stratoman Posted December 27, 2003 Share Posted December 27, 2003 http://www.bbc.co.uk/science/horizon/2003/timetripqa.shtml Time Trip - questions and answers How widely accepted is the theory that we can travel in time? The Future According to Professor Paul Davies "Scientists have no doubt whatever that it is possible to build a time machine to visit the future". Since the publication of Einstein’s Special Theory of Relativity in 1905, few, if any, scientists would dispute that time travel to the future is perfectly possible. According to this theory, time runs slower for a moving person than for someone who is stationary. This has been proven by experiments using very accurate atomic clocks. In theory, a traveller on a super high-speed rocket ship could fly far out into the Universe and then come back to Earth at a time hundreds or thousands of years in its future. Another consequence of special relativity is that gravity slows time down. So, another way of time travelling to the future would be to go and sit next to a black hole or a neutron star, both of which are very massive and have huge gravitational fields. When you went back to Earth, it would have aged more than you. The Past Time travel to the past is more problematic, but there is nothing in the known laws of physics to prevent it. It is accepted that if you could travel faster than light, you could travel to the past. However, it is impossible to accelerate anything to a speed faster than light because you would need an infinite amount of energy. But hope for prospective time travellers comes from Einstein’s General Theory of Relativity, considered to be the best theory of time and space that we have. In 1948 Kurt Gödel worked with general relativity to produce equations suggesting the possibility of time travel to the past. He showed that a rotating universe, consistent with Einstein’s theory, would allow you travel back in time. Gödel knew that his model was unlike the real universe we inhabit and also that even if we did live in such a universe, time travel would be practically unachievable because you would need a hugely powerful rocket in which to cover astronomical distances. Despite this, Gödel’s work was firm evidence that time travel to the past is, at least in theory, possible. Since then, numerous other scientists have come up with other solutions of general relativity that allow time travel to the past. Most rely on the prediction of the existence of 'closed time-like curves'. According to these scientists, there are ways of distorting space-time to make it curved in such a way that shortcuts through space-time exist allowing you to effectively travel faster than light and journey back into the past. Not all scientists like this idea and there are some scientists, like Professor Stephen Hawking, who insist that there must be something that prevents it. In 1990, Hawking proposed a Chronology Protection Conjecture which says that the laws of physics disallow time machines. Basically, such scientists argue that nature will conspire to prevent the building of a time machine - one possibility is that runaway surges in quantum energy would generate massive gravitational fields and turn any time machine into mush. There are no clear answers to the issue because quantum physics and gravity do not sit well together and there is not yet a unified theory of quantum gravity. Hawking and others have serious problems with the fact that time travel to the past would violate causality and this would have serious implications for our understanding of how the Universe works. A final answer to whether we really can travel back in time may have to wait until scientists find a way to bring quantum mechanics and general relativity together. What are the different possible time machines we could build? There are now a number of different proposals for time machines that have been put forward by well-regarded physicists, for example: Professor Frank Tipler In 1974, Professor Frank Tipler suggested that you could use an incredibly dense, spinning cylinder that was about 100 km long and 10 km wide. The cylinder would have to be incredibly strong and rigid so that it didn’t get squashed by its own gravity and so that it didn’t get torn apart by the centrifugal forces it would experience when spinning. Tipler pointed out these were 'just' practical problems which might be overcome by sufficiently advanced technology. To use a Tipler Time Machine, you would leave Earth in a spaceship and travel to where the cylinder was spinning in space. When you were close enough to the cylinder, where the space-time is most warped, you would orbit around it a few times and then fly back to Earth, arriving back in the past. How far back in the past would depend on how many times you went round the cylinder. During your journey, your watch would always work as normal, going forward. Tiplers work suggested that this could be done using a spinning black hole or neutron star. There are pulsars that have been observed which are spinning at a rate fast enough. However, the mathematics is not really conclusive as to whether such stars could be used for time travel or whether we would