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Progress in Physics INTRODUCTION We live in one of the most productive eras in the history of physics. This chapter highlights some advances and opportunities in physics that have been culled from the multitude of achievements reported in the accompanying volumes of the Physics Survey. Our discussion begins with a brief, nonspecialized summary of the highlights and a description of the unifying principles that join the different subfields of physics. Elementary-Particle Physics Elementary-particle physics, the science of the ultimate constituents of matter and their interactions, has undergone a remarkable develop- ment during the past two decades. A host of experimental observations made possible by the current generation of particle accelerators and the accompanying rapid convergence of theoretical ideas have led to a radically new and simple picture of nature. All matter in its infinite diversity has been found to be composed of a few basic constituents called quarks and leptons, which are structureless and indivisible at current limits of resolution. Great progress has also been made in understanding the character of fundamental forces that govern-natural phenomena. The weak and electromagnetic interactions have been 11

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12 PHYSICS THROUGH THE 1990s: AN OVERVIEW unified in a theory whose predictions have been verified by many experiments, culminating in the 1983 discovery of the W and Z particles, the mediators of the weak interaction The similarity among quarks and leptons and the mathematical resemblance among the theories of fundamental interactions spur bold attempts at unification in which all the fundamental forces are seen as different manifestations of a single underlying symmetry, a symmetry that is partially hidden. The new synthesis raises deep questions about family patterns of quarks and leptons and the origin of particle masses and invites speculation about the eventual compositeness of quarks and leptons themselves. These concerns motivate a broad program of experimentation at higher energies (and, equivalently, on shorter scales of distance and time) to test the emerging standard model and to uncover clues leading to more complete understanding. Nuclear Physics During the past decades, the building blocks of nuclei were thought to be protons and neutrons bound together by mesons. Today we know that protons and neutrons are made of quarks and that the forces between the quarks are created by particles called gluons. The new concept is based on theoretical advances in particle physics, but recent experimental work has demonstrated the importance of this description for nuclei also. The basic questions facing nuclear physics today involve detailed exploration of the quark structure in nucleons and nuclei and the strong many-body forces that confine quarks and gluons. Finding the answers represents an exciting frontier that may lead to more basic understanding of the strong forces and of nuclear structure and dynamics. Heavy ions have been used to probe nuclear dynamics under extreme conditions and to create new elements. Theoretical investiga- tion predicts the existence of a quark-gluon plasma similar to that which may have existed in the earliest moments of our universe. Studies are being conducted on new systems made in the laboratory called hypernuclei, in which a quark has been replaced by a strange quark. The use of high-energy electron scattering from nuclei is now revealing unprecedented levels of detail of nuclear structure, probing the electroweak interactions between nucleons and their underlying quark components. Finally, nuclear science continues to have great impact on our understanding of fundamental symmetries in physics, while also playing an ever increasing role in astrophysics and cosmol- ogy.

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PROGRESS IN PHYSICS 13 Condensed-Matter Physics Condensed-matter physics has continued its historic role as a major source of new concepts in fundamental science from the explanation of the behavior of neutron stars, through advances in our understanding of semiconductors, superconductors, and magnetisms, to prediction and discovery of a new state of matter, the superfluid phase of liquid helium atoms of mass 3. This is the area of physics that most directly fuels advances in technology, from jet engines to computers. Concep- tual advances abound, increasingly stimulated by the creation of totally new substances not found in nature. Some of these substances are produced by novel experimental techniques, such as exceedingly rapid cooling of liquids to the solid state or controlled deposition of atoms layer by layer. Others are produced by more conventional means on the basis of new theoretical hypotheses made possible by deepening theoretical understanding. Areas of great activity include studies of systems of one or two dimensions, studies of phenomena at surfaces, the role of interfaces between different materials, disordered systems, surprising new forms of ordered systems, the onset of turbulence in liquids, and the possibility of new forms of superconductivity. Atomic, Molecular, and Optical Physics This field has been revolutionized by the laser and modern optics. New atomic and molecular species have been created using laser light; spectroscopic resolution has been increased more than a millionfold. Lasers have made it possible to watch atoms as they collide and chemical reactions as they take place. Lasers are now being used to generate femtosecond light pulses and coherent soft x rays and to cool atoms to the submillikelvin regime. Optical-frequency counting meth- ods using laser light have become so precise that the meter is no longer defined in terms of the wavelength of light but as the distance light travels in a given time interval. Particle-trap techniques have led to ultraprecise studies of quantum electrodynamics and mass spectra; they have made it possible to study plasma liquids and to create new kinds of atomic clocks. Today's research opportunities include ultrasensitive tests of the properties of space and the symmetries in nature, studies of relativistic many-body theory and quantum electro- dynam~cs in heavy ions by advanced x-ray spectroscopy, new exper- imenta1 and theoretical approaches to the structure and interaction of atoms and molecules, and the creation of nonlinear optical techniques and new light sources.

