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  1. http://www.iaf.nl/Users/Meridian/time.htm What relations are there between the time of physics and that of human experience?
  2. http://www.iglou.com/homepages/emonk/dreams/Time.htm An Experiment with Time A tribute to the dream work of J.W. Dunne
  3. Hi Mokrie dela, thank you for your kind message :) ]"Did you ever see the Back to the Future movies?" Oh, yes, I did. That is a very, very good movie. It's a trilogy. And I am waiting for the next movie: Back to the Future IV... Clik here, please: http://www.bttf.com/bttf4.htm Just some thoughts about time travel: I think we usually think too linearly. If we are evolving till someday we transcend space and time, why wouldn't it be possible when this happens? Why wouldn't this be possible NOW, when we are free from this physical body in a state that time and space does not matter? I guess time/space itself is an illusion. And if time/space is really an illusion, I do believe we can transcend this illusion sometimes and travel in time, even to meet ourselves in other times... "Everything we don't know is to what we know at the same proportion of the oceon to a drop of water." (C. Flammarion, "Uranie"). So I do believe everything is possible. Indeed "the greatest illusion is that mankind has limitations." Arrivederci ;) , Marcelo. "Believing is NOT enough. You need to KNOW. And if you INSIST that only crawling is possible you will NEVER FLY" - Michel Desmarquet
  4. Hello, I'm back trying to correct some mistakes. I'm sorry, apparently my unhappy previous message was a whole disaster, hein?... Glup! :P I think there was a problem of communication. As I said before an another e-mail, I don't speak English very well (in fact very, very bad). So I think I didn't know to choose the correct 'words' to express the idea ("arrange", "meeting", "meet" or "encounter"?). I don't have a Teacher of English all day long. I went somewhat precipitate for send that message without an appropriate verification of the word's signification. So please don't take offence, okay? I'm not so bad after all... ;) "I'm a good boy";-) Marcelo. PS. But maybe the real problem be with the idea 'traveling back in time'(?). Maybe. I don't know... I think I'm going to edit the previous message. "There is nothing so big nor so crazy that one out of a million technological societies may not feel itself driven to do, provided it is physically possible." - Freeman J. Dyson
  5. Hi all, I have had an idea: Make/arrange a meeting with yourself, a meeting with a future self... literally. If you are a soul, if you are eternal, then you have all time the world to try traveling in Time, and so to back from the future! Did you understand? YOU WILL BE A TIME TRAVELLER IN THE REMOTE FUTURE! Then... back!... return from the future!... Think about it carefully. Untill we meet again :) , Marcelo. "We must expand our imaginations to include the truth." - Elia Wise
  6. Greetings, Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction Paul J. Nahin. Question: Do you read and have any commentaries on this book or any texts about or of this book, which you could to share with me/us? Is it really a good book? I don't read it yet...;P Sorry for my bad English, please... I do not speak/write your language very well... Hugs :), Marcelo. ------------ Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction Paul J. Nahin P. J. Nahin Format: Hardcover, 2nd ed., 628pp. ABOUT THIS ITEM From The Publisher "Time Machines" takes readers on an exhilarating journey through the intriguing theories of scientists and the far-flung imaginations of writers. It explores the ideas of time travel from the first account in English literature to the latest theories of physicists such as Kip Thorne and Igor Novikov. 75 illus. FROM THE BOOK Table of Contents A Sample of Things to Come Foreword What's New in the Second Edition Prologue to the First Edition Acknowledgments to the First Edition Ch. 1 An Overview of Time Travel 1 The Mystery of Time Travel 1 Machineless Time Travel Without Dreams or Drugs 13 Time Travel by Machine 18 H. G. Wells - Why His Time Machine Won't Work 22 Traveling to the Future 25 Traveling to the Past 30 Who Else Might Be Interested in Time Travel 35 Some Problems 40 Backward in Time - Can It Really Be Done? 43 The Problem of Paradoxes 47 The Fictional Origins of "Change the Past" 54 Ways to Avoid Paradoxes 57 Where Are All the Time Travelers? 66 Skepticism and Time Travelers 72 Einstein, Godel, and the Past 79 Quantum Mechanics, Black Holes, Singularities, and Time Travel 85 Tipler's Time Machine 92 Ch. 2 On the Nature of Time, Spacetime, and the Fourth Dimension 97 What Is Time? 98 Speculations on the Reality of Time 102 Has the Past Been for Ever? 109 Time and Clocks 115 Hyperspace and Wormholes 117 Monsters in Hyperspace 125 Space as the Fourth Dimension 130 Time as the Fourth Dimension 140 H. G. Wells on Space and Time 143 Spacetime and the Fourth Dimension 148 Spacetime, Omniscience, and Free Will 161 Does the Future Already Exist? Is the Past Still Around? 170 Ch. 3 The Arrows of Time 179 The Language of Time Travel 180 Does Time Have a Direction? 181 Cause and Effect 185 Backward Causation 191 What Does "Now" Mean? 198 Irreversibility 205 Worlds in Reverse 208 The Philosophy and Physics of Reversed Time 220 Entropy as Time's Arrow 227 Other Arrows of Time 236 Multidimensional Time 240 Ch. 4 Time Travel Paradoxes and (Some of) Their Explanations 245 Paradoxes 246 Early Science Fiction Speculations on Time Travel Paradoxes 251 Two Basic Time Travel Paradoxes 256 Can the Present Change the Past? Can the Past Be Un-Done? 259 Changing the Past vs. Affecting It 269 Why Can't a Time Traveler Kill His Grandfather? 