  We propose the development of X-ray interferometry (XRI), to reveal the
universe at high energies with ultra-high spatial resolution. With baselines
which can be accommodated on a single spacecraft, XRI can reach 100 $\mu$as
resolution at 10 \AA (1.2 keV) and 20 $\mu$as at 2 \AA (6 keV), enabling
imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby
accreting supermassive black holes (SMBH) and the SMBH `shadow'; SMBH accretion
flows and outflows; X-ray binary winds and orbits; stellar coronae within ~100
pc and many exoplanets which transit across them. For sufficiently luminous
sources XRI will resolve sub-pc scales across the entire observable universe,
revealing accreting binary SMBHs and enabling trigonometric measurements of the
Hubble constant with X-ray light echoes from quasars or explosive transients. A
multi-spacecraft `constellation' interferometer would resolve well below 1
$\mu$as, enabling SMBH event horizons to be resolved in many active galaxies
and the detailed study of the effects of strong field gravity on the dynamics
and emission from accreting gas close to the black hole.
