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Previously, I detailed how Albert Einstein’s special theory of relativity enables time travel to the future, while various interpretations of his general theory of relativity allow for travel to the past. One notable solution is attributed to Kurt Gödel, a distinguished friend and colleague of Einstein, whom I’ve discussed in earlier newsletters (here and here).
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In 1949, Gödel presented Einstein with a unique solution as a birthday gift, allowing for timelike curves. Traveling these paths would enable you to start at one point in spacetime, move backward, and eventually return to your original temporal point.
Gödel’s universe was an unusual one, filled with dust-like particles moving like a liquid in spacetime with a negative cosmological constant. This design was intentional to facilitate travel to the past.
Physicists initially considered timelike curves as peculiar anomalies within unrealistic solutions to general relativity. However, in 1988, Kip Thorne and his colleagues discovered traversable wormholes as a new solution to Einstein’s field equations. This discovery reignited interest in time travel.
Wormholes as a Time Machine
The possible existence of closed timelike curves presents intriguing questions. René Barjavel’s 1944 novel Future Times Three poses a classic paradox: What happens if a person travels back in time and inadvertently causes the death of their grandfather before he becomes a parent? This scenario would lead to the nonexistence of the time traveler, creating a paradox.
Russian physicist Igor Dmitriyevich Novikov addressed these paradoxes by proposing the self-conformity principle: a time traveler can influence but cannot alter the past. Consequently, the time traveler’s actions cannot change the future from which they originated. This concept has been explored in various TV shows and films, including Doctor Who, The Umbrella Academy, 12 Monkeys, and the German series Dark, where protagonists attempting to change the past end up causing the very events they sought to prevent.
To validate the self-conformity principle, Thorne and his students devised a model addressing Barjavel’s grandfather paradox. Picture a ball rolling through a wormhole into the past, emerging to collide with its future self, thereby preventing its entry into the wormhole. The question was whether for every initial condition—position and velocity of the ball—there exists a scenario where this does not occur. Could the ball always emerge from the wormhole in a manner that it collides with its future self, yet still finds its way into the hole? In 1991, Thorne’s team’s calculations appeared to support this possibility.
They found it impossible to establish initial conditions that would inevitably stop the ball from entering the wormhole. This reinforced their hypothesis: self-consistent solutions for time travel always seem possible, although proof was elusive.
In certain initial situations, multiple self-consistent scenarios were feasible, sometimes even infinitely so. This raised the question of which scenario would transpire. Adding to the complexity, Thorne and his students incorporated quantum mechanics, proposing the ball exists in a superposition of all consistent possibilities upon exiting the wormhole. This implies the ball doesn’t emerge at a single speed but at multiple speeds simultaneously. Their research indicated this version aligns with both wormhole physics and general relativity.
A Canceled Party for Time Travelers
While some physicists found this solution satisfactory, Stephen Hawking was not convinced. In 1992, he proposed the “chronology protection conjecture,” suggesting that fundamental physical laws would prevent macroscopic objects from traveling backward in time. He humorously remarked that there seems to be a chronology protection agency that keeps historians safe by preventing closed timelike curves.
This notion was not entirely novel and had been explored by Isaac Asimov in The End of Eternity, where a timeless organization named “Eternity” ensures that historical events occur as they originally did.
To illustrate his point, Hawking hosted an extravagant party with champagne, canapés, and balloons on June 28, 2009, but no one attended. Invitations were sent post-event, targeting time travelers. For Hawking, this absence was evidence against the possibility of traveling into the past.
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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