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The Scientific Excitement and Challenges

In 2005 the world celebrated the International Year of Physics.1 In part, this celebration commemorated the centenary of what has become known as Albert Einstein’s “miraculous year” of 1905, when he published four groundbreaking papers that laid a key part of the foundation of modern physics. It also honored other momentous discoveries in physics of the past century, including the development of quantum mechanics and the successful testing of what is known as the Standard Model of elementary particle physics—advances that have led to a new understanding of nature and to technologies that have profoundly influenced our lives.

In the sciences in general, the hundred years between 1905 and 2005 eventually could become known as the “miraculous century.” Greater understanding of the constituents and properties of materials resulted in an unprecedented array of new products and industrial processes. The discovery of the structure and function of DNA deepened our understanding of genetic inheritance and human development and gave researchers the ability to alter the genetic material of living organisms. The discovery of plate tectonics contributed to a new view of Earth as an integrated biological and physical system in which humans are playing an increasing role. In short, advances throughout the sciences during the 20th century revealed many of nature’s secrets and radically changed our view of the world.

In physics in particular, the advances of the 20th century were unprecedented.

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For additional information, see <http://www.physics2005.org/>.



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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics 1 The Scientific Excitement and Challenges In 2005 the world celebrated the International Year of Physics.1 In part, this celebration commemorated the centenary of what has become known as Albert Einstein’s “miraculous year” of 1905, when he published four groundbreaking papers that laid a key part of the foundation of modern physics. It also honored other momentous discoveries in physics of the past century, including the development of quantum mechanics and the successful testing of what is known as the Standard Model of elementary particle physics—advances that have led to a new understanding of nature and to technologies that have profoundly influenced our lives. In the sciences in general, the hundred years between 1905 and 2005 eventually could become known as the “miraculous century.” Greater understanding of the constituents and properties of materials resulted in an unprecedented array of new products and industrial processes. The discovery of the structure and function of DNA deepened our understanding of genetic inheritance and human development and gave researchers the ability to alter the genetic material of living organisms. The discovery of plate tectonics contributed to a new view of Earth as an integrated biological and physical system in which humans are playing an increasing role. In short, advances throughout the sciences during the 20th century revealed many of nature’s secrets and radically changed our view of the world. In physics in particular, the advances of the 20th century were unprecedented. 1 For additional information, see <http://www.physics2005.org/>.

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics One of Einstein’s 1905 papers described the special theory of relativity, which explained that moving objects become more massive as they approach the speed of light, clocks slow down, and objects flatten into pancakes. In 1916, Einstein published his general theory of relativity, showing that mass warps the structure of space and time, accelerating objects emit gravitational waves, and clocks slow down in a gravitational field. In the 1920s and 1930s, physicists developed the set of ideas known as quantum mechanics to explain the puzzling behavior of the subatomic world; these fundamental insights contributed to some of the most important technologies of the 20th century, including the semiconductors that have made possible the proliferation of modern electronic devices. Also in the 1920s and 1930s, astronomers produced evidence indicating that the universe is expanding, which suggests that all matter was created in an event known as the big bang, which took place more than 13 billion years ago. Studies of materials revealed new phenomena such as superconductivity, nuclear fission, and the coherent emission of light (leading to the development of the laser). These astonishing insights into the nature of the physical world produced new fields of physics (such as nuclear physics, condensed matter physics, and particle physics), generated knowledge that found applications throughout the sciences and in technology, and created a base of understanding that has helped remake our world. The field of elementary particle physics (or, simply, “particle physics,” which is the term used most often in this report) took shape in the first half of the 20th century as physicists began to study the fundamental constituents of matter and their interactions (Box 1-1). Both experimentation and theory have been critical to the advance of particle physics. For example, early in the 20th century, certain puzzling experimental results caused physicists to seek new and more fundamental explanations of the laws of nature. This search led to Einstein’s startling new theories of space and time and of gravity, as well as to the equally revolutionary development of quantum mechanics by physicists such as Max Planck, Niels Bohr, Werner Heisenberg, Max Born, and Erwin Schrödinger. The second half of the century witnessed a blossoming of particle physics as experiments tested existing hypotheses and inspired new ones. Many of those experiments involved particle accelerators, which convert matter to energy and back to matter again, as described by Einstein’s equation, E = mc2. In recent decades, accelerator experiments have become enormous undertakings involving thousands of scientists and engineers and intellectual and financial contributions from countries around the world. In addition, a spectrum of much smaller, less expensive, but also highly valuable experiments has measured the special properties of particles and particular interactions among particles. Most recently, astronomical data from satellites and ground-based facilities have produced extremely useful information for particle physics. The nascent field of particle astrophysics has brought a deeper apprecia-

