Time Travel or the Possibility of Global Causality Violation
by Ian Woolf

God cannot effect that anything which is past should not have been, it is more impossible than raising the dead" - Thomas Aquinas The biggest problem facing time travel is paradoxes. One way of avoiding them is to travel only on a one-way trip to the future. Relativistic time dilation has been a standard item in physics since Einstein demonstrated in 1905 that as an object approached the speed of light, the time in its frame of reference would slow down or dilate. This demonstrates that space travel is time travel. Time dilation was verified in 1971 with speeding clock by Hafele and Keating using a jet. Calling on general relativity, it has been demonstrated that time moves at a slower rate near gravity sources. This was demonstrated with clocks synchronized on Earth and then flown in satellites and aircraft. Even the difference between tall buildings and tunnels can be detected, tiny though it is. Thus travelling near a black hole will slow down your local time rate, allowing you to travel one-way to the far future. Both these techniques have been widely used in science fiction. Another method is suspended animation of the cold sleep, or stasis variety. Which have been used for one-way trips since Rip Van Winkle and HG Wells "The Sleeper Wakes", to Vernor Vinge's "Across Realtime", where stasis bubbles are global weapons and devices for travelling to the future. Real time travel means going back in time. Most people won't settle for less than going to the future and coming back to the present again. Which means travelling to the past. However an object appearing in the past violates the physical law of Conservation of Matter and Energy, regardless of the balance evening out some time in the future. This means that time travel into the past is impossible by current physics, but that doesn't necessarily stop us altogether. No conservation laws are broken if we can only observe the past with our time machine. In 1905 a story has a telescope that focuses light from Earth reflected by distant planets in "The time reflector". In 1904 Jean Delaire wrote "Around a distant star" where he has a spaceship travelling Faster Than Light across 2000 light years. They turn their telescopes to Earth to watch Jesus in Galilee. John Wyndham wroter "Pawley's Peepholes", where people in the future put in a ghostly tourist presence in a town and annoy the townspeople into retaliation. Asimov's "The Dead Past" shows that viewing the past means the end of privacy, for if you can view 1000 years ago, you can just as easily view 5 minutes ago. Michael Moorcocks "Dancers at the End of Time" has time travellors caught because they cannot travel back, so they live in "menageries" as pets of the all-powerful Dancers. The "Morphail effect" allowing jumps forward in time only. Einstein's relativity of simultaneity states that once you travel faster than light, you are travelling into the past according to some observers, and that therefore an FTL traveller can return before he leaves. As recorded in the limerick: There was a young lady called Bright Who travelled much faster than light she travelled one day, in a relative way, -- and returned home the previous night! This prediction is what worried many physicists about the announcement in April of photons travelling faster than light in a laboratory experiment. However, its is now thought that the observers who see you travelling into the past are seeing an illusion, and that there are other observers who will see you also travelling into the future. Science marches on, and it is possible that at some time a new physics may get around the Law of conservation of energy violation, but travel backwards is still fraught with difficulties. Changing the past is dangerous. "The main purpose of time travel is to change the past; and the prime danger is that the Traveler might change the past." -- Larry Niven Hopefully your time machine will deposit you at a different place to where you started, so that you don't try to occupy the same space and explode. Ray Bradbury's "Sound of Thunder" is a warning of the consequences of changing the past, and the earliest use of the butterfly effect. A time traveller accidentally steps on a Triassic butterfly and changes the results of an election in his own time. This view is where the travellers are the only people aware of any change, protected by being outside of time when history changes. In "The Brooklyn Project" by William Tenn (1948), nobody notices the changes, because their memories are altered as the past is changed. The reader watches the experimenters cycle through major and amusing changes, completely oblivious that their experiment has had a result. In "The Man from When" by Dannie Plachta a time traveller from the future reveals that the energy reuired to send him back destroyed the earth of his day, and he has travelled back a total of eighteen minutes. Saul-Paul Sirag suggests that the first time machines will have bugs that will create, unintentially, a series of wrinkles or weirdnesses in the time-flow, which rolling backwards will create the "occult" events that attracted many intelligent people in the late 60's and early 70's. "General relativity suggests that if we construct a sufficiently large rotating cylinder, we create a time machine", says Frank Tipler. By spinning a body of ultra-dense matter until the space-time continuum gives way in disgust..This requires vast amounts of energy, and lots of superdense matter travelling at near-light speeds. In the late 1980's Kip Thorne of Caltech conceived of a "wormhole" consissting of a pair of connected black holes, creating a tunnel held open by exotic matter. The tunnel's "mouth's" could, in theory be open to different times. Stephen Hawking ruled this out as a time machine because radiation would loop around and around the wormhole doubling its strength each time until it was destroyed by "boiling away". However, the Febuary 1995 issue of New Scientist reports that Li-Xin of the Chinese Centre of Advanced