### 5.4 Morris–Thorne wormholes

We consider a spacetime whose metric is of the form (69) with , i.e.,
where ranges from to . We call such a spacetime a Morris–Thorne wormhole (see [228]) if
such that the metric (112) is asymptotically flat for and for .
A particular example of a Morris–Thorne wormhole is the Ellis wormhole [102] where

with a constant . The Ellis wormhole has an unstable light sphere at , i.e., at the “neck” of the
wormhole. It is easy to see that every Morris–Thorne wormhole must have an unstable light sphere at some
radius between and . This has the consequence [152] that every Morris–Thorne
wormhole produces an infinite sequence of images of an appropriately placed light source. This infinite
sequence corresponds to infinitely many light rays whose limit curve asymptotically spirals towards the
unstable light sphere.
Lensing by the Ellis wormhole was discussed in [55]; in this paper the authors identified the region
with the region and they developed a scattering formalism, assuming that observer and
light source are in the asymptotic region. Lensing by the Ellis wormhole was also discussed in [271] in terms
of the exact lens map. The resulting features are qualitatively very similar to the Schwarzschild case, with
the radius values , , in the wormhole case corresponding to the radius values
, , in the Schwarzschild case. With this correspondence, Figures 15, 16,
and 17 qualitatively illustrate lensing by the Ellis wormhole. More generally, the same qualitative
features occur whenever the metric function has one minimum and no other extrema, as in
Figure 9.

If observer and light source are on the same side of the wormhole’s neck, and if only light rays in the
asymptotic region are considered, lensing by a wormhole can be studied in terms of the quasi-Newtonian
approximation formalism [181]. However, as wormholes are typically associated with negative energy
densities [228, 229], the usual assumption of the quasi-Newtonian approximation formalism that the
mass density is positive cannot be maintained. This observation has raised some interest in
lensing by negative masses, in particular in the question of whether negative masses can be
detected by their (“microlensing”) effect on the energy flux from sources passing behind them. So
far, related calculations [64, 292] have been done only in the quasi-Newtonian approximation
formalism.