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 1-1 What Is Elementary Particle Physics? Physics has demonstrated that the everyday phenomena we experience are governed by universal principles applying at time and distance scales far beyond normal human experience. Elementary particle physics is one avenue of scientific inquiry into these principles. What rules govern energy, matter, space, and time at the most elementary levels? How are phenomena at the smallest and largest scales of time and distance connected? To address these questions, particle physicists seek to isolate, create, and identify elementary interactions of the most basic constituents of the universe. One approach is to create a beam of elementary particles in an accelerator and to study the behavior of those particles—for instance, when they impinge upon a piece of material or when they collide with another beam of particles. Other experiments exploit naturally occurring particles, including those created in the sun or resulting from cosmic rays striking Earth’s atmosphere. Some experiments involve studying ordinary materials in large quantities to discern rare phenomena or search for as-yet-unseen phenomena. All of these experiments rely on sophisticated detectors that employ a range of advanced technologies to measure and record particle properties. Particle physicists also use results from ground- and space-based telescopes to study the elementary particles and the forces that govern their interactions. This category of experiments highlights the increasing importance of the intersection of particle physics, astronomy, astrophysics, and cosmology. In general, large, centralized infrastructure, such as large accelerators, telescopes, and detectors, plays a crucial role in enabling particle physics. Working together in large teams, particle physicists construct and operate these complex facilities and then share the results. Not all experiments are so large, however, and progress in particle physics depends on the combined efforts of large and small projects. tion of the fundamental connection between the study of elementary particles and such astronomical phenomena as active galactic nuclei, black holes, pulsars, and the overall evolution of the universe. Over the entire suite of experiments and observations spreads the umbrella of theory. Theoretical physicists seek to construct a coherent intellectual edifice that can encompass and explain what has been seen, using the power of mathematics to make their ideas precise and logically consistent. From these theoretical models emerge predictions that help define the critical experiments needed to test the current framework and extend today’s understanding to new phenomena. This sustained real-time interplay of experiment and theory has produced astonishing progress. In the first part of the 20th century, physicists learned that all matter here on Earth is built out of subatomic particles known as electrons, protons, and neutrons. In the second half of the century, they discovered that protons and neutrons are composed of more fundamental particles known as quarks, and that the quarks and electrons that constitute everyday matter belong to families that include heavier and much rarer particles. They learned that particles interact

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics through just four forces: gravity, electromagnetism, and two less familiar forces known as the strong force and the weak force. They developed a theoretical framework known as the Standard Model, which describes and predicts the behavior of elementary particles with extremely high levels of precision. The development and extraordinarily precise testing of the Standard Model have been among the crowning achievements of 20th century science. Yet considerable evidence suggests that the advances of the 20th century rather than ending the story have set the stage for a new era of equally exciting progress. Results from both experiment and theory suggest that the next few decades will produce information that could help answer some of the most basic questions scientists can ask: Why do particles have mass? What are the relationships between the forces observed in nature? What accounts for the structure and evolution of the universe, and what is its future? These questions are ripe for a new phase of investigation for a range of reasons. For decades, physicists have had strong reasons to think that great discoveries await experiments that can be conducted at what is known as the Terascale. “Tera” refers to the million million electron volts of energy that can be imparted to particles in the most powerful accelerators available. It has taken more than 75 years to develop the technologies needed to construct accelerators that can open this new frontier. At last, experimental facilities are being constructed that bring the Terascale within reach. Other experiments examining high-energy cosmic rays generated in the distant universe or neutrinos generated by solar fusion also promise to complement in extremely valuable ways the information generated by accelerators. Promising experiments currently under way at Fermi National Accelerator Laboratory (Fermilab) have begun to explore the lower reaches of the Terascale. In 2007 the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) is scheduled to begin colliding protons. This facility will for the first time provide physicists with the ability to carry out controlled laboratory studies at a broad range of energy levels within the Terascale range. Moreover, the prospect of further exploiting the Terascale with a new accelerator known as the International Linear Collider (ILC) has galvanized particle physicists from around the world to consider in detail how currently available technologies could be used to address compelling scientific questions beyond the reach of the LHC alone. CHALLENGES TO THE STANDARD MODEL Why is the Terascale so important? At the Terascale, two of the main forces in nature, the weak and electromagnetic forces, appear to join together to become a single entity. Exactly how this happens is a mystery. There is a proposal within the framework of the Standard

