Time Travel and the Laws of Physics An Exercise in Speculation by Joey Swails (c) 1992 Wou

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Time Travel and the Laws of Physics ---------------------------------- An Exercise in Speculation by Joey Swails (c) 1992 Wouldn't it be wonderful if travel to another time was possible. Time travel has been a mainstay of fantasy and science fiction for almost a century, from H.G. Wells to Star Trek. This short essay shall attempt to discuss the concept in the framework of what is currently held to be true in the realm of physical science, and how current theories might apply. I'm not a professional physicist, just a well read amatuer with a couple of underclassman physics courses to my credit. Nothing I have to say on the subject requires a doctorate in Theoretical Physics to comprehend. I've always been impressed by the fact that, unlike most of his collegues, Einstein felt that the average intelligent person could grasp the ideas of Relativity without the heavy mathematics; to him it was an indication of the fundimental truth of what he was working on. To that end, he wrote several highly readable "layman's" books on the subject himself. I gleaned EVERYTHING in this essay from other writers, Einstein, Hawking and Sagan among them. And Arthur C. Clarke, Larry Niven, and Isaac Asimov; these gentlemen, while best known for their science FICTION writing, are also excellent SCIENCE writers as well! And now, from high atop the shoulders of giants... Most people have an intuitive grasp of what is meant by "time travel", at least in a personal sense. One activates a machine or says the magic words and is "transported" bodily to a different "point in time", be it the past or future. Sounds simple. But in a universe that is observed to obey certain "rules of behavior", in practice it may not be as simple as it sounds. Why should we assume that certain laws of physics should apply to time travel? Aren't new theories of physics often proposed that supersede older theories, showing them to be incorrect? And therefore, couldn't even newer theories allow for time travel to be possible? The popular misconception is that when a new physical theory is proposed, it renders preceding one's obsolete. This may have been true in the case of Copernicus, where what is being superceded is basically a myth. In modern theoretical physics, the distiction is not so clear cut. The value of a theory is in it's power to predict the behavior of the universe by logical inference. Physical theories apply logical rules to analyzing a specific facet of the "universe of discourse", that reality which is accessable to everyone, either directly through the senses, or "second hand" through the use of insturments of measurement. To invoke logic, the proposed theory need only be logically self- consistent, even if what it descirbes applies only in very limited cases. When Einstein published his theories of Special and General Relativity, the planets did not suddenly alter their orbits. We still use Newtonian laws of motion to predict the trajectories of spacecraft, and they appear to work quite well WITHIN THEIR LIMITS. What happens is that the limits are expanded, and new theories are proposed to describe behavior outside the old law's limits. The old laws still hold; new laws usually apply only to new areas of observation. They build on, rather than eliminate, the old laws. So, let's speculate on some of the concepts of time travel. For the purposes of speculation, we must make some assumptions and see where they take us. For logic to work, the only requirement is that the assumptions are internally consistent, i.e. if something is true in case A(1) it is also true in case A(2) - no changing the rules as you go along. The overall assumption I'm making is that certain laws of the universe will always hold, and time travel can only be possible if NONE of them are violated in the process. Specifically, I refer to the Conservation of Matter and Energy; the Conservation of Motion; Entropy; and Causality. Perhaps I'm nitpicking, but not really - we'll leave Relativity and Quantum Mechanics aside for now and concentrate on predictive theories that have meaning across a broad spectrum of universal models, from Newton to Hawking. Also because Relativity tends to knock out time travel in the first round, which wouldn't be much fun; and Quantum Mechanics STILL can't find that damn tachyon particle (predicted to travel "backwards in time"), and at the quantum level, we can't even tell which way even NORMAL time is "moving" anyway... At least, not yet... The Basic Paradox of Time Travel -------------------------------- When anyone speculates about time travel, one of the first things encountered is known as the Grandfather Paradox, the origination of which is credited to Einstein. The basic idea is this: I invent a time machine. Since I always hated my nasty old grandfather, I travel back in time to when he was a young boy and murder him. The perfect crime! But I've gone and killed him before he met my grandmother, so therefore I was never born, and of course couldn't have invented a time machine. So I couldn't have killed him. So he sires my father, who sires me, who invents a time machine... Me and my time machine must both exist and not exist. There lies the paradox. Any process that interferes with the past in a self-cancelling way is broadly called a Grandfather Paradox. This is the big one: Causality. Our belief in ANY physical laws at all presupposes cause-and-effect; if this, then that. Maybe you don't WANT to kill your grandfather, but the possibility would exist that you COULD, and that's all that matters. The effect is made to come before the cause, and causes the cause not to effect the effect. Fundimentally illogical, and was reason enough for Einstein to consider time travel impossible. But it gets even stickier when we consider along with it the next law we will speculate about - Conservation of Matter and Energy. Conservation Laws and Time Travel --------------------------------- The law basically states that matter and energy can be converted into one another, but can NEVER be destroyed (as in totally eliminated) from the universe. Conversely, neither one can be created spontaneously out of nothing. A time machine that travels from now to 1000 years ago, can be said by any test available, to have appeared out of nothing, and can be said to also have dissappeared