A Guide To
SF CHRONOPHYSICS

Justin B Rye June the Sixteenth, 1997 AD
(animated edition)

TABLE OF CONTENTS

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 fuck 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.

This page now has a companion article entitled
A Primer In SF Xenolinguistics
and visitors to the past or future may also wish to learn the local lingo.

GLOSSARY

The following chronophysics jargon definitions can be revisited by clicking on the terms where they occur in the text.

METATIME:

If "formerly" history had a certain course (e.g. Hitler dying in 1945), then "later" I go back and alter things (in 1920), this implies a before-and-after temporal framework ("metatime") which is independent of the internal chronology of the alternative histories.

TIME TRAVEL:

Any process involving chains of cause-and-effect which are discontinuous and/or retrograde.  Nearly-As-Fast-As-Light starships or the like do not in my book count as "travelling into the future".

TIME MACHINE:

Any Time Travel device, including wormholes and chronoscopes.

TIME LINE:

Worldline, timestream, possible course of history, "parallel" universe (often a misnomer).

(TEMPORAL) PARADOX:

Any anomalous effect produced by Time Travel.  The term is traditional, but misleading as no contradictions are involved.  See also the detailed explanations of Closed Loops and Loose Ends.

(TEMPORAL) DOPPELGÄNGER:

Any version of yourself met via Time Travel (such as this one).

(TEMPORAL) SABOTEUR:

Anyone trying to alter history decisively by (as my standard example) shooting Hitler in 1920.  Any successful case of Führericide has four possible consequences; to wit...

• Divergent history: Nazism evaporates, Stalin invades Roumania, and there is a Russo-British nuclear war in 1952.  Or whatever.
• Parallel history: You can eliminate "our" WW2, but there is still some kind of global conflict leading to a US/Soviet Cold War, etc.
• Convergent history: Imagine Heinrich Himler stepping in as Führer and doing exactly the things Adolf would have... ideally this would lead to a "replacement" 1997 precisely identical to the "original".
• Reverting history: As above, but with the corrections taking place over metatime.  Saboteurs find life strangely confusing - perhaps they all paradox one another into oblivion...

All of these, by the way, should be kept distinct from the concept I'll call

• Fantasy history: Remarkably familiar even though it's set in a universe subtly unlike ours from the start, where (e.g.) the laws of physics permit the working of magic.

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.

TYPE SUMMARIES

There are four different possible sets of "laws of chronophysics" for Time Travel plots, although all but the most pedantic writers tend to blur the distinctions somewhat, and anything as confused as "Quantum Leap" is unclassifiable.  I suspect that the cosmos we're living in is a Type Zero Travelproof (AKA Recalcitrant) Time Line - the sort (excluded from consideration here as no fun) where it's just not possible to build a Time Machine.  But arguments about this are doomed to be mere axiom-juggling unless we can get experimental evidence... and even then it's almost impossible to disprove types Two and Four.

TYPE ONE	The Deterministic (AKA Permanent) Time Line.
Strict version:	History is utterly immutable.  Attempts to "ad lib" always turn out to have been scripted all along.
Lax version:	"Trivial" changes are allowed.
Metatime:	There is no metatime.
Paradoxes:	Closed Loops (phenomena created by their own effects).
Doppelgängers:	The elder self (a powerful ally).
Saboteurs:	Fated to fail (the gun misfires, you sneeze, or whatnot).
TYPE TWO	The Elastic (AKA Resilient) Time Line.
Strict version:	History has a preferred course/direction, and makes "corrections" (over time or metatime) for any interference.
Lax version:	The timestream can be permanently diverted.
Metatime:	Warped - histories revert and/or at least converge.
Paradoxes:	Both Closed Loops and Loose Ends (inconsistently).
Doppelgängers:	The replacement self (into which you may mutate).
Saboteurs:	Confused (likely to suffer Anachronist Amnesia etc).
TYPE THREE	The Overwriting (AKA Contingent) Time Line.
Strict version:	History is highly vulnerable.  Any Time Travel "erases" the original and replaces it with a freshly generated new version.
Lax version:	Histories may be reluctant to diverge.
Metatime:	Layered; "abolished" Time Lines are accessible only via meta-Time Travel!
Paradoxes:	Loose Ends (phenomena which prevent their own cause).
Doppelgängers:	The ultimate rival (one of you will end up abolished).
Saboteurs:	Good hunting - Hitler dies and stays dead.
TYPE FOUR	The Quantum-Forking (AKA Multi-Divergent) Time Line.
Strict version:	The "Many Worlds" interpretation of Quantum Physics: the cosmos constantly bifurcates into all possible alternatives.
Lax version:	Countable forkings (at "historic turning points").
Metatime:	Mindboggling (nothing is ever truly abolished).
Paradoxes:	Technically, neither kind can occur, but they can seem to.
Doppelgängers:	Hordes of microdiverged timetwins.
Saboteurs:	Futile - you can't get at the Hitler from your home Time Line.

