How much of “Doctor Who” might really be possible?

Science shows why Doctor Who is so special.

As Doctor Who’s 50th anniversary looms, time travel is everywhere – on the screen, at least. Famously, the Doctor can whizz through the years using a “dimensionally transcendental” machine, the TARDIS, and make changes to the past as and when he likes. But what is time travel – and how much of “Doctor Who” might really be possible?

A handy definition of time travel comes from philosopher David Lewis. Lewis says time travel involves a journey having different durations viewed from outside (in “external time”) or from inside (in “personal time”). Suppose you spend five minutes travelling aboard your machine, as measured by (e.g.) your watch and your memories. On arrival, you find 150 years have elapsed in the outside world. Congratulations, you have time-travelled. Five minutes of your personal time has covered 150 years of external time.

Odd as this sounds, Einstein’s theory of Special Relativity introduced such possibilities to physics in 1905. The theory says: the duration of a process varies with the relative velocity of the observer. The closer that relative velocity gets to the speed of light, the longer the travelling process takes.

Suppose you want to see the Earth a billion years hence, but worry you have only about 50 personal years left. Special Relativity specifies that if you travel very close to the speed of light relative to the Earth, your 50 personal years can cover one billion Earth years.

In backward time travel, personal and external time differ in direction, so journeys end in external time before, not after, they begin; you spend five personal minutes travelling 150 years into the external past. General Relativity suggests that the universe is essentially curved spacetime, which might allow such divergences of external and personal time.

Relativity treats space and time as aspects of a single entity: “spacetime”. One of the more remarkable features of General Relativity is that it allows time and space axes to be interchanged, so one observer’s space axis can be another observer’s time axis.

In 1949, Austrian mathematician Kurt Gödel used General Relativity to describe a universe where intrepid voyagers can go anywhere in (past or future) time without travelling faster than light. Gödel’s universe has no boundaries in space or time, and all the matter in it rotates. But our finite, non-rotating universe is not Gödel’s. Despair not though – simply spin an ultradense, very (maybe infinitely) long cylinder very fast. Spacetime should curve around the cylinder so the direction of the local future partially points into the external past. Such devices are called “Tipler Cylinders”, after physicist Frank Tipler.

Better yet, quantum theory suggests that “wormhole” connections between different spacetime points spontaneously form and break all the time. The chances are that natural wormholes are tiny - vastly smaller even than an electron, (and a billion trillion electrons can fit in a teaspoon). But you could perhaps find (or create) a wormhole big enough and durable enough to let you slip through into the past. Difficult, but theoretically possible.

No, you can’t kill your physics teacher

So perhaps you could travel into the past. But what about paradoxes? What is to stop you assassinating your grandfather or yourself as infants? One answer says: logical consistency.

Classical logic says you cannot consistently kill in infancy someone who achieves adulthood. But, Lewis says, time travel need not involve doing the logically impossible – provided travellers’ actions in the past are consistent with the history whence they come. So you could try killing your baby grandfather, but something would foil you – you would sneeze, or your gun would jam. Lewisian time travel is therefore (classically) consistent, but might look very strange, since seemingly possible actions (like shooting an unprotected infant) would prove impossible.

Another view says that backward time travel requires many worlds – that is, many different but equally real versions of physical reality. Physicist David Deutsch and philosopher Michael Lockwood argue that time travel must involve inter-world travel. If you travel backwards in time, you must arrive in a history different from your native one and so would be quite unfettered by your past once you get there. You could even kill this other history’s counterparts of your grandfather and yourself.

Both these concepts of backwards time travel may disappoint anyone wanting to change the “one and only” past. Conventional logic says time travellers would either help make history what it was (Lewis) or create a different history (Deutsch/Lockwood). However, quantum logic might let travellers change the actual (one-and-only) past.

Suppose we hold that quantum measurements determine (or change) quantities measured, even if those quantities lie in the past. Someone could travel back and “observe” history turning out differently from how it originally was, thereby retrospectively making actuality different from what it had been. What would happen to travellers who rebooted history is not clear, but this model seems closer to the time travel familiar from “Doctor Who” and other fictions. Beware, though, because quantum theory allows no predicting, and still less controlling, of the outcomes of changing the past. There would be no way to foresee the effect you would have on the present.

So classical logic, General Relativity and quantum theory all seem to permit time travel. Classical logic plus General Relativity suggest backward travellers face weird consistency constraints. Many-worlds travellers face no constraints, but get displaced into different histories. Quantum-logic travellers could change the (one and only) past without constraints, but they couldn’t predict or control what they would get.