285 Quantum Mechanics and Time Travel 294 Causal Loops 304 Sexual Paradoxes 319 Maxwell's Equations and Advanced Effects 323 Communication with the Past 327 Wheeler and Feynman and Their Bilking Paradox 332 Absorber Theory and Signaling to the Past 336 Tachyonic Signals, Spooky Actions, and the Bell Antitelephone 342 Epilogue 355 Notes and References 367 Tech Notes 1 What Time Is Now? 415 2 Time Dilation via the Photon Clock 423 3 The Lorentz Transformation 429 4 Spacetime Diagrams, Light Cones, Metrics, and Invariant Intervals 439 5 Proper Time, Curved World Lines, and the Twin Paradox 459 6 A High-Speed Rocket Is a One-Way Time Machine to the Future 467 7 Superluminal Speeds, Backward Time Travel, and Warp Drives, or Faster-Than-Light Into the Past 475 8 Backward Time Travel According to Godel and Tipler 489 9 Wormhole Time Machines 497 10 "Solving" the Einstein Gravitational Field Equations, Unphysical Mass-Energy, and the Cosmic String Time Machine 527 11 Time and Gravity 537 Glossary of Important Terms and Concepts 545 Bibliography 557 Bibliographic Adieu 618 Index
  7. Hi Folks ;), Here is a very good text about Robert A. Monroe and his fascinating experiences with astral travel and time travel. This text was received from the VML-Voyagers Mailing List, of the Monroe Institute, http://www.monroeinstitute.org/ <...He says he has seen the future, he has been there, although he will only describe it in sketchy terms.[bR> Nuclear war, for example, will not occur, says Monroe, at least not on a global scale: "There will be contamination, but not from bombs nuking all over the place. The contamination will be astronomical." People will be able to control their bodies, says Monroe, to the point where a handful of rice will provide sufficient calories for an entire day. In time, a kernel will suffice. Then, no food at all will be needed: "It's just a matter of stages."] "The original article appeared in the (Norfolk) Virginian-Pilot some years ago, and was reprinted in "Fast Takes," Mike D'Orso's first book, one of the first books Hampton Roads Publishing Company ever did. Enjoy."(Frank) ----------------- "The Well-Traveled Mind of Robert Monroe" <From *Fast Takes* by Mike D'Orso (copyright 1990)> The Blue Ridge forest shimmers in morning sunlight as Robert Monroe climbs behind the wheel of his black Subaru and points it up the mountain. A hawk swoops low over Lake Miranon, searching for bass. The sounds of saws and hammers echo from the woods as Monroe steers toward a ridge that marks the uppermost boundary of his 800-acre community. "This is an easy place to fall in love with," he says, ticking off the names of his neighbors as he passes their tree-shrouded homes. Down that drive is mystery novelist Phyllis Whitney. Up that one is Joseph Chilton Pearce, author of "Magical Child." Steve Pauley, a math professor at Purdue University, has a home among these pines, as does Florida social services executive Sharon Alley. There are psychologists living in these woods. And computer programmers. And there's newcomer Eleanor Friede, who edited Richard Bach's "Jonathan Livingston Seagull." Monroe likes to categorize the world in terms of hemispheres of the brain þ the right side being the seat of intuition and creativity, the left being the source of logic and rationale. Here in his own back yard, an eclectic mixture of righties and lefties has come to live, all, he says, with one thing in common. "All these people," pronounces Monroe, "in their own way are seekers after truth." When this former radio and television executive talks of truth, he is not referring to anything found through textbooks or prayers or drugs. The truth he has found and toward which he guides others is approached through sleep, through gateways of the mind, through altered states of consciousness. Amid the physical splendor of the Blue Ridge foothills, Monroe and his laboratory entourage spend their days among wires, microphones and EEG monitors, journeying inward, putting their bodies to sleep while their minds wander, literally, across the universe. For 28 years, Monroe has fiddled with electronically produced soundwaves, using them to trigger a release of the mind from the body, launching what are called out-of-body experiences, excursions of pure consciousness through and beyond time and space. More than 6,000 people have gone through the Monroe Institute's weeklong programs of psychic exploration. Not all have had the pleasure of an OOBE, as they call the cosmic trip. Most, however, come away more attuned to the unused portions of their brains. For those who cannot make it out the institute, Monroe's brain-tapping methods are available through a catalog of mailorder cassette tapes. No OOBEs are guaranteed, but the 90 tapes on the institute's menu do have their everyday uses. Rhode Island medical students use them to reduce anxiety at test time. Schoolteachers in Tacoma, Wash., use them to soothe, quiet and relax their first-graders. In Oak Ridge, Tenn., the tapes are used to reduce pain for surgery patients. It is these practical applications of his hemi-sync method short for hemispheric synchronization þ that Monroe is quick to emphasize make his work more than just another psychic parlor game to be lumped with fire-walking or bending spoons. "We think what we're doing here is much more to the point. We want to release all this from pure research to something usable, something valuable. I guess that's my left brain always asking `How can we use it?'" Somebody is using it. More than 22,000 of the institute's cassette tapes, which treat everything from insomnia to a shaky golf game, are sold through the mail each year. But Robert Monroe