Physics matters because it stands where wonder about the workings of the world meets our most practical demands. Like quicksilver, physics darts this way and that through the tangle of disciplines, making connections, building instruments, explaining why things work. Who would have expected that Maxwell's theory of electricity and magnetism, born in the midst of Victorian Cambridge, would a few decades later link continents by radio? That Einstein's relativity, launched on a wooden podium in a Swiss patent office, would astonish the world with radical new ideas of time and space and then be key to the understanding of fission, nuclear power stations, and weapons? Or that the sharing of data necessitated by the inquiry into quark dynamics in particle colliders would lead to the establishment of the World Wide Web?
Physics matters because issues of understanding and practicality rarely stay apart for long. In 1947, Bell Laboratories physicists working on a new amplifier built the first prototype transistor; subsequent exploration of semiconductors led to a dramatically new atomic understanding of condensed matter physics. That new atomic understanding, in turn, fed back into the massive industry that has placed microelectronics at the center of today's economy. Techniques that physicists developed as they grappled with the theory of magnetism aided in the understanding of superconductivity and then migrated to elementary particle physics, where they led to a new grasp of the unity of the basic forces that hold matter together.
Where is physics? It is not now and never was isolated in university departments. A generation ago, physicists made common cause with electrical engineers to build the radar facilities that played such a key role in World War II; they partnered with industrial engineers, chemical engineers, and metallurgists to construct the vast nuclear plants that have produced both electricity and weapons. More recently, physicists have been entering into new partnerships with industry as they construct the infinitesimal cir-
cuits and machines of nanotechnology. They have joined with biologists: Perhaps in the not-too-distant future they will use optical tweezers to rearrange the genetic code. They have begun to explore radical new ideas of quantum computing. Physicists are to be found in NASA, in industries, and, increasingly, on Wall Street as techniques born in the study of physical systems begin to play an important role in examining the dynamics of stocks, bonds, options, and hedge funds. Physics is large in the vast national laboratories of Los Alamos, Oak Ridge, Fermilab, the Stanford Linear Accelerator Center (SLAC), and Brookhaven. Physics is small in the start-up optics company or in the bench-top experiments that are reconfiguring the study of new materials, optical phenomena, biophysics, and magnetic media.
Precisely because physics is everywhere, from computer printers, copying machines, and laser-driven checkout counters to precision weapons, surgical instruments, airplane surfaces, and medical diagnostics, it is a tall order to survey the whole of it. One way to cast a first glance over the landscape is to think of the different distance scales of nature.
A few years ago, Sebastian Junger wrote A Perfect Storm, a moving account of a few small boats caught in the harsh fall Atlantic during the extraordinarily violent storm of October 1991. We learn of the killer waves, some almost a hundred feet high and roughly that long, that threatened the lives of anyone so unfortunate as to be at sea. Any small craft would eventually lose the power to face into the waves and once caught sideways would almost certainly be thrown keel up and sunk. Even on a calm day, the complexities of the sea seem impossible to understand, especially on discovering that there are waves of every conceivable size, from a fraction of an inch to giant ocean surges that stretch for a thousand miles. But there is a simplification available, one that in some ways undergirds the entirety of physics. It is this: Not all the ups and downs of a boat at sea are equally threatening. Waves much shorter in length than a boat make little difference; those an inch or a foot or even a couple of feet long do no damage. Much longer waves can equally well be ignored. A surge measuring a thousand feet from crest to crest merely lifts the boat gradually up and settles it back down. But waves roughly as long as the boat are potentially destructive. This simple fact—that of the myriad of interdependent waves, only those of roughly boat length are directly threatening—lies close to the heart of physics.
Take the string theorists. These are a group of physicists with the wildly ambitious goal of bringing together the various matter-binding forces (weak, electromagnetic, strong) with gravity. Using (and occasionally inventing) new mathematics, string theory has fastened on the unimaginably small
length scale of 10−33 cm. For at that distance, it is reckoned, the various forces no longer will be distinguishable. Instead of our familiar notion of particles—entities that resemble tiny BBs—the idea is that, looked at up close, even quarks and electrons exhibit the structure of tiny loops of string. These strings can vibrate, like plucked violin strings, at different harmonics. Each of these different “notes” corresponds to a different energy. We have known since Einstein that mass is related to energy (E = mc2), so a single string could, depending on what note was playing, correspond to different masses. So perhaps, the string theorists suggest, the variety of “elementary” particles might turn out to be just different vibratory states of a single string.
