  A space-based far-infrared interferometer could work synergistically with the
James Webb Space Telescope (JWST) and the Atacama Large Millimeter Array (ALMA)
to revolutionize our understanding of the astrophysical processes leading to
the formation of habitable planets and the co-evolution of galaxies and their
central supermassive black holes. Key to these advances are measurements of
water in its frozen and gaseous states, observations of astronomical objects in
the spectral range where most of their light is emitted, and access to critical
diagnostic spectral lines, all of which point to the need for a far-infrared
observatory in space. The objects of interest - circumstellar disks and distant
galaxies - typically appear in the sky at sub-arcsecond scales, which rendered
all but a few of them unresolvable with the successful and now-defunct 3.5-m
Herschel Space Observatory, the largest far-infrared telescope flown to date. A
far-infrared interferometer with maximum baseline length in the tens of meters
would match the angular resolution of JWST at 10x longer wavelengths and
observe water ice and water-vapor emission, which ALMA can barely do through
the Earth's atmosphere. Such a facility was conceived and studied two decades
ago. Here we revisit the science case for a space-based far-infrared
interferometer in the era of JWST and ALMA and summarize the measurement
capabilities that will enable the interferometer to achieve a set of compelling
scientific objectives. Common to all the science themes we consider is a need
for sub-arcsecond image resolution.