need to pile up a few of them on top of each other to form a cylinder. Professor Richard Gott Professor Richard Gott has shown that Cosmic Strings could be used for time travel. Cosmic strings are predicted to exist by about half the theories attempting to unify the different forces. They would be thin strands of high density material left over from the early universe. Cosmic strings have no ends so would be infinite in an infinite universe or be closed loops in a finite one. They should have a mass of about 10 million billion tons per cm and therefore they should warp the space-time round them. Gott has shown that if you have two such strings parallel to each other and moving past each other, they would warp space-time sufficiently to allow time travel to the past. Professor Kip Thorne Arguably the most likely method for time travel to the past is the wormhole time machine. This was invented by Professor Kip Thorne after he was asked to look into the idea by his friend Carl Sagan who used a wormhole as a plot device in his novel Contact. If time machines are possible, why haven’t we built one? Although the time machines suggested by physicists are theoretically possible, all of them would require massive amounts of energy and a level of engineering technology that we don’t have at the moment, and which we are unlikely to have for quite some time. What about the paradoxes caused by time travel, like going back and killing your grandparents? There are several problems that suggest that time travel is not possible. One of the arguments that is most frequently put forward is the so-called 'grandmother paradox': if you travel back to the past and kill your grandmother before your mother is born, you will not be born. Therefore, you could not have travelled back to the past to kill your grandmother, therefore you would be born! Physicists have managed to come up with solutions to this. Some have proposed the Principle of Self-consistency: you can visit the past but are physically unable to change it. So, if you tried to shoot your grandmother, the gun would jam or you would be prevented in some other way from killing her. This is well illustrated in the film Bill & Ted’s Excellent Adventure. This seems to go against notion of free will but philosopher David Lewis made the point that free will does not allow you to do something logically impossible, such as instantaneously turning yourself into a tomato. Another solution is suggested by Professor David Deutsch. He says that quantum mechanics tells us that parallel universes exist. So if you travelled back to past and killed your grandmother, you would simply end up being in a parallel universe where you had killed another version of your grandmother and were a time traveller. One of the most famous arguments against time travel is that if time travel is possible, why haven’t we been visited by lots of time travellers from the future? Again, people have come up with ways round this objection: we may be inundated with time travellers and not be aware of it. Maybe that's what UFOs are. Perhaps civilisations don’t last long enough to develop the knowledge and technology required to build a time machine. And most convincing of all, general relativity says that you can only go back to the time a time machine was created. Since no one has built a time machine yet, no one can come back to this time. 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Guest 979guy Posted December 27, 2003 Share Posted December 27, 2003 This was really interesting and almost "scientific", right until the point where he used "Bill & Ted’s Excellent Adventure" as a reference. :doh: Link to comment Share on other sites More sharing options...
flashback Posted December 27, 2003 Share Posted December 27, 2003 If time travel can exist, then it already does exist. Therefore, time travel is impossible. You're welcome! Link to comment Share on other sites More sharing options...
Om Posted December 27, 2003 Share Posted December 27, 2003 Thank you, flashback (nice handle in this context, no?). Except you apparently didn't get the memo whereby those of us who DO travel in time are bound by the Non-Interference and Disclosure Act of 2257? Oops. Link to comment Share on other sites More sharing options...
stratoman Posted December 27, 2003 Author Share Posted December 27, 2003 Has time travel been proven impossible?? If not then there is the possiblity that it can happen. Link to comment Share on other sites More sharing options...
ADF Posted December 27, 2003 Share Posted December 27, 2003 Not possible. Link to comment Share on other sites More sharing options...
codeorama Posted December 27, 2003 Share Posted December 27, 2003 There was a show on "Tech TV" that basically showed the guys in England that are working on time travel right as we speak. It was way over my head but they seem to believe it is not only possible but not too far off in the future. Link to comment Share on other sites More sharing options...
Guest 979guy Posted December 27, 2003 Share Posted December 27, 2003 Guys in England? Aren't they already stuck in the past anyway? Link to comment Share on other sites More sharing options...
