. Now suppose that the elementary particles making up the coin resemble not points but tiny bits of string. As they fall in, the strings grow continuously longer. They wind around until they encase the black hole in a giant spaghetti-like entanglement. Odd? An inevitable blend of black hole physics and string theory, says Leonard Susskind of Stanford University. The black hole warps the space-time around it so acutely that time stretches out as in a slow-motion movie-one microsecond for the coin seems to us to be several days or years. Even though the coin does fall into the black hole, we can only see it slow down and come to a stop at the horizon. Moreover, a string, like the wings of a hummingbird, is always vibrating. Most of the time such movement is just a blur. But catch it in a slow-motion movie, and the vibrating object suddenly looks opaque-and larger. So, too, a string; it grows longer if we are able to see it slowed down. Further, a string vibrates in many different ways. Thus, as it falls toward the black hole, and its microseconds stretch out into minutes or days, it seems from our point of view to elongate endlessly. This picture would be merely a curiosity if it did not promise to solve what Susskind calls 'a puzzle as deep as the constancy of the speed of light was' at the turn of the last century. The puzzle is the information paradox. First posed in 1974 by Stephen W. Hawking, the information paradox notes that objects such as encyclopedias or elephants can fall into a black hole, never to be seen again. What happens to the knowledge they carried, the details about the atoms they were made of? If, as Hawking believed, these are lost forever, then physics is in trouble. Whereas in practice information can be irretrievable, Gerard 't Hooft has explained, quantum mechanics dictates that in principle the information should still be there in some form. 'Theoretical physicists have been very thoroughly confused for some time,' says Edward Witten . One suggested way out of the paradox is that as the coin falls toward the black hole's horizon, its information is somehow scrambled and sent back to us as radiation. Still, the horizon can hold an infinite amount of ordinary matter. Within its finite lifetime, how can the black hole possibly emit the infinite amounts of information the matter must have carried in? This is where string theory holds out some hope. If strings make up matter, they will spread out and take up all the room at the horizon-allowing the black hole to ab sorb only a finite amount of material. Presumably information carried in could be encoded in radiation that the strings emit as they fan out. So is the information paradox solved? 'The scenario is plausible and attractive,' Witten says, 'but there is no smoking gun.' String theory is very far from being complete; no one can as yet do all the calculations needed to verify this solution. As Susskind puts it, 'Strings can't solve the problems of black holes until they solve their own first.' Spaghetti may be on the plate of theorists well into the next century. -Madhusree Mukerjee
Unbearable lightness (extract) Scientific American May 94
A new theory may explain why objects tend to stay put
Suffering from inertia? Gravity got you down? You are not alone. S Gravity and inertia are among the most fundamental attributes of anything possessing mass. But researchers have never attained a satisfactory understanding of the fundamental nature of gravity. inertia has proved an even more elusive problem Ever since Isaac Newton articulated his three laws of motion, scientists have simply accepted the existence of inertia as a given: bodies in motion remain in motion, and those at rest stay at rest, unless acted on by an outside force. Bernhard M. Haisch of the Lockheed Palo Alto Research Laboratory, Alfonso Rueda of California State University at Long Beach and Harold E. Puthoff of the Institute for Advanced Studies in Austin, Tex., think they may at last have a due to the process that gives rise to inertia. That process, Haisch argues, must be connected to gravitation as well, neatly unifying inertial and gravitational mass, the two ways that physicists define the mass of an object. Writing in the February issue of Physical Review A, the three researchers describe as the consequence of the subatomic happenings that take place ostensibly empty space. Quantum theory predicts that, on such tiny scales, random quantum fluctuations roil the vacuum, creating a soup of virtual particles. Those particles continuously pop in and out of existence before they can be detected. Haisch and his collaborators started by assuming the existence of such small-scale electromagnetic fluctuations, known as the zero-point field. They then examined the effects of the field on normal matter. In the mid1970s several researchers showed that an object accelerating through the zero-point fteld should be exposed to a glow of radiation stirred up from the vacuum. Haisch, whose background is in astrophysics, wondered whether that radiation would exert a 'pressure' opposing the acceleration, such a pressure exactly fits the description of inertia. Rueda cast those ideas in mathematical form and became convinced that Haisch was on to something. 'Intuitively, it made a lot of sense,' he says. "The only thing that can resist the accelerating agent is the vacuum-what else is there?' He notes that the zero-point field is present at all times and in all places, wtach would explain the instantaneous, universal nature of inertia. The two scientists soon teamed up with Puthoff, who had been exploring possible connections between gravity and the zero-point field. Although theorists have had considerable success understanding the other three forces of nature (electromagnetism and the two nuclear forces), gravity has always been the oddball," Haisch reflects. Puthoff, drawing on earlier work by the late Russian physicist Andrei Sakharov, seeks to explain gravity as a long-range effect of zero-point electromagnetic fluctuations' Linking gravity to the zero-point field automatically draws inertia into the eplanation and so naturally accounts for the equivalence of inertial and gravitational mass.
The ambitious, unconventional theory of inertia immediately faces a dubious audience.
The three researchers also look to observational support from an upcoming experiment at the Stanford Linear Collider which will measure the effect of electromagnetic radiation on the apparent mass of the electron. That phenomenon raises the highly speculative prospect that the proper electromagnetic field could eliminate the inertia of an object, thereby permitting levitation.