INTO nothing (from it's original starting point). This also goes for any energy that made the trip with it. As long as it remains translated out of it's original point in time, there's extra matter running around. But it doesn't stop there. Let's assume that I build a time machine in my workshop. Then I travel back to the past, visit the workshop, and remove or destroy some irreplacable part of the machine. How then would I be able to build the machine? Paradox again! Let's continue further along this line. I travel back in time to 1pm yesterday and remain there for one hour, then return. Upon returning, I again travel back to 1pm yesterday. I meet myself! BIG paradox! Some may want to believe in some "grand overall" theory of Conservation. It goes something like this - since the time machine disappeared in one time period, and reappeared in another, there is still only one time machine in any one place at a time, so some kind of "overall conservation" from the beginning of time to infinity, is preserved. There's no evidence that this should be the case, but no clear evidence that it could NOT be, so let's be gracious. Now, a problem arises when I time travel from various points in the future to the SAME point in the past. If I assume I could go on doing this indefinitely, a HUGE pile of doppelgangers begins to accumulate at a single point in time, seemingly replicating me and my time machine ad infinitum. Clearly a violation of logic as well as conservation. For MORE fun, let's say we all agree to return to the same point in future space/time. Now, how do we all occupy the same point in space/time? Now I here someone in the last row say, "OK, perhaps the process won't work if you try to go somewhere (or is it someWHEN?) that the time machine already is." Then the question arises, what constitutes "the time machine"? The collection of atoms and energy packets that make up it's physical structure? OK. But all of those components existed in SOME form before I built a time machine out of them. Why would matter behave differently once it was part of a time machine? Does it mean I can't travel back to my workshop to the time before I assembled the machine, when all I had were component parts? Or I can't travel to the time at the foundry where/when they cast the steel for the outer hull? A particle of matter or energy has no "knowledge" of what it is a "part of" at a given point of space/time (at least, an isolated particle has never been observed to behave differently due to where it "came from".) There's no clear place to "draw the line" either it's drawn with no exceptions (paradox) or it's not drawn at all (impossibility)! That leaves the Law of Conservation of Motion. Newton described it first, but Einstein was the first to offer a viable explanation for it. (I know, I promised "no Relativity", but this will be over in a second.) A rigid fourth dimensional continuum is required for his explaination to work; and it DOES work - If E=mc^2 was incorrect, thermonuclear bombs would not explode. I think we can all agree that they DO. For anything to move around in the time dimension, it must move faster than light, acquiring infinite mass and energy along the way. Once you EXCEED the speed of light, you're traveling backwards in time - but GETTING THERE could be hazardous to your health. Physical time travel clearly violates any law of motion, as motion ALWAYS relates to time. This effects conservation of motion, rules of kinetic energy, even the law of gravity.(At least, any law of gravity I can think of!) Entropy and Time Travel ----------------------- Finally, there is entropy (sounds like a philosophical statement, doesn't it?) - the tendency of matter and energy to move from order to disorder, embodied by the Second Law of Thermodynamics. This tendency toward randomization gives us the most obvious impression of "moving forward in time". It's not easy to observe the decaying orbit of a planet, or the breakdown of a lambda particle, but try to get an exploded bomb back into it's casing. That's entropy. It's happening all the time, to everything in the universe, and it's rate of decay for a given type of particle is steady in the extreme. This makes things like atomic clocks and Carbon- 14 dating possible. There's also the products of this breakdown to consider. A human body going through it's biological processes is a walking bundle of changing energy states, some changing to higher states, but more changing to lower states - always a net loss. Entropy again. So say I travel to the past, and while there I'm breathing the air of the past, breaking down the bonds of the O2 molocules and binding them to carbon, deriving energy for my metabolism from the process. So I'm exhaling particles with lower energy states than they had when I sucked them into my lungs (or less of them; a moot point, because matter and energy are the same thing ultimately.) When I return to my original time, how do I recover all of the exhaled particles that now have an "incorrect" energy state relative to all the other particles in their "new" time-frame? I'd HAVE to, in order to perserve any idea of universal entropy; if I leave them "behind" they stay there until the end of infinity, and we never "balance the books" of the total energy state of the Universe. Quantum Mechanics and Time Travel --------------------------------- I suppose we're going to HAVE to bring up tachyons. I know, it IS Quantum Mechanics, but the tachyon is the darling of time-travel enthusiasts, so in anticipation of it being brought up by SOMEBODY, let's discuss it a bit. Tachyons, though not yet detected, are predicted to exist by Quantum Theory. Let me dig out the physics text - what physicists are looking for is the decay of a lambda particle into a proton plus a pi meson. They predict they will find a nonzero value for the beta component of the spin of the proton (whew!), implying the release of a particle with reversed spin and charge - meaning it's moving "backward in time"! The problem is they have not found such a reaction to take place. Particle accelerators have been blasting away at the particles in their cloud chambers for years now, and the shy tachyon still refuses to "show itself". Most scientists studying the subject now believe they will find something wrong with the model that predicts the existence of tachyons. But let's assume that they DO exist. Can a machine (or a human body) "make like a tachyon" and