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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The City of London was never the same after the "Big Bang"

Michael Howard reviews Iain Martin's new book on the legacy of the financial revolution 30 years on.

We are inundated with books that are, in effect, inquests on episodes of past failure, grievous mistakes in policy decisions and shortcomings of leadership. So it is refreshing to read this lively account of a series of actions that add up to one of the undoubted, if not undisputed, successes of modern ­government action.

Iain Martin has marked the 30th anniversary of the City’s Big Bang, which took place on 27 October 1986, by writing what he bills as the inside story of a financial revolution that changed the world. Yet his book ranges far and wide. He places Big Bang in its proper context in the history of the City of London, explaining, for example, and in some detail, the development of the financial panics of 1857 and 1873, as well as more recent crises with which we are more familiar.

Big Bang is the term commonly applied to the changes in the London Stock Exchange that followed an agreement reached between Cecil Parkinson, the then secretary of state for trade and industry, and Nicholas Goodison, the chairman of the exchange, shortly after the 1983 election. The agreement provided for the dismantling of many of the restrictive practices that had suited the cosy club of those who had made a comfortable living on the exchange for decades. It was undoubtedly one of the most important of the changes made in the early 1980s that equipped the City of London to become the world’s pre-eminent centre of international capital that it is today.

But it was not the only one. There was the decision early in the life of the Thatcher government to dismantle foreign-exchange restrictions, as well as the redevelopment of Docklands, which provided room for the physical expansion of the City (which was so necessary for the influx of foreign banks that followed the other changes).

For the first change, Geoffrey Howe and Nigel Lawson, at the Treasury at the time, deserve full credit, particularly as Margaret Thatcher was rather hesitant about the radical nature of the change. The second was a result of Michael Heseltine setting up the London Docklands Development Corporation, which assumed planning powers that were previously in the hands of the local authorities in the area. Canary Wharf surely would not exist today had that decision not been made – and even though the book gives a great deal of well-deserved credit to the officials and developers who took up the baton, Heseltine’s role is barely mentioned. Rarely is a politician able to see the physical signs of his legacy so clearly. Heseltine would be fully entitled to appropriate Christopher Wren’s epitaph: “Si monumentum requiris, circumspice.”

These changes are often criticised for having opened the gates to unbridled capitalism and greed and Martin, while acknow­ledging the lasting achievements of the new regime, also explores its downside. Arguably, he sometimes goes too far. Are the disparities in pay that we now have a consequence of Big Bang? Can it be blamed for the increase in the pay of footballers? This is doubtful. Surely these effects owe more to market forces, in the case of footballers, and shortcomings in corporate governance, in the case of executive pay. (It will be interesting to see whether the attempts by the current government to address the latter achieve the desired results.)

Martin deals with the allegation that the changes brought in a new world in which moneymaking could be given full rein without the need to abide by any significant regulation. This is far from the truth. My limited part in bringing about these changes was the responsibility I was handed, in my first job in government, for steering through parliament what became the Financial Services Act 1986. This was intended to provide statutory underpinning for a system of self-regulation by the various sectors of the financial industry. It didn’t work out exactly as I had intended but, paradoxically, one of the main criticisms of the regulatory system made in the book is that we now have a system that is too legalistic. Rather dubious comparisons are made with a largely mythical golden age, when higher standards of conduct were the order of the day without any need for legal constraints. The history of insider dealing (and the all-too-recently recognised need to legislate to make this unlawful) gives the lie to this rose-tinted picture of life in the pre-Big Bang City.

As Martin rightly stresses, compliance with the law is not enough. People also need to take into account the moral implications of their conduct. However, there are limits to the extent to which governments can legislate on this basis. The law can provide the basic parameters within which legal behaviour is to be constrained. Anything above and beyond that must be a matter for individual conscience, constrained by generally accepted standards of morality.

The book concludes with an attempt at an even-handed assessment of the likely future for the City in the post-Brexit world. There are risks and uncertainties. Mercifully, Martin largely avoids a detailed discussion of the Markets in Financial Instruments Directive and its effect on “passporting”, which allows UK financial services easy access to the European Economic Area. But surely the City will hold on to its pre-eminence as long as it retains its advantages as a place to conduct business? The European banks and other institutions that do business in London at present don’t do so out of love or affection. They do so because they are able to operate there with maximum efficiency.

The often rehearsed advantages of London – the time zone, the English language, the incomparable professional infrastructure – will not go away. It is not as if there is an abundance of capital available in the banks of the EU: Europe’s business and financial institutions cannot afford to dispense with the services that London has to offer. As Martin puts it in the last sentences of the book, “All one can say is: the City will survive, and prosper. It usually does.”

Crash Bang Wallop is not flawless. (One of its amusing errors is to refer, in the context of a discussion of the difficulties faced by the firm Slater Walker, to one of its founders as Jim Walker, a name that neither Jim Slater nor Peter Walker, the actual founders, would be likely to recognise.) Yet it is a thoroughly readable account of one of the most important and far-reaching decisions of modern government, and a timely reminder of how the City of London got to where it is now.

Michael Howard is a former leader of the Conservative Party

This article first appeared in the 20 October 2016 issue of the New Statesman, Brothers in blood