Physicist Paul Davies responds to “Mr. Thursday,” Emily St. John Mandel’s short story about time travel.
The charm of time travel stories is that the narratives are at once easy to imagine and yet preposterous. As a theoretical physicist with a lifelong research interest in the nature of time, I am often asked by people: Can it really be done? The short answer is, yes—in a sense.
When Albert Einstein published his special theory of relativity in 1905, it made clear that travel into the future is not only possible, but a done deal. One merely has to move fast. Although any degree of relative motion will create temporal mismatches, it is the speed of light that sets the scale. If you travel close to light speed, time gets seriously warped. For example, suppose you want to reach Earth year 2100 in just one year. A yearlong spaceship journey (as experienced by you) at 99.993 per cent of the speed of light would do the trick. You would come back to Earth just one year older to find it was now 2100 at home. In effect, you will have been propelled 82 years into Earth’s future.
There is another way to leap ahead in time: go somewhere with a higher gravitational field. Clocks tick faster in space than on the Earth’s surface, for example. The effect can be directly measured using sensitive clocks but is today commonplace because the GPS satellite-based navigational system must factor in both the speed of the satellites and the lower gravity of orbit. Time warps are real—they’re a matter of practical engineering.
Of course, with existing technology, the time shifts are disappointingly small. Plane travel, for example, creates temporal dislocations measured in a few billionths of a second, which hardly makes for a Dr. Who–style adventure. The other snag is that it is a one-way journey. You can only go forward in time—that is, reach the future sooner. You can’t come back again. Neither can you visit the past, which from the science-fiction standpoint is where the real fascination lies.
Nothing in Einstein’s theory of relativity, however, specifically forbids travel into the past, but physicists are divided over whether to take the possibility seriously. Einstein himself found the notion highly disturbing. One thing on which physicists agree: Going back in time, if not outright impossible, would be stupendously hard to achieve. Proposals abound based on wormholes in space, cosmic strings, and colossal spinning cylinders, but they all seem to necessitate supertechnology and require massive amounts of energy.
Technological barriers and costs notwithstanding, visiting the past would also seem to unleash all manner of paradoxes, which is where the science and the fiction so productively meet. In Emily St. John Mandel’s engaging story, the time traveler, Mr. Thursday, tries to change the past and save a young woman’s life. The tale ends with a rueful discussion in a bar about the consequences of altering anything in history. “Even the smallest thing, you know, you walk through a door ahead of someone …” says the singer. The movie Sliding Doors starring Gwyneth Paltrow examined precisely this theme by presenting two very contrasting narratives flowing from a seemingly minor incident of this sort. In fact, it is easy to imagine more startling examples. It is merely necessary for a person to be displaced by a billionth of a centimeter to alter history, if in so doing a cosmic ray hits a key atom of DNA and induces cancer. If that person were, say, the young Adolf Hitler, the world today would be a very different place.
Causal loop paradoxes might seem to scupper the whole idea of visiting the past, suggesting that any interaction, however tiny, between the time traveler and the past world would be inconsistent with the future from which he had come. But there is a loophole: quantum mechanics.
Quantum mechanics is a description of all of nature, but its effects are most dramatic at the atomic level. The characteristic features are indeterminism and uncertainty: fire an electron at an atom and observe it bounce to the right. Repeat the process under identical conditions and next time it may bounce to the left. It is impossible to know in advance what is going to happen, although the relative probabilities can be accurately calculated. Quantum phenomena imply that the future is undetermined and open.
Less well known is that the inherent fuzziness of quantum mechanics applies to the past as well as the future. Contrary to common sense, there is no fixed and well-defined history stretching back from the present state of the world to the big bang origin of the universe. Rather, there is a multiplicity of contending histories, co-existing in a ghostly superposition of half-realities. Observations made today can nail down certain specific events in the past, like, for example, detecting a cosmic ray may determine its point of origin thousands of years ago. But most of the past, at least at the micro-level, remains not just unknown to us but intrinsically undetermined.
The existence of multiple parallel realities opens the way for time travel. If there is no “fact of the matter” about some aspect of the past, then there is no paradox if a person journeys back in time and resolves something hitherto fuzzy—that is, if the action of a time traveler in the past concretizes a history that would otherwise be left undetermined. It is important to understand that the past cannot be changed, but it can be made less vague, less ambiguous. Intriguingly, experiments with photons (in the present!) demonstrate how a measurement made at one moment affects the nature of reality at a past moment by resolving its quantum fuzziness, even though the measurement cannot alter the past state. So this weird aspect of quantum mechanics, closely related to what Einstein called “spooky action,” is well established.
But there is a caveat: Whatever the time traveler does in the past, it must be consistent with the state of the world from which she or he has come. Thus it is not possible to go back in time and kill your mother before you were born. But there may be no problem about going back to save a girl from being murdered, and having that girl become your mother, because it forms a self-consistent narrative.
Of course, this means the time traveler does not have unrestricted freedom to do as he pleases. He is hemmed in by the laws of physics. But that is nothing new. After all, I might want to walk on the ceiling, yet the laws of physics forbid it—I’ll simply fall. When causal loops are involved, the restrictions on what is or is not possible will be more severe. What would happen if you tried to kill your mother in the past? Would the knife fall down? Would you simply be unable to lift your arm? Many things could go wrong. But there is nothing strictly paradoxical about someone being part of his own past. So Mr. Thursday may not be able to prevent the accident, but he might have been able to exploit quantum fuzziness to tweak the details of how it happened.
According to Mr. Thursday, time travel would eventually become illegal. But it’s the laws of physics, not the laws of humans, that ensure the universe remains rational, even if it is indeterministic.