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 1-2 Einstein’s Dream After Albert Einstein published his general theory of relativity in 1916, he devoted much of his scientific work to a problem that consumed him until the end of his life in 1955: the unification of the fundamental forces of nature, including electromagnetism, gravity, and the forces active within the atomic nucleus. Einstein’s dream was to develop a unified field theory that would describe in a single set of equations all the seemingly distinct forces that act on particles. Though he worked on the problem until the day he died, he never solved it. Today physicists still have not achieved a unified theory of the fundamental forces. But new theoretical ideas and experimental results have resulted in extremely promising hypotheses. The discovery of phenomena unknown to Einstein, such as quarks, dark matter, and dark energy, means that physicists may be on the verge of realizing Einstein’s goal. The next generation of experimental facilities may bring Einstein’s dream within reach. Model, but it has never been tested and it raises baffling theoretical questions. Understanding how the weak and electromagnetic forces are unified is believed to be an important part of understanding the broader unification of particle forces, perhaps including gravity, in keeping with Einstein’s aesthetic dream of unifying all the laws of nature (see Box 1-2). How the weak and electromagnetic forces are unified is a question that can only be answered using accelerators. For example, it is not possible to make these measurements using cosmic rays, because the highest energy cosmic rays are too few and it is not possible to study them with enough precision. Scientists everywhere seek the simplest possible explanation of the phenomena they study that will survive scientific scrutiny. In physics, the development of a single coherent scientific framework that would explain the nature of matter, its mass, its evolution, and the forces associated with it has inspired the work and dreams of generations of physicists. Moreover, the scientific unification of seemingly diverse phenomena often generates great intellectual dividends, as occurred with the unification of electricity and magnetism in the 19th century. The next important step in this program of unification requires the direct investigation of the Terascale. Both theory and past experiments strongly indicate that new phenomena await discovery in this energy range. A world of new particles predicted by a hypothesis known as supersymmetry may be seen, and these new particles could provide essential information about already known particles. The particles that constitute the dark matter responsible for the formation of galaxies may appear at these energies. The Terascale may be the gateway to new dimensions of space beyond those we experience directly but that nevertheless can have an important impact on our world. New phenomena appearing at the Terascale could include a particle

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics called the Higgs boson, which is responsible for the mass of the known particles. Or, these new phenomena could take an entirely different form, including phenomena that are completely unexpected and not yet imagined. All of these possibilities can best be explored at accelerators. Exploring Terascale physics is the essential next step in addressing the most exciting scientific challenges in particle physics. Particle physics appears to be on the verge of one of the most exciting periods in its history. The Standard Model provides an excellent and carefully tested description of the subatomic world at the energy levels that currently can be studied in laboratories. However, at energy levels that physicists are only now beginning to access experimentally, the Standard Model is incomplete. This strongly suggests that exciting new discoveries loom in the years immediately ahead, especially as the LHC begins to probe this energy region. It also suggests that these impending discoveries may transform our understanding of the origin of matter and energy and the ongoing evolution of the universe. The limitations of the Standard Model are evident, for example, when trying to account for the force of gravity. The Standard Model incorporates the forces of electromagnetism and the strong and weak forces. But when physicists attempt to include gravity as a fourth force in the Standard Model, they run into severe mathematical inconsistencies. Thus, two pillars of 20th century physics—gravity (as described by Einstein’s general theory of relativity) and quantum mechanics— require some new theoretical framework that can include them both. Astronomical discoveries pose another severe challenge to the Standard Model. Astronomical observations have shown that protons, neutrons, electrons, and photons—which account for everything with which we are familiar—make up less than 4 percent of the total mass and energy in the universe. About 20 percent consists of some form of dark matter: massive particles or conglomerations of particles that do not shine and do not scatter or absorb light. Astronomers can detect dark matter by observing how it distorts the images of distant galaxies, an effect known as gravitational lensing, and they can map the distribution of dark matter throughout space. The composition of dark matter is not yet known; it may consist of a cloud of elementary particles of some unknown sort, though there are other possibilities. Yet we owe our existence to dark matter. Without the added gravitational attraction of dark matter, the stars and galaxies, including our own Milky Way, would likely never have formed, because the expansion of the universe would have dispersed the ordinary matter too quickly. More surprising still is the fact that most of the energy of the universe today consists of something else entirely—an ephemeral dark energy that gravitationally repels itself. A clump of ordinary matter or dark matter has an attractive gravitational force that draws matter together and slows down the expansion of the