Ignatius J. Posted December 27, 2003 Share Posted December 27, 2003 This article is not really bogus, but it is misleading. You can travel into the future. But then again, we already knew that. You're traveling into the future right now. The cool thing is that special relativity tells us that you can move so that your clock goes really, really slow compared to everyone else's clock, so that when only a minute goes by for you, thousands of years can go by on earth. But you just can't get back. We have never seen anything move faster than light. In fact, nothing massive ever moves even at the speed of light. So the hope of going back in time by going faster than the speed of light doesn't seem like the answer. The more interesting possibility for time travel comes from things called wormholes. If Einstein's theory of general relativity is correct than wormholes definately exist somewhere, and you can go back in time. However, there are two big problems with this: 1.) Going through the wormhole exposes any body to extreme forces which would rip any material to shreds, let alone a human being. 2.) We don't believe general relativity once we get to these places. We know that general relativity is wrong. It is an excellent approximation for macroscopic laws, but once you get to these regions of space where the curvature is EXTREMELY large, the theory might not be valid anymore. -DB Link to comment Share on other sites More sharing options...
stratoman Posted December 27, 2003 Author Share Posted December 27, 2003 Only time will tell?......(no pun intended) Link to comment Share on other sites More sharing options...
yank Posted December 28, 2003 Share Posted December 28, 2003 Relativity theory says that distance shortens as you approach the speed of light, necessary for time travel. The ability of human organisms, cells etc to exist in this shruken state is highly problematic. What I mean to say is, well, no ... Link to comment Share on other sites More sharing options...
NavyDave Posted December 28, 2003 Share Posted December 28, 2003 Cool I'll time travel back to when I was 17 and tell myself to invest more then have him time travel to the era of free love and the doors and jimi hendrix Link to comment Share on other sites More sharing options...
tex Posted December 28, 2003 Share Posted December 28, 2003 An old guy time travels back in time and kills his young self. If the younger pre-time traveling self is now dead how can the old self exist to travel through time in the first place? Sagan on Time Travel Carl Sagan, the astronomer, Pulitzer Prize-winning author, and legendary popularizer of science, gave this interview during the making of "Time Travel." True to form, he discusses arcane aspects of the field -- from how you define time to what it might look like inside a wormhole -- with flair and a refreshing dash of humor. Sagan was David Duncan Professor of Astronomy and Space Sciences and director of the Laboratory for Planetary Studies at Cornell University when he died in 1996. NOVA: Let's start with the crux of the matter. What for you is time? Sagan: Ever since St. Augustine, people have wrestled with this, and there are all sorts of things it isn't. It isn't a flow of something, because what does it flow past? We use time to measure flow. How could we use time to measure time? We are stuck in it, each of us time travels into the future, one year, every year. None of us to any significant precision does otherwise. If we could travel close to the speed of light, then we could travel further into the future in a given amount of time. It is one of those concepts that is profoundly resistant to a simple definition. NOVA: Do you think that backwards time travel will ever be possible? Sagan: Such questions are purely a matter of evidence, and if the evidence is inconsistent or insufficient, then we withhold judgment until there is better evidence. Right now we're in one of those classic, wonderfully evocative moments in science when we don't know, when there are those on both sides of the debate, and when what is at stake is very mystifying and very profound. If we could travel into the past, it's mind-boggling what would be possible. For one thing, history would become an experimental science, which it certainly isn't today. The possible insights into our own past and nature and origins would be dazzling. For another, we would be facing the deep paradoxes of interfering with the scheme of causality that has led to our own time and ourselves. I have no idea whether it's possible, but it's certainly worth exploring. NOVA: Would you like it to be possible? Sagan: I have mixed feelings. The explorer and experimentalist in me would very much like it to be possible. But the idea that going into the past could wipe me out so that I would have never lived is somewhat disquieting. NOVA: On that note, can you describe the "grandfather paradox?" Sagan: The grandfather paradox is a very simple, science-fiction-based apparent inconsistency at the very heart of the idea of time travel into the past. It's very simply that you travel into the past and murder your own grandfather before he sires your mother or your father, and where does that then leave you? Do you instantly pop out of