TYPE ONE PLOTS (deterministic)

Time-meddlers meet themselves leaving, fail in neatly dovetailing ways, and worry about free will.  Tales of abortive attempts to alter history (e.g. Terminator 1) often turn out when examined to be set in a probable Type One Time Line after all, which is rather anticlimactic.  It's less traumatic if you don't try to "interfere", so Wellsian time-tourist stories commonly use this backdrop.  For further examples see "Dragonflight" (McCaffrey), "The Anubis Gates" (Powers), and "Twelve Monkeys".

The special paradox associated with Type One chrononautics (almost unavoidable when retrograde Time Travel occurs in such a Time Line) is the Closed Loop:

• Causal loops - complicated deterministic dances in which any attempt to escape turns out to cause exactly what you were trying to prevent.
• Informational loops - if Shakespeare's plays were written by a Time Traveller, where did they come from?  (Answer: they were logically necessary!)
• Financial loops - Time Travellers can start an account using its own interest.
• Material loops - if I die in 1920, and bequeath to my future self a gold ring which I got as an heirloom from myself, how old is the ring?
• Personal loops - e.g. the classic Heinlein character who is his/her own father and mother.  Is he/she genetically human?  If so, why?

Such "paradoxes" are nothing to be afraid of - in fact they can be very handy.  Any intelligent person in a Type One plot can "bootstrap" godlike powers... indeed, even Bill and Ted can figure it out.  In any tricky situation, your elder self can bail you out; any seeming disaster can be negated by going back and converting it into a fake - so when you thought you witnessed your own death it was really an android, or a hologram, or (cheapest and simplest) a post-hypnotic suggestion!  Type One Time Travelling civilisations usually turn out to have been bootstrapped into existence in the first place.

TYPE TWO PLOTS (elastic)

This is essentially a muddled compromise between Types One and Three, and requires an implausibly purposive history-defending force.  But as it makes modifying the future more of a challenge, it is common in Time-Cop and Temporal-Imperialist scenarios such as "The Legion Of Time" (Williamson), "The End Of Eternity" (Asimov) or "The Big Time" (Leiber), as well as in comedies such as (recent) Red Dwarf.

Plots set in Type Two Time Lines make use of both Closed Loop and Loose End paradoxes, which are logically incompatible - hence the proliferation of unnatural causal glitches and duff chronophysics in such stories.  Ancestricides can never be sure what will happen; often they vanish or mutate to fit the new history, for no clearly apparent reason beyond literary tradition.  Part of the problem here is the vagueness of motivation in the change-resisting force.  What are its priorities?  Is it trying to minimise the degree or duration of the divergence, the improbability of its corrections, or the number of witnesses?  Is it allowed for instance to preserve recorded history by annihilating all incoming Time Machines via quantum miracle?

TYPE THREE PLOTS (overwriting)

Changes in the past produce an entirely independent new history.  According to the strict interpretation, the arrival of any retrograde Time Machine instantly and permanently abolishes its home future!  The commoner lax version assumes a replacement history implausibly like the old one, though it isn't clear how (e.g.) dice are meant to remember how they landed "last time".  Among the few clear examples are (in order of laxity) "Lest Darkness Fall" (de Camp), "Bring The Jubilee" (Ward Moore), "Yesterday's Enterprise" (ST:TNG), and "The Sound Of Thunder" (Bradbury).

The endemic "paradox" type (inescapable in a strict Type Three Time Line) is the Loose End: any Time Machine can quite happily abolish its own history of origin, and thus its own causal basis.  You can multiply gold bars, do "retakes", become World Dictator... but you can never go home, as even if history repeats itself, it ipso facto produces a new "you"!  Type Three doppelgängers are bad news: they will never become you, or vice versa; so either of you can kill the other without ill effects.  If you both have Time Machines, a single Time Line isn't big enough for both of you.  A shoot-on-sight policy is less crazy than it sounds!

TYPE FOUR PLOTS (quantum-forking)

Every "choice" causes the cosmos to bifurcate: each possible outcome occurs somewhere.  SF plots in Type Four Time Lines - e.g. "All The Myriad Ways" (Niven), "The Coming Of The Quantum Cats" (Pohl) - tend simply to show off their "alternative histories", as any actual Time Travel within them gets extremely confusing.  See for instance "The Time Ships" (Baxter).