is ultimately interested in more than ridding the world of bad sleep and bogeys. As his institute's glossy brochure describes it, Monroe wants nothing less than "to change constructively man's direction and destiny." A cosmic breakthrough is at hand, says Monroe. In a matter of decades, he says, the planet will be peopled by men and women whose control of mind over matter will free them from the bonds of their skin and bones. And they will not need Monroe's labs or his tapes to unleash their mental powers. "That's the crux of it all," he says, parking at the top of the ridge and looking down on a hillside of blossoming dogwoods. "Man has a great opportunity in the next 50 years to break out of our boxes, to be something far more than we've ever been." He is an unlikely-looking new-age frontiersman, this paunchy, 70-year-old, white-haired, white-mustached father of six. Wearing sunglasses, blue sportshirt, khaki slacks and white bucks, he looks more like a gentle grandfather which he is than a mind-tripping guru. But listen to him talk. Even in the most mundane, everyday conversation, Monroe tends to slip into laboratory lingo. A few examples from the morning outing: After discussing the removal of some old farm machinery from a nearby field, he tells a worker, "Now that I have this information to process, I will let you know." After haggling with a carpenter over the construction of his new ridge-top home and institute workshop, he sighs: "Carpenters are basically left-brained." Before turning in for a quick, midday nap, he announces: "I'm going to go ahead upstairs and recycle." Which is just what Monroe did that night in 1958 when he first spontaneously slipped out of his body and began joyriding the highways of inner and outer space. It was terrifying in the beginning, says Monroe, and it took him a year to convince himself he was not simply going out of his mind. It took him even longer until the 1971 publication of his first book, "Journeys Out of the Body" to tell the rest of the world about his experiences. In the meantime, he moved from New York to Virginia, where he established, headed, then sold his interest in the Charlottesville-based Jefferson Cable Corporation, and set up the Monroe Institute in rural Nelson County. Before the book, he kept his cosmic consciousness in the closet: "In a left-brain world, as president of a major corporation, I didn't quite feel the board of directors or the stockholders would look favorably on a wild guy doing this sort of stuff." After the book, he found he had plenty of company. More than 350,000 copies were sold, the book was printed in seven languages, and it established Monroe as more than a crackpot among the scientific community. It convinced death-and-dying expert Elisabeth Kubler-Ross to try Monroe's program and to incorporate life-after-death into her own research. Actor Jon Voight has also sampled the Monroe method. A good number of backpackers still show up at the institute's four-building complex, but most program participants are scientists, psychologists and doctors from clinics and universities around the world þ "as against the pure right-brainers and mystics," explains Monroe. Monroe has lectured at the Smithsonian. Papers on his work have been presented before the American Psychiatric Association. Both Mother Earth News and Omni magazine have analyzed the institute from their own divergent perspectives. Corporations have brought their executives to Monroe's hilltop headquarters for training in tapping the intuitive, creative, right-brain regions of their minds. A helicopter landing site is cleared in the weeds just beyond the institute's outdoor volleyball net. Credibility is no longer a concern for Monroe. Nor is viability. After more than a decade of pumping his own money into the institute þ "I'm not poor; I remember paying in a 90 percent tax bracket when there was a 90 percent tax bracket" þ the operation paid for itself in 1984 and became a nonprofit organization last year. That freed Monroe to finish his second book, "Far Journeys," which was published last October. He'll be packing soon for a European trip promoting British and French editions of both books. The institute, the books, the buildingþlately, Monroe has had a lot on his mind. Too much, in fact, to wander off to that region beyond the body. About 10 years ago, he tired of traveling through the space and time we all know. Leaving the body to sightsee on this or other planets, he says, soon became routine. "It got old," says Monroe. "There's a certain sameness to it." Hooking up with other consciousnesses þ "I now have a lot of nonphysical friends" þ he was guided to a realm beyond the time-and space-limited universe of our comprehension. It is these outer limits that are described in detail in "Far Journeys." After those outings, Monroe had no more interest in cosmic-sailing through the worlds of our comprehension. "Why bother with local traffic," he asks, "when you can take the interstate?" But even the "other reality system" he describes, where pure energy forms play God and create planets and solar systems just for kicks, is not enough to draw Monroe out of his body these days. "I've ceased to do this for my own self," he says. He explains that he's got all the information his mind can "process" in this body. He's got several more books' worth of experiences yet to sort out. So he is leaving it to the institute's volunteer "explorers" to continue bringing back data while he distills it and answers for others the questions he spent 10 years answering for himself. The question Monroe is most often asked is how he knows these excursions are real and not simply dreams? "It took me a full year to collect