BPA NEWS Board on Physics and Astronomy • The National Academies • Washington, D.C. • 202-334-3520 • national-academies.org/bpa • Winter 2004 EPP 2010: An Assessment of and Outlook for Elementary Particle Physics
s the first project in the Board on field telescopes will be needed to map dark collaborations at the European laboratory Physics and Astronomy’s decadal matter and study the acceleration of the CERN on the Large Hadron Collider (LHC), Asurvey of physics, the Elementary expansion of the universe, which appears to a facility currently under construction that Particle Physics 2010 Committee began its be caused by a mysterious dark energy, the will go into operation late in this decade. work in autumn 2004 with the ambitious nature of which may be elucidated by Beyond the LHC, the international commu- task of preparing a 15-year plan for the elementary particle physics. Other ques- nity is discussing the possibility of construct- future of elementary particle physics. tions can be addressed by observations that ing a very high energy electron-positron Fifty years of progress in understanding do not employ accelerators, including collider (the “Linear Collider”) as a possible the physics of matter, energy, space, and neutrino physics, proton decay, cosmic ray multinational project. Other accelerator- time have led the field to a new frontier of physics, the nature of dark matter particles, based experiments are under discussion that unprecedented breadth. The frontier is and various aspects of astrophysics. These address scientific problems that cannot be defined by deep questions. Are there more experiments also require a new generation addressed by the LHC or the Linear than the four dimensions defined by of observational apparatus and laboratories. Collider. Long-baseline experiments to Einstein in his general theory of relativity? Is To directly attack the questions of the search for charge-parity (CP) violation in there a unification of particles and forces? fundamental nature of space and time and the neutrino sector may require accelera- What is the nature of the dark matter and the nature of the particles and forces that tors with beam powers of megawatts. dark energy that seem to permeate the give rise to dark matter and energy will Studies of B-meson physics may require universe and account for most of its matter require a new generation of accelerator machines with luminosities of 10 to 100 and energy? These questions are linked experiments. Some experiments are now times those now attainable. together in complex ways and they make slated to be carried out in international See “EPP 2010” on page 8 contact with various other fields of research, including nuclear physics and astrophysics. Great progress has been made through new Bose-Einstein Condensation of Excitons at High theoretical ideas that promise deeper understanding of experimental observa- Magnetic Field tions. Such ideas have helped to frame these questions and will play a key role in James P. Eisenstein, California Institute of Technology understanding the results of future experi- ments. Professor James P. Eisenstein is a return- objects made up of an even number of Space-borne and ground-based wide- ing member of the BPA’s Solid State Sciences fermions can also often be regarded as Committee. He prepared this article based bosons. The prototypical example of such on an invited lecture he gave at the fall 2004 a composite boson is a 4He atom. In this issue: meeting of the Board on Physics and As- Bosons can undergo Bose-Einstein • EPP 2010. Page 1 tronomy. condensation (BEC), in which a macro- scopic number of particles begin to oc- • Excitonic Condensates. Page 1 lementary particles in nature are cupy the lowest energy quantum state divided into two classes: fermions available as the temperature is reduced • Highlights of the Fall BPA and bosons. Fermions are particles below a critical value. In the condensed Meeting. Page 2 E with spin angular momentum 1/2, 3/2, 5/2, state the many-particle quantum etc., times Planck’s constant h. Electrons, wavefunction of the system is coherent • High Magnetic Field Science. protons, and neutrons are all fermions. across macroscopic distances. At its root, Page 4 Fermions obey the Pauli exclusion prin- BEC is a purely statistical mechanical • Beckman Symposium. Page 5 ciple: no more than one fermion can phenomenon; no interactions between the occupy a single quantum state. Bosons bosons are required for the condensation however, have integer spin: 0, 1, 2, etc. to occur. The transition occurs when the • Physical and Life Sciences. Page π 7 times h/2 . Any number of bosons can thermal de Broglie wavelength of the occupy the same state. Elementary particles becomes comparable to their • Astronomy Before the Congress. bosons include photons, mesons, and separation. Page 10 gravitons. More importantly, bound See “Excitons” on page 12 2 BPA News • Winter 2004 Highlights of the Fall Meeting of the Board on Board on Physics and Astronomy Physics and Astronomy Burton Richter, Chair he Board on Physics and As- (Caltech) discussed his work with Stanford University tronomy met for its annual au- excitons to achieve Bose-Einstein con- Anneila Sargent, Vice Chair tumn meeting on November 6-7, densates. He pointed out that a key California Institute of Technology T 2004, at the Arnold and Mabel Beckman element of the research program of Elihu Abrahams Center of the National Academies in modern condensed-matter and materi- Rutgers University Irvine, California. Chair Burton Richter als physics is to understand how and Jonathan Bagger brought the meeting to order and intro- why “the whole is more than sum of its The Johns Hopkins University duced the new members of the Board parts.” The collective behavior of Gordon A. Baym University of Illinois at Urbana-Champaign (Ron Davidson, Andrea Ghez, Marc groups of interacting objectives, called Kastner, and Chris McKee). Dr. Richter emergent phenomena, is a crosscutting Ronald C. Davidson Princeton University briefly outlined the agenda for the meet- theme throughout much of physics, he William Eaton ing and thanked everyone for making said. Consider the neuron, he sug- 11 National Institutes of Health the trip to southern California. Board gested: you put 10 of them together Raymond Fonck director Don Shapero introduced David and suddenly you have a brain that University of Wisconsin at Madison B. Lang, who joined the BPA in Septem- exhibits very sophisticated properties Andrea M. Ghez ber 2004 as a research assistant. and behavior. And you cannot predict University of California at Los Angeles Deborah Jin (JILA) presented a any of them! He described his research Laura H. Greene discussion of her recent groundbreaking with doped bilayer semiconductors and University of Illinois at Urbana-Champaign observations of the condensation of magnetic fields to form novel excitons. Frances Hellman fermionic atom pairs in the BCS-BEC (See Dr. Eisenstein’s article elsewhere in University of California at San Diego crossover regime. With the use of a gas this issue for more details.) Erich P. Ippen of potassium-40 atoms at low tempera- BPA staff member Timothy Meyer Massachusetts Institute of Technology ture, her research has explored the then presented a short report on activi- Marc A. Kastner interactions of the fermionic atoms and ties related to the Board’s discussion of Massachusetts Institute of Technology the boson condensate simultaneously. opportunities at the intersection of the Christopher F. McKee Employing a 200-G magnetic field, the physical and life sciences at its meeting University of California at Berkeley molecular dimers of potassium atoms in autumn 2003. Dr. Meyer reported Julia M. Phillips can be formed such that the average that there had been significant develop- Sandia National Laboratories molecular size is much larger than the ments on the federal front, ranging Thomas N. Theis IBM Thomas J. Watson Research Center average spacing between atoms. Making from congressional report language to a condensates with fermions is well under- joint NSF/NIH workshop exploring the C. Megan Urry Yale University stood in the Bardeen-Cooper-Schreiffer possibilities for and challenges to Carl E. Wieman (BCS) theory of superconductivity, “bridging the sciences.” He also de- JILA/University of Colorado where fermions pair in momentum scribed a small meeting of experts space. Tuning the magnetic field sets the planned for December 2004 that would NRC Staff energy scale of the pairing energy per evaluate the case for and scope of a Dr. Donald C. Shapero, Director molecular dimer, allowing access to a potential NRC study in this area. (See Dr. Timothy I. Meyer, Program Officer Mr. Brian D. Dewhurst, Sr. Program Associate wide variety of regions on the overall article elsewhere in this issue for more Mr. David B. Lang, Research Assistant phase diagram. In her now-celebrated details.) BPA members endorsed the Ms. Pamela Lewis, Program Associate experiment, Dr. Jin used a gas of 500,000 concept and suggested that the full Ms. Van An, Financial Associate potassium atoms that was cooled to breadth of the physical sciences be ❧ temperatures below 50 nK and then included in the discussion, ranging from subject to a applied magnetic field. The chemistry and physics to materials The Board on Physics and Astronomy is a field coaxed the fermionic atoms to sciences and astrobiology. continuing interdisciplinary body with expertise spanning the various subfields of physics, match up into pairs, akin to the pairs of Ronald Davidson (Princeton Plasma astronomy, and astrophysics. It serves as a focal electrons that produce superconductiv- Physics Laboratory) presented a sum- point in the National Research Council for issues connected with these fields. The activities of the ity, the phenomenon in which electricity mary of the final report of the National Board are supported by funds from the National flows with no resistance. The Jin group High Energy Density Physics Task Force Science Foundation, the Department of Energy, the Department of Defense, the National detected this pairing and the formation that he chaired for the OSTP Physics of Aeronautics and Space Administration, the of a fermionic condensate for the first the Universe interagency working National Institute of Standards and Technology, time on December 16, 2003. group. The Task Force reviewed the and private and other sources. Following Dr. Jin, James Eisenstein scope of the high energy density physics BPA News • Winter 2004 3 area, building on the previous NRC research laboratories as embodied by report and some of the science identi- the Materials Research Science and Committees of the fied in Connecting Quarks with the Cos- Engineering Center program. Clearly Board on Physics and mos. As a sidelight, Dr. Davidson noted this study will offer a valuable prospec- Astronomy that the interaction of the different tive and retrospective review of this participants led to the planning of a joint program, he said, but it will be impor- Committee on Astronomy and Astrophysics RHIC and plasma physics workshop tant to find people with expertise and C. Megan Urry, Yale University, and later this year. The task force identified experience who are also free of conflicts Roger D. Blandford, California Institute