So far, however, it seems only the Doctor knows how to change the past at will.

Alasdair Richmond received funding from the Arts and Humanities Research Council, to fund one semester of a two-semester leave period that covered the academic year 2008-2009.

The Conversation

This article was originally published at The Conversation. Read the original article.

The eleven incarnations of the Doctor. Image: BBC/Matt Burlem

Alasdair Richmond is a Senior Lecturer in Philosophy at the University of Edinburgh.

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Broken and The Trial: From Sean Bean playing a priest to real life lawyers

A surprisingly involving depiction of a clergyman provides the saintly contrast to the sinner being judged by a real jury.

I was all set to scoff at Broken, Jimmy McGovern’s new series for BBC1 (30 May, 9pm). A drama about a Catholic priest and his impoverished parish in a “major northern city”, it sounded so hilariously McGovern-by-numbers (“Eh, lad, give us the collection bowl – the leccy wants paying”) that on paper it could pass for a spoof. Even funnier, Sean Bean, late of Game of Thrones, was to play the clergyman in question.

Naturally, I adore Bean, who comes from the major northern city that is Sheffield, as I do, and who is so terribly . . . virile (though when I interviewed him in a car park behind King’s Cross Station a few years ago, and a security guard in a high-vis jacket approached us furiously shouting the odds, he ran and hid in his trailer, leaving yours truly to face the music). But let’s face it: he’s not exactly versatile, is he? The idea of him in a cassock, or even just a mud-coloured cardigan, made me laugh out loud.

Settling down to watch the series, however, I soon realised that no scoffing would be taking place. For one thing, Broken is hugely involving, its Dickensian plot (no spoilers here) as plausible as it is macabre. For another, in the present circumstances, its script seems to be rather daring. Not only is Father Michael Kerrigan shown – cover my eyes with the collected works of Richard Dawkins! – to be a good and conscientious priest, but his faith is depicted as a fine and useful thing. If he brings his besieged parishioners solace, he is sure to be carrying vouchers for the food bank as well.

The flashbacks from which he suffers – in which his mammy can be heard calling him a “dirty, filthy beast” and a spiteful old priest is seen applying a cane to his hand – are undoubtedly clichéd. But they are also a device. Forty years on, he is happy to nurse his dying mother, and his love for God is undimmed: two facts that are not, of course, unrelated. How weirdly bold for a television series to set its face against the consensus that denigrates all things Christian as it never would any other faith.

I don’t for a minute buy Anna Friel as Christina, the gobby, broke single mother Kerrigan is determined to help. Even when covered in bruises – a bust-up at the betting shop – Friel manages to look glossy, and she never, ever quits acting (with a capital A), which is a drag. But Bean is such a revelation, I was able to ignore the voice in my head which kept insisting that a Catholic priest as young as he is – in this realm, “young” is a couple of years shy of 60 – would surely be Polish or African (I’m not a Catholic but I am married to one, for which reason I occasionally go to Mass).

He plays Kerrigan, whose overwhelming desire to be kind sometimes makes him cack-handed, with great gentleness, but also with an uninflected ordinariness that is completely convincing. Part of the problem (my problem, at least) with Communion is the lack of rhetorical passion in most priests’ voices, something he captures perfectly. One other thing: Line of Duty fans need to know that Adrian Dunbar – aka Ted Hastings – can also be seen here wearing a dog collar, and that he looks almost as good in it as he does in police uniform.

On Channel 4 The Trial: A Murder in the Family was an experiment in the shape of a murder trial in which the defendant – a university lecturer accused of strangling his estranged wife – and all the witnesses were actors but the lawyers and “jury” were real. Over five consecutive nights (21-25 May, 9pm), I found it pretty tiresome listening to jury members tell the camera what they made of this or that bit of evidence.

Get on with it, I thought, longing again for the return of Peter Moffat’s Silk. But I adored the lawyers, particularly the lead ­defence barrister, John Ryder, QC. What an actor. Sentences left his mouth fully formed, as smooth as they were savage, his charm only just veiling his mighty ruthlessness. Drooling at this performance – which was not, in one sense, a performance at all – I found myself thinking that if more priests came over like barristers, our dying churches might be standing room only.

Rachel Cooke trained as a reporter on The Sunday Times. She is now a writer at The Observer. In the 2006 British Press Awards, she was named Interviewer of the Year.

This article first appeared in the 25 May 2017 issue of the New Statesman, Why Islamic State targets Britain

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