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14 PHYSICS THROUGH THE 1990s: AN OVERVIEW Plasma Physics Most of the visible matter in the universe is made of plasmas- neutral gases composed of positive ions and unbounded electrons. Our understanding of stars, stellar winds, planetary magnetospheres, and galaxies is being spurred by advances in plasma physics. Spacecraft have probed the magnetospheres of the planets from Mercury to Saturn and soon will reach Uranus. The solar wind has been monitored by many spacecraft, from inside the orbit of Mercury to beyond Pluto. We may have data on galactic plasma beyond the influence of the Sun before the year 2000. The Earth's magnetosphere has been measured in great detail, and we are beginning to understand the complex phenom- ena seen there- its weather' so to speak. On Earth, our mastery of high-temperature plasmas has advanced remarkably. Today, one plasma-confinement approach is expected soon to achieve breakeven conditions for controlled fusion employing reactions similar to those that power the Sun and stars. Our under- standing of plasmas is having an impact on physics in many other ways. One example is the recent generation of electric fields of tens of millions of volts per centimeter in plasmas by the excitation of an electron-plasma oscillation. It is expected that the technique can be extended to give fields hundreds of times larger, i.e., as large as the electric field that holds electrons in atoms. The possibility of using these fields to accelerate particles to high energy is being explored. Another example is the development of the free-electron laser, which can generate coherent radiation from microwaves to the ultraviolet. Cosmology, Gravitation, and Cosmic Rays The study of the universe is being transformed by new eyes, such as x-ray and infrared telescopes in space and very-large-array radio telescopes on the ground. A vivid history of the universe has emerged, starting with a primordial explosion- the big bang about 15 billion years ago. Recent discoveries from elementary-particle theory are offering possible solutions to some of the profound questions in cosmology (for instance, why the universe appears to be so uniform, and why there is so much matter relative to antimatter). An intense search is under way for dark matter in the universe; such matter may dominate important processes such as the formation of galaxies and the ultimate fate of the universe. Will the universe expand forever or collapse to start anew in yet another primordial explosion? Fundamen-

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PROGRESS IN PHYSICS 15 tal theoretical advances in gravitational physics are leading to a better understanding of black holes and quantization of gravity, and a prediction of Einstein's General Theory of Relativity has been verified to 2 parts in 1000, using the Viking spacecraft on Mars. Basic theoretical and experimental research has now prepared the way for major efforts to detect gravitational radiation. This dramatic advance could take place in the next decade and open an important new window on the universe. Space-based studies of the enigmatic cosmic rays suggest that they originate in interstellar space, while ground-based instruments have discovered localized sources of gamma rays with 10'5 eV of energy. Interfaces and Applications Research at the boundaries between physics and neighboring areas such as chemistry, biology, materials science, and mathematics has blossomed with new ideas and new approaches. High-vacuum and surface science, the transition from orderly to chaotic motion, polymer and macromolecular structure, the origins of biological processes, microscopic control of structure and function in liquids and solids- these are but a few examples of physics' rapidly emerging interdisci- plinary advances, which are enriching all of science. The applications of physics are broad and affect virtually every area of society (see Figure 2.14. Our national programs in energy, the environment, medicine, and security depend critically on physics. Our industrial posture is linked to the flow of discoveries from physics that can lead to the creation of industries such as microelectronics and optical technology. THE UNITY OF PHYSICS The highlights in the preceding section hardly begin to portray current advances in physics and their effects on science and society. The following sections of this chapter provide a somewhat more detailed account; but for a comprehensive picture of today's research and tomorrow's opportunities, the reader is referred to the seven panel reports of the Physics Survey that accompany this overview volume. The scope of physics is so broad and its styles of research so diverse that it is easy to lose sight of the underlying unity that joins even the most disparate activities into a common enterprise. This unity is a fundamental source of the strength and vitality of physics: to under-