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics universe, but dark energy pushes itself apart and acts to speed up the expansion of the cosmos. Because most of the energy in the universe is dark energy, the expansion of the universe is accelerating. Thus, dark matter played a crucial role in the past by causing galaxies to form, and dark energy will play a crucial role in the continuing evolution of the universe. What dark matter and dark energy are and how they fit into the overall understanding of matter, energy, space, and time are among the most compelling scientific questions of our time. The predominance of matter over antimatter in the universe also poses problems for the Standard Model. In 1928, Dirac’s incorporation of Einstein’s special theory of relativity into quantum mechanics suggested that, for each kind of elementary particle, there is an antiparticle with the same mass and opposite charge. When a particle and its antiparticle come together, they are both annihilated and their mass is converted into radiant energy. Experiments using antimatter in high-energy physics laboratories show that the fundamental forces act nearly the same on particles and antiparticles except for small differences that can be explained using the Standard Model. However, the Standard Model cannot explain why the universe consists almost entirely of matter and almost no antimatter. This asymmetry is a good thing, since otherwise so much matter and antimatter would have been annihilated in the early universe that there would not have been enough to make stars and planets. Yet the cause of the large imbalance is a mystery. Many physicists believe that the imbalance was created by physical processes that occurred as the universe was cooling after the big bang. It may be possible to study some of the same physical processes by colliding elementary particles at high energies in accelerators. Another outstanding question involves the early evolution of the universe. Most cosmologists believe that the large-scale structure of the universe was created by a burst of “inflation,” a brief period of hyperaccelerated expansion during the first 10−30 second after the big bang, perhaps associated with interactions involving dark energy. This inflation could have rapidly smoothed out the distribution of matter and energy except for tiny lumps here and there that later became the seeds for galaxy formation. Recent observations of the cosmic background radiation have provided exquisitely precise corroborating evidence for this picture of inflation, but there remains a key missing component—the explanation for what drove the hyper-expansion. The Standard Model does not provide an answer, but new physical laws discovered using the next generation of high-energy accelerators may provide essential clues. New evidence about the properties of the elusive particles known as neutrinos also raises exciting new questions. Neutrinos are extremely numerous in the universe but interact very rarely with the basic constituents of ordinary matter— literally billions and billions of neutrinos pass unaltered through each of us every

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics second. A beautiful series of experiments has demonstrated that neutrinos, long thought to be without mass, instead have very small masses—approximately 1/200,000th the mass of the electron, which already has an extremely small mass by subatomic standards. Moreover, the neutrinos produced in nature are apparently not in states of definite mass. This phenomenon, which would baffle a classical physicist, is a typical effect of quantum mechanics. It has a peculiar consequence: Neutrinos can spontaneously change from one type to another, an effect known as “neutrino oscillations.” Neutrino masses do not fit into the Standard Model, so these new observations have necessitated the first major extension to the model in three decades. Exactly what further extensions are required will not be known until the completion of currently operating neutrino experiments as well as the next generation of experiments that are now being planned and initiated. Thus, at the start of the 21st century, particle physics experiments, astronomical observations, and theoretical developments in both particle physics and cosmology point to exciting new phenomena that are just on the verge of being observed. Combining quantum theory and general relativity, and understanding dark matter and dark energy, will require new ideas and new experiments. The technologies needed to conduct these experiments are now available. As a result, particle physics is poised on the brink of a scientific revolution as profound as the one Einstein and others ushered in early in the 20th century. There is every possibility that these Tersacale discoveries will have an equally important impact across the fields of science. RESPONDING TO THE CHALLENGES Physicists use a variety of natural phenomena to study elementary particles and their interactions. Extremely energetic particles are created in the distant cosmos and stream to Earth as cosmic rays, where they can be observed in special detectors. Studies of neutrinos generated within the sun were critical in establishing that neutrinos have mass. Nuclear reactors are sources of intense flows of neutrinos. Physicists will continue to observe and study these particles in a variety of laboratories, including laboratories embedded in ice or deep underground. However, most of the particles that physicists study are created in particle accelerators and observed in specialized detectors located at domestic laboratories and at laboratories in other countries. Such accelerators convert energy into particles that were abundant shortly after the big bang but are extremely rare today; accelerators also provide a window onto interactions among particles that are apparent only at high energies. Studying these particles under controlled laboratory conditions has been, and will continue to be, essential to understanding topics ranging from the origins of matter to the nature of the universe. In particular,

