INTRODUCTION Time is the dimension which is asymmetrical with regard to entropy. The way acceleration tilts your time-axis relative to the dimensions of space in Einsteinian physics may complicate things slightly, but anyone who thinks this definition isn't mysterious enough might as well **** off right now. (Sorry about that, but there are some real fruitcakes out there.) Many time travel plots derive their dramatic tension from some variant of the question Will this act change history, and/or will my home timeline survive? – the answer to which depends on the fictional world's temporal structure – and are thus (tacitly) experiments testing the paradox-proofing of hypothetical causal frameworks. Some authors like to hide technical details behind smokescreens of jargon; but I reckon SF is more fun when you can tell what's going on. Companion pages are now available on SF Xenolinguistics and Exobiology; visitors to the past or future may also wish to learn the local lingo. TABLE OF CONTENTS Introduction Glossary Quasiscience Axioms Type Summaries Type One Plots (Deterministic) Type Two Plots (Elastic) Type Three Plots (Overwriting) Type Four Plots (Quantum-Forking) Appendix Antescript QUASISCIENCE AXIOMS The following laws of time – commonly referred to in SF – can be very useful for plot purposes, and are often taken for granted, but they should not be mistaken for necessary facts about the chronophysics of the real world (personally, I reckon they're all nonsense; see Appendix, plus my Star Trek Rant for more about quasiscience). Contributions, including especially unwitting ones, are welcome. THE RIVER OF TIME: Time flows, and has all the properties you'd expect from a river: a velocity (of one second per second) relative to fixed banks; constant new supplies of water (passing, e.g., the 1873 milestone); inertia (so that it resists diversions – but also tends to meander); turbulence; fish; temperature; width; sediment; tributaries; depth; wetness; etcetera. THE MOVING ABSOLUTE-PRESENT-MOMENT: There is only one present moment which travels through history as a sort of wavefront, containing the only real life and consciousness. If you visit the past you will find it full of decaying automata, since the wavefront has already passed on; the future is empty, not yet populated. Of course, you have to carry a mini-wave of your own to live in. MULTIPLE PRESENTS: A variant of the above, which imagines many such wavefronts moving in synchrony, usually at intervals conveniently longer than a human lifetime. If for instance they're at precise hundred-year intervals it can be the Absolute Nineties – you could visit 1797 or 2397, but 1920 or 1066 would be tricky. THE GRAMMAR-AS-PHYSICS FALLACY: Forwards Time Travel is impossible, because the past already exists but the future hasn't happened yet. Alternatively, backwards Time Travel is impossible, because the future is still undecided but the past has already been set (this may sound more plausible, but it's equally daft). TIME AND IRRELEVANT DIMENSIONS IN SPACE: (added 28-Jan-05, with thanks to all the people who've mailed me their own theories!) Time Travellers never need to worry about where they'll end up, even if their launch point is on a speeding train on a spinning planet. If you're aboard a spaceship hurtling towards a black hole, when you decide in desperation to use your experimental warpdrive to hop one day forwards in time you can be confident that you'll reappear in whichever of the following locations is dramatically appropriate: A point one day's travel beyond the black hole. Somewhere solidly inside the black hole's event horizon (bad luck). The point you started from just outside the black hole. The same point taking into account the galaxy's proper motion (i.e., millions of miles away from the black hole). This phenomenon may be related to the way Time Machines built in the USA are liable to spontaneously switch to a more interesting continent as soon as you take them back beyond the eighteenth century. THE LAW OF ISOCHRONY: (extended 02-Mar-07) AKA Temporal Equivalence. If you travel back to the Jurassic for a week, then return, as many days will have elapsed in the present as you spent elsewhen. This also makes it possible for Time Travellers to report in to Mission Control about the current situation in the Jurassic, and for Time Management to order them to hurry up and un-cause all the paradoxes their monitors are displaying. Without the Law of Isochrony, urgent missions would be exactly the ones you'd spend the most time over! ARISTOTELIAN CAUSALITY: An effect will remain in existence only for so long as it is being pushed by a cause (just as, in dynamics, motion continues only while a force is being applied). Remove that, and the effect will vanish. But remember, a Time Machine arriving from the future never has a locally detectable cause, whether its origin is a self-cancelled Time Line or an umpteenth-century chronophysicist's practical joke. DOPPELGÄNGER EXCLUSION: AKA (in Doctor Who) the First Law of Time or the Blinovitch Limitation Effect. You can never meet yourself, let alone shoot yourself, because it's logically impossible (or in some versions just painful) to be