boogie backward in time? All you have to do is start moving backwards and wait for the past to catch up with you. The theory that predicts tachyons says that in order to do that trick, the mass in question would have to instantly reverse the spin and charge of every sub-atomic particle in it. But since the mass is NOT made of tachyons in the first place, to impose this on any other kind of particle will have the distressing side-effect of converting the entire mass into anti-matter. MAJOR fireworks... This points out the error in the logic of "if tachyons can move backwards in time, then why can't something ELSE do it too?". If you subscribe to the theory that allows for the existence of the backwards moving tachyon, you can't selectivly ignore any of the corrolaries that go along with the prediction. Tachyons are a LIMITED SPECIAL CASE, and QM is full of them. Sauce for the tachyon is not necessarily sauce for the proton, in a manner of speaking. It's this plethora of limited special cases that makes coming up with a Grand Unified Theory to cover ALL the cases so difficult to formulate. Relativity and Time Travel -------------------------- Before anybody gets upset, let me point out that the Relativity Laws provide the ONLY available means to travel in time that we know about. It's called time dilation, and it occurs during acceleration of mass. It's almost undetectable at speeds we normally deal with, but experiments with atomic clocks in space capsules have borne out the theory - it happens. An accelerating mass experiences the flow of time at a slower rate than a mass at rest. Travel to Proxima Centauri, a trip of 4.3 light years, and accelerate steadily to 9/10ths of lightspeed till you're halfway there, then de-accelerate down to rest; now turn around and go back. To you, the round trip took about 25 years. Returning to earth, you find that about 3000 years have elapsed while you were gone! The closer you approach lightspeed, the more drastic the effect. A one-way trip, and only to the future, but it works! Also, the mathematics of Relativity predict that some strange things may occur to a mass that manages to travel at a trajectory that passes the outer event horizon of a rotating black hole (look up Penrose diagrams in a text on Relativity for more about this effect.) Suffice it to say that it MAY allow a mass to travel outside it's normal space- time axis. The theory can't predict exactly HOW this effect, if it exists, would manifest itself; it could mean superluminal motion, it could mean time-travel, or it could mean emerging into a different universe entirely. It might also not follow the prediction at all (remember tachyons? Theory says we should find them, but we haven't.) Plus there are the practical problems. Converting enough energy for a controlled fly-by of a black hole would require a LOT of logs to throw on the fire, to say the least! Something like the output of your average star should suffice. And then there's the question of what to build the hull out of - something that can handle the hellish radiation spewing out of the event horizon, as well as the acceleration, gravity pull and tidal forces involved. A few gigatons of neutron star material should do quite nicely, I would think. And also the problem of controlling the trajectory that you take, which is what determines where/when you will end up, calculated to many fiendish decimal places. The tiniest of mistakes and you end up being INTIMATELY aquainted with a black hole... It may be fun to think about, but it's not likely that this "loophole" in Relativity will lead to a practical time machine! The Letter of the Law --------------------- I've assumed throughout all my speculations here that the physical laws involved cannot be broken. To be fair, I've accepted that there can even be some slight bending in the cases of Relativity and Quantum Mechanics, which really means only that our understanding of them is incomplete. I've been really stubborn, however, regarding Conservation Laws. Is this justifiable? I believe it is. One of Einstien's postulations was that the universe is the same everywhere. The rules governing the behavior of mass and energy are the same no matter where you are. And in ALL known cases, the Laws of Conservation hold rigorously, from the behavior of galaxies to the behavior of particles. They are what Quantum scientists use to find new quantum states. The existence of the neutrino was postulated using them, and this very elusive particle has been successfully detected, behaveing exactly as the Laws say it should. If we allow exceptions, we have some serious problems trying to find another way to explain all the phenomena occuring in our universe. Conservation is a FACT, as cast in immutable stone as anything like it can possibly be. Any theory is an exercise in speculation, and must start with assumptions. I chose the ones that allowed for the reality of ghost particles and hydrogen bombs. I think I'm in good company. But what truly matters is self - consistancy. Make you own assumptions and see where they take you. However, once you make them, stick to them, or you can't expect truth to emerge. A Few Direct Quotes ------------------- First, from the fictional character of Lazarus Long, the oldest living human, (when last seen, over 2000 years old!)created by Robert Heinlein: "What are the facts? Again and again and again - what are the FACTS? Shun wishful thinking, ignore divine revelation, forget what "the stars foretell", avoid opinion (especially your own), care not what the neighbors think, never mind the "unguessable verdict of history" - what are the facts, and to how many decimal places? You pilot always into an unknowable future, and facts are your single clue. GET THE FACTS!" Lastly, from Isaac Asimov, a NON-fictional character (but quite a character, nonetheless): "It is not required of The Laws of the Universe that they manifest themselves in a way that is convienient." In other words, "anything is possible" might simply be impossible. The universal dice just might be loaded in such a way that things like time travel and faster-than-light speeds are TRULY IMPOSSIBLE, now and forever, no matter how much we may wish it were otherwise. But take heart. There are enough wonderous, mind- boggling things left in the Universe to keep us entertained for a very long time to come... ...but please, no time machines.


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