### What's This Book All About? This book is like a window into a conversation that began long ago in the realm of science fiction but has since found a surprising home in the serious world of theoretical physics. It all started for the author with H. G. Wells's classic story, _The Time Machine_, and that iconic scene where the mysterious Time Traveller explains his theories about the fourth dimension to a group of Victorian gentlemen. The allure wasn't just the sensational title, but the possibility of time travel itself and the potential technology to make it real. Wells did a wonderful job painting the picture: the setting, the excited guests, a demonstration with a vanishing model machine. We even get a sense of what riding the full-scale machine feels like – like a frightening roller coaster ride, being hurled headlong through space. But one thing Wells famously left out was _how_ it actually worked. He just glossed over the technical details, which, while understandable for a work of fiction, left the author (as a twelve-year-old reader) a bit dissatisfied. The really exciting part, and the main subject of this book, is the discovery that this drawing-room conversation about the possibility and mechanics of time travel didn't end with Wells; it has been picked up and seriously explored by theoretical physicists and astrophysicists in recent decades. These scientists, often gathering in places that might remind you of that gaslit Victorian room, have been rigorously proposing, testing (through thought experiments!), and debating hypotheses about time travel, drawing upon complex ideas from relativity, quantum mechanics, and even quantum gravity. The book aims to trace this scientific history of the idea of time travel, offer a peek into the lives of these thinkers, and perhaps finally provide some of the "how" that was missing for that curious twelve-year-old. ### From Science Fiction to Science: A Big Shift For a long time, seriously studying time travel wasn't something many scientists were keen to do openly. Scientists have careers to build – dissertations, tenure, funding proposals – and worrying about time machines could easily be seen as trivial or lacking in respectability. While some physicists might have discussed it informally or published papers that hinted at it with phrases like "causality violation," the topic largely remained on the fringes of professional concern. But something significant changed in 1988. That's when a paper titled "Wormholes, Time Machines and the Weak Energy Condition" was published in the journal _Physical Review Letters_. Written by Kip Thorne, Mike Morris, and Ulvi Yurtsever, this paper was groundbreaking. It didn't claim anyone was building a time machine tomorrow – far from it! It acknowledged that doing so would require immense energy or manipulating exotic matter that hasn't even been directly observed. However, the paper made a clear proposal for a _means_ to undertake pastward time travel within the universe as we understand it. This publication was a game-changer. It sent a small shock wave through the theoretical physics community and, crucially, made the subject of time machines respectable. Suddenly, publications on the topic grew from a trickle to a steady stream, and time machines became a subject of discussion at professional meetings. Big names like Frank Tipler, Stephen Hawking, and Kip Thorne were among those driving this renewed inquiry. A major workshop dedicated to the subject was even held at the Aspen Center for Physics in 1992, allowing physicists to talk at length about the physical _and_ philosophical implications of time travel, including potential paradoxes, free will, and alternate universes. Interestingly, this shift wasn't just scientists working in isolation. The book notes a "creative cross-pollination" between science and science fiction. For example, Kip Thorne's serious work on time travel was directly sparked by a question from the famous science fiction author Carl Sagan, who wanted to get the science right for his novel _Contact_. Later, physicist and science fiction author Robert Forward consulted with Thorne, leading Thorne to learn about one of Forward's ideas that he would later cite. It seems that far from being separate worlds, science fiction has measurably influenced science in this area, inspiring new lines of inquiry. ### How Might It Work? Exploring Different Ideas The book delves into various ideas physicists have explored for making time travel a reality, starting with travel into the future, which is actually less complex than traveling to the past. **Futureward Time Travel (Time Dilation)** One of the implications of Einstein's theory of special relativity is something called time dilation. Basically, if you travel at very high velocities (close to the speed of light) or are subjected to a very strong gravitational field, time passes more slowly for you compared to someone in a weaker field or at rest. - **Relativistic Speed:** Imagine an astronaut, Gassendi, traveling to a distant star like Arcturus (about 36 light-years away) at near-light speed and then returning. From the perspective of someone on Earth (his assistant), the round trip might take over 72 years. But because Gassendi was traveling so fast, time would have slowed down significantly _for him_, and he might have aged only a few years. When he returns, his assistant is much, much older, and Gassendi has effectively traveled decades into the future relative to his starting point. This can work with linear motion (like Gassendi's trip) or circular motion at high speeds. - **Strong Gravity:** Time dilation also occurs in strong gravitational fields, like those