The Grandfather Paradox — Why Time Travel Could (and Could Not) Be PossibleAnalysing the Grandfather Paradox & theories & experiments that hold implications for time travel
The concept of time travel has long captivated human imagination, from H.G. Wells' literary works like The Time Machine to the flux-capacitor-fueled escapades of Marty McFly in Back To The Future or the all-action fight against Skynet in The Terminator. Yet, beneath these fictional narratives lies a perplexing conundrum that has both intrigued and confounded physicists and philosophers through time — the Grandfather Paradox. This thought experiment challenges our understanding of causality, free will, and the very fabric of reality. What is the Grandfather Paradox?Imagine you have a time machine and decide to travel back to a time before your parents were born. In your journey back through time, you inadvertently prevent your grandfather from meeting your grandmother (the classical concept suggests killing your grandfather, but let’s roll with preventing your grandparents meeting). So the immediate implication? — One of your parents and consequently, you would never be born. But if you were never born, how could you have traveled back in time to thwart their meeting in the first place? This self-contradictory scenario is the essence of the Grandfather Paradox, highlighting the potential inconsistencies pertinent to time travel. Kurt Gödel and Closed Timelike Curves (or CTCs)In 1949, the logician Kurt Godel presented a solution to Einstein's field equations that described a rotating universe containing closed timelike curves (CTCs). These CTCs are paths in spacetime that loop back on themselves, theoretically allowing an object to return to its own past. Godel's work suggested that, under certain conditions, time travel might be possible within the framework of general relativity. CTCs suggest that if spacetime is warped in specific ways—such as near a rotating black hole—time travel to the past could be possible. If an object could follow a CTC, it might experience time in a closed loop, enabling it to influence past events and potentially creating paradoxes like the Grandfather Paradox. CTCs are mathematically consistent with general relativity, but no experimental evidence for them has been found, leaving their practical existence and implications for time travel purely theoretical. However, the existence of CTCs raises profound questions about causality and the potential for paradoxes like that of the Grandfather Paradox. Gerald Feinberg and TachyonsIn 1967, physicist Gerald Feinberg proposed the existence of tachyons—hypothetical particles that always travel faster than the speed of light. According to special relativity, tachyons would experience time differently, potentially moving backward through time from our perspective. If tachyons exist, they could, in theory, allow information or objects to travel to the past, raising the possibility of time travel. But this creates a problem similar to the Grandfather Paradox—if a tachyon or signal traveling back in time changed the past in a way that stops it from ever existing, it would create a logical contradiction. Even though tachyons are just a theory, their introduction into the realm of time travel doesn't necessarily resolve paradoxes but rather complicates our understanding of causality. Quantum Superposition and Multiple RealitiesQuantum mechanics, with its principles of superposition and entanglement, offers a different lens through which to view time travel. In the quantum realm, particles can exist in multiple states simultaneously until observed—a concept known as superposition. This has led some to speculate whether time travelers might exist in a superposed state, both affecting the past and not affecting it, thus sidestepping paradoxes. However, applying quantum principles to macroscopic events like time travel remains a topic of debate and speculation. The Many-Worlds InterpretationOne of the most captivating ideas to emerge from quantum mechanics is the Many-Worlds Interpretation (MWI). Proposed by physicist Hugh Everett, MWI, in alignment with quantum superposition, suggests that all possible outcomes of quantum events actually occur, each in its own separate, branching universe. In the context of time travel, this implies that altering the past doesn't change the original timeline but rather creates a new, divergent one. So, if you did prevent your grandparents from meeting in one universe, you'd still exist in the original, unaltered timeline i.e. World A, while the altered timeline would exist in a separate parallel universe i.e. World B. This interpretation elegantly circumvents the Grandfather Paradox by proposing a multiverse where all possibilities play out independently. Time travel, as far as we know, remains frustratingly out of reach—perhaps because all successful time travelers are currently trapped in alternate timelines according to MWI. Seth Lloyd's Quantum Time MachineBut Seth Lloyd, a professor of mechanical engineering at MIT and a self-described "quantum mechanic," has been working on the next best thing. In 2010, Lloyd and his team devised a quantum simulation designed to mimic the principles of a time machine by combining closed timelike curves (CTCs) with quantum teleportation—a process where quantum information is transferred between two points using quantum entanglement. Essentially, this would simulate the effect of an object traveling through a CTC. The key twist was the addition of a post-selection mechanism, which allowed the quantum system to deterministically choose certain outcomes, bypassing the usual randomness of quantum mechanics. In effect, this setup created a controlled environment where quantum events could potentially mirror the effects of time travel. In this simulation, Lloyd sent photons a few billionths of a second backward in time, programming them with a mission to "terminate" their earlier states—similar to the Grandfather Paradox. The results were fascinating. The closer a photon got to succeeding in this self-destructive task, the more frequently the experiment failed. This suggests that time itself might have a built-in self-correction mechanism—if any action threatens to disrupt causality, reality seems to intervene and prevent it from happening. In other words, even if time travel were possible, attempts to create paradoxes might simply result in the failure of the journey itself. The universe, it seems, has safeguards against rewriting its own history. What the Grandfather Paradox Teaches UsThe intersection between time travel and the Grandfather Paradox forces us to confront profound questions about determinism, free will, and the nature of reality. If time travel is possible, does it imply a predetermined universe where actions are constrained to prevent paradoxes? Or does it suggest that every choice creates a new branch of reality, spawning endless parallel universes? Science doesn’t have clear answers yet, but just thinking about these possibilities makes us question the very nature of reality and free will. Granddad can rest easy—for now. Thank you for being a valuable subscriber to my newsletter Light Years! If you liked this post & found it informative, feel free to share this publication with your network by clicking the button below… I hope you found this post informative & it helped you in some way. As always, feel free to subscribe to my publication Light Years & support it & also share it if you’d like. 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Friday, 14 March 2025
The Grandfather Paradox — Why Time Travel Could (and Could Not) Be Possible
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