Stephen Hawking proposed a "chronology protection conjecture" - that the laws of physics somehow conspire to prevent time travel and protect causality. It was half-serious, half-joke, and completely unproven 30 years later.
Christopher Michel, CC BY-SA 4.0, via Wikimedia Commons
The problem with the past
Einstein's general theory of relativity allows for some strange possibilities. The mathematics that describes gravity and the curvature of spacetime also contains solutions with "closed timelike curves" (CTCs). A CTC is a path through spacetime that loops back on itself, allowing an object or person to return to their own past and potentially create paradoxes, like the classic grandfather paradox.
Physicists have conceived of several theoretical ways to create these CTCs. These include the Gödel metric, based on a rotating universe, and immense, infinitely long spinning cylinders of matter called Tipler cylinders. More famously, in the 1980s, physicist Kip Thorne and his colleagues explored the physics of traversable wormholes. Prompted by a request from Carl Sagan for his novel Contact, Thorne's work showed that these spacetime tunnels could, in theory, be manipulated to connect two different places, but two different times. This proliferation of physically plausible and technologically impossible time machines created a serious problem for causality.
A cosmic protection agency
In a 1992 paper, Stephen Hawking proposed a solution he called the "Chronology Protection Conjecture". His idea posits that the laws of physics themselves intervene to prevent CTCs from forming on a macroscopic scale. He humorously suggested the existence of a "Chronology Protection Agency" that "makes the universe safe for historians".
The proposed mechanism relies on the interaction between general relativity and quantum mechanics. According to the conjecture, as one attempts to warp spacetime to create a time machine—for example, by manipulating a wormhole—the threshold to the past would be flooded with energy. Quantum vacuum fluctuations, the sea of virtual particles that exist even in empty space, would enter the time loop. These particles would circle through the wormhole endlessly, becoming real and accumulating an infinite amount of energy. This runaway energy density, technically a divergence in the expectation value of the renormalised stress-energy tensor, would create a gravitational field so intense that it would destroy the wormhole right at the moment it was about to permit time travel.
The conjecture remains unproven and is debated. Subsequent research has suggested that in some models, the energy buildup might not become infinite, potentially allowing for a time machine to form. The construction of a traversable wormhole in the first place requires a huge amount of "exotic matter" with negative energy density to keep it open, something which may not be physically possible in sufficient quantities. An alternative idea, Igor Novikov's self-consistency principle, proposes that time travel might be possible, but only in ways that are not paradoxical; any attempt to change the past would be guaranteed to fail.
💡Fun Facts
On June 28, 2009, Stephen Hawking held a champagne party for time travelers at the University of Cambridge, but only sent out the invitations *after* the party had concluded. No one attended.
The idea for a "Chronology Protection Agency" was a playful reference to science fiction concepts like the Time Patrol from Poul Anderson's stories or Isaac Asimov's novel *The End of Eternity*.
Kip Thorne's serious work on traversable wormholes began after his friend Carl Sagan asked him for a plausible method of faster-than-light travel for the protagonist in his 1985 novel, *Contact*.
Hawking often cited the lack of tourists from the future as "strong experimental evidence" in favor of his conjecture.
The laws of physics operate 24/7. The University of Cambridge Department of Applied Mathematics and Theoretical Physics, where Hawking worked, keeps standard university hours.
Admission
Free to contemplate. Physical access to university buildings may be restricted to students and staff.
Accessibility
This is a theoretical concept. The physical location in Cambridge is generally flat with paved surfaces.
