Appendix C, the United States is a world leader in this area. To identify scientific forefronts, technological opportunities, and windows of future opportunity, broad input was sought from the AMO science community through letters to individual scientists, open sessions at professional society meetings, and other forums. That input identified the areas presented below as those meriting special discussion. Clearly, not all areas of significant progress can be included in such a brief presentation. Nevertheless, this summary will provide a glimpse of some of the scientific frontiers in AMO science.
One unique aspect of AMO physics, distinguishing it in all of science, is the capacity to make measurements with extraordinarily high precision. In suitably chosen systems, such precision measurements can probe physics far beyond the confines of what is customarily considered AMO science. For example, precision AMO measurements are testing our basic concepts of space and time, revealing new details about nuclear structure, probing the existence and properties of elementary particles, and exploring our fundamental understanding of the forces of nature. In this way, AMO science provides the unusual opportunity to explore the frontiers of physics without leaving the proverbial "tabletop." A common theme in much of this work is that one tests accepted theories at increasingly higher levels of precision until, at some point, a discrepancy is observed, which leads to important new insights.
One example is the testing of the ideas of space and time that are embodied in the theory of special relativity. The optical experiments of Michelson and Morley and of Kennedy and Thorndike provided important early tests of the isotropy of space and the speed of light. Recently, laser versions of these experiments have tested the isotropy hypothesis at a precision many orders of magnitude higher. Similarly, recent laser spectroscopy experiments have provided dramatically improved precision for the confirmation of the time dilation formula of special relativity.
The area of precision measurements that has made perhaps the largest contribution to the basic understanding of physics is the detailed examination of atomic structure. Historically, precision measurements of atomic and molecular spectra laid much of the groundwork for the development of quantum mechanics, and the high precision of these data provided an exceptionally rigorous testing ground for new theories as they were put forth. The microwave technology developed in World War II led to the precise measurement of the Lamb shift in hydrogen, which stimulated the modern development of quantum field theory. The rather radical concepts of renormalization and vacuum fluctuations gained quick acceptance because of the remarkable agreement between theoretical predictions