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 â€śyoungerâ€ť, 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 â€śyoungerâ€ť end would exit the â€śolderâ€ť end at a time when it was the same age as the

â€śyoungerâ€ť 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 â€śexotic matterâ€ť. 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 â€śRoman

ringâ€ť (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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