hardcore evidence, to validate that for myself," he says. The evidence ranged from traveling in a nonphysical form to distant friends' homes at preassigned times and later reporting correctly the things he saw, to pinching neighbors in their sleep and seeing the black-and-blue marks the next day. Which raises another question: Can shimmering forms of pure consciousness have physical properties? Absolutely, says Monroe, describing two psychiatrists who were lovers but who lived and worked in distant cities. After spending a week at the institute, they asked Monroe to help them rendezvous sexually in their out-of-body state. "I did," smiles Monroe, "and it worked beautifully. They met halfway between Kansas City and Denver every night." With all these minds meeting minds and minds meeting bodies, isn't there the danger of misuse? What about governments using the method for brainwashing or spying? What about the threat of cosmic rapists hovering through the bedrooms of the world? Monroe shakes his head, wearing the patient smile of a man who's heard it all before. "To be efficient at this," he says, "you have to develop na overview. With that overview, you are no longer interested in the things of this body-ruled world. You lose a sense of the body self, of local customs, of nationality. What we call morality changes radically. What's important and not important changes dramatically." The zone beyond, which Monroe describes as a realm of deep blackness, bright lights and exquisite joy, sounds much like what some religions might call heaven or nirvana. And Monroe admits it's hard to explain why anyone who has gotten there would ever come back. Why don't all those sleeping bodies lying in the institute's darkened cubicles remain vacant, deserted forever by consciousnesses playing hooky in the hereafter? "You'd be surprised at the attachment we have to the physical body," says Monroe. "It's addictive. As long as you have that body, the attachment is still strong. You have an urgency to get back." The call to return to the body, notes Monroe, is not always a deep philosophical or psychological one. "Ninety percent of the time," he smiles, "it's a full bladder." Lunchtime, and Monroe drives down the mountain to the nearby town of Nellysford for a cafe lunch and more questions. Over a burger, fries and iced tea, he talks about the past, the future and other lives. "Never mind religious beliefs, or whether you've been a bad boy or a good boy," he says of life beyond death. "You're going to survive physical death whether you like it or believe it or not. It's a fact." We have all been in other bodies in the past, says Monroe, and we will all be in other bodies in the future. Each consciousness is eternal, he says, while bodies come and go. The catch, he notes, is that when we're out of the bodies, we can't wait to get back, but while we're in these bodies, we are unaware that there is anything beyond. "While you're here, you forget you were ever anything but human," says Monroe. "You forget where you came from." And that, Monroe admits, begs the million-dollar question: Where did we come from? Who or what set up and guides this system, this planet, this universe? Monroe nods and fingers a fry. "Just somebody having some fun," he says. Somebody? "Some intelligent energy field." Energy field? "An energy being. Something the average human would call God, but it wasn't God." What it was, says Monroe, was one of many consciousnesses permanently unhooked from the cycle of death and life as a human. "There are graduates from this school of compressed learning we call Earth who are doing this in other places," he says. "It's a lot of fun, creating species, weaving just the right delicate balances for life on a planet." Monroe knows how much fun it can be, he says, because he has seen it. "Just a small demonstration," he says of the energy-being who escorted him during one of his OOBEs. The being, says Monroe, created a solar system for Monroe's viewing pleasure. "It was if someone was making it snow, and creating snowballs, and tossing them out like fiery suns, as it were. Nuclear snowballs. Just having fun." It was, in fact, just fun and games, says Monroe, that got the whole ball rolling here on Earth. The consciousness that created Earth, says Monroe, began tinkering with its toy, decided to try out its creation first-hand, in human form, and things soon got out of hand. "To really feel how it is, it had to get down into it. Only it didn't figure on the addiction of life as a human. It thought it was in for a quick shot and out. But things didn't happen that way." So all this is a result of an accident, of a great cosmic mistake? "Not a mistake,þ says Monroe, finishing his tea. ÞAn experiment." Back at the institute office, Monroe climbs out of his car as another vehicle comes up the drive. A woman hops out, clutching a copy of his latest book. She corners Monroe, gets his autograph and drives away as Monroe heads upstairs to recycle. Monroe's daughter Nancy, the institute's administrator, is downstairs, doing paperwork to the sound of a faint hum coming from a pair of speakers on her desk. The tape is called "Concentration," one of the institute's better sellers. "I can concentrate anytime I want," says Nancy. "But I happen to be lazy, so I put a tape on." Nancy, 33, says she has been having OOBEs since she was 16. But unlike her father's, her control is shaky. During one experiment, she slipped into an out-of-body state, and Monroe asked her to explore the "intervals between lives." Instead she slipped in and out of episodes in what she says were her previous lives þ as a man about to be hanged, as a woman about to be executed and as a soldier inside