In 1935, Albert Einstein and his postdoc Nathan Rosen discovered a hypothetical mathematical spacetime structure that bridges two separate regions of space, potentially allowing a shortcut between them. In theory, travel through this could allow much faster passage than travel in the familiar external spacetime that connects them. This is an example of a general class of spacetime structures with two mouths connected by a throat, more commonly known as a “wormhole.”
In 1962, Robert Fuller and John Wheeler demonstrated that the Einstein-Rosen wormhole was unstable and would pinch off before any particle could traverse it. Subsequently, a possible solution emerged when Stephen Hawking, Kip Thorne, and their collaborators later showed that exotic matter with a negative mass (energy) density could stabilize a wormhole and make it hypothetically traversable. Such a substance is known to exist: the “dark energy” whose repulsive gravity causes the accelerated expansion of the Universe.
However, to construct a wormhole, one would need to excavate dark energy from the cosmic reservoir and mold it into a wormhole. We do not know if that is possible since the nature of dark energy also remains unknown.
A traversable wormhole would allow an advanced civilization to travel back in time. This is because time progresses differently inside and outside the wormhole. As a result, synchronized clocks at either end of the wormhole remain synchronized for an observer passing through the wormhole. Imagine observers at one end of the wormhole that encountered time dilation and aged less by moving around or temporarily visiting a gravitational potential well. Such observers would be able to connect to the older end of the wormhole at the same time, thereby allowing the older version of themselves to meet their younger selves.
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