Tufts University

Project Title

Does general relativity permit time travel?

Project Summary

Is time travel possible? In general relativity, matter and energy curve spacetime. Perhaps the right matter and energy would curve spacetime back on itself so that an object could return to the same place in space at an earlier time than it left. But is there any matter and energy that would produce such curvature? It would have to have negative energy density. That is unlike nearly every kind of matter and energy that we know, but quantum mechanics can give rise to negative energy densities, for example between parallel plates in the "Casimir effect". But it is not enough just to have negative energy density in a few places. Rather, the energy density added up along the complete path of a light ray must be negative. Previous work showed that that would be impossible in flat spacetime. But of course flat spacetime is not curved into a time machine. What matters is whether there is a spacetime curved in a way to allow time travel which simultaneously allows the right distribution of negative of energy to produce the curvature of that spacetime self-consistently. The goal of this project is to determine whether or not this is possible.

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