# 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.

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# Oliver Stone on interviewing Vladimir Putin: "There are two sides to every story"

The director says his conversations with the Russian president, like all of his works, speak for themselves.

“You’re going to start with this blogging bullshit?” Oliver Stone raises his voice at a reporter, a look of fury on his face.

The director has been asked about the veracity of a video shown to him by the Russian president in his recent Showtime series, The Putin Interviews. The hapless Norwegian journalist who is asking the question notes that bloggers have taken exception to the footage’s true provenance.

What bloggers think of Stone's work, however, is clearly of no consequence to him. When another journalist asks if he’s afraid to be seen as Vladimir Putin’s "PR guy", though, he erupts.

“Do you really think I’m going to go and spend two years of my life doing a tourist guide book? You really think I’m that kind of a filmmaker? Do you have no respect for my work?”

Stone is on fiery form at Starmus science and music festival in Trondheim, Norway. His series on Putin was filmed over two years. The final four hours of footage were cut from an original 19 of recorded interviews, which covered such diverse topics as “Russia in the 1990s and the 2000s, the American expansion of Nato, the American support of terrorism in Central Asia, Syria from his point of view, Ukraine, nuclear arms…”

Critics, however, have termed it a hagiography, and argued it offers Putin a deferential platform to share his view. Others have dismissed Stone as a propaganda poodle.

Stone counters the criticism: “I researched it, I did the best I could, and I think it proves the old adage that there are two sides to every story.”

Whether because of naivety or professional courtesy, on the face of it, in the interview series the 70-year-old appears to buy into everything Putin tells him. "You catch more flies with honey than with vinegar," is all he'll say at the conference.

Later on, in the calm after the storm, we speak alone. “This was a special deal,” he tells me. “He was very congenial and articulate and willing to talk. He grabbed the moment.

“People need to keep something in mind. They said I was soft on him - that’s nonsense.

“You can’t have an interview where you’re asking hostile questions. He would have just tolerated it and said what he did, and then after that first interview he would have not have done a second or a third.

“I was interested in the long view. Nobody in the West has gone that far with him that I have seen.”

The long view is a speciality of Stone’s, as he reveals with his address at Starmus to a packed auditorium. As befits a science festival, he addresses the development of the atomic bomb and the modern digital arms race of cyber warfare.

In his view, “politics invariably gets a stranglehold on science and takes it in the wrong way”. He cites J Robert Oppenheimer, known as the father of the nuclear bomb, and computer analyst Edward Snowden’s life following his decision to turn whistleblower.

Stone directed the film Snowden, a task which involved navigating numerous obstacles, including gaining access to the real Snowden, by then in Russia, himself.

“Science gets slaughtered by politics,” he tells me.

In the shadow of the criticism on the Putin front, he admits that from an American perspective, for him to become involved with Snowden was, well… “beyond the pale".

But despite – or perhaps because of – the Academy Award-winning director’s commitment to the truth, he’s not letting go of various facts as he sees them.

“There is no evidence as far as I’m concerned for the Russian hacking allegations,” he says, adding that this was an “assessment” from the US security services which turned into a “farce”.

He has read the detail for himself, he says – and he also appears on film looking like he believes Putin when the president says it’s nothing to do with him.

Back at home, the American domestic political situation has him as appalled as ever. He is critical, not only of Donald Trump, but the system the US president operates in.

“It seems that the president does not have the power he thinks he has," he says. "You get elected, you think it’s a democracy, but there is this mechanism inside, this Deep State – intelligence agencies, military industrial, the generals, the Pentagon, CIA combined with other intel – which seems to have some kind of inner lock.”

Although Stone places characters at the heart of many of his films, he finds Trump hard to figure out.

“I don’t know what Trump’s mind is like, I think so few people do," he muses. "He says super-patriotic things suddenly like 'I love the CIA, I’m going to really support you, I love the military, I love generals, I love all that beautiful new equipment' – that he sold to Saudi Arabia.

“He also said, and it’s very disturbing, ‘the next war, we’re going to win’. As if you can win a war where you use cyber and nuclear and various weapons. He’s thinking this is a game like a child.

“The purpose of war is not to have one.”

Stone believes – as Trump initially seemed to profess – that Russia will be the chief ally in future for the United States: “They can be great partners in every walk of life, it’s crazy to have them as an enemy."

Nevertheless, he is not as slavish to the official Russian line as many have countenanced.

“I was able to shoot this documentary because of my reputation," he says. Some people say he pulled his punches, I counter.

“Gloves off, gloves on – the truth is, he sees things his way," Stone says. "I’m not there to change his mind, I’m there to show his mind.”

In his view, an observant watcher will learn about Putin just by watching him. "The camera doesn’t lie – the camera tells you things, body language, eyes – you can get a feel sometimes," he says. "I think if you watch all four hours you’ll see that we got an enormous amount of information."

Perhaps those who sit through those four hours will be satisfied that they know more about Putin – or about Stone himself. After all, if the camera doesn't lie, it doesn't lie for anyone.

As I leave the room, Stone raises his voice after me: “Don’t change my words.” He’s smiling broadly as he speaks.

Kirstie McCrum is a freelance journalist. Follow her @kirstiemccrum.

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