According to the passage, going back in time ----.

Time travel has been a favourite science fiction
theme ever since it was first used in H.G. Wells's
trailblazing novel “The Time Machine”. But not
everything it describes is science fiction: travelling
forward in time, for example, is a proven fact.
Einstein's theory of relativity predicts that an
observer moving relative to Earth can leap into
Earth's future, and the effect has been confirmed
using atomic clocks. Dramatic time warps require
speeds close to that of light, which is possible in
principle but would take a major feat of engineering,
not to mention a lot of money. Going back in time is
far more problematic. Relativity does not rule out an
observer being able to make a journey through
space-time and return to the past. But all scenarios
so far discussed require exotic circumstances.