near black holes. While early thoughts about black holes suggested you'd be torn apart by tidal forces before experiencing significant time dilation, later work indicated that very massive black holes (like those at the center of galaxies) have weaker tidal forces above their event horizon, making it theoretically possible to get close enough to experience extreme time dilation without being instantly destroyed. So, futureward time travel, in a sense, is already predicted by known physics, although achieving it would require technology vastly beyond our current capabilities. Pastward travel, however, is where things get really complex and require combining special and general relativity, alongside vast energies and currently unimaginable technologies. **Pastward Time Travel: Early (Unphysical) Ideas** Before the big shift in 1988, a few mathematicians and physicists made tentative inquiries into pastward travel. - **Van Stockum Cylinder:** In the 1930s, mathematician Willem Van Stockum solved Einstein's field equations for a theoretical object: an infinitely long cylinder made of rapidly rotating dust. He found that this cylinder's rotation would strongly drag spacetime with it, tilting the light cones (which represent the paths of light and causality) so severely that it would be possible to travel or send signals back in time along closed loops, called closed timelike curves (CTCs). While Van Stockum didn't seem to fully realize the time travel implications, his work described a mechanism for it. - **Gödel's Universe:** In 1949, mathematician Kurt Gödel, famous for his incompleteness theorems, discovered another solution to Einstein's field equations that allowed for time travel to the past. His solution described an entire rotating universe where CTCs were possible. - **Tipler Cylinder:** In the 1970s, physicist Frank Tipler revisited Van Stockum's idea, suggesting that a sufficiently large and fast-spinning _finite_ cylinder might also create CTCs. However, he later revised this, concluding that it would still require an infinite length, making it "unworkable" as nothing infinite exists in our known universe. These early ideas were intriguing but considered "unphysical" because they required objects or scenarios (like infinite cylinders or entire rotating universes) that don't seem to exist or be possible in reality. **Pastward Time Travel: More Modern Concepts** The post-1988 era brought new, potentially more "physical" ideas, even if they required technology indistinguishable from magic. - **Wormholes:** Sparked by Carl Sagan's novel, Kip Thorne and his colleagues explored the idea of traversable wormholes. Wormholes can be thought of as theoretical tunnels through spacetime that could connect two distant points, providing a shortcut. The challenge is keeping the wormhole "mouths" open, as they would naturally collapse. This seems to require "exotic matter" with negative energy density, which hasn't been directly observed and violates certain energy conditions that seem to hold true in the universe. However, if you could create and hold open a traversable wormhole, you could then turn it into a time machine. By moving one mouth at nearly the speed of light, you could cause it to age more slowly due to time dilation. When you bring the mouths back together, they would be in different temporal states, creating a pathway to the past (specifically, back to the moment the machine was created). Tiny wormholes might even exist at the quantum level, pulled from the "quantum foam," but enlarging them would also be a massive task. - **Cosmic Strings:** Physicist J. Richard Gott proposed another ingenious method in 1991 that didn't require exotic matter but utilized bizarre, hypothetical objects from the early universe called cosmic strings. These are incredibly dense, thin threads of energy. Gott realized that if two cosmic strings moved past each other at near-light speeds, they would warp spacetime in such a way that you could travel around them and end up back where you started, but earlier in time. Like the wormhole machine, travel would only be possible back to the moment the specific motion of the strings created the time-travel possibility. This proposal was exciting because it used materials believed to exist, suggesting an entire class of time machines might be possible. (Though some physicists later questioned if the total momentum of such a system would exceed the speed of light, potentially undermining the idea in certain universes). These models, while still requiring technology far beyond our reach (sometimes described as needing a "sufficiently advanced civilization" or technology "indistinguishable from magic"), opened the door to the possibility of pastward time travel being allowed _in principle_ by the known laws of physics, specifically general relativity. ### The Problem of Paradoxes Thinking about time travel, especially to the past, quickly brings up the thorny issue of paradoxes – logical contradictions that seem to make it impossible. - **The Grandfather Paradox:** The most famous example is the idea of traveling back in time and preventing your own birth (e.g., by killing your grandfather). If you succeed, then you wouldn't exist to travel back in time, which means you couldn't have prevented your birth, which means you _would_ exist, and so on, in a maddening loop. - **Causality Violation:** This is the broader term for any situation where an event in the past is altered by someone from the future, potentially leading to contradictions in the chain of cause and effect. Physicists thinking seriously about CTCs had to confront this "elephant standing in the parlor". - **Novikov's Self-Consistency