Physicist Claims Breakthrough on Time Travel’s “Grandfather Paradox”—But With a Surprising Twist!

A physicist is convinced he might have addressed the infamous “grandfather paradox,” indicating that time travel to the past might not be entirely dismissed by this specific area of physics.

To begin with, what does the grandfather paradox entail? Unlike the bootstrap paradox, which can get somewhat convoluted, the grandfather paradox is quite straightforward to clarify. Imagine possessing a time machine and a penchant for family slaughter; you could travel to the past and try to eliminate your grandfather before he had any offspring. If you succeeded, your parent would never exist, leading to your nonexistence and thus, you would not be able to travel back in time to kill your own grandfather.

It serves as a thought experiment, of course, but one suggesting that time travel to the past may be unfeasible, as it could result in contradictions within the universe. Due to this, Stephen Hawking proposed the chronology protection conjecture, or the notion that uncharted laws of physics may exist to prevent time travel from occurring.

Nonetheless, based on the physics we currently understand, time travel to the past is not yet dismissed. One theory stemming from Einstein’s work posits that “closed timelike curves” could potentially exist, where spacetime is warped (whether intentionally or naturally, such as around a supermassive black hole) in such a way that an object or observer traversing it would find themselves back at their original position.

“It is commonly assumed that, in a Universe with Closed Timelike Curves (CTCs), individuals can ‘travel to the past.’ On the surface, this appears to be a clear implication, since (on sufficiently large scales) one might consider a timelike curve as the worldline of an imaginary spacecraft moving through spacetime,” wrote Lorenzo Gavassino, a physicist at Vanderbilt University, in his new paper.

“If such a curve forms a loop, the spacecraft would return to its original spot, in its past. However, to validate that this is genuinely a journey to the past, we must first examine the effects on the passengers (i.e., on macroscopic particle systems) as they complete the roundtrip.”

The laws of physics are generally time-reversal invariant, meaning they would appear the same if played backward. This, however, doesn’t apply to the second law of thermodynamics, which is an observed statistical principle of the universe. In simple terms, everything tends towards chaos. Heat transfers from hotter regions to cooler areas, and in a closed system, entropy – representing disorder within that system – can only rise. The second law of thermodynamics provides us with a temporal direction. If you observe a system moving toward disorder, you can confidently assume it is advancing in time. You cannot reverse the cooking of an egg.

In this recent study, Gavassino endeavored to articulate what occurs thermodynamically if a spacecraft were to navigate a closed time-like curve, and emerge either when they set off or earlier. Throughout this journey, entropy must escalate towards thermodynamic equilibrium according to the second law of thermodynamics, but to maintain a consistent universe, it must also revert to the non-equilibrium state prior to entering the time-like curve. Taking a single unstable particle bouncing around a spacecraft as an example, he notes that the laws of physics as we comprehend them necessitate the particle returning to its original state.

“As expected, the particle spontaneously decays near τ = 0 and remains decayed for nearly the entirety of the journey. However, as τ approaches [the beginning of the loop], the particle is spontaneously reconstructed within the same duration it took to decay,” Gavassino clarifies. “This phenomenon directly results from the discretization of energy levels and does not compel us to finely tune the initial conditions.”

In his research, he posits that there exists a moment when entropy reaches its peak, “and the second law of thermodynamics begins to reverse.” All of this is intriguing, but here’s the twist; according to the findings, you would lose all recollection of any events occurring during the loop.

“Memory can be abstractly modeled as the outcome of an interaction, where an object leaves marks of its initial condition in the subsequent state of a ‘memory-keeper’, which could be a measurement device or a living entity,” he explains.

By modeling the memory collection process, he discovers that “any memory accumulated along the CTC [closed timelike curve] will be obliterated by Poincaré recurrence before the loop concludes.” You may traverse the loop, yet you will retain no new information; in a consistent universe, entropy will return you to your initial state unchanged. Regarding the grandfather paradox, the universe would evolve in such a manner that ensures it remains self-consistent, returning time travelers to their original conditions.

“Historically, most physicists and philosophers have contended that if time travel is a reality, nature will invariably devise a method to avert contradictory scenarios,” Gavassino stated in an interview with Live Science. “A ‘self-consistency principle’ has been proposed, indicating that everything should conform to construct a logically coherent narrative. My research marks the first rigorous derivation of this self-consistency principle directly rooted in established physics. Specifically, I applied the conventional framework of quantum mechanics – without additional assumptions or controversial propositions – and demonstrated that the self-consistency of history emerges naturally from quantum laws.”

Although it is an entertaining notion, he does not regard it as definitive evidence for the potential existence of closed time-like curves.

“Instead, the fundamental takeaway is that, within a hypothetical Universe with CTCs, time travel would not occur in the manner typically portrayed in science fiction. Indeed, in CTCs, thermal fluctuations disrupt macroscopic causation and erase all memories,” he concludes. “As often observed, Nature exhibits greater creativity than humankind.”

The paper has been published in Classical and Quantum Gravity.

[H/T: Live Science]