In the space described by Eq.10 there is a region within
one of the bubbles, both in the primed and unprimed coordi-
nates, in which the ‘‘forward’’ light cone runs backward in
time, and CTC’s occur. Suppose a passenger leaves S1 on a
spaceship traveling in the unprimed bubble, starting at
t=t'=0, moving along the line y=y0, and arriving at S2 at
t~=0, t'=T'~=-yBD, at which point the spaceship has
come to rest and the bubble has disappeared. He now travels
the short distance to y=-y0 at subluminal speed through
flat space, and accelerates to sublight velocity B, so as to be
at rest in the primed coordinates. [We assume that this pro-
cess requires negligible time since y<<D and, from Eq. ͑7͒,
B may be small when A is large.] The passenger can then
board a second spaceship bubble and travels back to S1 in the
primed bubble, arriving at t=T1=-BD, thus arriving home
before starting by a macroscopic time interval and carrying a
newspaper reporting on events which have not yet occurred.
This, of course, raises the problems with paradoxes always
associated with closed causal loops. It would appear pos-
sible, e.g., to arrange a mechanism which ensures that a
spaceship will depart from S1 at t=0 if and only if no news
of such an event has arrived from S2 at t<0. This does not
mean that a model of the type introduced in MA is ruled out
as being logically inconsistent, but it does mean that in such
a model there are restrictions placed on the initial conditions.
That is, apparently if superluminal travel through some
mechanism similar to that discussed in MA could actually be
realized, it would imply that the laws of physics include a
principle of consistency, as discussed by Friedman et al. ͓7͔,
which constrains the initial conditions on spacelike surfaces
at times subsequent to the creation of closed timelike curves,
so as to ensure in some way that no contradiction arises; for
example, the initial conditions might guarantee the failure of
the mechanism by which the previous arrival of news of the
spaceship’s departure prevents its later departure from occur-
ring. While not logically inconsistent, such theories appear to
enforce correlations which are certainly counterintuitive.
).