a tank about to be blown up. Asked why the experiment took her to moments of imminent death instead of post-death limbo, Nancy shrugs and smiles: "I kept missing." Nancy has worked for her father since 1974 after graduating from college and studying Zen philosophy in Japan. She and her husband, Joseph McMoneagle, a former Army warrant officer who now heads a consulting company called Intuitive Intelligence Applications, live just down the road. Monroe and his wife, Nancy, live upstairs, "over the store," as Monroe puts it. The office is busy, with several of the 14 staffers handling the correspondence generated by sales of the institute's assorted programs and products. The room also houses the only Xerox machine for miles around, and that's what brings Eleanor Friede through the door. Friede still has a home and office in Manhattan, where she edits and publishes books, always on the lookout for another "Jonathan Livingston Seagull." But since Christmas, when she moved into her new home on the institute's property, she's been shifting much of her work and time here. "I was simply curious," she says when asked why she signed up for a week-long institute program in 1982. The week, she says, was worth the $850 price tag. "I was just more aware. It didn't change my direction, but it made it clearer that I had to get away from the distractions, noise and dirt of New York. Now I've found I can have it both ways." And now that she's here, Friede says it will be only a matter of time before she moves from relaxation tapes to exploring life beyond the body. "I have a feeling that's why I'm here," she says. "I want to know how far we can go, to learn more about the universe we live in." Fully recycled, Monroe spends the afternoon giving a tour of the institute buildings, checking again on the mountaintop construction and explaining why he believes mankind is reaching what he calls "critical mass" in terms of the pressures on our collective consciousnesses. The threat of nuclear war, environmental holocausts and the latest specter of terrorism, says Monroe, are actually pushing people toward the same kinds of discoveries he has been making in his laboratory for years. "As anxiety levels rise worldwide," he explains, "as the rules and the game change, consciousness evolves to deal with all this. "The consciousness that can cope," he goes on, "is the developing consciousness. If man does not learn and pass this test and move to a mature consciousness, entropy will set in and man will no longer be the dominant species on this planet." Which, notes Monroe, would be no great disaster. "The future of man on earth," he says, "is an interesting, but relative term. If this compressed learning school that is human existence on Earth goes down the tubes, there are others." Man, however, will not go the way of the dinosaur, says Monroe. And he speaks from more than faith. He says he has seen the future, he has been there, although he will only describe it in sketchy terms. Nuclear war, for example, will not occur, says Monroe, at least not on a global scale: "There will be contamination, but not from bombs nuking all over the place. The contamination will be astronomical." People will be able to control their bodies, says Monroe, to the point where a handful of rice will provide sufficient calories for an entire day. In time, a kernel will suffice. Then, no food at all will be needed: "It's just a matter of stages." Communication, he says, will be nonverbal, requiring neither voices nor ears: "Mind to mind connection in its purest sense." The aim of all this development, says Monroe, and the object of his work at the moment, is erasing the fear of death. "When the fear is gone," says Monroe, "the fun begins." Part of his fun at the moment is conducting death-simulation experiments in his laboratory, and drafting a third book on what he calls "the ultimate frontier, the greatest unknown of all" þ death. Using the same hemi-sync methods that launch his explorers on OOBEs, Monroe and his staff are placing volunteers in a consciousness-free state in which they lower their own blood pressure, body temperature and pulse rate, "synthesizing death in slow motion, as it were," says Monroe. "We in essence reel them out close to death, pulling away all the mystical, superstitious trappings and trying to cross through the fears to what it's all about." There is no danger in the experiments, he says: "We can bring them back in instantly." And there is no possibility that death may look attractive after a close look: "It doesn't mean you want to die; you're just not afraid of it." In all his cosmic excursions, Monroe says he has avoided seeing how he will die at the end of this lifetime: "I don't want to spoil the fun." All things, he says, come with time. Eventually, we will "achieve escape velocity" and "go home, where we came from." Now and in the end, says Monroe, we are all in essence God. "But you've forgotten what you are. This heavy physical life experience has blotted it out." Monroe has been on both sides. The reason he is still here, "hanging around," as he puts it, are the simple pleasures of life, the "addictions" that for him include composing music, eating fresh trout and "occasionally playing a good game of cards." No out-of-body peeks at someone else's hand, of course. In all games we play, says Monroe, cosmic or otherwise, there is really no way to break the rules. No steps can be skipped. No lives left unlived. Each hand is played out all the way. And the dealer? "Something's driving," says Monroe, watching the sun set over his ridge, "but it's not the God of our childhood." -May 18, 1986 <This message has been edited by Marcelo (edited 25 June 2000).> <This message has been edited by Marcelo (edited 25 June 2000).>