Corey S Powell
A MASSIVE DISCOVERY Sci Am Aug 98 9
The weight of neutrinos offers clues to stars, galaxies and the fate of the universe
Future historians may took back on 1998 as the year that particle physics got interesting again. For decades, the search for the fundamental nature of matter has been reduced to a jigsaw puzzle. The Standard Model of particle physics provided the frame, with outlines of each of the two dozen elementary particles sketched in their proper places. When an army of almost 1,000 physicists discovered the top quark in 1995, the puzzle seemed to be complete. Only a bit of bookkeeping remained: to confirm that the three lightest particles-the electron-, muonand tau-neutrinos-indeed weigh exactly nothing, as the Standard Model predicts. But in June the 120 Japanese and American physicists of the Super-Kamiokande Collaboration presented strong evidence that at least one of the neutrinos (and probably all of them) weighs something. That neutrinos have a small mass is no small matter. It could help explain how our sun shines, how other stars explode into brilliant supernovae and why galaxies cluster in the patterns that they do. Perhaps most important, explains Lincoln Wolfenstein, a physicist at Carnegie Metion University, "once you accept that one neutrino has mass, you realize that the truth is something beyond the Standard Model." Nearly all neutrino physicists have accepted the conclusion, because the new data are supported by several years of similar observations at other detectors. John N. Bahcall of the Institute for Advanced Study in Princeton, N.J., says the evidence "seems completely convincing to me. It is simply beautiful!" The neutrino is certainly sublime in its subtlety. Quarks and electrons are impossible to miss; we and our world are made from them. The muon and tau, cousins of the electron, are unfamiliar because they die almost at birth. But neutrinos surround us perpetually, yet invisibly. Trillions zip through your body as you read this. Created by the big bang, by stars and by the collision of cosmic rays with the earth's atmosphere, neutrinos outnumber electrons and protons by 600 million to one. "If they have a mass of just one tenth of an electron volt [an electron, in comparison, weighs about 500,000 eV], then neutrinos would account for about as much mass as the entire visible universe," says Joel R. Primack, a cosmologist at the University of California at Santa Cruz. About 0.1 eV now seems to many physicists a likely mass for the muonneutrino. They can't be certain yet, because the only way to weigh particles that can zoom almost unhindered throtigii the earth at nearly the speed of light is to do so indirectly. By patiently watching a high-tech cistern buried 2,000 feet underneath a Japanese mountain, physicists working on the SuperKamiokande project could record faint flashes emitted on the exceedingly rare occasions when a muonor electron-neutrino collided with one out of the 50,000 tons of water molecules in the tank. Over time, traces from those neutrinos that had been created in the atmosphere started to reveal a pattern. Those arriving from above came in the expected proportion and number. "We even saw a hot spot toward the east caused by a well-known asymmetry in the earth's magnetic field" that creates more cosmic-ray collisions in that direction, says Todd J. Haines of Los Alamos National Laboratory. But too few muon-neutrinos ar rived from below. Two large groups of physicists worked inde pendently to explain why. Both ruled out all explanations save one: the three kinds of neu trinos are not different particles in the way that electrons and muons are. Each neutrino is in fact a mixture of three mass states. The mixture can change as the neutrino travels, transforming muonneutrinos created above South America into heavier tau-neutrinos by the time they reach the detector in japan. That is why too few muon-neutrinos appeared in the Super-Kamiokande tank; some had metamorphosed into another, undetectable type. Theorists figure that there is no way to have mass states without having mass. But so far all that Henry W Sobel, a physicist at the University of California at Irvine and spokesman for the collaboration, can say is that the difference between the mass of muon-neutrinos and whatever they are changing into is between 0.1 and 0.01 ev-definitely not zero. Wolfenstein points out that "this does not solve the solar-neutrino problems," the most baffling of which is the fact that only half the electron-neutrinos that theoretically should fall from the sun to the earth are actually detected here. But Bahcall adds that it does "strengthen the conviction of nearly everyone involved in the subject that the explanation of the solar-neutrino problems is oscillations" of neutrinos from one variety to another on their journey to the earth. "A little hot dark matter in the form of massive neutrinos may be just what is needed" to help reconcile another astrophysical accounting discrepancy, Primack says. Many lines of evidence suggest that there is about 10 times more matter in the universe than human instruments can see. Neutrinos will now fill in some of that missing matter. Whether neutrinos weigh enough to make a significant difference in the fate of the universe and the composition of matter remains a mystery. "We can't build theories on this without firmer data about the masses and transition amplitudes," says Steven Weinberg, one of the architects of the Standard Model and a professor at the University of Texas. "We're still far from that. But there are some very important experiments in the wings that may answer those questions." In January physicists will create a beam of muon-neutrinos in an accelerator near Tokyo and aim it at the SuperKamiokande detector. Within a few years, scientists at Fermi National Accelerator Laboratory in Batavia, lit., hope to send swarms of the particles flitting toward a detector deep in a Minnesota mine shaft. These controlled experiments may finally fill the last holes in the Standard Model and, if we are lucky, reveal it to be only a part of a much larger, more beautiful picture. -W Wayt Gibbs in San Francisco