neutron stars, EXIST will enable astrophysicists to study how radiation interacts with magnetic fields that can be a million to a billion times stronger than can be sustained in the laboratory.

A possible theory challenge for EXIST is

To solve the mystery of gamma-ray bursts.

The direct measurement of gravitational waves from astrophysical sources will open new investigations in both astrophysics and the physics of strong gravitational fields. Gravitational waves can probe the dense inner regions of astrophysical systems that are opaque to photons. The committee recommends the Laser Interferometer Space Antenna (LISA), a joint mission between the United States and the European Space Agency, to pioneer the study of low-frequency (periods of 10 to 10,000 seconds) gravitational waves from binary star systems in our galaxy and the coalescence of supermassive black holes. A mission of this type was recommended in the physics survey report Gravitational Physics: Exploring the Structure of Space and Time (NRC, 1999). LISA will complement the ground-based Laser Interferometer Gravitational-wave Observatory (LIGO), which is designed to study the much higher frequency gravitational waves from the coalescence of neutron stars and stellar mass black holes, as well as the core collapse of supernovae. The detection of low-frequency gravitational waves requires a space system with detectors several million miles apart whose separation is monitored with exquisite accuracy, to a precision a thousand times smaller than the size of an atom. Although much progress has been made in the technology for such a mission, including the ground-based laser interferometry for LIGO and the shielding of the reference mass detectors by the space-based Triad and Gravity Probe B programs, a dedicated technology mission in space is envisioned as a precursor to LISA. Three technical areas could benefit from an integrated test on such a precursor: the inertial reference mass, precision thrusters, and high-precision interferometry.

A theory challenge appropriate for LISA would be

To compute the expected gravitational waveforms from black hole mergers.

Front Matter (R1-R24)

Executive Summary (1-16)

1. Recommendations (17-50)

2. The Science Behind the Recommendations (51-94)

3. The New Initiatives: Building on the Current Program (95-136)

4. Benefits to the Nation from Astronomy (137-158)

5. The Role of Astronomy in Education (159-176)

6. Policy for Astronomy and Astrophysics (177-208)

References (209-212)

Appendix: Definitions (213-228)

Index (229-246)