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16 PHYSICS THROUGH THE 1990s: AN OVERVIEW PHYSICS SUBFIELD ASTROPHYSICS PLASMA PHYSICS CONDENSED~MATTER PItYSICS ATOMIC, MOLECULAR AND OPTICAL PHYSICS NUCLEAR PHYSICS ELEMENTARY-PARTICLE PHYSICS FIGURE 2.1 Physics subfields and some of their related applications. APPLICATIONS LOW-LEVEL X-RAY DETECTION, X-AAY TOMOGRAPHY FUSION PLASMA PROCESSING MATERIALS DEPOSITION, ETCHING, ETC. FREE ELECTRON LASERS SEMICONDUCTOR DEVICES INTEGRATED CIRCUITS, COMPUTERS. JUNCTION LASERS, OPTICAL DETECTORS SUPERCONDUCTORS HIGH-FIELD MAGNETS JOSEPHSON TECInINOLOGY MATERIALS MAGNETIC MATERIALS, OPTICAL FlaERS, QUARTZ RESONATORS LASER-QPTICS LASER SURGERY, OPTICAL COMMUNICATIONS, OPTICAL DATA PROCESSING, INTEGRATED OPTICS, LASER- ASSISTED PROCESSING, LASER PRINTII`IG ENVIRONMENTAL MONITORING SUPERSENSITIVE DETECTION, ANALYSIS, ATMOSPHERIC CHEMISTRY M ETROLOGY MAGNETOMETERS, ATOMIC CLOCKS, RING GYROSCOPES MAGNETIC RESONANCE IMAGING - NUCLEAR POWER NUCLEAR MEDICINE RADIATION THERAPY, ISOTOPE TRACERS SYNCHROTRON LIGlIT SOURCES, DATA PROCESSING, PARTICLE DETECTORS stand physics it must be appreciated. We present some examples below. A dramatic confluence of ideas from three diverse subfields illus- trates the unexpected connections in physics that suddenly occur. Insights from particle physics based on the quark-gluon model (the modern theory of the structure of protons, neutrons, and other subnuclear particles) have been combined with contemporary ideas from condensed-matter theory to portray the evolution of the universe in the earliest stages of the primordial explosion the big bang. This synthesis of thought allows us to understand important features of the

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PROGRESS IN PHYSICS 17 universe that we can observe today as consequences of elementary ideas about the structure and organization of matter. Physicists are drawing on techniques from nuclear, condensed- matter, and atomic physics to address another cosmological problem- that of the missing mass in the universe. A critical question is whether a particle known as the neutrino has a finite mass or whether it is massless like the photon. The most sensitive laboratory experiments appear to show that the neutrino's mass must be less than one ten-thousandth the mass of the electron, but the question of the neutrino's mass is not yet settled. If it is large enough, general relativity predicts that the universe will not expand forever but that it will eventually collapse. A further example of links between diverse areas is the renewed interest in the relation between regular and chaotic motion. Abrupt transitions from regular to chaotic behavior have been discovered in electrical, acoustical, and optical systems, in fluid flow, in chemical reactions, and in the behavior of simple differential equations. Cardiac arrest due to fibrillation of the heart is believed to be due to such an effect. Recognition of the universal nature of such transitions gives hope for understanding chaotic motion in more complex systems and, in particular, of understanding turbulence. This line of research can be . expected to have a deep influence on many areas of science and on problems ranging from aircraft and ship design to weather forecasting. The subfields of physics are joined by technical as well as conceptual bonds. Lasers, for example, have had a dramatic effect on science and technology. They have revolutionized spectroscopy by enormously increasing its sensitivity and precision and have opened the way to the creation of new types of atomic and molecular species. Femtosecond (a millionth of a billionth of a second) laser pulses make it possible to take "snapshots" of chemical reactions; nonlinear spectroscopy makes it possible to study reactions as they occur (for instance, in combustion hames). High-power lasers can create plasmas under unprecedented conditions and may provide a method for accelerating particles without the need for gigantic accelerators. The influence of lasers on science is too broad to summarize; perhaps it is sufficient to point out that lasers are now ubiquitous in laboratories of physics, materials science, chemistry, biology, physiology, and many of the other sciences. One of the most dramatic developments in condensed-matter physics is the opportunity to carry out spectroscopy from the optical to the x-ray region by using radiation of unprecedented intensity, many orders of magnitude brighter than was previously possible. The radia- tion is provided by a gift from particle physics: synchrotron light

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18 PHYSICS THROUGH THE 1990s: AN OVERVIEW sources. The fundamental technology of synchrotron sources is the technology of electron accelerators. At the same time, this gift has been reciprocated in that the high-field superconductors created by materials scientists in the 1960s have made proton accelerators in the trillion-electron-volt (TeV) range practical. Superconductor technol- ogy is also having a profound influence on health care, for super- conducting magnets are an essential element of the magnetic resonance imaging (MRI) technique. Countless other examples could be cited of the unifying ideas and techniques that link even the most disparate subjects in physics and, indeed, link physics to the other sciences and to the central technical and industrial needs of society. Appreciation of the unity of physics is essential in planning research and developing science policy. PROGRESS IN PARTICLE PHYSICS Quarks and Leptons as Elementary Particles The longing to discover the most elementary particles in nature is deeply rooted in physics. At the beginning of this century, physicists discovered that the atom is not a single particle but that it consists of electrons moving rapidly around a central nucleus; in the 1930s, it was discovered that the nucleus is not a single particle but that it con- sists of protons and neutrons tightly bound together. Initially, the protons and neutrons were assumed to be elementary, but during the 1950s and early 1960s a large number of similar particles, called hadrons, were discovered. Over 100 hadrons are now known. In the 1960s, it was suggested that the properties of all the hadrons could be explained by recognizing that they are not elementary particles but are composed of smaller particles, each with an electric charge of one third or two thirds that of the electron. These smaller particles are called quarks. During the past decade, the quark model has been experimentally verified. For example, jets of hadrons discovered in high-energy experiments have been explained in great detail by viewing the collision not as a collision of hadrons, but as a collision of the constituent quarks. In the earliest version of the quark theory, there were only three different quarks up, down, and strange. However, the discovery of the J/ particle in 1974 and of the Y particle in 1977 led to the addition of two new quarks the charm and bottom quarks. Definitive evidence for a sixth quark, the top quark, is now being sought (Figure 2.24. With these six quarks, the existence of all the hundred-plus hadrons could be explained.