of 15 different research thrust areas that of interest. Finally, the SSSC is also Technology, Co-chairs fell into four broad categories: astro- discussing the need for and scope of a physics systems, beam-induced plasmas, study of materials discovery and crystal Committee on Atomic, Molecular, and Optical Sciences stockpile stewardship, and ultradense, growth. At the autumn meeting of the Pierre Meystre, University of Arizona, Chair ultrafast laser science. Dr. Davidson SSSC, the committee discussed the briefly described the different thrust issues; the ability to supply high-quality Plasma Science Committee areas and some of the science that each crystals within the United States is very Cary B. Forest, University of Wisconsin at might cover. Although the Task Force limited, and so many researchers must Madison, Chair did not prioritize amongst the thrust team up with other groups offshore Committee on Radio Frequencies areas, the elucidation of a finite number simply to get access to materials. Karen St. Germain, Naval Research of these research areas was an intrinsic Heterostructures constructed with Laboratory, Chair prioritization of opportunities in the advanced molecular-beam epitaxy field. In closing, Dr. Davidson noted techniques are another example where Solid State Sciences Committee Marc A. Kastner, Massachusetts Institute of that the Task Force had explored a sample purity and availability have been Technology, Chair, and model for interagency coordinated a bottleneck in research pursuits. The Peter F. Green, University of Texas at investment, focusing on the potential SSSC resolved to prepare a study pro- Austin, Vice Chair role that medium-sized facilities and spectus before the next meeting in April centers might play (the new COBRA 2005. Committee on Smaller Facilities Robert Sinclair, Stanford University, Chair center at Cornell being a prime ex- Continuing its interest in providing ample). For instance, he said, the task guidance to the volumes of Physics 2010, Elementary Particle Physics 2010 force imagined that NSF could fund the Board organized a small roundtable Harold Shapiro, Princeton University research on the OMEGA laser at Roch- discussion among the four currently University, Chair, and ester. active elements of the series: Jonathan Sally Dawson, Brookhaven National Laboratory, Vice Chair Marc Kastner presented an overview Bagger for Elementary Particle Physics of the activities of the BPA’s Solid State 2010 (EPP2010; see article in this news- Committee on Review of Progress in Sciences Committee (SSSC). He de- letter); Pierre Meystre for Atomic, Astronomy and Astrophysics Toward scribed the progress of the two studies Molecular, and Optical Physics 2010 the Decadal Vision in progress: the Committee on Opportu- (AMO2010), Steve Cowley for Plasma C. Megan Urry, Yale University, Chair nities in High Magnetic Field Science Science 2010 (Plasma 2010), and Marc ❧ (led by Peter B. Moore of Yale Univer- Kastner for Condensed-Matter and sity) and the Committee on Smaller Materials Physics 2010 (CMMP2010). More information on BPA committees may Facilities (led by Robert Sinclair of Of the four volumes under consider- be found on the BPA Web site at Stanford). Both groups are drafting ation, only EPP2010 and AMO2010 have
the Large Hadron Collider (LHC) at CERN, Opportunities in High Magnetic Field Science as well as those contemplated for the Inter- Peter B. Moore, Yale University national Thermonuclear Experimental magnetohydrodynamics, and high-field Reactor (ITER), depend on magnet technol- Peter Moore chaired the BPA’s Committee magnets also play an increasingly important ogy developed in the United States (al- on Opportunities in High Magnetic Field role in industry. though the magnets are in fact being manu- Science (COHMAG) with support from the High magnetic field science and technol- factured overseas), as do the magnets National Science Foundation. Under his ogy are thriving in the United States today, installed in several user facilities overseas. leadership, the committee met three times in and the prospects are bright for future gains On the other hand, in the area of nuclear seven months and released its final report in from high-field research. Recent accom- magnetic resonance (NMR), which is a prepublication form in January 2005. Here, plishments include the development of major component of high-field science, the he writes about the committee’s findings and functional magnetic resonance imaging, United States is competitive but not domi- recommendations. which is revolutionizing neuroscience; nant. About half of the instrumentation optically pumped magnetic resonance used by NMR spectroscopists in the United ecause of the scientific importance of techniques, which allow visualization of new States, and virtually all of the magnets in high-field magnets and the possibility quantum phenomena in semiconductors; their spectrometers, were manufactured B that new breakthroughs might and ion cyclotron resonance mass spectros- abroad. Further, many of the most impor- develop, the Committee on Opportunities in copy, which is becoming an important tool tant recent advances in NMR have been High Magnetic Field Science was formed to for exploring the chemical composition of made overseas, and, in general, European assess the current state of, and future complex systems. High-field research has and Japanese companies have been ahead of prospects for, high-field magnet science and led to the discovery of new states of matter U.S. companies in commercializing magnet technology in the United States, taking into in low-dimensional systems. It has also technology developments. Finally, Europe is account international trends and particularly provided the first indications of how high- far ahead of the United States in equipping promising multidisciplinary areas of re- temperature superconductors evolve into its synchrotron light sources and neutron search and development. The committee’s unconventional metallic alloys in the ex- scattering centers with instruments for task was to identify key scientific and tech- treme quantum limit. Outstanding work studying the X-ray and neutron scattering nological challenges and opportunities, not continues to be done in the area of magnet properties of materials in high magnetic to make specific programmatic recommen- engineering, the discipline on which all these fields. It also worth noting that several key dations. The committee met three times activities depend. There is every reason to facilities in Japan have made important between September 2003 and April 2004 and believe that new developments as interesting contributions to the development of the its final report was released in as those mentioned above will occur in the technologies required for the generation of prepublication form in mid-January 2005. decades to come, especially if magnets are the highest steady-state and pulsed mag- In general, the committee found that high built that deliver higher fields than those netic fields. magnetic field science in the United States is available today. For instance, pulsed fields High-field magnet science is intrinsically healthy and broadly multidisciplinary. offer the opportunity to explore the highest multidisciplinary. The construction of high- However, there are some important oppor- magnetic fields in ways that can take re- field magnets has always been motivated by tunities that will be missed unless attention is search in new directions. Additionally, the science that could be done with them, paid to them soon. In this article, I present a advances in high-speed electronics, instru- and in recent decades, physics, chemistry, summary of the committee’s findings and mentation, and miniaturization also offer biology, and medicine have all benefited recommendations. the potential to allow greater experimental from advances in magnet technology. Even A magnet is “high field” if its field access to higher fields. the technology of high-field magnets is strength is high enough to test the limits of The United States is a leader in many cross-disciplinary. Materials science and the mechanical and/or the electromagnetic areas of high-field science and technology, engineering make dominant contributions, properties of the materials from which it is but further investment will be required to but several branches of physics contribute built. High-field magnets have been—and make the United States competitive in some as well. continue to be—used for research in many critical areas. There are many indicators of U.S. scientists will be unable to access a scientific disciplines, including medicine, the strength of the U.S. effort in high mag- wealth of science opportunities if high chemistry, and condensed-matter physics; netic field research. For example, con- magnetic field instrumentation is not pro- they are also enabling for fields such as densed-matter physicists and materials vided at the Spallation Neutron Source and plasma science and high-energy physics. researchers from other parts of the world at the nation’s third-generation light Research that could only have been done routinely travel to the National High Mag- sources. The scientific opportunities avail- with such magnets has produced important netic Field Laboratory (NHMFL) to per- able to those able to study the neutron and insights in a host of areas ranging from brain form experiments that they are unable to do X-ray scattering properties of materials at function to high-temperature superconduc- at home, but U.S. scientists seldom travel high magnetic fields are attracting growing tivity. High magnetic fields are of great abroad for that reason. In addition, the attention around the world. Certain aspects interest in areas such as astrophysics and superconducting magnets being installed in See “High Magnetic Fields” on page 6 BPA News • Winter 2004 5
