The possibility of time travel has fascinated humanity for centuries, fueling both literary imagination and scientific debates. In a study published in Classical and Quantum Gravity, physicist Lorenzo Gavassino explores the thermodynamic and quantum foundations that would sustain life and physical processes in a universe with closed timelike curves (CTCs). Through rigorous analysis, the author presents a surprising scenario that combines quantum mechanics and relativity to demonstrate that, although possible, time travel would be drastically different from what fiction has portrayed.

In simple terms, a CTC is a path in spacetime that allows an object to return to its starting point on its own timeline. In the universe described by Einstein’s general relativity, these curves can arise in certain models, such as Gödel-type universes. In these, the geometry of spacetime has unusual properties that allow for the existence of circular paths where time flows as a closed dimension.

Gavassino uses this framework to analyze what happens when a physical system—such as a spacecraft—travels in a CTC. According to the author, the thermodynamic and quantum implications of such travel generate fascinating phenomena, such as the spontaneous inversion of the time’s entropy arrow and the impossibility of classical paradoxes, such as encountering younger versions of oneself.

A key pillar of Gavassino’s work is the self-consistency principle. This principle holds that, in a universe with CTCs, all events must occur in a way that does not create contradictions. This eliminates famous paradoxes like the “grandfather paradox”—where a time traveler could prevent their own birth by altering the past. According to Gavassino, the laws of quantum mechanics automatically guarantee this self-consistency.

The paper mathematically demonstrates that any quantum system traveling in a CTC must return to its initial state when completing the loop. This result, derived from fundamental theorems such as Wigner’s, establishes that the energy levels of a system in a CTC are discretized spontaneously. This means that the possible energies of the system are quantized, separated by precise intervals that depend on the duration of the temporal curve.

Time travel paradoxes
Time travel would look nothing like science fiction movies. Credit: Universal

Thermodynamics, and particularly entropy, plays a central role in Gavassino’s analysis. Entropy, a measure of disorder in a system, tends to increase over time, marking the difference between the past and the future. However, in a CTC, entropy must return to its initial state when the loop is completed, implying a spontaneous reversal of the time’s arrow.

The author illustrates this phenomenon with a simple model: an unstable particle that disintegrates into lighter particles while traveling through the CTC. Although statistically improbable, the quantum laws dictate that, upon completing the loop, the light particles must spontaneously recombine to form the initial particle again, thus reversing the entropic process.

One of the most intriguing aspects of the work is the idea that biological and mental processes, such as the formation of memories, are also affected by the properties of CTCs. Gavassino argues that any memory or record created during the journey must be erased before the system returns to its initial state. This process ensures that no contradictions occur in the timeline, but it also raises profound questions about the nature of causality and personal identity in a universe with time travel.

Additionally, the author introduces the concept of the “minimum entropy event”, a point in the CTC where entropy is lowest and where order in the system seems to arise spontaneously without apparent cause. According to Gavassino, this event challenges our macroscopic notions of causality, as complex structures, like brains or books, could appear “out of nowhere” at these points due to statistical fluctuations.

A common question in science fiction is whether it would be possible to meet a younger version of oneself during a time travel journey. According to Gavassino, the quantum and thermodynamic laws make this scenario extremely unlikely. Any “future version” of a time traveler appearing in a CTC would, at best, be an illusory copy or “clone” generated by statistical fluctuations, not a direct causal extension of the traveler themselves.

Although Gavassino’s paper does not directly advocate for the existence of CTCs, it offers a fascinating view of how they could operate in a hypothetical universe. Beyond the mathematical complexities, his work highlights the ability of modern physics to address questions that once seemed relegated to philosophy or fiction.

The main message of the author is clear: if time travel were possible, it would be nothing like the popular representations in literature or film. Instead, it would be governed by the strict rules of quantum mechanics and thermodynamics, with consequences as strange as they are surprising. Ultimately, time is not merely a linear dimension we can manipulate at will, but an entity deeply intertwined with the fundamental laws of the universe.

SOURCES

L Gavassino, Life on a closed timelike curve. Class. Quantum Grav. 42 015002. DOI 10.1088/1361-6382/ad98df

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