Doctor Who raises questions of the nature of time – if his assistant dies in the past and again in the future, why as a Time Lord can he not travel backward and forward to prevent both occurrences? There are fixed points in time, moments unchangeable, but intervention in a single life would surely not upset the entire sequence?

Einstein would have doubted the Doctor’s capacity to make any difference. Einstein conceived of space and time as one, it was not a question of space existing in time, but space-time being a single continuum; the whole of time exists in a single moment that embraces the whole of space.

Conventional scientific thought believes the universe began 13.5 billion years ago with a massive explosion of matter and space to fill the nothingness that previously existed, or didn’t exist as the case may be, nothingness having no existence.

Since that moment, space has been getting bigger, and time, being part of a single continuum, has expanded with space.  Human lives have been led in a single direction, beginning at the cradle and ending at the grave.

But, some thought suggests, there will come a point when the outward momentum of the universe comes to an end, when inertia will slow down, and eventually halt, the progress of the galaxies.  Eventually, the gravity that holds the universe together will exert a pull that is stronger than the last remnant of momentum left from the Big Bang and the universe will begin to contract.

As the process of the creation of space is reversed, time, being one with space, will also be reversed.  At some point, perhaps billions of years hence, space-time will be at the point where it is at this moment, except the journey will be a return one rather than an outward one, and time, as part of reversing space, will presumably be also reversed: life will begin at the grave and end with our birth.

The logical way to behave, if such a theory proved correct, would be to put everything into the early part of one’s life and not to worry about the later years because when time is reversed life will then go from the bad to the good.

Of course, there are theories that the universe goes through a series of expansions and contractions, but if there are tens of billions of years between each of our appearances, there will be plenty of time to work out the best strategy. Unless the Doctor has changed everything by then.