Wormhole Shadows
  We propose a new method of detecting Ellis wormholes by use of the images of
wormholes surrounded by optically thin dust. First, we derive steady solutions
of dust and more general medium surrounding the wormhole by solving
relativistic Euler equations. We find two types of dust solutions: one is a
static solution with arbitrary density profile, and the other is a solution of
dust which passes into the wormhole and escapes into the other side with
constant velocity. Next, solving null geodesic equations and radiation transfer
equations, we investigate the images of the wormhole surrounded by dust for the
above steady solutions. Because the wormhole spacetime possesses unstable
circular orbits of photons, a bright ring appears in the image, just as in
Schwarzschild spacetime. This indicates that the appearance of a bright ring
solely confirms neither a black hole nor a wormhole. However, we find that the
intensity contrast between the inside and the outside of the ring are quite
different. Therefore, we could tell the difference between an Ellis wormhole
and a black hole with high-resolution very-long-baseline-interferometry
observations in the near future.
