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 did Britain's first road atlas take you to Aberystwyth?

Alan Ereira's new The Nine Lives of John Ogilby tells the story of a remarkable book – and its remarkable creator.

John Ogilby was a talented dancer with a bright future. Performing at White Hall Palace in February 1619, the 18-year-old leapt higher than ever to impress the watching James I and his queen. But then, crashing to the floor with a torn ligament, Ogilby never danced again. It was one of many misfortunes he overcame in a remarkable life. He went on to become a theatrical impresario, the deputy master of the revels in Ireland, a poet, a translator and a publisher of ancient classics. He even organised the public celebration of Charles II’s coronation. He was also an accomplished soldier, sailor and spy, as Alan Ereira reveals in this entertaining account of his “lives” and times.

It was a remarkable collection of lives for a man born in Scotland in 1600 and raised in poverty, the illegitimate son of an aristocrat. Yet Ogilby’s greatest achievement was to put Britain on the map when he was appointed “His Majesty’s Cosmographer and Geographick Printer” in 1674. His Britannia is the first detailed road atlas ever made. It opens with a map of England and Wales showing, he wrote, “all the principal roads actually measured and delineated”. It contains a hundred or so beautifully engraved plans of roads as winding ribbons sliced into sections. Rivers, forests, villages and bridges are included as landmarks.

Embracing the new science of measurement and experiment championed by the Royal Society, Ogilby’s surveyors used a wheel with a circumference of 16ft 6in and a handle that allowed it to be pushed along, as well as a clock face that recorded journey distances. With no universally agreed length of a mile, Ogilby chose 1,760 yards. Britannia led to the accurate measurement of almost 27,000 miles of tracks, paths and roads, though only about 7,500 are depicted in the atlas at one inch to the mile.

Britannia was published in September 1675. There were few who could afford it, at £5 (roughly £750 in today’s money), and it was too heavy to carry. Instead, travellers found their way around the country by following printed itineraries, with lists of the towns to pass through on any particular journey.

Britannia is not, as Ereira explains, an atlas of commercially useful roads of the day. The first journey is an odd one, from London to Aberystwyth, then a town of fewer than 100 houses and a ruined castle. Some of the roads chosen were no longer in use, while important routes such as those to Liverpool and Sheffield were left out.

But the choice of roads in Britannia begins to make sense as being those necessary for the royal mastery of the kingdom. The London to Aberystwyth road led to mines nearby. In the days of Charles I those mines contained lead and silver that helped the king pay his soldiers during the civil war. Britannia was a handbook, Ereira explains, for a conspiracy leading to a new kingdom under a Catholic king.

Ever since the start of the Reformation, Europe had been rumbling towards a religious war. When it came on the mainland it lasted 30 years and left millions dead. The subsequent Peace of Westphalia led to a new map of Europe, one of countries and defined frontiers instead of feudal territories with unclear borders and independent cities. England was not included in the peace but shared in its vision of separate sovereignty. This led to different results in different places. In France, the king became an all-powerful despot; in England it was the ruler who lost power as parliament emerged triumphant.

In 1670 Charles I’s son Charles II decided to throw off the restraints he had accepted as the price of his restored monarchy. He wanted to be the absolute master in his land. To achieve this, he entered into a secret treaty with the French king Louis XIV. Charles needed money, an army, allies to execute his plan, and detailed knowledge of the kingdom; Louis was willing to bankroll the venture as long as Charles converted to Catholicism. Britannia was a vital part of Charles’s strategy to assert military control: he would use it to help land and deploy the 6,000 French troops that Louis had promised him to assist his forces. The pact remained a well-kept secret for nearly a century, even though it soon fell apart when the French and British got bogged down in a war with the Dutch.

No matter. Ogilby died in September 1676 and in 1681 Charles II dissolved parliament for the last time during his reign. “Britannia provided an extraordinary grasp over the business and administration of the 399 communities that it identified in England and Wales, and the crown took a grip on them all,” Ereira writes.

In this way, the atlas played a significant part in enabling the king’s revenue to grow by one-third within a few years. No longer needing financial help from Louis, Charles ruled by divine right, exercising absolute power until his death in 1685. The lesson of Britannia was that whoever controls the map controls the world.

Manjit Kumar is the author of “Quantum: Einstein, Bohr and the Great Debate about the Nature of Reality” (Icon)

This article first appeared in the 12 January 2017 issue of the New Statesman, Putin's revenge