in two places at once (sometimes due to mass conservation or even due to Pauli's exclusion principle). DOPPELGÄNGER CONFUSION: (added 02-Mar-07) Where the above doesn't apply, people who are due to become Time Travellers never recognise their strangely familiar visitor as their elder self. This may be because they would automatically find themselves blurting out something moronic like Hang on – you aren't you, you're me!, which is such an embarrassing thing to hear yourself say that even the cosmic force of temporal determinism itself isn't enough to make them go back and live through it again. METATEMPORAL NARRATION: Saboteurs will experience bizarre phenomena such as gradually acquiring memories and features from the replacement Time Line. The process in a reverting history technically takes place over metatime, not over time, but this way of narrating it is far easier. HIMES' COROLLARY: (from contributions by Dennis Himes, 01-Jan-98, and a Mysterious Benefactor, 22-Oct-98) When time runs backwards, everything happens in reverse except the central character's mental processes. Similarly, in cases when time gets reset to an earlier moment and everything in the universe returns to its original state, an exception is made for the parts of the human brain which retain memories. Actually, in some cases the exceptions are so wide-ranging that you can only tell the timewarp's happening by its effects on timepieces. MACFLY'S LAW: Autoinfanticides vanish, regardless of the solidity of their flesh and blood. Sometimes they disappear suddenly, leaving a real – noisy – vacuum; other times (e.g. Back To The Future) they slowly evaporate, which is yet stranger (how do you walk when your legs are half unreal?). The bullet in the baby's head may or may not follow suit. Note that this is also a case of metatemporal narration (above). ANACHRONIST AGEING: People caught in freak Time Travel accidents will be left older (I wonder – does an egg turn into a rotten egg, a chicken, or a KFC Bargain Bucket?), or occasionally younger (just as the victims of stepladder accidents end up two feet tall, no doubt). ANACHRONIST AMNESIA: If you were displaced in time you'd begin to forget things that haven't happened yet. The classic case is the episode of Star Trek (also cited in my Trek Rant) in which they put an awkward witness back at the point in space-time where they first found him (but isn't that point already occupied?) so he forgets it all. (Strangely, this effect never adds memories…) ETERNITY: AKA the Space-Time Vortex, The Strat, etc. There is a region outside time (with its own internal chronology), from which Time-Cops operate and in which the Tardis can park. EDDIES IN THE SPACE-TIME CONTINUUM: When a saboteur steals a Time Machine and sets off to alter history, bystanders have a few moments' grace to set off in pursuit before the change-wavefront (or timequake) reaches them and their Time Line collapses. THE JONBAR HINGE EFFECT: It requires a great effort (or at least, a conscious human decision) to make a historically significant change, but not to modify something trivial – so, for instance, it's easier to accidentally cause the chromosomal fluke that turns Blackie the Cat into Ginger than to turn Adolf Hitler into Adela. GUEST STAR SYNDROME: Even if history had gone differently, people from our version would still have recognisable equivalents in the other. See for example Harry Harrison's A Trans-Atlantic Tunnel, Hurrah!, in which the Moors win a battle in 1212, so Spain stays Islamic, so Columbus never sails, etc. But George Washington still leads an attempted American rebellion, and the twentieth century even contains an Arthur C Clarke. A daft axiom, but very popular. THE OSCILLATING PARADOX HYPOTHESIS: Our Time Line is stable because paradoxes naturally edit themselves out. If I shot myself as a baby, I would vanish – but this would destroy the causal basis of the baby's death, and I would reappear again. I would oscillate in and out of existence (with random variations each time) till a consistent Time Line arose. Note that this is neither a true Closed Loop (which could no more vary than can the beads on a necklace) nor a Loose End (which need imply no further metatemporal layering) but a confused amalgam of the two; and a paradox in which a baby is both alive and dead is assumed to be worse than one where I just come back and look at it, leaving the kid both observed and unobserved. Compare the following: NIVEN'S THEOREM: In any universe where Time Travel is easy, it will never be discovered, as any Time Line involving Time Machines will be prone to self-undermining and get abolished. (Meta-)sooner or later a Time Line must arise in which it happens not to be developed; and being stable, this final version will persist. This is relatively clever, but I have some reservations: It assumes we're living in a Type Three Time Line (and in the final history). The final Time Line must contain at least one Time Machine – the one which abolished the penultimate version (maybe it crash-landed on the professor's cot). There may well also be stacks of ancient wrecked Time Machines lying about, and indeed any number of machines used only for forward hops.