If traversable wormholes exist, they might allow time travel.
[30] A
proposed time-travel machine using a traversable wormhole might
hypothetically work in the following way: One end of the
wormhole is accelerated to some significant fraction of the speed of
light, perhaps with some advanced propulsion system, and then
brought back to the point of origin. Alternatively, another way is to
take one entrance of the wormhole and move it to within the
gravitational field of an object that has higher gravity than the other
entrance, and then return it to a position near the other entrance. For
both these methods, time dilation causes the end of the wormhole
that has been moved to have aged less, or become “younger”, than
the stationary end as seen by an external observer; however, time
connects differently through the wormhole than outside it, so that
synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer
passing through the wormhole, no matter how the two ends move around.[35]:502 This means that an
observer entering the “younger” end would exit the “older” end at a time when it was the same age as the
“younger” end, effectively going back in time as seen by an observer from the outside. One significant
limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of
the machine;
[35]:503
it is more of a path through time rather than it is a device that itself moves through
time, and it would not allow the technology itself to be moved backward in time.
[49][50]
According to current theories on the nature of wormholes, construction of a traversable wormhole would
require the existence of a substance with negative energy, often referred to as “exotic matter”. More
technically, the wormhole spacetime requires a distribution of energy that violates various energy
conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions.
However, it is known that quantum effects can lead to small measurable violations of the null energy
condition,[11]:101 and many physicists believe that the required negative energy may actually be possible
due to the Casimir effect in quantum physics.
[51] Although early calculations suggested a very large amount
of negative energy would be required, later calculations showed that the amount of negative energy can be
made arbitrarily small.
[52]
In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference
could not be brought together without inducing quantum field and gravitational effects that would either
make the wormhole collapse or the two mouths repel each other,
[53] or otherwise prevent information from
passing through the wormhole.
[54] Because of this, the two mouths could not be brought close enough for
causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex “Roman
ring” (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric
polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical
quantum gravity theory rather than proof that causality violation is possible.
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