Science and Technology in Beijing has calculated a work-around to avoid the energy build up, using mirrors and relativity. Li's working time machine requires two wormhole mouths each about 10 kilometres in diameter. Li adds a perfectly reflecting sphere, about the same size as each mouth, between the two and exactly on a line joining their centres. The wormhole mouths would then be moved close to, but not touching the mirror(Physical Review D, vl 50, p R6037). Any radiation leaking from one wormhole mouth into the tunnel itself will be reflected away into the Universe at large, harmlesly. The mirror would have to be a perfect reflector of every type of radiation, but any civilsation able to build wormholes should find this easy. Whatever the practicalities, it appears that there are situtations described by Einstein's equations, under which stable time machines can exist. To me this suggests that General relativity is due for replacement. Very strange possibilities are opened up, as closed causal loops become possible. Imagine the following: One morning I come into my lab. At 11:59 a small two-minute time machine appears on the bench. To test it, I set it to jump back two minutes at 12:01. At 12:01 it disappears. To understand this, a Minkowski world-line diagram is useful. In figure174 you can see the loop clearly. There is no contradiction here, but its certainly a weird situation. Nothing is actually moving, there is simply a circular loop. According to quantum mechanics empty space seethes with liitle matter-antimatter loops. Energy, carried by a photon can be briefly conveted to mass, then reconverted back to energy. At any one point, one might have an electron and a positron emerging out of nothing, only to bump into each other and disappear. Positrons are sometimes thought of as electrons that travel backwards in time. Positrons have the same mass, spin, size, etc as an electron. The only difference is that electrons have negative charge, and positrons a positive charge. Whenever they meet up they disappear in a flash of light, this is called mutual annihilation. The other side of the coin is that when you create an electron out of nothing, you also create a positron at the same time. This process is called pair production. Robert Heinlein wrote the two definitive closed causal loop stories "All you Zombies" and "By his bootstraps". In the first, a man is able via time travel and a sex change to be his own mother, his own father, and his own daughter. In the second, a man meets interacts with many future and past selves going over the same experiences from a different perspective each time. Backwards living is best expressed in Phillip K. Dick's "A little something for us Tempunauts" and "Counter Clock World". People are disinterred from the cemetary and live their lives backwards to the womb. This was also the subject of an award-winning episode of the TV series "Red Dwarf". Unfortunately anyone attempting this in a universe with foward time-flow like ours, will become antimatter and explode. The biggest problem with time travel is that it leads to physical paradoxes, to contradictions in the fabric of reality. The major example being the Grandfather Paradox, which is about travelling into the past and smothering your grandfather in his cradle. This results in you never being born and so never travelling back to kill him. Therefore you are born and do kill him. If you kill him then you don't. Thus he is both dead and alive. Changing the past is full of Grandfather paradoxes. Seeing the future and then changing the present is just as bad. If one travels to the future, or sees in the future that a big war is coming, so you return to the present to prevent it. Thus the war never happens, so there was no evidence of war for your earlier self to find. When he appears, he finds nothing and gives no warning, so the war does happen. Another Grandfather paradox. One way of resolving the paradoxes is the proposal that travel sideways in time to alternative universes is what happens when you attempt time travel. Many authors have explored the theme of alternative histories, however this isn't really time travel. The other solution is that the Universe has a law of Conservation of Reality that prevents global causality violation on any but a small scale. In Isaac Asimov's classic "The endochronic properties of resublimated thiotimoline" a new chemical powder dissolves BEFORE the water is added, acting on future knowledge. After lots of clever ideas are explored, then scientists try to fool the powder by not adding water after it dissolves. A Conservation of Events is invoked, and a tidal wave ensures that the dissolved powder gets wet on schedule. Many authors have invoked a law of Conservation of Events to heal breaks in time, for instance in Fritz Leiber's "Change War" stories. A soldier tries to prevent a man's death by gunshot, and when he finally succeeds, a small meteorite speeds through the window and kills him with a bullet-sized hole in the head. Thus paradoxes get resolved. Kill your grandfather and you will likely take his place, after all you're already carrying his genes, and you're now an extra on the stage of history. In Michael Moorcock's "Behold the man" a time traveller visiting Jesus Christ becomes forced to impersonate him. Larry Niven pointed out that the end result of such smoothings by every Tom, Dick and Harry time travelling meddler, would be a universe in which no time machine was ever invented. Thus he propounds Niven's Law of Time Travel which states that in any Universe where time travel is possible, it will never be invented. References: Great mambo Chicken and the Transhuman Condition by Ed Regis "Time Travel: its all done with smoke and mirrors" by John Gribbin, New Scientist 4 February 1995 Profiles of the Future by Arthur C. Clarke The Theory and practice of Time travel by Larry Niven, All the Myriad Ways 1971 Cosmic Trigger by Robert Anton Wilson The fourth dimension and how to get there by Rudy Rucker The Visual Encyclopaedia of Science Fiction
edited by Brian Ash