Time Travel. A History

Let’s say that one day in 1811, in the town of Teplice, northwestern Bohemia, a man named Ludwig inks a note on a stave in his sketchbook. On an evening in 2011, a woman named Rachel blows a horn in Boston Symphony Hall, with a measurable effect: the air in that room vibrates with a predominant wavelength of 444 cycles per second. Who can deny that, at least in part, the note on paper caused the atmospheric vibrations two centuries later? Using the laws of physics, the path of influence from those molecules in Bohemia to the molecules in Boston would be challenging to compute, even given Laplace’s mythical “intelligence which could comprehend all the forces.” Yet we can see an unbroken causal chain. A chain of information, if not matter.

Russell did not end the conversation when he declared notions of causality to be relics of a bygone age. Not only do philosophers and physicists continue to wrangle over cause and effect, they add new possibilities to the mix. Retrocausation is now a topic: also known as backward causation or retro-chronal causation. Michael Dummett, a distinguished English logician and philosopher (and reader of science fiction), seems to have given this branch its start with his 1954 paper, “Can an Effect Precede Its Cause?” followed ten years later with his less tentative “Bringing About the Past.” Among the questions he raised was this. Suppose he hears on the radio that his son’s ship has sunk in the Atlantic. He prays to God that his son should be among the survivors. Has he blasphemed by asking God to undo what has been done? Or is this prayer functionally identical to praying in advance for his son’s safe passage?

What might inspire modern philosophers, against all precedent and tradition, to consider the possibility that effects might precede causes? The Stanford Encyclopedia of Philosophy offers this answer: “Time Travel.” Indeed, all the time-travel paradoxes, births and murders alike, stem from retrocausality. Effects undo their causes.

The first main argument against the causal order being the temporal order is that temporally backwards causation is possible in cases such as time travel. It seems metaphysically possible that a time traveler enters a time machine at time t₁, thus causing her to exit the time machine at some earlier time t₀. Indeed, this looks to be nomologically possible, since Gödel has proved that there are solutions to Einstein’s field equations that permit looping pathways.

Not that time travel settles the matter. “A variety of incoherencies might be alleged here,” the encyclopedia cautions, “including the incoherency of changing what is already fixed (causing the past), of being both able and unable to kill one’s own ancestors, or of generating a causal loop.” Brave writers are willing to risk an incoherency or two. Philip K. Dick ran the clocks backward (as it were) in Counter-Clock World, and so did Martin Amis in Time’s Arrow.

We do seem to be traveling in circles.

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“THE RECENT RENAISSANCE of wormhole physics has led to a very disturbing observation,” wrote Matt Visser, a mathematician and cosmologist in New Zealand in 1994 in Nuclear Physics B (the forking path of Nuclear Physics devoted to “theoretical, phenomenological, and experimental high energy physics, quantum field theory and statistical systems”). Evidently the “renaissance” of wormhole physics was well established, though these supposed tunnels through spacetime remained (and remain) entirely hypothetical. The disturbing observation was this: “If traversable wormholes exist then it appears to be rather easy to transform such wormholes into time machines.” It was not just disturbing. It was extremely disturbing: “This extremely disturbing state of affairs has led Hawking to promulgate his chronology protection conjecture.”

Hawking is, of course, Stephen Hawking, the Cambridge physicist who by then had become the world’s most famous living scientist, in part because of his dramatic decades-long struggle with an inexorably paralyzing motor neuron disease and in part because of his flair for popularizing the knottiest problems of cosmology. No wonder he was attracted to time travel.

“Chronology Protection Conjecture” was the title of a paper he wrote in 1991 for Physical Review D. He explained the motivation as follows: “It has been suggested that an advanced civilization might have the technology to warp spacetime so that closed timelike curves would appear, allowing travel into the past.” Suggested by whom? An army of science-fiction writers, of course, but Hawking cited the physicist Kip Thorne (yet another Wheeler protégé) of the California Institute of Technology, who had been working with his graduate students on “wormholes and time machines.”