• information processors: people and computers. Information can be in the form of data, symbolic knowledge, or control specifics.³

A virtual world, or virtual reality, is a precise re-creation of a real-world environment via multisensory data and computer graphics that allows interaction between humans and synthesized objects. It consists of a set of multisensory devices employed as both actuators and effectors. "Virtual" is often used synonymously with computer-generated or synthetic.

A synthetic environment is a reconstructed multipurpose environment with a mix of real and computer-synthesized (simulated) objects under computer control. It allows interaction between combinations of real and synthesized objects. A synthetic environment consists of a digital and analog representation of a physical environment with specified fidelity and complexity and is scalable to any size and degree of complexity.

The grand challenge areas selected by the panel should be understood in the context of the usage of that term in the High Performance Computing and Communications (HPCC) 1991 Initiative of Alan Bromley, President Bush's Science Advisor, stated as follows: "The HPCC Program is driven by the recognition that unprecedented computational power and capability is needed to investigate and understand a wide range of scientific and engineering grand challenge problems. These are fundamental problems whose solution is critical to national needs."⁴

The panel tailored this concept to its work in the following manner: Grand challenge areas are those fundamental problem areas to which the application of scientific and engineering resources will yield much-needed improvements in capabilities and performance. They also serve to identify key scientific and engineering issues and opportunities.

In selecting the grand challenge areas, the panel applied the following two constraints:

• Only leading-edge technologies were considered, and
• Research and technology application communities had to believe that the grand challenges were susceptible to resolution and that their resolution would provide demonstrable value-added to nontrivial user groups.

The panel chose the following six grand challenge areas:

• Representation and modeling of complex systems,
• Collaborative problem solving,
• Machine learning and adaptive systems,
• Reasoning under uncertainty,
• Virtual worlds (reality), and
• Neurophysiological models of cognition.

Panel members concurred that organizing their observations and findings concerning the NRL priority topics in the terms of reference (see the preface) according to the grand challenge areas selected would bring focus to their work and would permit easy presentation of highlights, issues, and high-value applications. Table 1.1 summarizes the relationship of the NRL priority topics to the grand challenges.

Table 1.1 can be read as follows: Software production, particularly for increasingly complex systems, presents challenges in the extensive collaborative and sometimes