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PROGRESS IN PHYSICS 19 GENERATION TOP QUARK QUARKS LEPTONS I }3 BOTTOM QUARK CHARM QUARK STRANGE QUARK - UP QUARK .-- DOWN QUARK ~ TAU TAU NEUTRINO MUON MUON NEUTRINO | ELECTRON ELECTRON NEUTRINO ~2 }3 2 1900 1940 1980 FIGURE 2.2 Quarks and leptons are the basic particles of matter. Most of them were discovered in the past two decades. There is not yet definitive evidence for the existence of the top quark. A startling feature of quarks is that, as far as we know, there is no possibility of isolating one of them. Quantum chromodynamics, a theory of the strong interactions between quarks, accounts for this by predicting that the energy to separate two quarks grows continuously as they are separated owing to the creation of a gluon string between them. Quantum chromodynamics is now so far advanced that theorists can apply it to calculate numerically the masses of the hadrons. The gluon-mediated interaction between quarks becomes weaker the closer the quarks are together. This effect, called asymptotic freedom, makes it possible to view hadron collisions as a series of collisions between individual quarks. In addition, it eliminates some of the internal in- consistencies that plagued previous theories. The family of quarks was discovered relatively recently. There is a second family of elementary particles, the leptons, some of whose members have been known for many decades. The first of these, the electron, was discovered at the turn of the century; two others- the neutrino and the muon were predicted in the 1930s and discovered experimentally in the following two decades. The electron and muon are charged; the neutrino is electrically neutral. In 1963, it was found

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20 PHYSICS THROUGH THE 1990s: AN OVERVIEW that there are at least two kinds of neutrino, one associated with the electron, the other with the muon. In 1975, a third, very heavy, charged lepton, the a, was discovered. Soon thereafter, evidence was found for a third type of neutrino associated with the a. Thus, we now know of six leptons, which form three groups. A decade of experimental and theoretical research on the quark and lepton families has led to the realization that many of their properties can be explained by two simple ideas. First, the particles can all be classified into pairs by their properties and interactions. Each charged lepton is paired to a unique uncharged neutrino. Among the quarks, the up and down quarks pair together, as do the charm and strange quarks. Second, the quark family and lepton family are related: each quark pair is uniquely related to one lepton pair by a simple arrangement called the generation model. We do not understand why the generation model works, nor do we know if there are more generations. In fact, we do not know why the quarks and leptons are related at all. To find the answers to these questions, experiments with higher-energy accelerators are being planned to explore the internal structure and dynamics of the known particles, to search for new particles, and to provide the data essential to constructing new theories. Unification of the Forces of Nature There are four fundamental forces in nature. Two have been known for centuries: the force of gravity and the electromagnetic force. In the period between the World Wars, two other forces were identified: the strong force, which holds the nucleus together, and the weak force, which is responsible for many types of radioactivity. Since the days of Einstein, it has been the dream of physics to develop a unified theory, a theory that describes all these forces with a single set of equations and concepts. In the last two decades, the dream has been partially realized. The electromagnetic and weak forces have been combined in a single theory: the photon (a particle of light) carries or mediates the electro- magnetic interactions, whereas the weak forces are mediated by massive charged particles called the W+ and W- and by a neutral particle called the Z. Interactions mediated by the neutral particle, called neutral currents, have been discovered in experiments with high-energy neutrino beams and in studies of how electrons and positrons annihilate each other. In addition, atomic physicists have detected minute effects due to neutral currents in the spectra of cesium, bismuth, and thallium, adding to evidence for the theory. In one of the