existence because you were never made? Or are you in a new causality scheme in which, since you are there you are there, and the events in the future leading to your adult life are now very different? The heart of the paradox is the apparent existence of you, the murderer of your own grandfather, when the very act of you murdering your own grandfather eliminates the possibility of you ever coming into existence. Among the claimed solutions are that you can't murder your grandfather. You shoot him, but at the critical moment he bends over to tie his shoelace, or the gun jams, or somehow nature contrives to prevent the act that interrupts the causality scheme leading to your own existence. NOVA: Do you find it easy to believe the world might work that way -- that is, self-consistently -- or do you think it's more likely that that there are parallel universes? Sagan: It's still somewhat of a heretical ideal to suggest that every interference with an event in the past leads to a fork, a branch in causality. You have two equally valid universes: one, the one that we all know and love, and the other, which is brought about by the act of time travel. I know the idea of the universe having to work out a self-consistent causality is appealing to a great many physicists, but I don't find the argument for it so compelling. I think inconsistencies might very well be consistent with the universe. NOVA: As a physicist, what do you make of Stephen Hawking's chronological protection conjecture [which holds that the laws of physics disallow time machines]? Sagan: There have been some toy experiments in which, at just the moment that the time machine is actuated, the universe conspires to blow it up, which has led Hawking and others to conclude that nature will contrive it so that time travel never in fact occurs. But no one actually knows that this is the case, and it cannot be known until we have a full theory of quantum gravity, which we do not seem to be on the verge of yet. One of Hawking's arguments in the conjecture is that we are not awash in thousands of time travelers from the future, and therefore time travel is impossible. This argument I find very dubious, and it reminds me very much of the argument that there cannot be intelligences elsewhere in space, because otherwise the Earth would be awash in aliens. I can think half a dozen ways in which we could not be awash in time travelers, and still time travel is possible. NOVA: Such as? Sagan: First of all, it might be that you can build a time machine to go into the future, but not into the past, and we don't know about it because we haven't yet invented that time machine. Secondly, it might be that time travel into the past is possible, but they haven't gotten to our time yet, they're very far in the future and the further back in time you go, the more expensive it is. Thirdly, maybe backward time travel is possible, but only up to the moment that time travel is invented. We haven't invented it yet, so they can't come to us. They can come to as far back as whatever it would be, say A.D. 2300, but not further back in time. Then there's the possibility that they're here alright, but we don't see them. They have perfect invisibility cloaks or something. If they have such highly developed technology, then why not? Then there's the possibility that they're here and we do see them, but we call them something else -- UFOs or ghosts or hobgoblins or fairies or something like that. Finally, there's the possibility that time travel is perfectly possible, but it requires a great advance in our technology, and human civilization will destroy itself before time travelers invent it. I'm sure there are other possibilities as well, but if you just think of that range of possibilities, I don't think the fact that we're not obviously being visited by time travelers shows that time travel is impossible. NOVA: How is the speed of light connected to time travel? Sagan: A profound consequence of Einstein's special theory of relativity is that no material object can travel as fast as light. It is forbidden. There is a commandment: Thou shalt not travel at the speed of light, and there's nothing we can do to travel that fast. The reason this is connected with time travel is because another consequence of special relativity is that time, as measured by the speeding space traveler, slows down compared to time as measured by a friend left home on Earth. This is sometimes described as the "twin paradox": two identical twins, one of whom goes off on a voyage close to the speed of light, and the other one stays home. When the space-traveling twin returns home, he or she has aged only a little, while the twin who has remained at home has aged at the regular pace. So we have two identical twins who may be decades apart in age. Or maybe the traveling twin returns in the far future, if you go close enough to the speed of light, and everybody he knows, everybody he ever heard of has died, and it's a very different civilization. It's an intriguing idea, and it underscores the fact that time travel into the indefinite future is consistent with the laws of nature. It's only travel backwards in time that is the source of the debate and the tingling sensations that