At length scales much longer than 10−33 cm, physicists do well by ignoring the ultrasmall-scale phenomena of these strings; much of elementary particle physics is concerned with the order of the world revealed at about 10−16 cm. Small as that may seem, it is nearly a billion billion times larger than the world depicted by string theory. At this scale there are the observable objects, the so-called elementary particles, that leave tracks in cloud chambers, bubble chambers, or the vast electronic detectors that populate Fermilab, SLAC, and the European Organization for Nuclear Research (CERN). These particles include not only the electron, the familiar particle that governs the properties of atoms, but also heavier versions of the electron. Quarks, too, figure among the elementary particles, as do the force-carrying particles that hold matter together. And none have recently captured more interest than the elusive neutrinos, created in accelerators, our atmosphere, and stars. Through a tightly woven collaboration between experimenters and theorists, elementary-particle physicists assembled a “standard picture” of these basic particles that explained and predicted a wealth of observed effects and entities. Left out of this synthesis were certain basic questions that stubbornly haunted the community: Why do the elementary building blocks of matter—the elementary particles—have the masses they do? Why do their forces have the strengths they do, and how are they related? These are some of the fundamental questions that will be addressed in the new decade. Their ultimate resolution will involve connections to the physics of much smaller distance scales, possibly even those at play in string theory.
Moving to lengths ten thousand times larger, in the region of 10−12 cm, we are in the midst of nuclear physics. This is the scale at which the binding together of protons and neutrons, the hard core of an atom, is salient. Nuclear physics deals with the dynamics of the core itself, how quantum mechanics can be applied to it, how its parts undertake certain collective motions up to and including fission. It lies at the base of our understanding
of fission and fusion processes, which is central to the construction of nuclear arsenals and the nuclear power industry, as well as our attempts to explain the evolution of our Sun. More recently, the Jefferson Laboratory in Newport News, Virginia, has begun investigating the nucleus in a new way, seeking to elucidate its structure in terms of the quarks in the protons and neutrons. And the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, another large new facility, has begun to explore the interiors of nuclei under conditions of vastly higher energies and pressures, conditions that offer first steps toward an experimental replication of the early universe or the interior of neutron stars. But nuclear physics has more familiar and more practical consequences as well. Using electromagnetic fields to flip nuclei led to nuclear magnetic resonance, a technique that definitively sorts different substances one from the other. It has become not only a key instrument for chemistry and biochemistry but also, under the name MRI, perhaps the single most powerful medical diagnostic advance of the last half-century.
Magnetic resonance is a good jumping off point to the larger distance scale of atoms themselves. At 10−8 cm, atoms are ten thousand times the size of their nuclear cores. It is here that the phenomena of everyday life begin to enter. The ductility of metals, the transparency of glass, the conduction of electricity, the physical properties of matter depend crucially on the ways in which electrons move among the atoms. And it is here, in this intermediate scale between the very small and the very large, that physics has recently had some of its most astonishing advances. Over the last decade or two, atomic physicists, molecular physicists, condensed-matter physicists, and optical physicists have developed an array of techniques for observing and controlling atoms. Suddenly it becomes imaginable to design circuits by arranging the matter atom by atom. Using lasers, physicists can stretch an individual strand of DNA and other macromolecules to examine their physical properties. New materials like fullerenes, high-temperature superconductors, and magnetic materials for computer memory have tumbled from the laboratory, presenting both abstract questions about the underlying physics and an array of industrial and practical challenges. Will a deeper understanding of quantum effects like the Bose-Einstein condensate lead, for example, to a new generation of superfast quantum computers?
At the human scale, the physical world offers up new phenomena. Vortices shape the turbulence and smooth flow of both air around airplane wings and high-temperature plasmas in fusion reactors. Cracks splinter the Earth in quakes, tornadoes swirl down from the clouds, and magnetic levitation hoists trains off tracks—all at the scale of the world we measure in feet and miles. But being able to grasp something with our senses is only the
first step in understanding, and a host of new physics techniques from computer simulations to chaos theory have begun to spread light on corners of nature still shaded in darkness only a generation ago.
Astrophysics and gravitational physics have in their sights the largest scale of all. For in this domain researchers are seeking order at the size of planets and stars: What are their dynamics, how do they evolve, how are chemical elements produced, how do stars exchange mass-energy with the surrounding interstellar media? Astrophysicists continue into the domain of cosmic sizes beyond planets and stars as they confront the zoo of novel phenomena discovered over the last decades: gas disks, cosmic jets, pulsars, quasars, black holes, gamma-ray bursts. Explaining these phenomena draws on physics from across the specialties, and studying them observationally has demanded the full-scale collaboration of ground-based and space-based stations. How do galaxies form? Why do they group into clusters, and clusters of clusters? What mechanism in deep space accelerates cosmic rays? What are the origin and the fate of the universe as a whole?
At all distance scales, physicists are attacking challenging problems, and as they deploy new instruments, novel concepts and deeper puzzles emerge. The physical sciences have entered a period of tremendous excitement. No one area dominates the whole, as astronomers, optical physicists, and string theorists all grapple with a vast new array of unfamiliar objects to study and an altered landscape of collaboration, along with shared instruments and techniques. Among the new collaborations is that between physics and biotechnology.