an enormous advantage,” Roberts said, Instrumentation for a Better Tomorrow “except that infrared spectroscopy is easier A Symposium in Honor of Arnold O. Beckman to understand.” Leroy Hood, president of the Institute for Systems Biology, then altech professor of chemistry John ments currently produced by the company presented a discussion of the future of D. Roberts, Institute of Systems that Dr. Beckman started. The Chemical molecular and system biology, arguing that CBiology president Leroy Hood, and Heritage Foundation provided an interactive today’s research problems require interdis- Chemical Heritage Foundation president multimedia display that chronicled several of ciplinary teams and tools and that tradi- Arnold Thackray were among the distin- Dr. Beckman’s achievements and allowed tional organizational structures for re- guished scientists, engineers, and research- visitors to tinker with a computer-simulated search programs are often inadequate to ers who participated in a 1-day symposium version of the Dr. Beckman’s DU spectro- meet these challenges. Gabrielle Long, an on November 15, 2004, to honor famed photometer. associate director of the Advanced Photon inventor, entrepreneur, and philanthropist After introductory remarks by Wm. A. Source at Argonne National Laboratory, Arnold O. Beckman. The symposium Wulf, president of the National Academy of described the evolution of X-ray scattering focused on the role of instrumentation in Engineering, and a welcome from Ralph instruments from the original benchtop scientific research and the important influ- Cicerone, chancellor of the University of tools to today’s enormous national syn- ence Arnold Beckman had on the develop- California at Irvine and president-elect of the chrotron user facilities. ment of laboratory instrumentation. Speak- National Academy of Sciences, Dr. A lunch keynote address was presented ers discussed the evolution of instrumenta- Beckman’s daughter Pat Beckman shared by Chad Mirkin, professor of chemistry at tion in several fields of research and how some personal reflections about her father’s Northwestern University and a past recipi- historical trends position us for the future. philosophy. She described the enormity of ent of the Young Investigator Award of the The sophistication of instrumentation in his legacy, from instrumentation and re- Arnold and Mabel Beckman Foundation. research has grown immensely since Arnold search to philanthropy, education, and He detailed some of the dramatic frontiers Beckman, then a professor at the California raising a family. of chemistry at the interface with nano- Institute of Technology, marketed his first The morning keynote address was scale science and technology. He de- commercially successful instrument in delivered by Arnold Thackray, president of scribed how a new toolkit developed by his 1935—an electronic meter designed origi- the Chemical Heritage Foundation. He group at Northwestern, dip pen nally to measure the acidity of lemon juice. discussed Arnold Beckman’s intuitive grasp nanolithography, provided a new regime of Today, the conduct of most research is of the “sweet spot of opportunity,” and control over the assembly of nano-struc- essentially inseparable from the develop- described Dr. Beckman’s life and work in tures with profound implications for ment and use of reliable, high-performance, terms of his inventive recklessness, his medical research. and integrated research tools. Indeed, contributions to chemists’ tools and the new Michael L. Roukes, professor of phys- instrumentation has become so important biology, and, finally, his role in the electronics ics, applied physics, and bioengineering at in research that instrument development revolution. Dr. Thackray closed by quoting the California Institute of Technology, has itself become the subject of research, from a printed advertisement promoting discussed the frontiers of nanotechnology creating a positive feedback loop that has Beckman Instruments, Inc., in 1960: “Since and its intersection with chemistry, physics, accelerated the pace of scientific and tech- the year one there has been no change in the and biology. “The underlying principle is nological progress. scientific method. Only the tools are differ- that we can make devices so small and The National Academies sponsored the ent. Our job—providing them...One day the See “Beckman Symposium” on page 13 symposium in honor of Arnold O. present science of electronics will be supple- Beckman, the renowned inventor and mented or replaced. Still newer technologies philanthropist who died in 2004 at age 104. will need even more advanced instruments to The symposium was entitled “Instrumenta- implement them. Our catalog for the fu- tion for a Better Tomorrow,” and over the ture?—We’re working on it now.” course of the day the symposium partici- The program then shifted to a discussion pants were treated to a wide-ranging and of instrumentation in different fields of inspiring overview of the role that research research in science, engineering, and medi- instrumentation has played—and will cine. John D. Roberts, Institute Professor of continue to play—in improving our lives. Chemistry, emeritus, at the California Insti- More than 60 people attended the sympo- tute of Technology, described the evolution sium and were treated to special enhance- of nuclear magnetic resonance (NMR) and ments of the standing heritage exhibits its role in analytical chemistry. As a young (supported by the Arnold and Mabel faculty member at MIT, the first instrument Beckman Foundation) at the center. he bought was a Beckman DU spectrometer. Beckman Coulter, Inc., set up a display Then, just a few years later, NMR spectrom- featuring two modern laboratory instru- eters were introduced. “Clearly, NMR had 6 BPA News • Winter 2004
High Magnetic Fields magnets themselves. Thus it is vital that a magnetic resonance and magnetic resonance national laboratory equip its magnets with the imaging systems, and manufacturers of ad- (continued from page 4) best possible supporting instrumentation and vanced materials, such as high-strength materi- of magnetism and high-temperature super- personnel. In addition, ways to maximize the als and superconducting wire. The sharing of conductivity have already been elucidated return on capital invested in the national information and resources within that larger overseas at scattering center laboratories that laboratory should be explored, such as longer community, which is now fragmented into have high-field instrumentation. Nevertheless, hours of operation and flexible scheduling. components that communicate poorly, would the United States does not currently plan to The laboratory should undertake a cost- accelerate the rate at which solutions are found increase the high magnetic field instrumenta- benefit analysis to identify the optimal balance to the fundamental problems they all confront. tion at its national radiation laboratories. between addressing user demand and the COHMAG proposes that the involved com- Unless steps are taken to rectify this situation, increased operating costs associated with munities cooperate to establish a consortium the nation is sure to lag behind in key areas of longer hours of operation. For instance, the whose objective would be the development of condensed-matter and materials physics. nation’s synchrotron light sources and neutron the technology necessary to pursue several There are important issues relevant to the scattering centers provide access 24 hours a aggressive goals that may have different advancement of magnet technology that could day, 7 days a week when in full operation; this timescales. Some groups might frame their be more efficiently addressed if the interested schedule allows visiting researchers to use their goals in terms of application-specific require- constituencies interacted more strongly, time at the facility to the best advantage. The ments for magnet performance, such as the communicated more fully, and coordinated trade-offs for expanding access to the NHMFL development of a 30-T superconducting high- their activities better. One striking characteris- need to be identified and weighed carefully, resolution magnet for NMR, a 60-T DC hybrid tic of all the sciences that use high magnetic especially in constrained budget situations. magnet, or a 100-T long-pulse magnet. Others, fields is how constrained they are by the New instruments for studying the neutron such as the high-energy physics and fusion limitations imposed by magnet technology. and x-ray scattering properties of materials in science communities, might focus explicitly on Nevertheless, despite a shared need to over- high magnetic fields should be developed in the the materials problems intrinsic to enabling come the same set of fundamental problems, United States. Nowhere in the domestic high-volume production of quality conductors each constituency has historically tended to research program is the gap between the for a variety of magnet systems. develop the magnets it needed without much instrumentation available for experimentation Agencies supporting high-field magnetic reference to the others. The reasons are at zero field and that available for high-field resonance research should directly support the several and obvious. The various communities experimentation wider than in the areas of development of technology and instrumenta- that use high-field magnets have different neutron and X-ray scattering. This gap in tion for magnetic resonance and magnetic missions, and the magnets they need are capability is unfortunate, because scattering resonance imaging. Without the concomitant specific to the mission of each. In addition, experiments provide a powerful means for development of ancillary technologies, the these communities are supported by different elucidating atomic and magnetic structure, as construction of higher-field magnets for funding agencies, each of which has had its well as determining the nature of the spatial magnetic resonance will not produce the own perspective. A coordinated approach to and dynamical correlations in materials. scientific dividends it should. While federal magnet technology based on the pooling of Development of new high-field capabilities at funding for the application of existing technol- resources and talent would be beneficial. X-ray and neutron scattering centers in the ogy and methods to specific scientific problems Based on these observations, the commit- United States could have an enormous scien- has generally been good, federal funding for tee has several recommendations that it offers tific impact. the development of novel technology and in the order of the most important first. A consortium should be established to methodology has been poor. Magnetic The United States should maintain a foster the development of magnet technology. resonance and MRI instrument manufacturers national laboratory that provides its scientific Rather than supporting an all-out, brute-force have done a good job of advancing the sup- community access to magnets operating at the effort to build higher-field magnets using porting technologies relevant to these tech- highest possible fields. A national high-field current technology, it makes sense to find new niques when the commercial markets for their magnet facility is essential to the vitality of many approaches that will make it easier (and products justified their doing so. However, important scientific disciplines. NHMFL has cheaper) to build the magnets needed for there are many areas where technological successfully fulfilled the need for high-field research. Essential to this enterprise will be the advances are sorely needed, but where the magnets for about a decade, and its activities development of both resistive and supercon- commercial market is not large enough to have done much to foster the leadership ducting materials with improved electrical, attract the attention of instrument manufactur- position that the United States currently enjoys magnetic, and mechanical properties. Scien- ers. Likewise, because higher fields result in in many areas of magnetic science and tech- tists and engineers from all the communities significant changes in the relative strengths of nology. It is important to understand that at working today on magnet technology should the interactions that determine how nuclear any high-field magnet laboratory, the capabili- be brought together: the magnet engineers at magnetic moments evolve, improvements in ties of the devices available for controlling the the NHMFL, academic researchers, the pulse sequences and methodologies will be environment in which a sample is tested and magnet designers in the high-energy