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics comprehensive exploration of the Terascale will require the use of accelerators to elucidate nature’s underlying physical principles. The most powerful accelerator in existence today is the Tevatron at Fermi National Accelerator Laboratory outside Chicago. Before the end of the decade, when it is scheduled to be shut down, the Tevatron will explore the lower reaches of the Terascale and may make important new discoveries about the Higgs boson and the possible existence of new particles predicted in some extensions of the Standard Model. However, the next major set of discoveries is likely to come from a very exciting set of experiments at a new accelerator, the LHC in Geneva, which is scheduled to begin operating in 2007. This machine will enable physicists to explore energy regions inaccessible to Fermilab’s Tevatron. The LHC is a project of CERN, the international laboratory established in 1954 as a joint venture of 12 European countries; CERN currently has 20 member states, all in Europe. The LHC will make CERN the most important center in the world for particle physics over the next decade. The United States has participated both in building the accelerator and in the large collaborations that are building the detectors. U.S. participation has been an important contributor to this tremendous scientific opportunity. The experimental facilities required to reach the Terascale and record the necessary data are exceedingly complex and costly. As the activities at CERN have demonstrated, some of the most advanced experimental facilities, especially those exploring the energy frontier under controlled conditions, are beyond the resources that any single country, or even a single region of the world, can be expected to commit to particle physics. Moreover, these technologically complex facilities require the contributions of many scientists and engineers from throughout the world with different mixes of skills. These factors have caused experimental particle physics to become a truly international activity. No matter what future program of particle physics the United States supports, international collaborations of various kinds will become more essential than ever to the advance of particle physics and to the vitality of the U.S. program in particle physics. In one sense, all of science is becoming increasingly internationalized. New information flows easily and quickly around the world and is shared, almost in real time, with interested scientists wherever they are located. Such information flows also characterize the world of particle physics. However, particle physicists also need to assemble geographically, often in international teams, at national or regional laboratories to jointly plan and carry out particular experiments. Moreover, such experiments typically take 5 to 10 years or more from the initial set of ideas to the full analysis of the results. As a result, the field of particle physics has developed its own distinctive sociology, which is characterized by a great deal of movement of scientists, engineers, and students across international borders and a full accep-

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics tance of the interdependence of the scientific world. The capacity to welcome scientists from abroad as full partners, wherever the key experimental facilities are located, is an essential requirement for the field of particle physics. No nation or region can provide all the experimental facilities to meet the full needs and interests of its community of particle physicists; as a result, international partnerships of various kinds have been developed to solve this problem.2 THE ROLE OF THE UNITED STATES IN PARTICLE PHYSICS For the last 50 years the United States has been at the forefront of particle physics. That leadership position has had an immense impact on this country. It has inspired generations of young people to become members of the strongest scientific workforce in the world. It has attracted outstanding scientists from abroad to come to the United States and contribute to the nation’s intellectual and economic vitality. The novel technologies developed to carry out particle physics experiments have had widespread applications in other areas of science and industry (see Box 1-3). Despite its historic accomplishments, the U.S. program in particle physics is at a crossroads. Great scientific opportunities lie immediately ahead, but the challenge of mobilizing the U.S. program to exploit this special moment is significant. In fact, it is not at all clear that the United States can continue to occupy a leadership position in the worldwide particle physics community. There are several reasons for this situation. First, despite the growing sense of scientific excitement and opportunity within particle physics, and despite a decade of strong national economic growth, no additional resources have been devoted in recent years to the U.S. program in particle physics (see Box 1-4). This stand-still budget contrasts strongly with the situation abroad, where Europe and Japan are both making new commitments to take advantage of exciting scientific opportunities.3 In addition, 2 The emerging dominance of large-scale facilities for science has attracted considerable attention. In a 2003 report, the German Science Council made the following observations: (1) the success of basic research in natural sciences often is based on the use of complex and costly large facilities, (2) large facilities should be initiated by a broad scientific user community, and (3) a central role of the operation of a large facility lies in the close connection of top-level scientific research and the education of young scientists and their integration into international research collaborations. See German Science Council, Theses on the Significance of Large-Scale Facilities for Basic Scientific Research, 2003. 3 It is the committee’s rough estimate that Europe, through local spending by member states and through the collective national funding of CERN, invests about twice as much each year in particle physics as does the United States, and that Japan alone invests about half as much as does the United States.