It is as evident as the daily headlines that biotechnology has altered both the medical and economic landscape. From the first days of the cyclotron, medicine and physics were bound together. Berkeley's Ernest Lawrence not only developed the earliest cyclotrons, but also used them right away to produce radioisotopes for medical purposes. And while often forgotten, Lawrence and his brother used one of their early machines to treat, successfully, their mother for a brain tumor. Since those days, the use of waves and particles to image the body and to treat tumors has become one of the essential features of modern medicine. Indeed, now that advanced computer technologies have made it possible to reconstruct electronic images of scattered waves and particles, we can see in a wide variety of ways. Scattered sound waves make ultrasound images, key not only for obstetrics but also for the examination of tumors and the study of the beating heart. Images of blood flow reveal blockages in the heart and are now providing a wealth of neurological information. CAT scans—x-ray-generated, three-dimensional cross sections of the body—go far beyond the sharpest pictures
produced by traditional radiology. And once the tumor is detected, physicians now have at their disposal the full panoply of particle acceleration techniques developed over several generations. They can, depending on the location and type of pathology, irradiate the threatening growth with electrons, x rays, neutrons, protons, mesons, and heavy nuclei. Together these various techniques have propelled a vast physical-medical industry.
But the alliance between physics and the biomedical sciences goes deeper still, beyond scattering to see and scattering to kill tumors. Over the last 20 years, physicists have come to see biological matter as territory that can be explored not only in its larger structures but also atom by atom. What, we can now ask, is the precise way in which an antigen binds to an antibody? How can physics methods be used to address the complex ways in which proteins fold, and in particular how do the large biological molecules like DNA take the particular form that they do? With a precision unimaginable a generation ago, it is now routine for physicists and biologists to work together in sorting out dynamical biological processes: Individual clusters of molecules can be marked, tracked, excised, altered, inserted, or moved. Soon to come are new uses for these biomaterials in molecular motors, DNA computers, and biological elastics produced in macroscopic quantities.
Reflecting on the new relation between physics and biology, Harold Varmus, the former director of the National Institutes of Health (NIH), summed up some of the key directions for future research: an improved use of micro-manipulative methods like optical tweezers, a new and vastly more sophisticated form of data analysis closer to the work astrophysicists undertake in their deep space searches than to traditional biological research, and, finally, a biophysical attack on the signaling pathways by which cells tell each other how to respond—a task that will draw on the experience physicists have with the feedback mechanisms of complex machines. Concluding his remarks on the widening alliance between physics and biology, Dr. Varmus added:
The NIH can wage an effective war on disease only if we—as a nation and a scientific community, not just as a single agency—harness the energies of many disciplines, not just biology and medicine. These allied disciplines range from mathematics, engineering, and computer sciences to sociology, anthropology, and behavioral sciences. But the weight of historical evidence and the prospects for the future place physics and chemistry most prominently among them. 1
1 Harold Varmus, plenary talk at the centennial meeting of the American Physical Society, Atlanta, Ga., March 22, 1999.
Modeling, imaging, miniaturizing, and controlling complex systems: These are the themes that draw together “pure” and “applied” physics in an ever-tightening weave. In national defense, many of these same themes recur. In the absence of nuclear testing, modeling the realistic characteristics of nuclear weapons is key to reliability of the arsenal. In addition to stockpile stewardship, the reduction of the global nuclear danger involves nonproliferation and arms control and the restoration of environments damaged by the production and testing of nuclear weapons. Beyond the nuclear domain, there remain vital issues of national defense, issues that if anything have gained importance in the post-Cold War epoch: cryptography, remote sensing, precision warfare, missile defense, and the development of new materials. And these are just a few of the principal areas of current research and development.
Take one arena where the interests of physicists are congruent with military importance: the Global Positioning System (GPS). Used widely in the Gulf War for directing precision weapons to their targets, the 24-satellite GPS system is based on the atomic clock. GPS had spawned a vast industry, estimated at some $2.3 billion by 1995, and generated an estimated 100,000 jobs by the end of the millennium. In a dynamic typical of physics, the GPS incorporates corrections predicted by general relativity, and military applications have diffused into the wider economy. Pilots, sailors, hikers, drivers, and surveyors all make use of what has become a device costing no more than a decent radio.
The economic fruits of physics research are visible all around us. Quantum mechanics shaped our understanding of the transistor and the electronics that pervade our world. New forms of wireless and optical technologies are shifting the way we compute, communicate, and store information. Not only is the “old” computer revolution of the postwar period now built into the infrastructure of the economy, but new waves of development continue to break, while others are just beginning to rise: biologically based computation, optical switching, and even single-molecule devices.
It is the constant exchange between understanding and application, between civilian and military, between university- and industry-based research that marks physics at the beginning of the 21st century. To prepare for the coming years in which this constant realignment of physics will no doubt continue, our older expectations of education, funding, and international cooperation all need to be reassessed. We will need a physics education and curriculum that aggressively reflect the manifold links between physics and the wider world. We will need a funding structure that captures