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 Autumn Statement proved it – we need a real alternative to austerity, now

Theresa May’s Tories have missed their chance to rescue the British economy.

After six wasted years of failed Conservative austerity measures, Philip Hammond had the opportunity last month in the Autumn Statement to change course and put in place the economic policies that would deliver greater prosperity, and make sure it was fairly shared.

Instead, he chose to continue with cuts to public services and in-work benefits while failing to deliver the scale of investment needed to secure future prosperity. The sense of betrayal is palpable.

The headline figures are grim. An analysis by the Institute for Fiscal Studies shows that real wages will not recover their 2008 levels even after 2020. The Tories are overseeing a lost decade in earnings that is, in the words Paul Johnson, the director of the IFS, “dreadful” and unprecedented in modern British history.

Meanwhile, the Treasury’s own analysis shows the cuts falling hardest on the poorest 30 per cent of the population. The Office for Budget Responsibility has reported that it expects a £122bn worsening in the public finances over the next five years. Of this, less than half – £59bn – is due to the Tories’ shambolic handling of Brexit. Most of the rest is thanks to their mishandling of the domestic economy.

 

Time to invest

The Tories may think that those people who are “just about managing” are an electoral demographic, but for Labour they are our friends, neighbours and the people we represent. People in all walks of life needed something better from this government, but the Autumn Statement was a betrayal of the hopes that they tried to raise beforehand.

Because the Tories cut when they should have invested, we now have a fundamentally weak economy that is unprepared for the challenges of Brexit. Low investment has meant that instead of installing new machinery, or building the new infrastructure that would support productive high-wage jobs, we have an economy that is more and more dependent on low-productivity, low-paid work. Every hour worked in the US, Germany or France produces on average a third more than an hour of work here.

Labour has different priorities. We will deliver the necessary investment in infrastructure and research funding, and back it up with an industrial strategy that can sustain well-paid, secure jobs in the industries of the future such as renewables. We will fight for Britain’s continued tariff-free access to the single market. We will reverse the tax giveaways to the mega-rich and the giant companies, instead using the money to make sure the NHS and our education system are properly funded. In 2020 we will introduce a real living wage, expected to be £10 an hour, to make sure every job pays a wage you can actually live on. And we will rebuild and transform our economy so no one and no community is left behind.

 

May’s missing alternative

This week, the Bank of England governor, Mark Carney, gave an important speech in which he hit the proverbial nail on the head. He was completely right to point out that societies need to redistribute the gains from trade and technology, and to educate and empower their citizens. We are going through a lost decade of earnings growth, as Carney highlights, and the crisis of productivity will not be solved without major government investment, backed up by an industrial strategy that can deliver growth.

Labour in government is committed to tackling the challenges of rising inequality, low wage growth, and driving up Britain’s productivity growth. But it is becoming clearer each day since Theresa May became Prime Minister that she, like her predecessor, has no credible solutions to the challenges our economy faces.

 

Crisis in Italy

The Italian people have decisively rejected the changes to their constitution proposed by Prime Minister Matteo Renzi, with nearly 60 per cent voting No. The Italian economy has not grown for close to two decades. A succession of governments has attempted to introduce free-market policies, including slashing pensions and undermining rights at work, but these have had little impact.

Renzi wanted extra powers to push through more free-market reforms, but he has now resigned after encountering opposition from across the Italian political spectrum. The absence of growth has left Italian banks with €360bn of loans that are not being repaid. Usually, these debts would be written off, but Italian banks lack the reserves to be able to absorb the losses. They need outside assistance to survive.

 

Bail in or bail out

The oldest bank in the world, Monte dei Paschi di Siena, needs €5bn before the end of the year if it is to avoid collapse. Renzi had arranged a financing deal but this is now under threat. Under new EU rules, governments are not allowed to bail out banks, like in the 2008 crisis. This is intended to protect taxpayers. Instead, bank investors are supposed to take a loss through a “bail-in”.

Unusually, however, Italian bank investors are not only big financial institutions such as insurance companies, but ordinary households. One-third of all Italian bank bonds are held by households, so a bail-in would hit them hard. And should Italy’s banks fail, the danger is that investors will pull money out of banks across Europe, causing further failures. British banks have been reducing their investments in Italy, but concerned UK regulators have asked recently for details of their exposure.

John McDonnell is the shadow chancellor


John McDonnell is Labour MP for Hayes and Harlington and has been shadow chancellor since September 2015. 

This article first appeared in the 08 December 2016 issue of the New Statesman, Brexit to Trump