physics required if magnetic resonance research is to for measuring a sample’s properties are almost and fusion communities, commercial vendors take full advantage of high-field magnet ■ as important as the field strengths of the of superconducting magnets, including nuclear development. BPA News • Winter 2004 7 Opportunities at the Intersection of the Physical and Life Sciences David B. Lang, BPA Research Assistant compelling question of science has multidisciplinary research; in the latter a greatly from a rapid closed loop between long been “How does life emerge from group of researchers, each with their own theory and experiment, whereas many theories A a network of chemical reactions?” In expertise, come together to each tackle a in life science lack general predictive power fact, the interagency guidance memo from the part of a problem entangled with many owing to the difficulty of conducting definitive White House’s Office of Science and Technol- different areas of science. This exposure to experiments. Also, physical and life scientists ogy Policy and its Office of Management and other fields often invites scientists to tend to have different views about basic Budget identified the molecular-level under- broaden their horizons and to integrate new research tools such as instrumentation and standing of the basis of life as a priority area for science with their own disciplines. facilities. Perhaps this difference also explains federal research in 2004. The answer to this One area identified at the meeting as the difficulty in publishing across the disciplines. question lies beyond the traditional capabilities having great potential is the understanding One barrier to developing and promoting of the life sciences and begs for integrating the biomolecular machines. Molecular biology interdisciplinary research is the program creativity of researchers in many fields of theory now requires creating models of how processes involved. Budget strategies are often science beyond biology. proteins move on a multitude of length and not tailored for research across fields, and so The idea of pursuing interdisciplinary time scales. Creating such models would researchers at this crossroads often find research at the interface between the physical greatly improve understanding of how themselves passed from program to program and life sciences has long been brewing. biomolecular structures self-assemble. To or from agency to agency. This situation is also Within the past few years, this vision has understand the machinery of cells demands due to the more speculative and innovative become more concrete: several workshops the development of a new branch of statistical nature of the research. At most academic involving the National Academies, the Ameri- physics at the nanoscale. New synthetic institutions, hiring, tenure, and promotion are can Chemical Society, the National Science materials are emerging to allow the design of controlled by individual departments aligned Foundation, and the National Institutes of new molecules and materials by using new with traditional disciplines, which dissuades Health took place in 2004, bringing the excite- technologies which are able to probe this young researchers from working “outside the ment of this research area to a boil. length scale. lines.” This attitude results in a lack of overlap The Board on Physics and Astronomy Similarly, the mechanics and spatial struc- in professional training between physical and and the Board on Life Sciences hosted a ture of the cell as an organized environment life sciences for students, tracing back to even workshop on December 11–12, 2004, at the may become accessible with new tools of before undergraduate education. The gap Stanford Linear Accelerator Center in single-molecule physics and chemically driven widens as researchers continue their education. Menlo Park, California, to discuss the cell structure and function redirection. The The workshop participants also felt that the spectrum of research problems that lie at structure of a cell is an incredibly complex evolution from multidisciplinary research the interface of life and physical science and fabrication and is not only reliant on spontane- collaborations to interdisciplinary research will to evaluate how the National Research ous self-assembly, but also on the history of the play an important role in characterizing this Council might contribute. In response to cell; the cell is far out of equilibrium, yet fantas- area. Pursuing this interface demands taking the need to frame the discussion broadly, tically organized. Only recently has theoretical larger risks, so research centers with mixed the Board on Chemical Sciences and Tech- physics begun to examine systems far from programs that can balance more or less nology (BCST) and the Board on Math- equilibrium, and with the inclusion of this speculative research may play an important ematical Sciences and Their Applications physics, it may be possible to physically inter- role. Also, instead of research proposal review (BMSA) were invited to participate in the vene at a subcellular level. For astrobiology heavily weighting potential outcomes, reviews workshop. The meeting, chaired by Ken- and molecular evolution, life’s creation from a should include other aspects, such as the past neth Keller of the University of Minnesota, fortunate chemical amalgamation is of central performance of the researcher. A better included experts in chemical engineering, interest. But, there are physical problems with review process coupled with a redesigned, not science policy, biology, biophysics, physics, organizing these chemicals into retrofitted, funding engine will enable many and biochemistry, as well as researchers nonequilibrium structures. Understanding the young researchers to travel down this new who had crossed over from one discipline to origin of self-replicating systems would be a avenue less encumbered. another. This article discusses some of the major step forward in putting the vision of The workshop concluded that an NRC discussions at the meeting and in subse- nanotechnology to practical use. study could add significant value to the devel- quent activities. Of course, what is opportunity without opment of research at the interface of the Research is truly interdisciplinary when obstacles? The NRC workshop also recog- physical and life sciences. The NRC looks researchers operate and think outside nized the barriers that must be overcome in forward to examining the opportunities and traditional disciplinary boundaries, enabling order to see interdisciplinary research flourish. barriers in this interdisciplinary research area a synthesis of ideas from many areas of At the core of the problem lies a basic differ- and, with the guidance of both the life and science to form new ones. Interdisciplinary ence in the cultures of the life and physical physical science communities, hopes to help research is to be distinguished from science communities. Physical science benefits further this exciting vision. ■ 8 BPA News • Winter 2004
EPP 2010 Harold T. Shapiro, president emeritus of gate the frontiers, and—most important, he Princeton University and a famed econo- said—describing the emergence of connec- (continued from page 1) mist and expert on bioethics. The simple tions between problems heretofore thought to A variety of experimental approaches fact that persons outside particle physics be distinct and separately soluble. That is, the will be required to answer the questions that agreed to volunteer for this study says a lot crosscutting nature of the apparent solutions is define the field. Thus the time is ripe to about the broad appeal of particle physics. an indication that we are close to fundamental define priorities among the science ques- The first meeting of the EPP2010 com- discoveries. Dr. Drell discussed experimental tions that could be addressed by new ex- mittee was held in Washington, D.C., strategies for these discoveries, describing the periments. There is a recognized need for a November 30–December 1, 2004. At the role of both observatories (of all sorts) and realistic plan to implement these priorities beginning of the meeting, chair Harold accelerators. She then outlined three ex- that looks ahead 15 years. Shapiro observed, “People here at the amples: dark matter, antimatter, and the In recognition of the current context, Academies and in the government believe mysteries of the vacuum. the National Science Foundation and the that we need to do this [study], and they The committee also heard from Department of Energy requested a Na- were insistent about the need to include several representatives of the government, tional Research Council assessment of people outside the field. We want to set including Michael Turner from NSF, and plan for the field of elementary par- priorities and set the agenda in a way that is Robin Staffin from DOE, and J. Patrick ticle physics. As Robin Staffin, associate exciting to many people in order to capture Looney from the Office of Science and director of Office of High Energy Physics, their attention—and commitment—for a Technology Policy. Christopher McKee, said, “What we want to know is: What is program going forward.” Vice chair Sally co-chair of the most recent astronomy the human significance of the current Dawson, a particle theorist from and astrophysics decadal survey, de- moment in particle physics?” In response Brookhaven National Laboratory, agreed, scribed the procedure that his community to this request, the Board on Physics and adding, “I want to really encourage the non- of scientists uses to identify compelling Astronomy formed the Elementary Par- particle physicists to really speak up if we new directions and to develop a consen- ticle Physics 2010 Committee (EPP2010) in lapse into jargon—or slogans! Together, we sus-based priority list of the most impor- early autumn 2004 to address the follow- want to craft a better message.” tant new initiatives. Abrahram Seiden, ing charge: Presentations at the first meeting fo- chair of the joint DOE-NSF Particle 1. Identify, articulate, and prioritize the cused on introducing the science of particle Physics Project Prioritization Panel, briefly scientific questions and opportunities that physics. The committee also discussed how described the panel’s hoped-for function define elementary particle physics. to set scientific priorities in general. The and role. He admitted that the panel’s 2. Recommend a 15-year implementation meeting led off with a trio of physics talks by most significant project prioritization plan with realistic, ordered priorities to realize Joe Lykken and Chris Quigg of the Fermilab overview activity so far had been the high- these opportunities. theory department and Persis Drell of energy physics contribution to DOE’s In developing plans for the project, the SLAC. They described the “coming revolu- Office of Science 20-year facilities outlook BPA was cognizant of the long history of tions” in particle physics, with an emphasis planning process. In a final formal ad- planning documents developed by the on the pivotal nature of the questions now dress, Barry Barish, co-chair of the most elementary particle physics community. In being studied in particle physics. Dr. Quigg recent HEPAP