Time  travel's been    one  of man's   wildest fantasies  for centuries. It's   long    been  a popular  trend in movies and fiction ,   inspiring everything 
from  Charles  Dickens'  A Christmas Carol to H.G. Wells'The Time Machine  to the Interstellar.

         Travelling   forwards in time is surprisingly easy. Einstein’s   special   theory of relativity, developed in 1905, shows that time passes at different rates for people who are moving relative to one another - although the effect only becomes large when you get close to the speed of light.

           If one were to leave Earth in a spacecraft   travelling  at an  appreciable fraction  of lightspeed, turn around   and    come back, only a  few years might  have passed on board but many years could   have  gone  by on   Earth.  This is known  as the “twins paradox” ,   since a  traveller undertaking such a journey would   return to   find herself   much   younger   than   her   twin.
There’s only one problem from anyone wishing to get a glimpse of the future – getting back. It would mean travelling faster than light – and that’s not possible.


            But  there may  be an out to be   found in general   relativity , Einstein’s  theory of gravity that unites space and  time as “spacetime”, which  curves in the presence  of mass. It allows  for the possibility of wormholes –a kind of tunnel through spacetime connecting otherwise very distant parts of the universe.
If  the “mouths” of the wormhole are moving relative to one another, then traversing the bridge between different points in space would also take a traveler  to different point in time to that in which she  started.


                  However it would still be impossible to go back further in time than the point at which the wormhole was created, limiting the options for travel somewhat - and possibly explaining why we haven’t encountered any visitors from the future. If any natural wormholes were formed in the Big Bang, it might be possible to travel to a limited number of  points in the past and in the distant universe, but wouldn’t enable one to flit around the cosmos at will  as the Doctor seems to do.


            More restrictively still, theoretical work by Kip Thorne  of  Caltech using a partial unification of general relativity with quantum physics suggested that any wormhole that allows time travel would collapse as soon as it formed.


            Thorne did, however, resolve an apparent issue that could arise due to by time travel (within the confines of general relativity). The “grandfather paradox” involves going back in time and accidentally killing one’s grandfather before one’s father is conceived - preventing one’s own birth, making it impossible to go back in time and kill one’s grandfather. Thorne found that for point masses traversing a wormhole, no initial conditions create this type of paradox.


             That’s good news for anyone worried about people going back and changing the past willy-nilly, but bad news for any Whovians hoping to reverse the decision to cancel the show in 1989 and prevent a 16-year hiatus. That would probably be beyond even the Doctor  himself.