physicists and science-fiction readers delight in. NOVA: In your novel Contact, your main character Eleanor Arroway travels through a wormhole. Can you describe a wormhole? Sagan: Let's imagine that we live in a two-dimensional space. We wish to go from spot A to spot B. But A and B are so far apart that at the speed of light it would take much longer than a generational time or two to get there as measured back on world A. Instead, you have a kind of tunnel that goes through an otherwise inaccessible third dimension and connects A and B. You can go much faster through the tunnel, and so you get from A to B without covering the intervening space, which is somewhat mind-boggling but consistent with the laws of nature. And [the theoretical physicist] Kip Thorne found that if we imagine an indefinitely advanced technical civilization, such a wormhole is consistent with the laws of physics. It's very different from saying that we ourselves could construct such a wormhole. One of the basic ideas of how to do it is that there are fantastically minute wormholes that are forming and decaying all the time at the quantum level, and the idea is to grab one of those and keep it permanently open. Our high-energy particle accelerators don't have enough energy to even detect the phenomenon at that scale, much less do anything like holding a wormhole open. But it did seem in principle possible, so I reconfigured the book so that Eleanor Arroway successfully makes it through the center of the galaxy via a wormhole. NOVA: What do you think it would be like to travel through a wormhole? Sagan: Nobody really knows, but what Thorne has taught me is that say, for example, you were going through a wormhole from point A to point B. Suppose point B was in orbit around some bright star. The moment you were in the wormhole, near your point of origin A, you would see that star. And it would be very bright; it wouldn't be a tiny point in the distance. On the other hand, if you look sideways, you would not see out of the wormhole, you would be in that fourth physical dimension. What the walls of the wormhole would be is deeply mysterious. And the possibility was also raised that if you looked backwards in the wormhole you would see the very place on world A that you had left. And that would be true even as you emerged out of the wormhole near the star B. You would see in space a kind of black sphere, in which would be an image of the place you had left on Earth, just floating in the blackness of space. Very Alice in Wonderland. NOVA: Your inquiries about space travel for Contact sparked a whole new direction in research on time travel. How does that make you feel? Sagan: I find it marvellous, I mean literally marvellous, full of marvel, that this innocent inquiry in the context of writing a science-fiction novel has sparked a whole field of physics and dozens of scientific papers by some of the best physicists in the world. I'm so pleased to have played this catalytic role not just in fast spaceflight but in the idea of time travel. NOVA: How do you feel being responsible for bringing time travel perhaps a step closer? Sagan: I don't know that I've brought time travel a step closer. If anyone has it's Kip Thorne. But maybe the joint effort of all those involved in this debate has at least increased the respectability of serious consideration of the possibility of time travel. As a youngster who was fascinated by the possibility of time travel in the science-fiction novels of H.G. Wells, Robert Heinlein, and others, to be in any way involved in the possible actualization of time travel -- well, it just brings goose bumps. Of course we're not really at that stage; we don't know that time travel is even possible, and if it is, we certainly haven't developed the time machine. But it's a stunning fact that we have now reached a stage in our understanding of nature where this is even a bare possibility. Link to comment Share on other sites More sharing options...
Ignatius J. Posted December 28, 2003 Share Posted December 28, 2003 yank, no problems come up. The cells shrink too. -DB Link to comment Share on other sites More sharing options...
panel Posted December 28, 2003 Share Posted December 28, 2003 It IS possible to travel forword into the future, (I'm going it right now), and it is possible to accelerate that process through relitivity, but I garentee you, that there is NO WAY, to go BACK into time. Link to comment Share on other sites More sharing options...
IowaSkinsFan Posted December 28, 2003 Share Posted December 28, 2003 If time travel is possible, why have we never encountered a time traveler? Link to comment Share on other sites More sharing options...
Mickalino Posted December 28, 2003 Share Posted December 28, 2003 Originally posted by IowaSkinsFan If time travel is possible, why have we never encountered a time traveler? We have indeed. For example, Michael Vick is from the year 2025. And Marty Schottenheimer, from the year 1963. Link to comment Share on other sites More sharing options...
Guest 979guy Posted December 28, 2003 Share Posted December 28, 2003 tex, :applause: for posting that Carl Sagan bit. Fascinating as always. Link to comment Share on other sites More sharing options...