long-range planning sub- part to bring in new perspectives on the started by stating that the greatest lesson of committee, outlined his views on what important issues, the EPP2010 committee 20th century science was, perhaps, that the challenges lie before the committee and was convened with an unusual balance of human scale of time and distance is not lessons learned from his experience. The membership. Rather than relying uniquely privileged—not even sufficient—for under- first day of the meeting wrapped up with a on the expertise of the practitioners, mem- standing the nature of the universe. He then public-comment session organized by bers with expertise outside particle physics articulated four “coming revolutions in representatives of the American Physical were sought. Fully half of the committee particle physics”: (1) understanding the Society’s Division of Particles and Fields. has significant expertise outside traditional everyday (Why are there atoms, chemisty, Twelve members of the community particle physics, in areas such as condensed- conditions for life?); (2) the meaning of shared their excitement about the future matter and materials physics, atomic, identity (What makes things the way they of particle physics and offered short molecular, and optical physics, and observa- are?); (3) unifying quarks and leptons descriptions of their research. tional astronomy. Three members are not (What do quarks and leptons have in The committee met for a second half- even traditional physicists, bringing expertise common? Why are atoms neutral?); and (4) day in executive session. Committee mem- from the aerospace industry, molecular new conceptions of space-time (Are there bers discussed the broader context of their genetics and biomedical research, and more dimenions to space? How does gravity work, including comments about interna- economics. The chair of the committee rejoin particle physics?). In his presentation, tional perspectices and connections, a embodies the effort to bring new perspec- Dr. Lykken argued that “this is a special historical view of federal investment in basic tive and fresh energy to the task of articulat- time in particle physics” by outlining pro- research and in the field of elementary ing a vision for the future of the field and vocative questions that have new urgency, particle physics in particular. An aggressive framing the best strategies for achieving it: describing the advent of new tools to investi- See “EPP2010” on page 14 BPA News • Winter 2004 9
BPA Meeting flexibility to handle new developments observed that the single largest concern and changes in the outlook for the for NSF is probably the lack of staff— (continued from page 3) future; teasing apart the scientific and while the grants programs have grown project priorities was identified as a significantly over the past 20 years, the Bagger described the appointment of difficult but important task. number of program managers and sup- economist and Princeton president Megan Urry (Yale University) shared a port staff have remained relatively con- emeritus Harold Shapiro as chair of the report on the activities of the BPA’s stant. This situation needs to be ad- EPP2010 committee because of the need Committee on Astronomy and As- dressed, everyone agreed. for an impartial view and fresh perspec- tronomy (CAA) of which she is the co- In a final session, committee chair tive. Dr. Meystre echoed these senti- chair. She briefly outlined several short Donald Backer (University of California ments and commented on the AMO letter reports that had been prepared at Berkeley) discussed the plans of the 2010 plan of selecting two co-chairs, one under the CAA’s auspices. At NASA’s BPA’s standing Committee on Radio from within the field and one from request, the CAA convened a committee Frequencies (CORF). Prof. Backer outside, as well as including several non- to review the science goals of the current described several of the recent filings AMO scientists on the committee. Dr. Terrestrial Planet Finder (TPF) project as before the Federal Communications Cowley agreed that involving leaders well as NASA’s plan for acquiring the Commission and the National Telecom- from outside the traditional community necessary precursor knowledge to suc- munications and Information Adminis- could add value; since the Plasma 2010 cessfully meet those goals. The commit- tration. For instance, on the issue of committee has not yet been formed, tee published its findings in a letter report licensing the transmission of broadband however, no specific plans have been on September 23, 2004. (See article else- Internet services over electrical proposed. Finally, Dr. Kastner explained where in this issue for details.) The CAA powerlines, CORF asked for protection that the SSSC was attracted to the idea of also oversaw a short scientific review of of remote radio astronomy observato- a well-qualified and prestigious outside the AEOS telescope in Hawaii. Finally, the ries. Similarly, it responded to a request chair but that some members worried CAA has convened a special panel to for changing rules governing Channel 37 that a lack of disciplinary expertise might address progress toward realizing the (a portion of the radio spectrum near undermine the authority of the chair with decadal vision of astronomy and astro- 600 MHz) that is currently allocated for the community. That is, different disci- physics. The CAA has been considering radio astronomy and other passive plines face different needs, and if a com- the need for several research briefings listening activities. CORF is engaged in munity viewed itself as in need of an that would make some of the latest sci- an educational effort and plans to visit affirmation of identity and unity, an ence developments since the 2001 decadal the FCC and NTIA after the administra- outside leader for a survey of the field survey more accessible to a broad audi- tion teams settle into their second term. might not be the appropriate. ence. The Board speculated on some of Because radio astronomy needs have The roundtable participants also the challenges in NASA’s future as the changed radically over the years, CORF commented on how they planned to exploration vision is implemented. The is interested in reanalyzing the system respond to the Board’s advice on setting delay of elements of the Beyond Einstein for spectrum management by scientists. scientific priorities. Although the temp- program beyond the budget planning The Board suggested trying to include tation to prioritize large facilities was horizon was particularly troubling to international perspectives and interests strong, they agreed that the objective some, especially in areas where NASA was in the project, as some important radio was to identify compelling science cooperating with other agencies in sup- astronomy observatories are located in themes and then describe the appropri- port of a project. The Board also dis- other countries. It was also suggested ate routes for pursuing them. Commu- cussed potential new CAA business with that OSTP be involved in some of the nity outreach and buy-in to the process Dr. Urry. discussion because of its commitment to will be a critical component, however. BPA members Gordon Baym, Christo- fostering cooperation and coordinations The BPA encouraged each of the study pher McKee, and Elihu Abrahams then between agencies on scientific issues. planners to include town meetings and reported briefly on a recent workshop at Turning its attention to the next meet- coordination with the professional NSF sponsored by the Mathematical and ing, the Board identified several topics for societies in their plans. Each decadal Physical Sciences directorate that exam- consideration. Hearing from OSTP about survey should also take into account ined the state of theory. The workshop the interagency working group process other recent reports that cover some was tasked to give NSF advice on how to was one priority item, as was a report similar issues; for instance, the CMMP nurture advances in theory and how to from a BPA member about the NRC’s 2010 study should examine the findings strengthen all aspects of this part of the newly formed Board on Science Education of the Committee on Smaller Facilities, general research enterprise. The work- that he now chairs. With a warm thank and EPP2010 should consider the recent shop was large, with over four dozen you to everyone for participating, Dr. report Quantum Universe. The Board invited participants and almost as many Richter adjourned the meeting in time for also discussed the challenge of setting observers, they said. The final report is a light meal in the Beckman Center’s convincing priorities while allowing still in preparation. Board members spacious dining room. ■ 10 BPA News • Winter 2004 Congressional Testimony on Astronomy and Astrophysics Joseph H. Taylor, Princeton University Professor Joseph H. Taylor is the James S. Hubble Origins Probe or a similar rehost that make the process possible is the McDonnell Distinguished University Professor of mission is launched? remarkable success of the surveys. The Physics at Princeton University. He shared the 2. How important are the contribu- National Science Foundation and NASA 1993 Nobel prize in physics. He was a member of tions that would be expected from ex- have used the survey reports as the basis the NRC’s Committee on Assessment of Options tending the life of the Hubble Space of their planning processes, and the vast to Extend the Life of the Hubble Space Telescope; Telescope when compared to advance- majority of recommended projects from he is also a former member of the Board on ments expected from other astronomical previous surveys have been completed Physics and Astronomy and was co-chair of the programs at NASA to be launched in the — even if they have sometimes National Research Council’s Astronomy and next decade, such as the James Webb stretched over the boundaries from Astrophysics Survey Committee. On February 2, Space Telescope? decade to decade. The completed 2005, the Committee on Science of the U.S. House 3. Should either a Hubble servicing projects have much to do with the of Representatives held a hearing to explore mission (whether by robot or by Shuttle) leadership position of our national options for the future of the Hubble Space or a new telescope such as the Hubble enterprise in the astrophysical sciences. Telescope. The hearing will help guide Congress’ Origins Probe be a higher priority for The process of priority setting is decisions regarding the telescope’s fate. Dr. funding than other astronomical pro- based on a set of assumptions. For the Taylor provided the following testimony as an grams at NASA? purposes of this hearing, the most expert witness. In the balance of my testimony I shall important of these is that priorities from address all three questions. previous decades should be completed. r. Chairman, Ranking Minority Until recently, the NRC decadal For example, the year 2000 Survey Member, and members of the survey was an activity unique to the reaffirmed the importance of complet- Mcommittee: thank you for discipline of astronomy and astrophysics. ing the Atacama Large Millimeter Array inviting me here to testify today. My The most recent survey involved the that had been recommended in the 1991 name is Joseph Taylor and I am the James direct participation of 124 astronomers; Survey.