SO the bookies say Tilda Swinton, our resident White Witch of Nairn, leads the betting on who will become the new Doctor Who.

For this viewer – best described as a brooding former fan – the prospects of this are instantly pleasing. First, there’s the scene where the reincarnated Doctor starts to gingerly lay hands on his (now, hopefully, her) new form.

For connoisseurs of those moments when TV captures a moment of social change, there are great possibilities here.

Swinton is already the very definition of androgyny. I imagine the post-2005 (and post-Russell T Davies) generation of Doctor Who writers would relish this opportunity – to journey the lands of gender ambiguity, as it reverberates over a few series.

Secondly, Swinton is mutating into David Bowie’s representative on this planet – the girl who fell to Earth, in terms of her friendship and even her look. Indeed, ever since her earliest movies (say Orlando), Swinton has easily conveyed an other-worldliness in any of her parts. It’s not difficult to imagine her as an undying alien with two hearts.

It would be a proper reset of the show to have Swinton stride around the universe like a pallid stick insect, her massive eyes observing everything anew.

Instead of being one more rumpled older (or younger) white male from the same old thespian filing cabinet, taming a procession of bristling monsters with their twinkly charms.

But there you are – that’s what Doctor Who turns you into, if you linger with it long enough: a fan-fiction writer, over-investing in its success. My own engagement has matured from a childhood watching from behind the sofa, just like everyone else. But there are deeper themes moving through its massive archive that increasingly fascinate me. Nowadays, I note the British post-imperial angst and mist that wreaths every episode. Indeed, this is built into the basic structure.

The Doctor is a renegade from the “Time Lords”, an oligarchic race who have the technology to move back and forth in time.

To ensure that no “time distortions” or “paradoxes” exist, they carefully intervene in the lives of species and civilisations, keeping history continuous and natural. What beneficent rulers they are!

How noble their legacy!

In this framing, the Doctor – who broke from the Lords, stole their Tardis and plots his own careering path through time and space – becomes a travelling bohemian, a moral freelancer. He may be a Gallifreyan master of space and time. But he always gets wrapped up in the suffering fates of those he is supposed to wisely rule over.

The Doctor learns their languages, stands in their space shoes. He risks disrupting “the course of history” in order to respond with empathy, or a sense of injustice, to particular cases.

Yet all this is done with playfulness and eccentricity, the heroism wrapped in wackiness. It’s also decorated with the most comforting and nostalgic British styles – the consistent Victoriana of all the Doctors’ clothing, the blue police box of the Tardis.

To me, the underlying message is clear. We used to rule everything; we’ve given that up now; we still want to do good and make things happen; we’ll do it with charm, wit and lightness; and who knows, maybe the future of the planet – nay, the universe – may still depend on us!

Like James Bond (though much more attractively than that tuxedoed psychopath), Doctor Who is a healing salve for the post-imperial scars of Britishness. How the political emotions of Brexit – defensively patriotic, alien-averse – filter into the next few series will be intriguing to watch.

There’s another deep theme in Doctor Who that the years have revealed to me, particularly as the pop-science books have accumulated on my shelves.

It’s just how wobbly time travel is as a science-fiction device.

The Gallifreyan “Laws of Time” might say “don’t intervene to change the course of history”. But for the sake of a plot device (or a distressed damsel), our scarf-wrapped hero has broken all of them regularly.

As the geeks would put it, I am a “hard-SF” man. What I love about science fiction is the way it projects our social and technological possibilities forward, in a desirable or cautionary way. Unlike artificial intelligence, or bio-science, or new political systems, however, time-travel is extremely impractical.

Though it’s not impossible, say the physicists – that is, if you’re happy to only head into the future. If a spacecraft could approach the speed of light, then Einstein’s theory of relativity predicts that time would tick much more slowly for the astronauts on board than for those waiting for their return on Earth.

If they could find a way to get back to Terra, they’d find many generations had passed away, while they stayed young.

I’d be satisfied with that. The past is done, the future’s open. HG Wells once wrote: “Things have been, says the legal mind, and so we are here. The creative mind says we are here because things have yet to be.”

Recently, the American science writer James Gleick has produced a tour-de-force recently on the subject of time travel. He notes that HG Wells was the first ever to imagine the concept of a time-travelling machine – and that includes antiquity and mythology.

As a self-consciously “modern” man, Wells would have been looking for a device that could show how humanity’s actions fully determined its fate. The time machine itself was, in his words, only “an apparatus of pseudo-logic”.

Yet even beyond Doctor Who, time-travel narratives still abound in popular culture. The profound SF movie Arrival is most notable – but it’s everywhere on TV.

In The Man In The High Castle, Nazism has won and Hitler survives; in 11/22/63, James De Franco is transported back to the point where he can prevent Kennedy’s assassination. In Outlander, or Harry Potter and the Cursed Child, magic enables transport through the centuries.

I could give many more examples from 2016. But why?

Gleick makes a brilliant point that our digital and interactive lives already make us super-aware of our “timelines”. A voice message on a mobile phone could be left there for years. Or, if we want, we can time our posts to appear at a date in the near, or far, future.

If we spend our waking hours in computer game worlds, any era can be jumped into, or mashed up with another era. Let alone our ability to slow or speed up the pace of each game. So under digital conditions, in a way, we are all our own mini “time-lords”.

Yet as we should know from our recent political and cultural upheavals, we don’t always handle well the invitation to be “open” and “flexible”. The reaction is as likely to be one of recoil and defence, conservation and repair, as much as it is an excited embrace.

Take the revived Doctor Who of the last 10 years, still roaming freely back and forth across the light years of the universe. However, most of these episodes’ basic pleasures come from seeing our suburban lives and historical traditions subjected to the most gruesome horrors. Yet Earth (and its garden hedges) survive, essentially undamaged and intact, for the next series.

I guess that’s why Doctor Who sits at the heart of “family” entertainment. You may be hiding behind the sofa – but at least there’s a sofa you can come out and sit on, after it’s all over.

So on second consideration, maybe Tilda Swinton is a little too avant-garde to be the new Doctor Who.

A figure who is also, always, Doctor Who-Are-We? And perhaps the answer to that question, for the next few years, demands another long, comforting woollen scarf.

James Gleick’s Time Travel: A History is published by Fourth Estate this month