Physics: The time lord and fellow travellers

As television's time-bending Doctor Who turns 50, Andrew Jaffe explores time travel in fiction and science.

In 1963, an extraterrestrial burst from his time machine and onto British television screens to the strains of a deliciously eerie electronic theme tune, courtesy of the BBC's pioneering Radiophonic Workshop. Doctor Who was born. The series' doughty and eccentric time lord has been zipping from past to future ever since in his TARDIS (Time and Relative Dimension in Space). Regenerated 11 times by new actors — including the latest, Peter Capaldi — the Doctor is going stronger than ever since the show's 2005 reboot.

Generations of fans in numerous countries have fallen for his world-saving, time-hopping antics. But in much science fiction the possibilities of time travel are taken more seriously: its creators try to build a coherent set of rules. Because once you allow the capacity to change past or future, in the real world or in fiction, anything seems possible. For more than a century, time travel has been a rich vein for science-fiction writers and even some scientists — especially those willing to travel farthest from the known laws of physics.


Robert Heinlein's short story “—All You Zombies—” is noted for its rigorous internal logic: all the main characters are the same individual at different times in his/her subjective life. The character travels back and forth through time, changing sex and becoming both of his own parents. In a sense, the story is completely self-consistent: cause and effect seem to be preserved, from the protagonist's point of view; life progresses, albeit with science-fictional sex changes and time travel. But something — that is, someone — has been created from nothing, seemingly violating the local laws of physics (and biology).

Amazingly, this kind of time travel is not obviously forbidden by the laws of physics on a global scale. Einstein's general theory of relativity allows 'closed timelike curves' in which a particle can travel back to the same space-time event at which it began. Travelling along such a curve, everything seems fine. But to other observers, something or someone seems just to pop into and out of existence.

This makes physicists very uncomfortable, so theoretical physicist Igor Novikov and collaborators proposed a 'self-consistency principle' in which time travel is possible. Such a trip must be free of paradoxes, and have a single, coherent four-dimensional view of space and time: travel into the past can happen only if it occurred in the Universe's past! Mathematically, we would lose the ability to make predictions in such a Universe, or parts of it: we don't have enough information about the future to know whether a time traveller will emerge in the present. (This also makes physicists very uncomfortable.)

The US television series Lost codified the self-consistency principle as 'Whatever Happened, Happened': even an atomic bomb exploded by the castaways cannot change the past and bring them home. The 1995 film 12 Monkeys (or its 1962 progenitor La Jetée) similarly plays with the chronology of a single event: a character sees himself release an apocalyptic virus and usher in the very future he was sent back in time to prevent.

But maybe time travel can occur in other ways; perhaps it is possible to change the past after all. The Star Trek episode 'City on the Edge of Forever' has Dr McCoy travelling back in time to Depression-era America. There he saves the life of a woman, thus changing the future so that the Enterprise is never built. So Kirk and Spock travel back in time and change the past, to save the present, at the price of annihilating the woman with whom Kirk has fallen in love.

In Charles Stross's 2009 sci-fi novella Palimpsests (much influenced by Isaac Asimov's The End of Eternity and 1930s sci-fi pioneer Olaf Stapledon's grander cosmic visions), each intervention in the past revises present and future. In this scenario, when you kill your ancestor, the Universe becomes one in which you were never born. So you never went back in time, so you didn't kill your ancestor, so you were born, so you were able to travel through time, so you did kill your ancestor, so .... Stross makes a virtue of this: the initiation into his Stasis, a sort of universal time-police, is to go back and kill your own grandfather.

The other change-the-past trope is to make today's world a better place by getting rid of some of its more evil past denizens — Desmond Warzel's short story Wikihistory ( is the funniest version of this I've seen: newbie time travellers keep killing Hitler, so the gurus have to go back and fix the past each time. Or perhaps each intervention cleaves off a new Universe, as in the so-called many-worlds interpretation of quantum mechanics (itself a rich source of scientific and science-fictional ideas, as discussed previously in these pages; Indeed, it seems that understanding the possible (or impossible) physics of time travel will require a full understanding of the 'theory of everything', marrying general relativity and quantum mechanics.

In any scenario, it seems impossible to have time travel without paradoxes or violations of physical laws. So some physicists have theorized that a corollary to the fundamental laws may be that time travel is effectively impossible. In some varieties of the principle, any time machine is censored, hidden inside a black hole formed as a side effect to its creation, walled off from the rest of the Universe by an event horizon. Stephen Hawking has come up with a stronger version, the 'chronology protection conjecture': the laws of physics, relativistic and quantum-mechanical, conspire to prevent time machines' construction (or natural occurrence) in the first place.

Some take a more nuanced, if less physically plausible, approach. Stephen King's book 11/22/63 is premised on attempts to change the history of the day on which President John F. Kennedy was shot. “There's a kind of a rule that you'd express as a ratio,” King told Wired magazine. “The more potential a given event has to change the future, the more difficult that event would be to change.” But not all fictional time travel needs to involve material bodies and the problem of causality. In Kurt Vonnegut's Slaughterhouse-Five, it is Billy Pilgrim's consciousness that has “come unstuck in time” and travels between upstate New York, the planet Tralfamadore and the firebombed city of Dresden.

More than a century ago, writers were already using time travel for dramatic ends. Mark Twain's A Connecticut Yankee in King Arthur's Court skewers the American technophiles of the 1880s as much as the early-medieval Brits whose world he enters. And H. G. Wells's The Time Machine is a polemic on the social stratification of late-Victorian Britain, couched in the language of extra dimensions that would inform Einstein's relativistic merger of space and time in the following decades.

In 1899, the playwright Alfred Jarry leapt off from Wells's ideas to make time travel part of his knowingly absurd 'pataphysics', in his pamphlet Commentary and Instructions for the Practical Construction of the Time Machine. As part of the Beyond Entropy project with the Architectural Association in London, architect Shin Egashira and I tried to realize some of Jarry's machine. Alas, our success, if any, was aesthetic rather than technological.

And then, of course, there is the time lord himself. Paradoxes rarely trouble the Doctor. Time travel serves mostly as a plot device allowing him to visit humans (much easier on the special-effects budget than aliens) in different circumstances, from the recognizable past to the distant future, defeating his enemies again and again. More recently, however, the show has attempted some sort of cross-temporal continuity, even when this means retroactively changing the past and future to bring his nemeses, the Daleks, back from the dead.

The creators of Doctor Who have tended to favour thrills and chills over scientific (or pseudoscientific) precision. But they have also inspired millions to ponder profound questions about the nature of space and time and our movements through them. Here's to the next 50 years.

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Jaffe, A. Physics: The time lord and fellow travellers. Nature 502, 620–621 (2013).

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