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Last week I explained how Albert Einstein’s special theory of relativity makes time travel into the future possible—and how varied solutions to his general theory of relativity open the door to traveling into the past. One of the most memorable solutions comes from a brilliant friend and colleague of Einstein whom I’ve written about in this newsletter before (here and here): Kurt Gödel.
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In 1949, as a birthday present for Einstein, Gödel developed a solution that would permit timelike curves. As mentioned in the last newsletter, if you were to travel along these curves, you would start at one point in spacetime, move into the past and ultimately end up back at your temporal starting point.
The universe described by Gödel’s solution was bizarre, however. It consisted of dustlike particles that moved like a liquid in a spacetime with a negative cosmological constant. He specifically designed it this way to make travel into the past possible.
This led physicists to assume that timelike curves were a kooky artifact of unrealistic solutions to general relativity. But in 1988 future Nobel laureate Kip Thorne and his colleagues found a new kind of solution to Einstein’s field equations: traversable wormholes, which would allow time travel in realistic models of the universe. This rekindled scientists’ interest in time travel.
Wormholes as a Time Machine
The possible existence of closed timelike curves raises many questions. Consider one posed in French writer René Barjavel’s 1944 novel Future Times Three: What if a person travels into the past and causes the death of their grandfather before he could father any children? In that case, the time traveler cannot exist, creating a paradox.
Russian physicist Igor Dmitriyevich Novikov grappled with such questions and formulated his self-conformity principle: a time traveler can influence the past but cannot change it. Thus, the time traveler’s intervention cannot transform the future into a different one from which they started. This principle has been explored in many TV shows and films, including Doctor Who, The Umbrella Academy, 12 Monkeys and the German TV series Dark. The protagonists attempted to alter the past, but their actions ultimately caused the very events they originally sought to prevent.
To support the self-conformity principle, Thorne and several of his students developed an exemplary model of the grandfather paradox described by Barjavel. Imagine a ball rolling through a wormhole and into the past that emerges in such a way that it collides with its future self, thus preventing it from rolling into the wormhole in the first place. The question that occupied the physicists was: For every initial condition—position and initial velocity of the ball—is there a solution in which the described scenario does not occur? In other words, could the ball always roll out of the wormhole in such a way that it would collide with its future self but the future self would still find its way into the hole? Indeed, the calculations by Thorne’s group from 1991 seemed to confirm precisely that.
They were unable to construct suitable initial conditions that would allow the ball to inevitably prevent its future self from rolling into the wormhole. This conclusion allowed them to strengthen their hypothesis: self-consistent solutions for time travel always seem possible. But they could not prove it.
As they discovered, for certain initial situations, several self-consistent scenarios were possible—in some cases, even infinitely many. But this led to the question of which scenario would actually occur. As if that weren’t complicated enough, Thorne and his students also utilized quantum mechanics: they assumed that the ball is in a superposition of all consistent possibilities at the moment that it exits the wormhole. This means that the ball doesn’t fall out at precisely one speed or another but rather at all of them simultaneously. The researchers were still able to show that this version is also compatible with the physics of the wormhole and thus with general relativity.
A Canceled Party for Time Travelers
Although this solution satisfied some physicists, it wasn’t enough for the late Stephen Hawking. In 1992 he formulated his “chronology protection conjecture,” according to which physical laws of a (still undefined) fundamental theory would prevent macroscopic objects from traveling backward in time. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” he wrote.
This idea, too, was not entirely new and can be found, among other places, in Isaac Asimov’s work The End of Eternity. There, an organization outside of time, “Eternity,” intervenes to ensure that all historical events actually occur as they already have.
To underscore his point, Hawking hosted a lavish party with champagne, canapés and balloons on June 28, 2009—but nobody showed up. The public invitation was only sent out after the party took place and was therefore aimed at time travelers. For the physicist, this was a clear indication that travel into the past is impossible.
This article originally appeared in Spektrum der Wissenschaft and was reproduced with permission. It was translated from the original German version with the assistance of artificial intelligence and reviewed by our editors.
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