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PROGRESS /N PHYSICS 21 PROG RESS ~ N UNIFICATION OF FO RC ES KNOWN BASIC FO RC ES GRAVITATIONAL FORC E WEAK FORCE ELECTROMAGNETIC FORCE STRONG OR NUCLEAR FORCE 1900 1940 1980 PART! CLE CARRY! NG THE FORCE _ NOT DISCOVERED W AN D Z PARTICLES I ) - CONF I RM ED PHOTON ., ~ THEORY PARTICLES YEAR WHEN THE PARTICLE CARRYING THE FORCE WAS DISCOVERED SEVERAL THEORIES, ) NEED HIGHER ENERGY EXPERIMENT ~ TO TEST J FIGURE 2.3 The four known basic forces expected to be carried or mediated by an elementary particle. The particles carrying two of the forces have been discovered in the past decade. Since the days of Einstein, physicists have wanted to unify the forces so that they can all be derived from ~ single basic equation. That has been accomplished for the weak and electromagnetic forces. most ambitious experiments ever undertaken, the W+, W-, and Z particles have recently been observed, eliminating any further doubts about the origins of the weak force (Figure 2.3~. Efforts to unify the electroweak forces and the strong force are leading to exciting new challenges in theoretical and experimental physics. Theories have been proposed predicting that the proton is not stable but will decay and that magnetic monopoles exist. Testing these predictions is one of the many opportunities for particles physics in the coming decade. PROGRESS IN NUCLEAR PHYSICS The challenge to understand the diverse arrangements of protons and neutrons in the nuclei of atoms has fascinated physicists since the 1930s. In the 1960s, the model of the nucleus as a simple collection of protons and neutrons evolved into a more complex picture in which the strong nucleon-nucleon interactions arose from the exchange of me- sons; now this picture is being replaced by the rich portrait that emerges from recognition of the underlying quark-gluon nature of nucleons. One day it should be possible to explain the entire nucleus as a many-body system of interacting quarks and gluons. The experimen- tal and theoretical challenge is enormous, but so is the reward of understanding nuclear matter (Figure 2.41.

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PROGRESS IN PHYSICS 33 Alternatives to toroidal confinement systems are also being devel- oped. One approach is the mirror configuration, which employs a linear magnetic-field geometry that pinches the ends to form mirrors for the plasma particles. The tandem mirror concept, in which the electric fields are generated Flora magnetic field lines, has been introduced to suppress plasma leakage through the ends. Results point toward the possibility of mirror confinement systems adequate for fusion-reactor applications. Multimegawatt neutral-beam sources have been devel- oped to fuel the mirror machine and heat plasmas to fusion tempera- tures. Two-hundred-million-degree plasmas at fusion-plasma densities have been achieved. In toroidal devices, 80-million-degree tempera- tures have been obtained (Figure 2.7~. _ 1 \ ~ 1 ' ' ' ~ ' - 3/85 \ \ Ignition j lol4 a) 1013 llJ c lol2 A Icator C ( 1983) Alcator A ~ JET ( 1978) ( 1 9842. -~e TFTR AIcator A ~ (1985) ( 1976) D111 _ ~1 984 ) ~ ( 1~484 ) ( 1 979) T 3 (1968) 0.1 Thermalized \ Breakeven - - TFR (1975) ST ~ (1971) . , , , , ,,,,1 , , , POX (1981) PLT - (1978) ,,, ,,1 , . . ..... Tj(O)(keV) 10 100 FIGURE 2.7 Plot of Lawson confinement parameter n(O)~,: versus central ion temper- ature The) for several tokamaks. Here' n(O) is the central density and ~~ is the energy confinement time. The year of the result is indicated in parentheses. The JET (Joint European Torus) and TFTR (Tokamak Fusion Test Reactor) tokamaks, with auxiliary heating, are expected to operate in the 10- to 15-keV range during 1985-1987. The thermalized breakeven and ignition curves refer to an equidensity fuel mixture of deuterium and tritium plasma with Maxwellian ions.

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34 PHYSICS THROUGH THE 1990s: AN OVERVIEW Space Plasmas The interactions of the Sun's wind and its magnetic field with the magnetic fields of the planets are but one example of the plasma phenomena that occur throughout our solar system. The magnetic field and plasma surrounding each planet define a region known as the magnetosphere. The Earth has a relatively quiet magnetosphere, which we are understanding in increasing detail. For example, we are beginning to understand how the reconnection of the Earth's magnetic field lines is related to auroral activity. Data from the Pioneer and Voyager space missions have yielded a detailed picture of planetary magnetospheres and the electromagnetic activity in the solar system (Figure 2.8~. Today, space probes are providing data on collective oscillations, shocks, particle acceleration, and instabilities. Outside the solar system, plasma behavior in extreme astrophysical environments can give rise to such bizarre phenomena as the jets of particles in opposite directions that have been observed to be ejected from pulsars. FIGURE 2.8 The magnetosphere of Jupiter. The insert shows a Voyager photograph of lo's sulfur plasma torus. (Courtesy of NASA.)

