London asked Neil deGrasse Tyson:

If traveling back in time were possible, what do you think that would involve? Would we have to utilize black holes? Would we have to figure out a way to travel faster than light?

Neil deGrasse Tyson Time travel makes especially fertile storylines in science fiction. One that passed under the radar, but a story well told, was the 1980 film "Somewhere in Time". And since this is Goodreads, I get to tell you it's based on the 1975 Novel "Bid Time Return" by Richard Matheson.

There are ways to travel through time that don't violate Einstein's relativity, but they involve trajectories and combinations of black holes and worm holes. To make a wormhole requires negative matter -- or more generally, negative gravity. So we know how to make one, we just do not have the material to build it. And the material might not exist in the universe.

If you want to get deeper, my friend and Princeton Colleague J. Richard Gott III wrote a 2001 book "Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time".

If you go missing, we'll know why.

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