Tarhog Posted December 28, 2003 Share Posted December 28, 2003 Originally posted by flashback If time travel can exist, then it already does exist. Therefore, time travel is impossible. You're welcome! I'd like to respond, but my mother taught me never to talk with my mouth full, and I'm not done chewing my obfuscation yet This thread reminds me of one of my favorite flicks...'A Perfect World'...where Kevin Costner tell's a little boy he's kidnapped that they're not riding in a car, they're in a time machine. 'Look back there...see, that there is the past. Right here, thats the present...and up there ahead, well, thats the future'. Link to comment Share on other sites More sharing options...
Om Posted December 28, 2003 Share Posted December 28, 2003 I can think half a dozen ways in which we could not be awash in time travelers, and still time travel is possible.NOVA: Such as? Sagan: First of all, it might be that you can build a time machine to go into the future, but not into the past, and we don't know about it because we haven't yet invented that time machine. Secondly, it might be that time travel into the past is possible, but they haven't gotten to our time yet, they're very far in the future and the further back in time you go, the more expensive it is. Thirdly, maybe backward time travel is possible, but only up to the moment that time travel is invented. We haven't invented it yet, so they can't come to us. They can come to as far back as whatever it would be, say A.D. 2300, but not further back in time. Then there's the possibility that they're here alright, but we don't see them. They have perfect invisibility cloaks or something. If they have such highly developed technology, then why not? Then there's the possibility that they're here and we do see them, but we call them something else -- UFOs or ghosts or hobgoblins or fairies or something like that. Finally, there's the possibility that time travel is perfectly possible, but it requires a great advance in our technology, and human civilization will destroy itself before time travelers invent it. I'm sure there are other possibilities as well, but if you just think of that range of possibilities, I don't think the fact that we're not obviously being visited by time travelers shows that time travel is impossible. This is why Carl Sagan should well be considered perhaps the single most important mind of our times. The man not only "gets it" scientifically, but articulates it for the lay person. For those who haven't already, do yourselves the favor of reading his masterpiece The Demon Haunted World. For many years up to his death, this was the guy I'd have put up as the representative to the human race when and if the day came that one was required. I haven't yet decided who's Next. Link to comment Share on other sites More sharing options...
Guest 979guy Posted December 28, 2003 Share Posted December 28, 2003 Originally posted by Om This is why Carl Sagan should well be considered perhaps the single most important mind of our times. The man not only "gets it" scientifically, but articulates it for the lay person. For those who haven't already, do yourselves the favor of reading his masterpiece The Demon Haunted World. Fully agree. Om (and anyone else), have you read his "Billions and Billions"? Science, lots of philosophy and deeply personal thought, some really funny some really scary, but all very inspiring and read as if he was right there, across from you, talking. I treasure it. Link to comment Share on other sites More sharing options...
Om Posted December 28, 2003 Share Posted December 28, 2003 I've read everything the man wrote, bro. At this point though, I only really hawk Demon. I figure nobody likes to be told what to read by someone they've never met, so I try to limit my schill to just the one I consider his seminal work. Link to comment Share on other sites More sharing options...
Guest 979guy Posted December 28, 2003 Share Posted December 28, 2003 Originally posted by Om I've read everything the man wrote, bro. At this point though, I only really hawk Demon. I figure nobody likes to be told what to read by someone they've never met, so I try to limit my schill to just the one I consider his seminal work. Think nothing of it. Have time, will read. Thanks. Link to comment Share on other sites More sharing options...
redman Posted December 29, 2003 Share Posted December 29, 2003 Originally posted by DrunkenBoxer yank, no problems come up. The cells shrink too. -DB Yikes! Shrinkage! BTW- Sagan was obviously a dumbass! Link to comment Share on other sites More sharing options...
stratoman Posted December 30, 2003 Author Share Posted December 30, 2003 Carl Sagan did not say that time travel into the past or future was impossible. He stated that time travel into the future, given what we now know, is more probable. He didn't state that going back in the past was impossable but not as probable as into the future. He also used his version of (Ted and Bill) by stating that if you went to kill your Grandfather something would happen like the gun jamming to stop it from happening. Great Carl Sagan piece Tex! :applause: Link to comment Share on other sites More sharing options...
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