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PROGRESS IN PHYSICS 35 Fluid Physics The physics of fluids is far from understood because fluid motion involves many degrees of freedom and is inherently nonlinear. Under- standing fluid flow, whether the fluid is a gas or a liquid, is essential for applications such as weather prediction, flight and transportation, plate tectonics, combustion and chemical reactions in flames, and biological problems such as blood flow in cardiovascular systems. Thus, any advance in our understanding of flows, particularly turbulent and unsteady fluid flows, can be expected to have enormous technological impact. For example, recent advances in the theory of acoustic damping and turbulent flow, applied to jet noise, led to a thousandfold reduction in the acoustic energy emitted by aircraft, providing a major reduction in perceived noise levels. There is increasing interest in the so-called nonideal fluids. New constitutive models for fluids, based on molecular structure, have led to a better understanding of the striking flow properties of polymer solutions and drag-reducing agents. These fluids have an interesting application in fire-fighting equipment; the addition of minute quantities of very long macromolecules greatly increases water flow and reduces steam backup. Progress toward understanding the onset of turbulent flow is encour- aging. Developments include an increased understanding of chaotic behavior in simple systems, new methods for observing fluid behavior near onset, and new techniques for analyzing the data. The advances are due in large part to modern large-scale computational capabilities. Among the potential applications of the work is the prediction of global-scale flows for both short- and long-term weather forecasting. PROGRESS IN GRAVITATION, COSMOLOGY, AND COSMIC-RAY PHYSICS Gravitational Physics The best-known prediction of Einstein's General Theory of Relativ- ity is that gravity bends light, but the theory predicts other equally startling ejects. They are all so small, however, or so hard to observe, that testing general relativity presents a formidable challenge. During the past decade, there has been a breakthrough due to space tech-

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36 PHYSICS THROUGH THE /990.s: AN OVERVIEW piques. For example, the propagation time for light passing the Sun has been monitored using a spacecraft. General relativity predicts that light is not only bent by the Sun's gravity but also slowed by it; a beam grazing the Sun is delayed by 250 microseconds. By using the Viking Lander spacecraft on Mars, this tiny delay has been measured to an accuracy of 0.1 percent. Space techniques have provided other tests of gravitational theory. A hydrogen-maser atomic clock in a rocket was compared with a similar maser on the ground, allowing the minute effect of the Earth's gravity on clock rate (the gravitational redshift) to be measured to an accuracy greater than 1 part in 10,000. The results agreed with the value predicted by the equivalence principle, which relates the effects of acceleration and gravity. Cosmological arguments have opened the possibility that Newton's gravitational constant G may not really be a constant but that it changes as the universe ages. Ranging measure- ments using the Viking Lander on Mars have been combined with other solar-system data to set a limit on the possible change: it is no more than I part in 10" per year. One of the most dramatic quests in gravitational physics today is the search for gravitational radiation. The radiation from known sources is predicted to be so weak that detecting it requires highly innovative experimental techniques. One method attempts to sense the passing of gravitational waves by their eject on the length of a large aluminum bar that is cooled to liquid-helium temperature and carefully isolated from vibration. A strain level (fractional change in length) of 10-'8 can be detected. The extreme sensitivity of this measurement can be appre- ciated by noting that a strain of 10-'8 in a 1-meter-long bar is a change of length by 0.1 percent of the diameter of an atomic nucleus. Further improvements are under way. Another approach uses laser inter- ferometers whose mirrors are mounted on inertial platforms. These detectors are expected to reach strain levels of 10-23 when baselines of several kilometers are achieved. interest in gravitational radiation goes beyond its role In gravitational theory; the waves can reveal sources, like black-hole formation, that are invisible to us now. The discovery of gravitational radiation would truly open a new window on our universe. A compelling demonstration of the reality of gravitational radiation has been provided by careful observations of a system of two compact objects, one of which is a pulsar that emits regular pulsed signals. The 8-hour orbit has been studied in exquisite detail by clocking the radio pulses. Since 1975, the orbit has decayed owing to the loss of energy by

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PROGRESS IN PHYSICS 37 gravitational radiation; theory and observation agree to within 1 percent. Fundamental advances in relativity theory have accompanied these experimental advances. The positive energy theorem has proved that, in general relativity, any isolated system must have positive total energy. This is by no means obvious, because gravitational binding energy is negative. In another milestone discovery, theoretical relativ- ists have shown that black holes evaporate by emitting thermal radiation, the temperature being inversely proportional to the mass. They have also shown that black holes have well-defined entropy and that a generalized form of the Second Law of Thermodynamics is valid even for systems containing black holes. Cosmology Confidence in the theory of the primordial explosion the big bang- continues to increase. New measurements of the spectrum of the 3-K radiation that fills the universe, as well as recent measurements of the cosmic abundances of the light elements, match the big-bang predic- tions. Equally important, whatever direction one looks in, the 3-K radiation is found to be remarkably uniform: the universe is apparently isotropic to better than 0.01 percent. This isotropy confirms Einstein's assumption of cosmic homogeneity (the cosmological principle), but it presents a puzzle in causality. The regions of space being viewed had not yet been connected by light signals at the time of emission; so lacking any possibility of communication, how could these regions "know" the temperature elsewhere? The extreme temperatures predicted for the earliest moments of the big bang correspond to energies far beyond the wildest possibilities for elementary-particle accelerators, but they can nevertheless be con- ceived in the imagination of theorists. If we calculate the cosmological consequences of theories like the Grand Unification Theory (Figure 2.9) and compare them with observations, both particle physics and cosmology are advanced. Possible explanations are being found not only for the large-scale causality puzzle but for problems such as why the ratio of baryons to photons in the universe, only 10-9, iS SO small. Theoretical cosmology is advancing rapidly on many fronts. Impor- tant progress has been made toward understanding the formation and evolution of large-scale structure galaxies and clusters of galaxies, for example. Such studies may lead to an estimate of mass distribution early in the life of the universe. Theoretical ideas for possible dark-