[5] Along the same lines, the S. McDonnell Distinguished University moreover, the direct participants received most recent Survey was based on the Professor of Physics and former Dean of input from many hundreds more of their expectation that a shuttle servicing the Faculty at Princeton University. I colleagues. Altogether, a substantial mission would install in the Hubble appear today in my capacity as co-chair of fraction of the nation’s astronomers were Space Telescope new instruments called the Astronomy and Astrophysics Survey in some way involved in the creation of the Cosmic Origins Spectrograph and Committee. the report. By gathering such broad Wide Field Camera-3, and would refur- As you know, the Astronomy commu- community input, the survey process bish the satellite in other ways so that nity has a long history of creating, through creates a document that reflects the Hubble would continue to operate until the National Research Council (NRC), consensus opinion of the researchers in 2010 — about the time that the infrared broad surveys of the field at ten-year the field. The value of this activity to James Webb Space Telescope (JWST) is intervals. These surveys lay out the NASA and the NSF has been demon- planned to become available.[6] We community’s research goals for the next strated in many ways, and most recently were told that this mission, now referred decade, identify key questions that need by NASA’s request for the NRC to con- to as SM-4, would cost $350 million, and to be answered, and propose new facilities duct similar surveys for planetary sci- it was one of the considerations that led with which to conduct this fundamental ence,[2] solar and space physics,[3] and to the final shape of the priority list. research. The most recent decadal sur- earth science.[4] There are a number of strong argu- vey, entitled Astronomy and Astrophysics The feature of the decadal As- ments for keeping the Hubble telescope in the New Millennium, was released in the tronomy Survey that distinguishes it operational until JWST is ready. The new year 2000.[1] I have been asked to answer from summaries of other fields of instruments will expand Hubble’s reach the following questions from my perspec- science is the prioritized list of missions farther into the near-infrared region of the tive as the co-chair of the committee that and facilities that are recommended for spectrum. This capability will enable the produced that report: construction. This list is put together selection of potentially interesting targets 1. To what extent, and in what ways, very carefully; many worthy projects do that will form much of the basis of the was the Decadal Survey premised on the not make the list, while others are initial JWST research program. The Hubble Space Telescope having additional deferred to the next decade. I can Hubble Space Telescope is still in the instruments that were to be added by a assure you that the decision-making prime of its scientific life. Even with some servicing mission? Would the loss of the process is very thorough and sometimes temporarily reduced capacity, astrono- Hubble cause you to entirely rethink your leaves some “blood on the floor,” meta- mers are using it to observe objects that priorities? Would that change if the phorically speaking. One of the factors were thought to be beyond any telescope’s BPA News • Winter 2004 11 capability. Hubble is also important to the made. In this context, it is difficult to following the wise decisions made in that nation for reasons beyond its immediate say that the premature loss of the way. A project similar to the Hubble scientific contributions. According to a Hubble telescope would significantly Origins Probe could easily be included in recent NRC study, nearly one third of all alter the Survey’s priority list. It is pos- the next Astronomy Survey, and would federal support for astronomy research is sible that the committee would have likely be a strong contender then. As you tied to the Hubble telescope and its affili- given a stronger priority to the Space know, I am also a member of the Com- ated research programs.[7] NASA, in Ultraviolet Observatory (SUVO), which mittee on Assessment of Options to consultation with the community, plans to was omitted from the final priority list; Extend the Life of the Hubble Space transfer these programs to the James but I do not believe that the rest of our Telescope. I heartily endorse that Webb Space Telescope when it becomes list would have been very different. committee’s recommendation that NASA operational; but the premature loss of Mr. Chairman, the scientific promise should pursue a Shuttle servicing mission Hubble would threaten the continuity and of JWST and other Survey priorities lies to Hubble so as to accomplish the objec- vitality of this research enterprise, and this in the future, while your committee is tives of the planned SM-4 mission. How- source of highly trained technical person- grappling with decisions that need to be ever, I do not favor such a plan, much less nel for the nation. made very soon. Accounting methods the launch of a new satellite to host We all love Hubble. It is truly a and other changes that have taken place Hubble’s replacement instruments, if it remarkable instrument. That said, the at NASA since the completion of the would require major delays or re-ordering object of my committee’s decadal sur- Survey now make it seem very unlikely of the Survey Committee’s science priori- vey was to look ahead and identify the that a Shuttle servicing mission would ties. With such a course of action, I tools that would be needed to continue cost the science mission directorate as believe that NASA would squander the answering deep questions about the little as $350 million. However the excellent reputation for scientific judg- Universe and the most fundamental Hubble telescope is serviced, present ment and leadership that it has so rightly laws of Nature. In the Survey cost estimates seem to run to at least $1 earned over the years. committee’s judgment, in the present billion—roughly equivalent to that of a I should stress that these opinions are decade answers to these questions are second JWST. Such a cost, if borne by my own, informed by my work on the more likely to be found in regions of the the science program, will likely delay a survey and other advisory committees and spectrum outside the Hubble telescope’s number of other missions that are under by conversations with many colleagues. capabilities. Top Survey priorities such development, including those ranked Thank you for your attention, and I as JWST and the Constellation X-Ray highly in NRC decadal surveys across all would be pleased to answer questions. (Con-X) observatory will open large of space science. spectral windows on the universe that One option that I have not yet men- Endnotes are simply not available to instruments tioned is to host the Hubble replace- [1] Astronomy and Astrophysics in the on the ground. While we can never be ment instruments COS and WFC3 on a New Millennium, NRC, 2001. sure where the next scientific break- new satellite like the proposed Hubble [2] New Frontiers in the Solar System, through will arise, the future with these Origins Probe (HOP). According to the NRC, 2003. missions seems very bright. JWST will team proposing HOP, the cost for such [3] The Sun to the Earth – and Beyond, be able to observe and examine the very a mission would also be roughly $1 NRC, 2003. first galaxies that formed in our Uni- billion, and the telescope would be [4] Study underway - http:// verse, and to study the era when the first ready by 2010. The proposal also calls qp.nas.edu/decadalsurvey stars ignited. Con-X will be able to for an additional wide-field imaging [5] The Decade of Discovery in As- observe how matter and energy behave camera. Such a satellite offers signifi- tronomy and Astrophysics, NRC, 1991. near black holes — an extreme environ- cant promise; however, to start work on [6] The James Webb Space Telescope ment in which the laws of physics have it would in essence insert a new priority (then referred to as the Next Generation not yet been well tested. into the mission queue, without benefit Space Telescope) was the highest priority The Survey does not neglect the of the kind of comparative review un- recommendation of Astronomy and optical region of the spectrum. Two of dertaken in the survey. From the point Astrophysics in the New Millennium. the Survey’s top three recommendations of view of the survey committee, I [7] Federal Funding of Astronomical for ground-based facilities are for new believe that neither a $1 billion servicing Research, NRC, 2000, pg 54. optical telescopes that will observe the mission nor a $1 billion rehosting satel- [8] The Giant Segmented Mirror universe in new and different ways.[8] lite should be a higher funding priority Telescope and the Large Survey Tele- While Hubble can do some things that than the astronomical science priorities scope. are unmatched by telescopes on the recommended by the survey committee. ground, the choice to move space astro- Our nation’s science enterprise has Additional information about the physics into the infra-red and X-ray been well served by having open, hearing may be found at the House regions of the spectrum was one of the broadly based mechanisms for setting Science Committee’s Website, http:// difficult decisions that the committee priorities in astronomy, and by closely www.house.gov/science/welcome.htm. ■ 12 BPA News • Winter 2004