  8. Hello group :), Here is a very interesting text about time travel: http://www.arts.unimelb.edu.au/amu/ucr/student/1996/m.joyce/timetrav.htm Copyright © Michael Joyce 1996 10 Is time travel to the past logically possible? Is it logically possible for you to travel back and have a conversation with your former self? Could you kill your former self? Could you do anything to anyone, or could they do anything to you, which you had not already done or had done to you? Would you, or anyone else, in such circumstances, be a free agent? Introduction Long a favourite topic of science fiction, the notion of time travel is one that raises an enormous number of philosophical problems and quandaries regarding causation, identity and the nature of time itself. While it is fascinating to study these merely as hypotheticals, research in the context of relativity has suggested some circumstances under which time travel, to the past as well as the future, might be possible. Views of time Linear The traditional view of time has been of something flowing inexorably forwards, and in three distinct stages: past, present and future. The past is fixed and unchangeable, the future is unwritten and does not exist yet, and the present is what we are experiencing now. This picture is still the most intuitive and natural way of viewing time. Since the theory of relativity has become more widely accepted, time is no longer seen as being so unique and separate, but as part of a four-dimensional framework. It is still different to the three space dimensions in many important aspects, but it can still be altered by factors such as speed and gravity. It is relativity that gives a little credibility to the possibility of time travel.1 Many-worlds Another view of time that I will mention is the "many-worlds" or parallel universe view, in which a time traveller may not actually be visiting their own past, but is actually travelling to a universe similar to his or her original universe. Although it might be seen as an extreme extension of some principles of quantum mechanics, the many-worlds hypothesis is a useful way of sidestepping many of the logical and philosophical problems of time travel. This is often thought of in terms of an extra dimension of time, with different time-lines branching off because of the different events in each system.2 Time travel is not a physical impossibility A good simple description of time travel was given by David Lewis3 as being when the personal time of the time traveller differs from the global and historical time of the rest of the world. Lewis makes the analogy of a personal time as a winding mountain railroad and external time as being the distance as the crow flies to demonstrate that this does need a second time dimension, but can fit in with our more usual picture of space-time as a four-dimensional system. The railroad might take a meandering path, even crossing over itself, but it is still contained within the same three dimensions as a straight line between the railroad's origin and destination. So too, a time traveller's personal time might different to the global, external time of the rest of the world, but is still contained within that global time. Possible types of time travel Future The type of time travel that is easiest to explain is travel into the future, which could be accomplished by a spacecraft travelling at relativistic speeds4. As the ship gets faster, time dilation slows time on board the spacecraft relative to the planet it has left behind. For the viewpoint of those remaining on earth, the astronaut has travelled into the future. Travel in this manner is not usually thought of as time travel, since the astronaut's personal time continues normally, but it would have the same effect as a classic time machine (although it would be disappointing for the astronaut to learn that there is no reverse gear on the time machine). It also differs from the normal description of time travel because, since the time traveller cannot return to his or her natural time, his or her path does not cross over or duplicate itself. The "Twins Paradox" described by travelling at relativistic speeds is not one of time travel, but of time dilation and frames of reference. The notion of time flowing at different paces is unexpected and difficult, but not as counter-intuitive and confusing as the possibilty of travelling backwards in time. Past Like travelling into the future, travelling into the past would also require several gross manipulations and incredible engineering feats in order to exploit some of the more unusual effects of the theory of general relativity. Many physicists are willing to ignore the possibility of time travel to the past because it is so unlikely to occur and the energy required to complete it render it extremely improbable5. A number of schemes have been proposed to travel backwards in time. Some involve a time that loops around itself6, others would use a rotating black hole7, an accelerated wormhole8 or a cylindrical object so massive that time would be bent right around and into the opposite direction9. Although all of them differ to time travel as usually suggested in popular fiction in that they also involve travel through space, the logical complications are much the same. Information Another type of time travel that might be much more possible, although a little less dramatic, would involve tachyons. Tachyons are controversial particles that travel faster than light, and, as a consequence of their superluminal speed, backwards in time10. An ability to control these would lead to the much the same philosophical problems of causation as a