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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Notes from a small island: the fraught and colourful history of Sicily

Sicily: Culture and Conquest at the British Museum.

When a gun was fired a hundred metres or so from the Sicilian piazza where we were eating, my reaction was to freeze, fall to my knees, and then run for cover in a colonnade. As I peered back into the square from behind a column, I expected to see a tangle of overturned chairs and china but I watched instead as the freeze-frame melted into normality. I retrieved my shoe from the waiter.

I should not have been surprised by how coolly everyone else handled what I was inclined to call “the situation”. The Sicilians have had 4,000 years in which to perfect the art of coexistence, defusing conflict with what strikes outsiders as inexplicable ease, rendering Sicily one of the most culturally diverse but identifiable places on the planet. Still, having visited “Sicily: Culture and Conquest” at the British Museum, I feel vindicated. There may be no Cosa Nostra in this exhibition, which charts the island’s history from antiquity to the early 13th century, but that doesn’t mean there is no simmering conflict. Like Lawrence Durrell, who described Sicily as “thrown down almost in mid-channel like a concert grand” and as having “a sort of minatory, defensive air”, I felt the tension beneath the bliss that has characterised Sicily for many centuries.

The “barbarians”, wrote the Greek historian Thucydides, moved to Sicily from Iberia (Spain), Troy and Italy before the Phoenicians and Greeks settled there in the 8th century BC – the time of Homer, whose Odyssey provided a useful guide to some of the more threatening features of the landscape. The giant, sea-lying rocks off the east coast were the boulders that the one-eyed Polyphemus hurled at Odysseus’s ship; the phrase “between Scylla and Charybdis” referred to the Strait of Messina that divides Sicily from the mainland; Lake Pergusa, in the centre of the island, was the eerie spot whence Hades snatched Persephone and carried her down to the underworld.

It is a delight to behold the British Museum’s case full of terracotta figurines of Persephone, Demeter and their priestesses, some of thousands uncovered across Sicily, where the Greeks established the cult of these goddesses. The Phoenicians introduced their
own weather god, Baal Hammon, and the indigenous Sicilians seem to have accepted both, content that they honoured the same thing: the island’s remarkable fecundity.

The early Sicilians were nothing if not grateful for their agriculturally rich landscapes. As early as 2500 BC, they were finding ways to celebrate their vitality, the idea being that if the soil was fertile, so were they. On a stone from this period, intended as a doorway to a tomb, an artist has achieved the near impossible: the most consummate representation of the sexual act. Two spirals, two balls, a passage and something to fill it. The penis is barely worth mentioning. The ovaries are what dominate, swirling and just as huge as the testicles beneath them. We see the woman from both inside and out, poised on two nimble, straddling legs; the man barely figures at all.

Under the Greeks in the 5th century BC, it was a different story. Although many of Sicily’s tyrants were generous patrons of the arts and sciences, theirs was a discernibly more macho culture. The second room of the exhibition is like an ode to their sporting achievements: amid the terracotta busts of ecstatic horses and the vase paintings of wild ponies bolting over mounds (Sicily is exceptionally hilly) are more stately representations of horses drawing chariots. These Greek tyrants – or rather, their charioteers – achieved a remarkable number of victories in the Olympic and Pythian Games. Some of the most splendid and enigmatic poetry from the ancient world was written to celebrate their equestrian triumphs. “Water is best, but gold shines like gleaming fire at night, outstripping the wealth of a great man” – so begins a victory ode for Hiero I of Syracuse.

But what of the tensions? In 415BC, the Athenians responded to rivalries between Segesta and Syracuse by launching the Sic­ilian expedition. It was a disaster. The Athenians who survived were imprisoned and put to work in quarries; many died of disease contracted from the marshland near Syracuse. There is neither the space nor the inclination, in this relatively compact exhibition, to explore the incident in much depth. The clever thing about this show is that it leaves the historical conflicts largely between the lines by focusing on Sicily at its height, first under the Greeks, and then in the 11th century under the Normans – ostensibly “the collage years”, when one culture was interwoven so tightly with another that the seams as good as disappeared. It is up to us to decide how tightly those seams really were sewn.

Much is made of the multiculturalism and religious tolerance of the Normans but even before them we see precedents for fairly seamless relations between many different groups under the 9th-century Arab conquerors. Having shifted Sicily’s capital from Syracuse to Palermo, where it remains to this day, the Arabs lived cheek by jowl with Berbers, Lombards, Jews and Greek-Byzantine Sicilians. Some Christians converted to Islam so that they would be ­exempt from the jizya (a tax imposed on non-Muslims). But the discovery of part of an altar from a 9th-century church, displayed here, suggests that other Christians were able to continue practising their faith. The marble is exquisitely adorned with beady-eyed lions, frolicsome deer and lotus flowers surrounding the tree of life, only this tree is a date palm, introduced to Sicily – together with oranges, spinach and rice – by the Arabs.

Under Roger II, the first Norman king of Sicily, whose father took power from the Arabs, the situation was turned on its head. With the exception of the Palermo mosque (formerly a Byzantine church, and before that a Roman basilica), which had again become a church, mosques remained open, while conversion to Christianity was encouraged. Roger, who was proudly Catholic, looked to Constantinople and Fatimid Egypt, as well as Normandy, for his artistic ideas, adorning his new palace at Palermo and the splendidly named “Room of Roger” with exotic hunting mosaics, Byzantine-style motifs and inscriptions in Arabic script, including a red-and-green porphyry plaque that has travelled to London.

To which one’s immediate reaction is: Roger, what a man. Why aren’t we all doing this? But an appreciation for the arts of the Middle East isn’t the same thing as an understanding of the compatibilities and incompatibilities of religious faith. Nor is necessity the same as desire. Roger’s people – and, in particular, his army – were so religiously and culturally diverse that he had little choice but to make it work. The start of the Norman invasion under his father had incensed a number of Sicily’s Muslims. One poet had even likened Norman Sicily to Adam’s fall. And while Roger impressed many Muslims with his use of Arabic on coins and inscriptions, tensions were brewing outside the court walls between the
island’s various religious quarters. Roger’s death in 1154 marked the beginning of a deterioration in relations that would precipitate under his son and successor, William I, and his grandson William II. Over the following century and a half, Sicily became more or less latinised.

The objects from Norman Sicily that survive – the superb stone carvings and multilingual inscriptions, the robes and richly dressed ceiling designs – tell the story less of an experiment that failed than of beauty that came from necessity. Viewing Sicily against a background of more recent tensions – including Cosa Nostra’s “war” on migrants on an island where net migration remains low – it is perhaps no surprise that the island never lost its “defensive air”. Knowing the fractures out of which Sicily’s defensiveness grew makes this the most interesting thing about it. 

Daisy Dunn’s latest books are Catullus’ Bedspread and The Poems of Catullus (both published by William Collins)

“Sicily” at the British Museum runs until 14 August

This article first appeared in the 28 April 2016 issue of the New Statesman, The new fascism