Excitons the conduction band. The electron is field, electrons execute circular cyclotron attracted to the hole it left behind and an motion. Since the electron motion is (continued from page 1) exciton results. If a suitably dense collec- confined to 2D planes, quantization of this tion of excitons cools down sufficiently circular motion leads to a discrete energy Superfluidity in liquid helium is the following optical excitation, excitonic BEC spectrum, analogous to Bohr orbits in an most dramatic phenomenon associated should occur. atom. The individual orbitals, known as with Bose-Einstein condensation. Super- Optically generated excitons are Landau levels, are highly degenerate and fluidity, and its cousin superconductivity, unstable. In a bulk semiconductor like can accommodate many electrons. The refer to the dissipationless flow of the GaAs, excitons survive for only about 1 ns degeneracy is proportional to the mag- condensate around obstacles and imper- before the electron and hole recombine netic field and thus it is possible to adjust fections in the system. Although not into a photon. This is a serious obstacle in the field so that only the lowest Landau always appreciated, unlike BEC itself, the search for exciton BEC. Nowadays level is occupied at low temperatures. In superfluidity does require interactions advanced crystal growth techniques, like what follows we shall assume that the among the particles in the system. In molecular-beam epitaxy (MBE), have lowest Landau levels in each layer are half liquid 4He these interactions are so strong been used to create layered quantum well filled with electrons. that conventional BEC theory does not structures. Via appropriate optical If the two layers of electrons are far provide an adequate description of the excitation it is possible to create excitons apart, they behave independently. The system. in which the electron resides in one quan- combination of confinement to a single Superconductivity in metals arises tum well and the hole in another. Since Landau level and strong Coulomb repul- from the condensation of pairs of elec- sive forces between particles allows the trons. These Cooper pairs may be re- Double Layer 2D Electron Gas electrons in each layer to avoid one an- garded as composite bosons and the onset other very effectively. In spite of this, at of superconductivity a type of Bose con- half filling of the lowest Landau level the densation. Interactions, however, are of electron gas remains compressible, i.e., it paramount importance in superconduc- is possible to inject an additional electron tors, and the transition temperature of a at very little energy cost. superconductor is set not by bosonic If the two layers are close together statistical mechanics but by the details of (i.e. the layer separation is comparable the interaction between the constituent to the mean spacing between electrons electrons. In fact, in a typical supercon- the two quantum wells are spatially sepa- in either layer), the situation changes ductor the size of a Cooper pair greatly rated by a barrier layer, the recombina- dramatically. Now electrons must avoid exceeds the average distance between tion time of such indirect excitons is much not only their neighbors in the same electrons in the material. Cooper pairs longer than 1 ns. Several recent experi- layer but those in the opposite layer as are thus far removed from the pointlike ments have revealed intriguing collective well. This additional requirement bosons one normally envisages as central behavior of cold indirect exciton gases[2]. suggests that an electron in one layer to BEC. Recent experiments with BEC, however, has not yet been observed would like to position itself between ultracold gases of fermionic atoms have in such systems. electrons in the other layer and remain probed the crossover from conventional A different type of cold exciton gas is so positioned at all times. Such in- BEC (when two fermionic atoms bind to provided by a system consisting of two between positions exist in each layer form a bosonic molecule) to the more parallel layers of electrons. Two identical because the lowest Landau level is only tenuous Cooper pair condensate[1]. quantum wells, separated by a thin barrier partially occupied. Furthermore, since In analogy to superconductivity, layer, are created by MBE. By suitable each Landau level is half filled, the physicists have long speculated that doping it is possible to produce sizeable, number of unoccupied states, or in- excitons in semiconductors could un- equivalent, and virtually disorder-free between positions, in one layer precisely dergo a similar kind of BEC. An exciton populations of electrons in the ground equals the number of filled states in the consists of an electron bound to a hole. A state of each quantum well. These two other layer. As a result, every electron hole is not an elementary particle but a two-dimensional (2D) electron gases, in the double-layer system can accom- vacancy, or unfilled orbital, in an other- which exist only in the conduction band, modate itself directly across from an wise nearly filled band of electrons. Usu- are stable and in thermodynamic equilib- empty state in the opposite layer. This ally but, as we shall see, not always, the rium. (See the figure for a cartoon repre- commensurability heavily restricts the hole is in the valence band of the semi- sentation of such a system.) form of the many-body wavefunction conductor, while the electron is in the It is a remarkable fact that such a and renders the electron system incom- conduction band. Excitons are readily bilayer electron gas can be rendered pressible, i.e., a finite energy gap must created by shining light on a semiconduc- equivalent to a bilayer electron-hole be overcome in order to inject an addi- tor. The incoming photons promote an system simply by applying a magnetic field tional electron. electron from the filled valence band into perpendicular to the layers[3]. In such a The association of electrons in one BPA News • Winter 2004 13 layer with empty states in the other of this new state of matter and how well The analogy between excitons and brings excitons and exciton condensa- do they support the picture given above? Cooper pairs suggests that some kind of tion to mind. A particle-hole transfor- Experiments in the early 1990s demon- superfluidity ought to exist in the double- mation makes this connection rigorous. strated clearly that a phase transition does layer electron gas at half-filling per layer. A This transformation is the same as the occur when two 2D electrons layers, each uniform flow of excitons should result in one used to map empty valence band at half filling of the lowest Landau level, little or no energy dissipation, in analogy states into holes for understanding are brought close together[4]. The signa- with superconductivity. But unlike Cooper conventional optically generated exci- ture of this transition was the onset of the pairs, excitons are charge-neutral. Their tons. Here, however, it is applied to one quantum Hall effect (QHE). The ordinary flow carries no net electrical current. Gen- of the two electron layers to map empty Hall effect refers to the development of a erating and detecting such a flow presents conduction band Landau level states voltage VH perpendicular to the current I experimental difficulties not faced in experi- into holes. Since each layer contains a flowing through a material in a magnetic ments with superconductors. half-filled Landau level, the result of the field. This voltage is due to the Lorentz In the QHE experiments mentioned transformation is a half-filled Landau force on a moving charge and is propor- above, equal electrical currents were level of holes in one layer and a half- tional to both the sign and magnitude of driven in the same direction through both filled Landau level of electrons in the the charge. In the QHE, the Hall resis- layers of electrons. If excitons are present other. If the two layers are close enough tance RH = VH/I exhibits a series of pla- in the system, they could not contribute to together, the negatively charged elec- teaus versus magnetic field which are such currents because they carry no net trons in one layer bind onto the posi- given exactly by the quantum of resistance charge. However, if currents of equal magni- tively charged holes in the other to h/e2 ~ 25,813 Ω divided by certain rational tude but opposite direction could be set up in create interlayer excitons. These exci- numbers p/q. Each plateau signals the the two layers, excitons could take part. Estab- tons are not independent; the strong existence of a gap in the electronic energy lishing such a counter-flow configuration Coulomb forces in the system engender a spectrum and the number p/q indicates requires the ability to make separate electrical global quantum coherence of the many- the total number of Landau levels that are connections to the individual 2D layers, in spite body wavefunction. The situation is occupied by electrons. For the case of of the fact that they are typically only a few 2 similar to superconductivity, only the interest here, RH = h/e and p/q = 1. This is hundred angstroms apart. Fortunately, such Cooper pairs in this system are charge- the appropriate value since there are two separate contacts can in fact be made, using a neutral excitons. Since the average size of layers of electrons each filling one-half of novel technique whereby one or the other these excitons is roughly the same as the the lowest Landau level. This was an layer is completely depleted of electrons just in distance between them, this example of exciting observation because a single 2D the vicinity of the contact[5]. In the bulk of the Bose condensation lies somewhere in the electron layer at half-filling does not sample electrons occupy both layers with equal middle of the crossover between the exhibit QHE. Despite this excitement, the (and yet controllable) densities. condensation of pointlike bosons and that QHE does not demonstrate that the Recent experiments with oppositely of highly overlapping fermion pairs. system is equivalent to a Bose condensate directed currents in the two layers have pro- What are the experimental signatures of excitons. See “Excitons” on page 14
Beckman Symposium scientist at Beckman Coulter’s Advanced as education, the twilight of major industrial (continued from page 5) Technology Center, discussed the technique of research labs, and the importance of inter- flow cytometry and its impact on diagnosing, disciplinary research to foster innovation monitoring, and understanding disease. In and creativity. In general, instrumentation sensitive that we can resolve individual binding describing the development of flow ctyometry, and research have a symbiotic relationship. events,” said Dr. Roukes. “We can follow the he said, the first step is to define what the goals Scientific and technological advances lead to stochastic chemistry in real time.” Forensic of an instrument are. The users of an instru- new instruments, while important scientific science and technology was identified by ment “need to talk to people who are building and technological problems stimulate the Robert Gaensslen, head of the forensic science an instrument and say, ‘This is what we need. development of new instruments. Instru- program at the University of Illinois at Chicago, Build us an instrument that will answer those ments developed for one area of research as one of the emerging societal impacts of questions.’ ” Otherwise, said Dr. Shankey, often find application in other areas, both in analytical chemical instrumentation. He “companies frequently build machines to the research enterprise and in the broader described the challenges in forensics and how answer questions that you don’t have.” society. modern scientific tools such as the gas chro- The symposium wrapped up with a A man of charity, wit, humility, and matograph and mass spectrometer are helping panel discussion that included the sympo- curiosity, Arnold O. Beckman was also a to address them. “Instrumental analysis is sium speakers and William Ballhaus, Sr., man of great strength and vision. At this deeply interwoven with the analysis of chemi- former president of Beckman Instruments, symposium, participants celebrated his cal, trace, and biological evidence” in forensic Inc. The panelists discussed the future of many legacies and the promise they hold for science, he said. T. Vincent Shankey, a clinical instrumentation, highlighting themes such the future. ■ 14 BPA News • Winter 2004