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 1-3 Particle Physics in Science and Society The world’s most powerful accelerators, which are among the largest and most technologically sophisticated experimental devices ever built, are tremendously impressive machines that involve remarkable feats of engineering. They have also generated waves of technological innovations and applications throughout the sciences and society. One notable example in recent years was the development of the key protocols that underpin the World Wide Web. Building on the backbone of the already existing Internet, this new way of sharing information has revolutionized the way the world communicates and does business. These protocols were initially developed by a researcher at CERN seeking better ways for large groups of particle physicists to share information and collaborate on experiments. The small accelerators used in hospitals to generate x rays for radiation treatment come from designs developed for particle physics. These designs have been improved and refined as research on accelerator technologies for forefront science continues to be applied to medical accelerators. Roughly 100,000 patients are treated every day in the United States with radiation from electron beam accelerators. Accelerators also are used to produce radioisotopes for treatment, diagnostic tools, and research, and technologies developed for detecting particles in high-energy physics experiments have had important applications in medical imaging. When energetic charged particles pass through curved paths in a magnetic field, they generate radiation. The ability of accelerators to produce powerful beams of x rays or photons of differing energies has generated applications across a broad range of science. Each year as many as 40,000 U.S. researchers from many different scientific disciplines use these powerful light beams to conduct experiments. Accelerator x-ray sources provide, for example, the ability to decipher the structure of proteins and other biological macromolecules and to find trace impurities in the environment or on the surface of a silicon chip. The science produced by these experiments has found applications throughout industry and medicine. In general, particle physics contributes to—and depends on—advances in other areas of physics (such as nuclear physics and condensed matter physics) and in many other scientific fields, including materials science, computing, biology, chemistry, and nanoscience. The health of science requires support of all parts of this interlocking web. Technical challenges faced by particle physicists—such as processing millions of signals quickly, using distributed computers to solve complex problems, and generating electromagnetic fields to accelerate and confine charged particles—have led to many spinoff technologies. Particle physics also has contributed in important ways to mathematics, even as mathematics has been used to understand the theoretical structures describing particles. In industry, accelerators are used for R&D, manufacturing, testing, and process control. For example, beams from accelerators are used to alter the composition of materials and to improve the characteristics of products. Uses of accelerators range from the dating of archaeological samples to the simulation of cosmic rays to determine the impact of radiation on space-based electronics. Finally, because particle physics addresses some of the deepest questions that humans can ask, it resonates strongly with the public at large. The science shelves of bookstores teem with popular expositions of the current understanding of these issues, and many students are attracted to science because they are interested in issues addressed by particle physics.

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics BOX 1-4 Federal Investments in Particle Physics over the Past Decade Two federal agencies have been the main source of funding for elementary particle physics: the Office of High Energy Physics in the Department of Energy’s (DOE’s) Office of Science and the Physics Division in the National Science Foundation’s (NSF’s) Directorate for Mathematical and Physical Sciences. Although support from the NSF plays a crucial role, it is DOE that provides by far the largest share of resources for particle physics and maintains the key national laboratories (see Figure 1-4-1). In addition, the National Aeronautic and Space Administration (NASA) and NSF provide important support for a range of astronomical experiments that are related to particle physics. Support from DOE for particle physics has averaged about $720 million per year over the last 5 years. This funding supported the nation’s flagship accelerator facilities at the Stanford FIGURE 1-4-1 Federal investments in elementary particle physics research at DOE and NSF have remained relatively constant in inflation-adjusted FY2006 dollars. The lower dashed line extends from the FY1995 level of DOE investment to the current levels (projected) for FY2006. The upper dashed line extends from the FY1995 total federal investment to FY2006 and indicates that, as a whole, the federal portfolio has maintained a nearly constant effort over the past decade. The NSF construction investment is dominated by the building of the IceCube neutrino observatory. Note that new construction initiatives have decreased significantly and that DOE operations costs have become a larger percentage of the total.