person actually travelling backwards. While there would be no problems of physically meeting oneself in a previous time, it might be possible to communicate with oneself with tachyons, or even to use those tachyons to cause (or at least attempt to cause) some events in the past. Meeting a previous self One scenario of backwards time travel that frequently raises concerns is that of a time traveller travelling back and meeting themselves at an earlier stage in life, often with the intention of giving themselves advice in order to improve their lives or avoid a particularly traumatic event. The first issue here is of the problem of duplication: is it possible to have two "copies" of the same person at the same place and the same time, and if so what is the difference, if any between the two people? The second is the problem of the inconsistencies between the traveller's memory of the past and the events that transpire once he or she goes back to the past. In David Lewis' description of time travel, the time traveller's world line might have discontinuities in relation to the rest of the "normal" timeline11, so he has to go to great pains to explain the continuing identity of a time traveller as not only having the same physical characteristics as the time traveller's previous existencein their personal time, but also an element of causal connection. The methods of time travel mentioned above involving travel through space as well do not have these discontinuites, so personal identity is easier to ascertain. Identity If a time traveller did travel back to a time that he or she had already lived through, would we be able to say that the two people are one and the same? There would obviously be some physical differences in age and appearance and mental differences in the older time traveller's extra memories and experiences, but these differences would not normally lead us to make a distinction between a young and old version of an everyday non-time-travelling person as being different people. Although time travel seems more complicated, there is no reason why a time traveller's personal time line could not cross over itself like a mountain railroad crosses over itself on a bridge. Inconsistency with the past The second problem of meeting oneself as one travels back in time is really just a simpler version of the grandfather paradox detailed below. If the time traveller does meet with him or herself in the past, then it follows that the meeting will be a part of his or her own memories and past. Unless the two-dimensional time framework is introduced, the time traveller must have a memory of being visited by an older version of himself in his or her youth. The possibility of a time traveller acting in a way that was inconsistent with the known past raises a few interesting questions. The Grandfather Paradox As the most extreme example of changing the past, a hypothetical is often envisaged whereby the time traveller visits the past with the intention of killing his or her own grandfather or grandmother so that the time traveller will never be born. This is usually known as the Grandfather Paradox12. Numerous variations on the theme exist, but the basic question remains the same: if time travel to the past were possible, would it be possible to change the past, and moreover to change it in such a way as to make the time traveller's existence impossible? The simplest way to avoid this paradox is by invoking the many-universes theory of time. In this case, the time traveller actually kills the person corresponding to his ancestor in a different time, leaving his own ancestor (and thus himself) untouched13. To many, the two-dimensional-time solution is a cop-out, since the time traveller is not visiting his or her own past, just a past that is remarkably similar to it. Even though the time traveller can perform actions that are inconsistent with the past as he or she know it, these actions will still have to be consistent with the timeline of that particular universe. Lewis reminds us not to confuse "logically impossible" with physically incapable14. If we look at the time traveller as just another occupant of that time armed with a gun and all the skill required to kill his target, we would not say that it is impossible for him to complete the task. However, this ignores the important fact of the time traveller's past. We know that the time traveller's grandfather did not die at that stage, so the assassin could not have succeed. We can say the same thing of any past event: John Hinckley could have killed Ronald Reagan when he shot him in 1981, but didn't. Doing anything else inconsistent with history The case of killing your own grandmother or grandfather is a particularly interesting one because it involves not only a contradiction with the past as the time traveller knows it happens, but also a paradox that shows why such an inconsistency is illogical. Although the existence of the time traveller's grandfather is of obvious importance to the time traveller, it is not significantly different from any other act that is inconsistent with the time traveller's past, whether he or she knows about it or not. The time traveller's past exists not just for the time traveller, but also for everyone else alive in his or her "natural" time. If the time traveller killed someone else or gave them the blueprints for an invention, the records of the time traveller's natural time would have to show events that correspond with the actions of the time traveller in the past, regardless of whether the time traveller knew if that was how things would pan out or not. Free will The worrying aspect of looking at time travel in this way is that it eliminates free will