  Intensity interferometry -- the correlation of spatially separated light
intensities -- has historically been an important tool for precision optical
astronomical observations. However, due to the extremely narrow field of view,
its scope has been limited to studies of the morphology of very bright emission
regions, primarily determinations of angular diameters of nearby hot stars. We
propose adding an adjustable path extension into the detector optics which
creates a primary interference fringe for widely separated sources, allowing
maximum source separations parametrically larger than the angular resolution.
This Extended-Path Intensity Correlator (EPIC), augmented with advances in
single-photon detectors and spectroscopic gratings, would enable ground-based
astrometry at microarcsecond-level precision in a field of view as large as
several arcseconds. EPIC has the potential to revolutionize astrophysical and
cosmological observations requiring high-precision differential astrometry on
sources of high surface brightness. We outline how EPIC can be employed to
detect the astrometric wobble of Earth-like planets around Sun-like stars at
tens to hundreds of parsecs, and expect that EPIC's larger field of view will
expand the power of intensity interferometry to a broad range of astronomical
applications.
