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.

Photo: Getty
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Out with the old: how new species are evolving faster than ever

A future geologist will look back to the present day as a time of diversification, as well as extinction.

Human population growth, increased consumption, hunting, habitat destruction, pollution, invasive species and now climate change are turning the biological world on its head. The consequence is that species are becoming extinct, perhaps faster than at any time since the dinosaurs died out 66 million years ago. This is an inconvenient truth.

But there are also convenient truths. Britain has gained about 2,000 new species over the past two millennia, because our predecessors converted forests into managed woodlands, orchards, meadows, wheat fields, roadsides, hedgerows, ponds and ditches, as well as gardens and urban sprawl, each providing new opportunities.

Then we started to transport species deliberately. We have the Romans to thank for brown hares and the Normans for rabbits. In the 20th century, ring-necked parakeets escaped from captivity and now adorn London’s parks and gardens.

Climate warming is bringing yet more new species to our shores, including little egrets and tree bumblebees, both of which have colonised Britain in recent years and then spread so far north that I can see them at home in Yorkshire. Convenient truth No 1 is that more species have arrived than have died out: most American states, most islands in the Pacific and most countries in Europe, including Britain, support more species today than they did centuries ago.

Evolution has also gone into overdrive. Just as some species are thriving on a human-dominated planet, the same is true of genes. Some genes are surviving better than others. Brown argus butterflies in my meadow have evolved a change in diet (their caterpillars now eat dove’s-foot cranesbill plants, which are common in human-disturbed landscapes), enabling them to take advantage of a warming climate and spread northwards.

Evolution is a second convenient truth. Many species are surviving better than we might have expected because they are becoming adapted to the human-altered world – although this is not such good news when diseases evolve immunity to medicines or crop pests become resistant to insecticides.

A third convenient truth is that new species are coming into existence. The hybrid Italian sparrow was born one spring day when a male Spanish sparrow (the “original” Mediterranean species) hitched up with a female house sparrow (which had spread from Asia into newly created farmland). The descendants of this happy union live on, purloining dropped grains and scraps from the farms and towns of the Italian peninsula. Some of those grains are wheat, which is also a hybrid species that originated as crosses between wild grasses in the Middle East.

This is not the only process by which new species are arising. On a much longer time scale, all of the species that we have released on thousands of islands across the world’s oceans and transported to new continents will start to become more distinct in their new homes, eventually separating into entirely new creatures. The current rate at which new species are forming may well be the highest ever. A future geologist will look back to the present day as a time of great diversification on Earth, as well as a time of extinction.

The processes of ecological and evolutionary change that brought all of Earth’s existing biological diversity into being – including ourselves – is continuing to generate new diversity in today’s human-altered world. Unless we sterilise our planet in some unimagined way, this will continue. In my book Inheritors of the Earth, I criss-cross the world to survey the growth in biological diversity (as well as to chart some of the losses) that has taken place in the human epoch and argue that this growth fundamentally alters our relationship with nature.

We need to walk a tightrope between saving “old nature” (some of which might be useful) and facilitating what will enable the biological world to adjust to its changed state. Humans are integral to Earth’s “new nature”, and we should not presume that the old was better than the new.

“Inheritors of the Earth: How Nature Is Thriving in an Age of Extinction” by Chris D Thomas is published by Allen Lane

This article first appeared in the 20 July 2017 issue of the New Statesman, The new world disorder