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38 PHYSICS THROUGH THE 1990s: AN OVERVIEW o lol4 e 108 IC US 2 103 107 109 OK TIME _> 1 C, _ ~ l _ z ~ _O l l STRONG ELECTRO WEAK INFLATION ELECTRO WEAK //// ~ A D. QUARK CONFINEMENT NEUTRINO FREON our ........... .. NONNUCLEAR FREON OUT : NUCLEAR . ::~::~ ~ ~ YNTHESIS' / /` E ~ M FREEZE OUT ~ -LEPTON to. ~^ c /]GALAxlE ;/////////% ~ .; . OCR for page 11
PROGRESS IN PHYSICS 39 to, but significantly different in detail from, either that found in the solar system or that resulting directly from nucleosynthesis in super- novae. Information from deep-space probes has increased our under- standing of the interaction of galactic cosmic rays with the local interplanetary environment. The highest-energy cosmic rays are observed using ground-based and underground detectors. Cosmic rays with energies of as much as a joule have been observed, and the isotropy in the arrival direction of these highest energy particles has indicated that they have an extra- galactic origin. In addition, discrete sources of gamma rays with energies up to 10~5 eV have been observed, and they indicate a few localized cosmic-ray accelerators with enormous power. Understand- ing the acceleration mechanisms for such energetic particles is a major goal of current research. INTERFACES AND APPLICATIONS Physics is woven so tightly into the fabric of science and society that the boundaries separating it from neighboring sciences are hardly discernible. In the wider context of society as a whole, hardly any aspect of modern life remains unaffected by the discoveries of physics. Interface Activities The basic concepts of physics are incorporated in virtually every area of science. In addition, physics interacts directly with many sciences by exchanging theoretical approaches and experimental tech- niques. CHEMISTRY The interface between physics and chemistry is among the best developed interdisciplinary areas in science. Advances in spectros- copy, including laser spectroscopy, nonlinear optics, extended x-ray absorption fine structure (EXAFS), the advent of synchrotron light sources, and molecular beams are playing increasing roles in chemis- try. These advances have already had a significant influence on molecular physics and surface chemistry. They are also finding appli- cations in other areas of growing physical-chemical interest (such as the study of polymers and liquids) and in photochemistry and photo- chemical processing. Order-disorder transitions a central problem in physicsare being studied in the chemical arena using media such as

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40 PHYSICS THROUGH THE 1990s: AN OVERVIEW micellar and colloidal crystals, microemulsions, and liquid crystals. Condensed-matter physics is benefiting from the new classes of mate- rials with unique physical properties that are being created by organic chemistry. For example, the discovery of organic conductors of electricity in the early 1970s has led to the creation of effectively one-dimensional conductors that display the drastic fluctuation effects predicted by one-dimensional statistical mechanics. By manipulating the molecular architecture, one can pass systematically from insulators to semiconductors to metals. We can look forward to numerous applications of these discoveries, particularly in materials science and electronics. BIOPHYSICS Physics encompasses increasingly complex problems as its experi- mental and theoretical powers expand. In biophysics, for example, new frontiers have been opened by the development of methods to observe the conductance of single molecular channels through biolog- ical membranes. Hundreds of different channels have been identified, with more surely to be discovered. Each channel is switched in a quasi-random process while it is simultaneously modulated by mech- anisms such as binding of neurotransmitter molecules. One can look forward to working out the finest details at the molecular level in the next few years. Molecular genetic technology will assist this research by creating mutants that are specially engineered to probe the molec- ular mechanisms. The secrets of protein structure and enzymatic processes can be expected to unfold as they are probed with synchro- tron radiation, x-ray and electron-beam crystallography, laser Raman spectroscopy, magnetic resonance, and other tools from physics. The new, powerful techniques of biophysics and molecular genetics prom- ise to deepen our understanding of molecular processes in biology at all levels. GEOPHYSICS Experimental and theoretical methods of physics are contributing to geophysical understanding of the Earth's three phases atmosphere, oceans, and solid planet. Today, solid-earth geophysics is dominated by the concepts of plate tectonics. Modern seismology and innovative techniques of metrology, particularly those using lasers and atomic clocks, have made it possible to monitor terrestrial motions with

