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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Marvel has moved past the post-credits teaser, and it's all the better for it

Individual stories are suddenly taking precedence over franchise building.

The lasting contribution of 2008’s Iron Man to contemporary cinema comes not from the content of the film itself, but in its Avengers-teasing post-credits scene featuring an eyepatch-sporting Samuel L. Jackson. While post-credits scenes were not invented by Marvel, their widespread adoption in other blockbusters is a testament to Marvel using them to titillate and frustrate.

Fast forward nine years and Marvel’s direction has significantly altered. Having moved to a three-film-a-year structure ahead of next year’s climactic Infinity War, their two releases this summer have featured less explicit connective tissue, using post-credits scenes that are, in typical Marvel fashion, self-reflexive and fun – but this time with no teases for films to come.

Where previous Marvel Cinematic Universe (MCU) films have trailed characters donning superhero mantles, confrontations to come, or more light-hearted team ups, Guardians of the Galaxy Vol. 2 decided to lovingly poke fun at Marvel grandmaster Stan Lee, leaving him stranded on a godforsaken space rock in the outer reaches of the stars. Spider-Man: Meanwhile Homecoming targeted filmgoers who had stayed until the end in expectation of a tease, only to receive a Captain America educational video on the virtues of “patience”.

That isn’t to say that connective tissue isn’t there. Marvel seems to be pursuing world building not through post-credits stingers, but through plot and character. In the past, teasing how awful big bad Thanos is ahead of the Avengers battling him in Infinity War would have been done through a menacing post-credits scene, as in both Avengers films to date. Instead Guardians of the Galaxy Vol. 2 uses character as a tool to explore the world at large.

Nebula’s seething rage is, rather than just a weak excuse for an antagonist’s arc, actually grounded in character, explaining to Sean Gunn’s loveable space pirate Kraglin that Thanos would pit his daughters, her and Gamora, against each other, and replace a part of her body with machine each time she failed – and she failed every time. It’s effective. Thanos’ menace is developed, and you feel sympathy for Nebula, something Marvel has historically failed to do well for its antagnoists. Her parting promise – to kill her father – not only foreshadows the events of Infinity War, but also hints at the conclusion of a fully formed arc for her character.

In the high-school-set Spider-Man: Homecoming, the stakes quite rightly feel smaller. The inexperienced wall-crawler gets his chance to save the day not with the galaxy at risk, but with an equipment shipment owned by Iron Man alter-ego and billionaire inventor Tony Stark hanging in the balance. While such a clear metaphor for widespread change in the MCU might be a little on the nose, the set-up is effective at plaing the film at street level while also hinting at overall changes to the structure of the universe.

Stark gifting Peter a new (and oh so shiny) suit is a key set piece at the end of the film, whereas in 2015's Ant-Man’s Hope Pym inheriting her mother’s own miniaturising suit it is relegated to a teaser. Peter’s decision to turn it down not only completes Peter’s transition past seeking the approval of Stark’s unwitting father figure, but it also leaves the Avengers in an as-yet unknown state, still fragmented and incomplete after the events of 2016’s Civil War. To anticipate Spider-Man joining the Avengers proper is to anticipate the forming of the team as a whole – keeping our collective breath held until we stump up for tickets to Infinity War.

With this happy marriage of the macro and the micro, individual stories are suddenly taking precedence in the MCU, rather than being lost in the rush to signpost the foundations for the next instalment in the franchise. It’s a refreshingly filmic approach, and one which is long overdue. To suggest that Marvel is hesitant to overinflate Infinity War too early is supported by their refusal to share the footage of the film screened to audiences at the D23 and San Diego Comic Con events in recent weeks. Instead, the limelight is staying firmly on this November’s Thor: Ragnarok, and next February’s Black Panther.

Stan Lee, at the end of his Guardians of the Galaxy Vol. 2 post credits scene, cries, “I’ve got so many more stories to tell!”, a hopeful counterpoint to a weary Captain America asking “How many more of these are there?” at the end of Homecoming. With Disney having planned-out new MCU releases all the way into 2020, entries in the highest-grossing franchise of all time won’t slow any time soon. We can, at least, hope that they continue their recent trend of combining writerly craft with blockbuster bombast. While the resulting lack of gratuitousness in Marvel’s storytelling might frustrate in the short term, fans would do well to bear in mind Captain America’s call for patience.