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics Linear Accelerator Center (SLAC) and the Fermi National Accelerator Laboratory (Fermilab) (see Figure 1-4-2). Smaller accelerators have operated at other national laboratory and university sites, and many other universities and research organizations have programs in particle physics supported by the DOE and NSF. The direct contribution from NSF, including construction funding, has ranged from $60 million to $125 million per year over the last decade. For the past 10 years, the nation’s investments in elementary particle physics have remained relatively constant in inflation-adjusted dollars. However, this flat budget has been achieved only because declining support from DOE has been counterbalanced by what may be a short-term increase in NSF support, particularly for the construction of the large antarctic neutrino observatory IceCube. The President’s proposed FY2007 American Competitiveness Initiative, with its increase in real funding for particle physics (about 8 percent), could enable many of the exciting opportunities described in this report. FIGURE 1-4-2 Laboratory scientific staff working in particle physics at Argonne National Laboratory (ANL), Fermi National Accelerator Laboratory (Fermilab or FNAL), Stanford Linear Accelerator Center (SLAC), and Brookhaven National Laboratory (BNL) have all declined in recent years (measured in full-time-equivalent employees), with the decrease at BNL from FY2002 to FY2003 being driven partly by the termination of the high-energy physics program of the Alternating Gradient Synchrotron (AGS). Only at Lawrence Berkeley National Laboratory (LBNL) did the staff grow from FY2003 to FY2004, owing to work on the proposed Supernova/Acceleration Probe (SNAP), a satellite designed to study dark energy through the discovery and measurement of thousands of distant supernovas.

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics the investments required to construct new facilities or to operate existing ones effectively have grown because of the increasing sophistication of accelerator and detector technologies. Second, the future of experimental facilities for particle physics in the United States is uncertain. Several of the country’s flagship particle physics experiments are scheduled to be shut down within the next few years, and at least one major facility (the accelerator at the Stanford Linear Accelerator Center in California) is being redirected toward other scientific areas. Some new small-scale projects related to particle physics are under construction, but no approved plans are in place for new initiatives in the United States that could capitalize on the exciting scientific challenges that have recently crystallized. Again, in other regions of the world—especially in Europe and Asia—programs in particle physics are being expanded and new experimental facilities are under construction. Third, there has been insufficient investment in R&D for the tools and facilities of particle physics—namely, accelerators and detectors. More generally, not enough resources have been deployed in the United States to carry out smaller scale but critical experiments or to undertake critical initial explorations of the new technological frontiers for accelerators and detectors that would help enable future experiments. To continue to explore the energy frontier, more efficient and less expensive technologies are essential, and smaller scale science will remain a critical component of the national program. Finally—and most important—since the cancellation of the Superconducting Super Collider (SSC) in 1993, the U.S. program in particle physics has not had an overall strategic plan informed by a long-term vision that would give it a unique shape, direction, and excitement. In retrospect, many feel that the failure to proceed with the SSC was a lost opportunity both for the U.S. particle physics program and for the entire U.S. scientific enterprise. Moreover, it delayed scientific progress in particle physics by at least a decade. To grasp the opportunities now available in particle physics, a new vision is needed—a vision that can mobilize the creativity, excitement, and leadership of students and scientists and generate the public commitment needed to maintain U.S. scientific and technological leadership in particle physics. The United States, despite not being a member state of CERN, has made and continues to make substantial commitments of both intellectual and financial resources to the LHC. Many U.S. students and scientists are participating in the upcoming experiments at the LHC. Within the next few years, more than half of U.S. experimental particle physicists will be focused on experiments occurring at CERN rather than in the United States. If the United States maintains its present course and chooses not to seize the opportunity to explore the Terascale with distinctive U.S.-based, next-generation