for the time traveler. Although the time traveller believes that he or she is able to kill a particular person, they are destined to fail since the events of that time have already been fixed, regardless of whether the time traveller knows of them or not. We think of the future as being variable and determined by the actions we take in the present. The presence of a time traveller in our time would shatter this illusion. Our future is the time traveller's past. If he cannot change it, then neither can we. Whatever is written in the history books of the time traveller is what we in the present will do. If the future is fixed as well as the present, we might look at time as like a movie film strip. To everyday people, the film moves past the projector at a steady rate and we see only the present. We know what has already happened in the movie, but can only guess at what will happen next. To a time traveller, the movie is unravelled and spread out on the floor. The whole of the movie is fixed, regardless of which frame the time traveller believes he or she is in. At no stage can anyone alter the course of events once the film has been processed and fixed in its nature. In a two-dimensional description of time, there is no paradox in a time traveller killing what appears to be their own ancestor, so it at first looks as though free will still exists. Although the time traveller might seem to be able to do whatever they want, whatever they do will still have to be consistent with the events that transpire within the timeline that the time traveller now occupies. Imagine two unrelated time travellers both travelling to the same timeline: one is travelling to his or her own past (Traveller-A) and the other to a time-line that is merely similar to his or her own past (Traveller-B). Traveller-A will be in exactly the same situation as if he or she were in a one-dimensional time situation, and will be unable to do anything inconsistent with his or her own past. Similarly, Traveller-B will also be unable to do anything that would be inconsistent with Traveller-A's past. Since the events forward of the time when Traveller-A and Traveller-B meet are fixed for that timeline, the situation is no different to a one-dimensional time situation. Changing the past in fiction Depictions of time travel in science fiction commonly allow the protagonists to intervene with and interfere with the past, although the travellers usually take enormous pains to avoid doing so. Two-dimensional depictions of time apart, the most logical of these depictions is in stories where the memories and records of the past that the time travellers have change to reflect the past: usually instantaneously, but sometimes as a flow-on effect of the change. Such a notion is plausible, so long as the changes they make do not logically rule out the fact that the travellers were able to make these changes. In such a situation, free will is preserved in all aspects but those that will make it impossible for the travellers to exist at that point in time. Of course, the problem with this is in determining what changes would rule out this existence: killing your own grandfather is obvious, but what of merely preventing two of your distant ancestors from meeting, or preventing the time machine from being invented? The more usual story depicted in science fiction is one where the traveller's past does change, but he or she retains her memory of the past as it was. This story is inconsistent from a logical and philosophical point of view, but makes for a more interesting and conventionally comprehensible narrative. Conclusions For a time traveller, the distinctions between past, present and future disappear, so that the future may be fixed and unchangeable. If this is the case, free will is gone too, not only for the time traveller, but also for all the other inhabitants of his or her past. This conclusion is so extreme that we may be better to reject the notion of time travel and keep our precious free will than to accept this consequence of the general theory of relativity. References Davies, Paul (1995) About Time Penguin Books Horwich, P. (1987) Asymmetries in Time: Problems in the Philosophy of Science, MIT Press, US Lewis, D.K. (1976): 'The Paradoxes of Time Travel', American Philosophical Quarterly 13 pp 145-152. Reprinted in Source Materials 136-220, 1996 Footnotes Davies, Paul (1995) About Time Penguin Books, p 33 Lewis, D.K. (1976): 'The Paradoxes of Time Travel', American Philosophical Quarterly 13 pp 145-152. Reprinted in Source Materials 136-220, 1996 p 231 Ibid p 137 Davies p 234 Ibid p 244 Ibid p 246 Ibid p 244 Ibid p 246 Ibid p 245 Ibid p 234 Lewis p 136 Lewis p 141-146 Ibid p 146 Ibid p 143 Copyright © Michael Joyce 1996.Created by Michael Joyce [email protected] Last Modified October 25, 1996 <This message has been edited by Marcelo (edited 24 June 2000).>
  9. Hi list :). Interesting texts. Clik here: http://www.time.com/time/magazine/articles/0,3266,42371,00.html Will We Travel Back (Or Forward) In Time? Einstein proved we can travel forward by moving near light speed. Backward requires a wormhole, cosmic string and a lot of luck BY J. RICHARD GOTT III -- http://www.sciencenews.org/20000610/fob7.asp Light pulses flout sacrosanct speed limit P. Weiss Five years ago, a wave of discontent swept away the 55-mile-per-hour U.S. speed limit. Nowadays, some physicists are taking a hard look at the 670-million-miles-per-hour speed limit of light in a vacuum, or c. --- Marcelo. "Wendy, Wendy, when you are sleeping in your silly bed, you might be flying about with me saying funny things to the stars." - James Barrie, "Peter Pan".
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