Principle:** Russian physicist Igor Novikov, who had thought about CTCs for years, proposed that the laws of physics would somehow prevent paradoxes from happening. This "Principle of Self-Consistency" suggests that if you try to do something in the past that would alter history (like killing your grandfather), some natural process or chance event would inevitably intervene to stop you. The outcome would always be self-consistent with existing history. A simple illustration involves a billiard ball entering a wormhole time machine set up to knock its past self off course, thus preventing it from entering the wormhole. Novikov argued that the only possible outcome would be one where the ball emerges at just the right angle, perhaps glancing off its past self, but still entering the wormhole, ensuring the self-consistent loop. This principle found some support from quantum theory through the idea of the "sum over histories," suggesting nature might favor self-consistent paths. - **The Bootstrap Paradox (Jinn):** Another type of paradox, sometimes called the "causal loop" or "bootstrap paradox," doesn't involve changing history but rather something existing _without_ an apparent origin. Imagine someone completing a manuscript, then taking it into a time machine and delivering it to their past self, who then copies it and later takes it into the time machine to deliver it to their _past_ self (who is the original person). Where did the manuscript come from? The future self didn't write it; they received it from their _own_ future self. It seems to exist in a loop, created from nothing. Physicists Andrei Lossev and Igor Novikov explored this, calling such paradoxically existing entities "jinn". They even imagined a complex scenario where a spacecraft design and knowledge of wormhole locations could exist as jinn, created from nothing within a time loop to enable the very discovery of the time machine used to create them. This "getting something from nothing" aspect is, understandably, met with some skepticism. - **Entropy and Jinn:** The idea of a jinn manuscript also runs into a problem with the second law of thermodynamics, which states that entropy (disorder) always increases over time. A manuscript traveling back in time should show signs of aging and decomposition. For the loop to be perfectly self-consistent, the manuscript returning to the past must be in the _exact_ same state as the one that left. Novikov and Lossev suggested a jinn might "self-organize" during its journey, temporarily decreasing its entropy by drawing energy or matter from its environment to arrive back in time in the necessary condition. This, too, adds layers of complexity and strangeness to the idea. ### Challenges and Potential Roadblocks Beyond the logical paradoxes, physical challenges remain. - **Negative Energy:** As mentioned, traversable wormholes seem to require exotic matter with negative energy density. This is a major hurdle, as such matter hasn't been directly observed in the necessary quantities, and some physicists suspect it might even be prohibited by as-yet-undiscovered laws. Stephen Hawking argued that any time machine constructed in a finite region of spacetime would require negative energy. - **Quantum Fluctuations:** A serious concern is the effect of quantum fluctuations – tiny, unpredictable bursts of energy that exist everywhere in spacetime. Physicists like Robert Geroch, Robert Wald, William Hiscock, and Deborah Konkowski raised the possibility that as these fluctuations travel through the warped spacetime of a time machine (like a wormhole), they might build up in intensity, creating a "back reaction" that could destroy the machine or prevent CTC formation. While early analysis by Thorne and Sung-Won Kim suggested a "defocusing effect" might prevent this buildup, the issue remains a significant challenge. - **Chronology Protection Conjecture:** Stephen Hawking famously proposed the "Chronology Protection Conjecture," suggesting that the laws of physics somehow conspire to prevent the appearance of closed timelike curves, making time travel to the past impossible. He wryly noted the lack of tourists from the future as experimental evidence for this conjecture. This conjecture remains a major point of debate and research in the field. ### Alternative Perspectives: The Multiverse Some physicists, like David Deutsch, approach the problem of time travel and paradoxes from a different angle, one that involves a different interpretation of quantum mechanics: the Everett-DeWitt "many universes" or "multiverse" interpretation. This interpretation suggests that every quantum event causes the universe to split into multiple copies, each representing a different possible outcome. - **Paradoxes in the Multiverse:** In this view, paradoxes like the grandfather paradox simply don't arise in the same way. If you travel back in time intending to kill your grandfather, you succeed, but your action occurs in a _different_ universe (or a different branch of the multiverse) than the one you left. The original universe continues on its course, with your grandfather living and you being born. You haven't changed the past of _your_ universe; you've just moved to a different one. Similarly, a jinn manuscript or clever spacecraft design wouldn't be created from nothing in _your_ universe; it would have been created through normal means (writing, engineering) in another universe and then transported to yours via the time machine. - **Time Travel Across Universes:** From this perspective, time travel might involve moving "across" these different universes or branches of spacetime. This offers a potential way for time travel to be possible