In the news: Physicists Simulate Sending Particles of Light Into the Past, Strengthening the Case that Time Travel Is Possible. Actually, this was back in May, but it begs repeating.
From the article:
Closed timelike curves (CTCs) are a necessary concept to understand this experiment.
CTCs are used to simulate powerful gravitational fields, like the ones produced by a spinning black hole, that could theoretically (based on Einstein’s theory of general relativity), warp the fabric of existence so that space-time bends back on itself. This creates a CTC, almost like a pathway to travel back through time.
The source of time travel speculation lies in the fact that our best physical theories seem to contain no prohibitions on traveling backward through time. The feat should be possible based on Einstein’s theory of general relativity, which describes gravity as the warping of spacetime by energy and matter. An extremely powerful gravitational field, such as that produced by a spinning black hole, could in principle profoundly warp the fabric of existence so that spacetime bends back on itself. This would create a “closed timelike curve,” or CTC, a loop that could be traversed to travel back in time.