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PROGRESS IN PH YSICS 41 unprecedented precision. Extended movements of the solid-earth crust can now be interpreted with such high precision that one can begin to account for fine details like mineralization. In the atmosphere, the analysis of the turbulent fluid flow associated with global weather patterns is being revolutionized by advances in large-scale computing. The fundamental theory of turbulence, however, has yet to be created; the lack of such a theory continues to limit progress. In the ocean, underwater seismology, sound-propagation tomography, and satellite sensing are providing data on the ocean's temperature, its level, the state of the sea, and the strength of the currents, all of which are essential for basic understanding of this intricate system. The problems of geophysics present a formidable challenge because of their inherent complexity. However, it is vital that progress continue, because geophysics is involved in the utilization of every type of energy resource and is an essential element of any attack on the global energy problem. MATERIALS SCIENCE The systematic study of materials and the development of techniques to control, modify, and create materials are central goals of this interdisciplinary subject. To this effort, physics contributes fundamen- tal theory, such as the rapidly developing theory of disordered mate- rials. It also contributes new experimental tools, such as synchrotron light sources and the free-electron laser. Microelectronics would not have been possible without the techniques for making ultrapure semiconductors developed by materials scientists; a strong underpin- ning of physics and chemistry was essential to this advance. Con- versely, the precisely controlled materials fabricated by materials scientists constitute an invaluable resource for condensed-matter phys- ics. The development of methods for making artificially structured materials is one of the many areas in which condensed-matter physics and materials science are moving forward together. These materials provide a testing ground for basic physical theory and a likely arena for discovering novel physical behavior. At the same time, they offer the possibility of creating materials with unique properties for applications such as ultrahigh-speed microelectronics. The development of low-loss optical fibers and the creation of the fiber-optics industry are examples of the dramatic advances made possible by the confluence in materials science of basic science, applied science, and technology.

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42 PHYSICS THROUGH THE 1990s: AN OVERVIEW Applications The most profound technological advances from scientific research have often been unanticipated: no one could predict that experiments on amber and lodestone would lead to the replacement of water wheels by electrical power or that research on the magnetic interactions of nuclei in matter would lead to a revolutionary technique for medical imaging. Nevertheless, much of today's scientific research in the United States is motivated by the need to solve specific problems facing the nation. Physics plays an important role in this task. The applications of physics research to societal needs are too broad even to summarize, but the following are a few examples. ENERGY AND THE ENVIRONMENT Society's welfare depends on its energy supply. Physics is involved in almost every aspect of research on the generation and efficient use of energy. Progress in fusion energy has been described; fission and other technologies from physics (such as photovoltaics) are expected to play increasingly significant roles in the future. For the present, combustion remains our major source of energy. New diagnostic tools and new materials can make combustion more efficient, and even a small increase in efficiency will have an enormous economic impact. Every manufacturing process and every technology for producing energy somehow affects the environment. Understanding and control- ling these effects are crucial to our future well-being. To this task physics brings essential data, analytical techniques, and theoretical tools for monitoring the earth, the oceans, and the atmosphere. MEDICINE Physics is addressing increasingly complex problems of biophysics and physiology. In addition to basic research, however, it contributes directly to the quality of medical care by providing new analytical tools, diagnostic techniques, and therapies. X-ray tomography has had a major impact on x-ray diagnostics; magnetic resonance imaging (described in Chapter 1) is widely regarded as a revolutionary advance in medical diagnostics. Ultrasonic imaging is yet another of the noninvasive diagnostic tools from physics. Lasers are finding increas- ingly widespread applications in medicine. Laser surgery replaces some highly delicate or traumatic operations with simple and straight-

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PROGRESS IN PHYSICS 43 forward procedures. Fiber-optic endoscopes exemplify the many new diagnostic instruments. Combined with lasers, the endoscopes can be used to provide new therapies that may replace elaborate surgical procedures. These examples represent but a few of the many new instruments and technologies from physics that are today enhancing the quality of health care in this nation and elsewhere. NATIONAL SECURITY Physics is without peer as a source of discoveries that have an impact on national security strategies and tactics. The profound effects of physics are apparent in the development of weapons systems and strategic defense systems and in the complex process of arms control. Lasers, for example, are now widely used for communications, guid- ance, and surveillance. The free-electron laser and cyclotron-reso- nance maser are capable of providing intense coherent radiation from microwave through ultraviolet wavelengths, with a potentially major influence on radar technology, particularly "stealth" technology, and on optical countermeasures. The ability to respond appropriately to a rapidly changing situation can depend on the real-time ability to acquire and interpret vast amounts of data: new optical and informa- tion-processing techniques are at the heart of this effort. In these and countless other ways, the discoveries from basic physics in former years, combined with the efforts of scientists and engineers working today, are helping to assure that the nation can meet its national security goals. INDUSTRY Physics contributes broadly to industry through the creation of new materials, instruments, and technologies. Beyond these, discoveries in basic research can lead to the creation of industries such as microelec- tronics and laser optics. Microelectronics has made possible the information revolution that is transforming society. Laser optics is revolutionizing communications and printing. Laser-assisted manufac- turing, particularly when applied to robotics, is spreading through every kind of industry. As described in Global Competition, the Report of the President's Commission on Industrial Competitiveness (U.S. Government Printing Office, Washington, D.C., January 1985), ". . . basic research in the Nation today is a critical factor in our long-term preeminence."