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

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

Photo: Getty
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# The End We Start From imagines London underwater

Megan Hunter's fictional apocalypse is a tender one.

It is six months after the flood. The nameless narrator of The End We Start From is a new mother and a refugee, and by the midpoint of the novel we have followed her and her baby from the “Gulp Zone”, where their London flat was swallowed, to a safe house that proved to be not safe enough, and then refugee camps, every move stripping life a little closer to the essentials. First what can be fitted in a car as you flee to safety, then what can be carried in your arms; first porridge, then only gruel.

Halfway through, the narrator and her baby make it to an island under the guidance of another new mother she befriended in the camps. Here, a family has established a small life of plenty. The narrator has left behind a “place of not-enough”, but here there is food to spare. Seeds grow into vegetables. The baby “likes to eat butter in chunks”. But where has the butter come from? There’s no mention of cattle on the island, no bucolic descriptions of churning. We’re told there is no electricity. So how do they have butter and why is it not rancid?

It’s a small thing, but an outsize irritant in a book whose prose is pared back to match the minimal existence it describes. Every detail feels weighted with significance because it was chosen over something else. Megan Hunter is a poet (this is her first novel), and her poetic instincts are underlined by the TS Eliot-referencing title, borrowed from Four Quartets: “What we call the beginning is often the end / And to make an end is to make a beginning. / The end is where we start from.”

Apocalypse and rebirth are central to Hunter’s story. Butter aside, it invokes a thoroughly plausible end of the world. Like Emily St John Mandel’s luminous Station Eleven, or Margaret Atwood’s MaddAddam trilogy, you read it with the conviction that this is what it would be like. (These stories are told from the perspective of the resourceful fortunates who make it through. Apocalypse literature kindly dodges the reality that, if it came to it, most of us would die whimpering in a dirt hole.)

But realism is not the only dictate here. The End We Start From is also deeply invested with symbolism. It begins with the narrator going into labour: “Finally I am waterless, the pool of myself spreading slowly past my toes.” Maternity is a kind of apocalypse, an end to being one kind of self who lives one kind of life, and the beginning of another. Names, like everything else here, are cut back to the barest essentials, becoming just initials. The narrator’s husband is R, her in-laws are N and G, and her baby Z – an alphabetical end who is at the beginning of his life. Anyone who has welcomed the catastrophe of a newborn into their lives is likely to feel sympathy for this parallelbetween infant and Armageddon.

There is a cost to the allegory, though, and it comes through in moments when Hunter sacrifices the merciless logic of calculating survival in favour of giving play to her metaphor. Milk is, as it would be for a new mother, a theme. The milk in the narrator’s breasts that keeps her baby alive becomes an analogue for all sustenance: “As for food, I have started to think of it all as milk,” she says. “I wonder how long we would survive, how quickly human milk runs out in famine.” Perhaps it’s inevitable, then, that the unexpected gift of security and nourishment the narrator and Z find on the island should be represented through dairy; but it also punctures a world you could otherwise believe in utterly.

Hunter’s apocalypse is a tender one. There is violence and disorder at the start: one of the most affecting uses of Hunter’s spare style is when the narrator’s mother-in-law fails to return from a brutal trip to gather provisions, and the narrator simply announces: “No G.” But while R chooses isolation and suspicion of others, leaving his wife and child to make his own way, the narrator chooses humanity. She tells us how she “falls in love”, deep and quick, with those with whom she forms alliances. To borrow again from Four Quartets, “The houses are all gone under the sea” – but The End We Start From promises the possibility of life afterwards.

The End We Start From
Megan Hunter