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics experimental facilities, leadership in particle physics will move to Europe, Japan, and elsewhere. Indeed, such a migration of leadership has already begun. The committee believes that if an exciting forefront particle accelerator is not available in the United States in coming decades, fewer of our young people will be attracted to particle physics and to science in general. Without such a facility, U.S. scientists and engineers will have to travel abroad to work on forefront accelerator experiments, as has already begun happening with the LHC. Universities will be increasingly reluctant to appoint particle physicists to their faculties, knowing that they will have to spend large portions of their careers working abroad. Leading scientists and engineers from other countries will no longer travel to the United States in large numbers to participate in high-energy accelerator experiments, where in the past they contributed in so many ways to the nation’s scientific, cultural, and economic vitality. Eventually, particle physics in the United States will lose its vitality, with most of the important advances occurring in other parts of the world. The committee has concluded that the price the United States would pay by forfeiting a leadership position in particle physics is too high.4 Leadership in science remains central to the economic and cultural vitality of the United States.5 To fuel the innovation economy of the 21st century, to maintain national security, and to produce the knowledge needed to ensure our well-being in the face of an uncertain and challenging world, the United States needs more than ever to have a strong base of science and technology. A strong scientific enterprise attracts ambitious and talented students to science. It also makes the United States a desirable place for excellent scientists from abroad to pursue some of the most important challenges on the scientific frontier. Particle physics contributes greatly to the strength of U.S. science and technology while allowing U.S. students and scientists and engineers to participate in and benefit from a worldwide scientific activity. More generally, leadership in particle physics can serve, as it has in the past, as an important symbol of leadership in science and technology. In the flat world that is taking shape, leadership in particle physics no longer consists of single-handed efforts to maintain dominance in a particular subfield. Rather, leadership emerges from the creativity and initiative needed to organize international teams of collaborators to pursue projects that are beyond the capability of any one country. Such leadership requires making investments both at home and abroad in order to participate in and benefit from developments across 4 House Committee on Science, Unlocking Our Future: Toward a New National Science Policy, September 1998. Available online at <http://www.house.gov/science/science_policy_report.htm>. 5 NAS, NAE, and IOM, Rising Above the Gathering Storm: Energizing and Employing America for a Brighter Economic Future, Washington, D.C.: The National Academies Press, 2005 (Prepublication).

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Revealing the Hidden Nature of Space and Time: Charting the Course for Elementary Particle Physics the broad scientific and technology frontier. When such investments are joint ventures with colleagues from abroad, all partners participate directly in both the costs and benefits of the enterprise. However, the capacity to deploy new discoveries across a broad spectrum of economic activities depends on the structure, incentives, and capacity of individual economies to adapt and encourage change. Careful studies in the United States indicate that investments in high-quality science and scientific leadership repay those investments many times over.6 Chapter 2 reviews the most important and exciting scientific questions in particle physics within the context of some of the significant milestones in the development of particle physics between 1950 and the present. Chapter 3 discusses the types of experimental facilities, scientific approaches, and devices that will be needed to explore the questions posed in the preceding chapter. In addition, it describes the evolving international framework within which decisions in particle physics must be made. Chapter 4 highlights the strategic framework within which the committee believes decisions on priorities for the future of particle physics should be made. Chapter 5 focuses on the findings and recommended action items of the committee. Particle physics is a discovery-based science that probes the deep secrets of nature. What are the characteristics of space and time? How did the universe evolve, and how will it evolve into the future? Is nature understandable, or are there fundamental limits to knowledge? These are questions that capture the imagination of people everywhere and that great nations should strive to answer. Indeed, the nations that lead the way in answering these questions will occupy a special place in human history. If the United States is to continue to be a leader in particle physics, it must provide appropriate support for scientists and students working at the scientific and technological frontiers in particle physics, leverage resources by pursuing joint efforts with international partners, and—above all—adopt a strategic framework and an associated set of priorities to maximize the impact of the resources that are available. The administration’s proposed budget for FY2007 takes up this challenge and begins to provide the necessary resources for the physical sciences and mathematics to sustain their vitality and the vitality of U.S. science. This report charts a path toward the future for particle physics that will make the field’s tremendous potential a reality. 6 See, for example, NAE, The Impact of Academic Research on Industrial Performance, Washington, D.C.: The National Academies Press, 2003; Council on Competitiveness, Innovate America: Thriving in a World of Challenge and Change, Washington, D.C.: Council on Competitiveness, 2004.