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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Why a man soiling himself was one of my Olympic highlights

The joy of the Olympics is how easy it is to drop in and form strong opinions about the best way to win in any discipline.

There used to be a rumour that a newspaper (now defunct) had in its possession some compromising photographs of the wife of a beloved TV entertainer (now dead) romancing a chihuahua. I mention this because I think John Inverdale must have a similar hold over BBC Sport bosses. How else does he get such great gigs? At the Olympics, if he wasn’t being corrected by Andy Murray about the existence of women, he was having water droplets “accidentally” shaken over him by a sour-faced Steve Redgrave as he aired out his umbrella.

Then again, perhaps Inverdale’s continued employment is the salt in the caramel, or the Tabasco in a Bloody Mary: a small irritant, designed to give a kick to what would otherwise be bland niceness shading into enforced cheeriness. The rest of the Olympic presenters (grumpy Sir Steve possibly excepted) were a bunch of lambs: the sweet Helen Skelton, and the even sweeter Mark Foster and Rebecca Adlington, hosting the swimming; Matt Baker from The One Show and Beth Tweddle doing the gymnastics; that poor bloke they put on the beach so that leery passers-by and lecherous drunken couples could get into his shot. With 306 events over 19 days, I felt as if Clare Balding had moved into my spare room, we were spending so much time together. (The fact I didn’t want to smash my screen every time she came on is proof that she’s worth every penny of her £500,000 salary.)

The time zone difference could have made these Olympics a washout for British viewers, but the BBC used its red-button technology sensibly, and the presenters (mostly) coped with pretending they didn’t know what was going to happen while hosting the highlight reels. Someone at New Broadcasting House even grew a pair as the first week went on and stopped news programmes from intruding on the medal action. Earlier in the week, viewers had been forced to hop from BBC1 to BBC4 to BBC2 to follow their favourite events, the change sometimes occurring at an inopportune moment.

The joy of the Olympics is how easy it is to drop in and form strong opinions about the best way to win in any discipline. Unlike football, say, where true enjoyment requires memorising rafts of statistics and forming strong opinions about the transfer market, all Olympics coverage is designed for people who couldn’t tell one end of a derny bike from the other five minutes ago. Who really understands the rules of the omnium? Luckily, it turns out you don’t need to.

I thought I was going to hate the Olympics, which took place in the shadow of controversies over drug testing, the US swimmer Ryan Lochte’s faked robbery and Caster Semenya’s hormone levels. For all the guff about the international hand of friendship, the Games are a ruthless commercial enterprise, and one in which global inequalities are harshly self-evident. Are Americans just better athletes than the rest of the world? Clearly not. Money buys success. Could most of us, even given a trainer, dietician and acres of free time, qualify for any of these sports? No. Genetically, most of us are Morlocks compared to these people.

Nonetheless, all the natural (and artificial) advantages in the world can’t win you a gold medal if you sit on your sofa and eat Pringles all day. One of my favourite competitions was the gymnastics, where Simone Biles of the United States seemed to dominate effortlessly. Yes, being 4ft 8in clearly helps her – her shorter steps allow her to pack in more tumbles – but she’s still willing to do a somersault on a bar four inches wide. (The dangers of the discipline became clear when the French gymnast Samir Aït Saïd snapped his leg landing off the vault on the first day of qualifying rounds.) In the 50-kilometre race walk, Yohann Diniz pooed himself, collap­sed twice – and still finished in eighth place.

These are the Olympic moments I cherish. Usain Bolt makes it look too easy, which is boring. Without a narrative, sport is little more than a meaningless spectacle – a Michael Bay film or the latest Call of Duty. Luckily, Team GB seemed to heed the call for drama, delivering us a penalty shoot-out victory in the women’s hockey (and a team with a married couple in it); a comeback for Mo Farah after the allegations against his coach Alberto Salazar; and a surprising failure for Tom Daley in the 10-metre dive. We also got to see Laura Trott and Jason Kenny’s races through each other’s eyes.

In other words, bring on Tokyo 2020, so I can grouse about the money and the drugs and the inequality right up to the moment the first person shits themselves – and still finishes the race. Truly, human endeavour is a beautiful sight to behold. 

Helen Lewis is deputy editor of the New Statesman. She has presented BBC Radio 4’s Week in Westminster and is a regular panellist on BBC1’s Sunday Politics.

This article first appeared in the 25 August 2016 issue of the New Statesman, Cameron: the legacy of a loser