Culture: Why DCI Hunt may disappear up his own investigation

Among philosophers, the concept of time travel is considered fraught with difficulties. For one thing, it isn't clear that there's anywhere to travel to. Then there's the danger that a time machine embarking on a journey into the past would collide with itself. Finally, there's the problem of reverse causation — namely, that an individual could travel back in time and commit an action that would alter the present.

For those of us who worry about such things, the debut of BBC1's Ashes to Ashes (pictured) on Thursday was deeply troubling. I don't mean that the pilot episode was poor, only that the very existence of a sequel to Life on Mars – particularly one starring Philip Glenister as DCI Hunt – is at odds with the conceit of the first series. ( Life on Mars was a Bafta-winning drama documenting the adventures of Sam Tyler, a present-day police officer who found himself trapped in the 1970s after suffering a near-fatal accident.) The writers of the original series sidestepped the philosophical landmines associated with time travel by leaving open the possibility that the parts of the story set in the 1970s were simply a coma-dream taking place in the head of the unconscious Tyler.

For example, at various points, Tyler made contact with members of his family living in the past, including his mother and father. Now, as any reader of Pawley's Peepholes by John Wyndham will know, such behaviour is extremely reckless, since interfering in the past risks endangering your own existence. In Life on Mars, however, we were asked to assume that the world Tyler left behind was unaffected by his actions in the past – and if anyone objected to this, the programme's defenders could always claim that the time-travel element was a figment of Tyler's imagination. After all, since when have dreams been logical?

But the writers of Life on Mars have now discarded this defence. Ashes to Ashes concerns the adventures of a new character – a policewoman from 2008 called Alex Drake – who, like Tyler, is plunged into a coma and encounters exactly the same set of characters Tyler had to deal with in the first series. Since Tyler doesn't appear in the sequel, we're forced to conclude that DCI Hunt and his cronies really do exist. In effect, the writers have cleared up the central mystery of the first series — and, in the process, saddled themselves with a whole host of philosophical problems.

Of course, only a tiny fraction of the audience will care about such things. The vast majority will simply delight in the fact that DCI Hunt has moved from Manchester to London and traded in his Ford Cortina for an Audi Quattro.