Excitons extent. The first such indication came from EPP2010 (continued from page 13) studies of the rate at which electrons quantum (continued from page 8) mechanically tunnel from one layer to the vided compelling evidence for exciton conden- other[8]. The tunneling rate was observed to work plan for the committee was developed sation[6,7]. The most dramatic effect is also increase explosively at the onset of exciton that includes three more meetings (each with the simplest: the Hall voltage in the two layers condensation. Although quantitative theoreti- its own public-comment session), several (measured separately) vanishes at low tem- cal analysis of this effect has proven difficult[9], informal particle physics tutorial sessions peratures when the layers are brought close the experiments strongly suggest that coher- between members of the committee, and together and the magnetic field is adjusted to ence over a spatial extent encompassing presentations at future meetings of the joint produce the half-filling per layer condition. roughly 100 excitons exists. A more obvious DOE-NSF High Energy Physics Advisory Panel This is expected if the counterflowing electrical sign of coherence, and the superfluidity that it and the American Physical Society. currents are carried by excitons. The Lorentz suggests, has been provided by additional The second meeting of the EPP2010 forces on the electron and hole within each counterflow transport experiments. These committee, held at the Stanford Linear exciton cancel exactly. To fully appreciate the experiments have clearly shown that in addi- Accelerator Center in Menlo Park, Califor- significance of this result, consider what is tion to the Hall voltage, the ordinary resistive nia, on January 31 and February 1, 2005, expected if the two layers are far apart or the voltage parallel to the current in either layer focused on a survey of the broad research magnetic field is not at the magic half-filling vanishes as the temperature goes to zero in the areas in elementary particle physics. The value. In that case there will be large Hall exciton condensed state. This result demon- committee heard presentations from Ian voltages in each layer. The signs of these strates that energy dissipation accompanying Hinchliffe, Hitoshi Murayama, and JoAnne voltages will be opposite simply because the exciton flow becomes extremely small at low Hewett on the science motivation for current is flowing in opposite directions in the temperatures. While the microscopic pro- exploring TeV-scale physics and the science two layers. Indeed, in actual experiments this is cesses that prevent truly infinite conductivity capabilities of the Large Hadron Collider precisely what is observed at magnetic fields of the excitons at finite temperature are far and a linear collider. Robert Cahn pre- away from the half-filling condition and at all from understood, there is now compelling sented an overview of research studying the fields when the layers are too far apart. But evidence that a closely spaced bilayer 2D B meson with precision experiments. Boris when the field is adjusted to the proper value electron gas at an appropriate high magnetic Kayser discussed studies of neutrino physics and the layers are close, the Hall voltage in both field provides a concrete example of the with the committee and outlined several key layers drops precipitously, vanishing as the long-sought Bose-Einstein condensate of points from the recent American Physical temperature heads toward absolute zero. (The excitons. ■ Society multidivisional study of this area. figure schematically illustrates the experimental Steve Kahn described the emerging connec- setup and the basic observation.) References tions to cosmology, astronomy, and astro- [1] C.A. Regal, M. Greiner, and D. Jin, physics, underlining the need for particle Phys. Rev. Lett. 92, 040403 (2004). physics to understand and include particle [2] D. Snoke, Science 298, 1368-1372 astrophysics in thinking about the future. In V H (2002). the final plenary session, SLAC director Exciton BEC I [3] See J.P. Eisenstein and A.H. Jonathan Dorfan shared a vision for the MacDonald, Nature, in press, and the refer- future of SLAC. Capping off the day was a ences cited therein. town meeting session organized by the [4] S.Q. Murphy et al., Phys. Rev. Lett. 72, American Physical Society’s Division of 1.0 728 (1994). Particle and Fields. Participants presented ) 2 [5] J.P. Eisenstein, L.N. Pfeiffer, and K.W. brief comments on their own work and on 57 how the EPP2010 committee should think (h/e West, Appl. Phys. Lett. , 2324 (1990). xy
R 0.5 [6] M. Kellogg, J.P. Eisenstein, L.N. Pfeiffer, about the breadth of the field. The commit- and K.W. West, Phys. Rev. Lett. 93, 036801 tee met in executive session for the second (2004). day and reviewed the presentations of the day before. They also began discussing a 0.0 [7] E. Tutuc, M. Shayegan, and D.A. Huse, 0 0.5 1.0 1.5 2.0 Phys. Rev. Lett. 93, 036802 (2004). framework for the final report and agreed Magnetic Field (Tesla) [8] I.B. Spielman, J.P. Eisenstein, L.N. on plans to address the need for interna- Pfeiffer, and K.W. West, Phys. Rev. Lett. 84, tional perspectives on the future of the field. The vanishing of the Hall resistance in 5808 (2000) and Phys. Rev. Lett. 87, 036803 The committee’s third meeting , sched- counterflow demonstrates that excitons are (2001). uled for May 16–17, 2005, will be held at the present. But it is not obvious from this that the [9] L. Balents and L. Radzihovsky, Phys. Fermi National Accelerator Laboratory in excitons are behaving coherently, like the Rev. Lett. 86, 1825 (2001); A. Stern, S.M. Girvin, Batavia, Illinois. Please visit the committee’s Cooper pairs do in a superconductor. Addi- A.H. MacDonald, and N. Ma, Phys. Rev. Lett. public Website for more information, tional experiments have, however, established 86, 1829 (2001); and M. Fogler and F. Wilczek, including copies of presentations made to that such coherence exists, at least to a limited Phys. Rev. Lett. 86, 1833 (2001). the committee. ■ BPA News • Winter 2004 15 BPA Mission BPA Update: Emerging Projects The Board on Physics and Astronomy (BPA) • Plasma 2010: An Assessment of and Outlook for Plasma Science.* Since publication was created in 1983 as the successor to the Na- of the previous decadal study of this area in 1995, the field has undergone rapid ad- tional Academy of Sciences Office of Physical vances and significant changes. A committee of about 15 members with broad exper- Sciences. Several standing committees were tise in plasma science will be convened to address the following tasks in a report that assigned at that time to the BPA, including the will communicate well to policymakers and scientists in other fields: (1) Assess the Committee on Atomic, Molecular, and Optical Sciences, the Solid State Sciences Committee, and recent progress and achievements of plasma science; (2) Identify the new opportunities the Committee on Radio Frequencies. Later, the and the compelling science questions for plasma science, frame the outlook for the Committee on Astronomy and Astrophysics and future, and place the field in the context of physics as a whole; (3) Evaluate the opportu- the Plasma Science Committee were created in nities and challenges for the applications of plasma science to fusion and other fields; response to requests from the scientific commu- and (4) Offer guidance to the government research programs and the scientific com- nity. Since its inception, the BPA has published munities aimed at addressing these challenges and realizing these opportunities. more than 40 reports, workshops, and collabora- tive activities, including two surveys of physics • Review of the Science Requirements for the Atacama Large Millimeter Array and two surveys of astronomy. (ALMA). In response to a request from the National Science Foundation, a committee of The important questions in physics and astronomy change as we learn more about nature, about 10 experts will be formed to review the technical requirements for the Atacama Large and that rate of change has been increasing. The Millimeter Array (ALMA) and prepare a report analyzing the impact on its performance and BPA seeks to inform the government and the scientific merit of a reduction in the number of elements in the array. public regarding important scientific opportuni- ties and issues as well as the changing nature of • The Future of Materials Research Laboratories. A committee of about 15 members science. It builds bridges between the evolving will be formed to assess the Materials Research Science and Engineering Center subdisciplines of physics and astronomy and with (MRSEC) program at NSF’s Division of Materials Research. The assessment will evalu- other areas of science. The BPA is successful if it ate the program’s performance and impact and frame an outlook for its future. Factors helps the science community and society under- to be considered will include the role of the MRSECs in enabling multidisciplinary stand what is needed to advance physics and research, supporting instrumentation development, providing research and education astronomy and why doing so is important. Every activity of the BPA is aimed at accom- infrastructure, and facilitating collaboration and cooperation between researchers and plishing one or more of the following goals: industry. The committee will complete its work over the course of 18 months. • Monitor the health of physics and astronomy. • Identify trends in research and new develop- • Biomolecular Materials and Processes. The goal of this study is to review current ments at the scientific forefronts. achievements, assess the compelling science at the interface between biology and mate- • Foster interactions with other fields and rials, and recommend actions to realize the identified opportunities. In particular, the cooperation among academic disciplines. study will identify the most compelling questions and the emerging scientific opportuni- • Strengthen connections to technology. ties at the interfaces between biology and condensed matter and materials research and • Facilitate effective service to the nation. suggest strategies to best address the identified opportunities. • Improve public understanding of science. • Encourage cooperation among federal agencies, government laboratories, and *Elements of Physics 2010 universities involved in research in physics and astronomy. Approaches for achieving these objectives include the following: BPA Update: Upcoming Meetings in 2005 • Periodic assessments of major fields. By April 2005 setting priorities, these surveys provide 4/1-2 PLSC meeting, Washington, D.C. programmatic guidance to agencies. 4/7-8 SSSC meeting, Washington, D.C. • Response to particular needs and requests from federal agencies, both those that have 4/27-28 CORF meeting, Washington, D.C. programs of research and those that play an 4/29-30 BPA meeting, Washington, D.C. administrative role. May 2005 • Continuing surveillance of scientific progress 5/16-17 EPP2010 meeting, Batavia, Illinois and identification of issues and problems in August 2005 various fields. Several standing committees 8/2-3 EPP2010 meeting, Ithaca, New York. are focused on this task. Sept 2005 • Cross-disciplinary studies of special areas that 9/24-25 PLSC meeting, Irvine, California lie at the intersection of several disciplines. Oct 2005 • Many scientific assessments address the benefits that accrue to society through 10/20-21 SSSC meeting, Irvine, California technology development that follows from Nov 2005 the pursuit of science. 11/6-7 BPA meeting, Irvine, California
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