without violating causality within any single timeline, although it assumes the reality of the multiverse, which is itself a subject of ongoing debate and interpretation among physicists. ### Why Haven't We Seen Time Travelers? If time travel to the past is potentially allowed by physics, even under extreme conditions, why aren't we overrun by tourists from the future? The book explores several possibilities: 1. **Chronology Protection:** The most direct answer, as suggested by Hawking's conjecture, is that time travel to the past is simply impossible because some undiscovered law of physics prevents it. 2. **Limited Travel Range:** Most proposed time machine models (wormholes, cosmic strings, etc.) only allow travel back to the moment the machine was created. So, if no time machine has been built yet, no one can travel back to our current time. 3. **Travelers Go Elsewhere:** Maybe time travel is possible, but travelers go to other times (future or past before our era) or are simply few in number or hard to detect. 4. **They Hide:** Time travelers might be very careful not to be seen, perhaps observing from a distance, using sophisticated disguises, or even being miniaturized. 5. **Inter-Universe Travel:** If the multiverse interpretation is correct, time travelers from our future who travel to the past might end up in _different_ universes, not ours. 6. **Future Civilization Constraints:** Perhaps future civilizations capable of time travel place ethical restrictions on interfering with the past, including minimizing contact. Or, from the vast perspective of cosmic time, our era might not be a particularly interesting destination, or they might only be able to travel back to their own distant origin point, far later than our present. There was even a fun, low-tech suggestion: publishing an article in a widely archived journal with a request for future time travelers to contact the authors on the publication date. While seemingly simple, even this thought experiment highlights the challenges: you need a time machine to exist _now_ (or reachable from now) and a cooperative traveler willing to respond. As the book notes, like calling spirits from the vasty deep, the question isn't whether you _can_ call, but whether they _will_ come. ### Beyond Time Travel: What Else Do We Learn? Thinking about time machines, even theoretical ones, forces physicists to confront fundamental questions about the nature of spacetime, gravity, quantum mechanics, and causality. This inquiry is pushing the boundaries of our understanding and helping to probe ideas like the possibility of negative energy or shedding light on the search for a unified theory of physics (like quantum gravity). The concept of jinn and bootstrap paradoxes, while strange, prompts questions about information, causality, and whether complex things can truly arise without a clear starting point. The "clever spacecraft" scenario, while wild, highlights how self-consistency might be exploited and challenges our intuition about predetermined futures and getting "something from nothing". The book also provides a fascinating look at the human side of science – the personalities, the collaborations (the "consortium" of physicists working on CTCs), the debates, the worries about professional respectability, and the sheer curiosity and impatience that drives scientists to explore even the most far-fetched ideas. The image of physicists discussing vacuum stress energy or Lorentzian manifolds at a microbrewery captures this spirit well. ### Ideas and Questions to Explore Further: This journey through the book leaves us with plenty to ponder! - **What is the true nature of time?** Is it an objective reality, or is it, as Gödel suggested, potentially just a human construct? - **Is negative energy possible, and if so, in what quantities?** This seems like a crucial physical hurdle for many time machine ideas. How could we test for it or potentially create it? - **Does the Chronology Protection Conjecture hold true?** What would be the single elegant physical mechanism that prevents time travel to the past in all possible scenarios? Why would nature have such a rule? - **If the Multiverse is real, what are the implications for our understanding of reality, free will, and identity?** If a version of you exists in countless other universes, including ones where you made different choices, what does that mean for "you" in this universe? - **Can information or complex things truly exist in a causal loop (as "jinn")?** How does the idea of self-organization overcome the second law of thermodynamics in such a scenario? - **How might a "sufficiently advanced civilization" actually manipulate spacetime or energy on the scales needed for time travel?** While described as "magic," are there specific physical processes that might eventually allow such feats? - **Beyond the paradoxes discussed, what other logical or physical contradictions might arise from time travel?** The book touches on some, but what other unforeseen issues might appear? - **What other ways might science fiction inspire scientific inquiry?** Are there other "sensational titles" or imaginative concepts waiting to cross the border into respectable science? The story told in "The New Time Travelers" is one of boundless curiosity, pushing the very limits of what we believe might be possible according to the laws of physics. It's a testament to the power of asking "what if," even when the answers lead to strange places or reveal that the universe might be stranger than we imagine. It's a conversation that continues, fueled by the spirit of those Victorian gentlemen and the incredible minds working at the frontiers of physics today.