If every collapsing wave-function (i.e. every non-deterministic event) produces forkings in the Time Line, far more copies of 2020 exist than 1920s.  This hardly matters before Time Travel is invented, but then it's shattering.  Imagine setting your chronoscaph's controls for a spot from which to shoot Hitler.  As you hit "Launch", some particle somewhere decays (or not).  Now there are two of you heading for the same grassy knoll.  Or more likely, zillions of you - not to mention assassins after his chauffeur, time-tourists etc... all appearing at that same point in space-time.  KAPOW!  Was that the Berlin in our past you just nuked?!  Limiting Time Machines to interbranch rather than intrabranch Time Travel doesn't stop double hops (USA 2020 to Byzantium 1970 to Berlin 1920), and even if you can only travel through "probabilities", not time (USA 2020 to Byzantium 2020), your doppelgängers will follow you.

In the strict quantum-physical version, there is a solution: the unpredictable (non)appearance of a Time Machine is itself a world-forking event, so there is a Time Line (ours) where it didn't happen; one where only you appear; and others for every mathematical combination of arriving Time Travellers, including all the quantum-miracle Time Machines from nowhere.  This makes the universe even more alarmingly uncontrollable; furthermore, it means you can't kill the original Hitler, or return to your home Time Line by Time Machine.

The Type Four Time Line cannot contain genuine Closed Loops or Loose Ends (though it can seem to), but it can still induce bafflement.  The main problem with a cosmos where everything that's even remotely possible happens somewhere is that it undercuts the concept of probability, and of "causing" or "preventing" anything (thus destroying any narrative tension).  Many-Worldsists have technical fixes for this, but they're inadequate when a Time Machine can visit any Time Line regardless of its probability.

APPENDIX

The "Time And Motion" Rant

Some of my favourite SF stories depend heavily on the metaphor of time as an "ever-rolling stream", but it always surprises me that people take it so literally.  Time and a river have at most two features in common: each has two distinct "ends", and an inbuilt asymmetry of direction.  Both these features also apply to, say, an armadillo; whereas rivers also have misleading characteristics like a rate of flow (see Axioms).  Just what speed is this river meant to be moving at?  The usual answer is "one second per second", but what detectable difference would it make if it was -9 s/s?  We're not only applying the term "speed" where no displacement is involved, we're assuming a Newtonian yardstick of Absolute Time to measure against!  Saying that "Mount Everest is moving into the future at a speed of 1 s/s" makes no more sense than saying "the road passes my house at a speed of 1 m/m", or "my TV varies in mass at a speed of 1 kg/kg"!  You aren't "moving from birth to death" at any speed; you just extend into the future, in the same way that you extend a metre or two in height off the floor, and roads extend from A to B.  It may sometimes be appropriate to use "time ratios" to compare two relativistic frames of reference; but other uses of "seconds per second" are gibberish.

Now, it can be hard to understand how an immobile dimension of time (added to three of space) is adequate to explain our experience of motion, change and "free will"... but it's not impossible.  Those who fail frequently assume that hypothesising an extra dimension of "hyper-time" (over the course of which the Absolute Present Moment "moves") will help somehow.  But if adding an immobile dimension didn't work the first time, how can you stop short of an infinite regress, which at least pushes your failure of imagination out of sight?

This whole topic is - for obvious reasons - hard to discuss in everyday language, which takes for granted the imagery of "moving through time".  In Isaac Newton's diary, the word "now" meant 1697; in mine, "now" is 1997.  But if this is evidence for a moving Present Moment (something I have heard people try to argue), then his use of "I" to mean Isaac Newton must be evidence of reincarnation!  Likewise, metaphors like "in days gone by" or "it came to pass" are no more literally justified than "sunrise" or "from the heart".  European languages such as English, or indeed Esperanto, treat tense - which is really a context-dependent "pointer" like "this", "on your left", "here" etc - as if it was an essential, objective feature of the event, just as plurality is an attribute of objects.  And tense marking on verbs is obligatory, no matter how redundant or meaningless this is - whether the situation described is tenseless ("time is a dimension", "seven is prime"), tensed ("I was born in 1967", "Hitler shot himself") or indeed metatensed ("I will kill Hitler", "this Time Line has become unstable")!  The best solution isn't the Hitch Hiker's Guide one of inventing special tenses for time travellers; it's the normal non-European approach of ignoring tense unless it's worth explicitly mentioning.

ANTESCRIPT

Most of this section was actually published - in an Edinburgh University SF society newsletter - several years before I compiled the rest of this essay, but for contorted reasons it's the last part to be added to this HTML version...

Twenty Fun Things To Do With A Time Machine

1. Take half a critical mass of plutonium back to meet itself.
2. Infest the timestream with time-beavers.
3. Shoot the gunsmith.
4. Take one end of a space-time wormhole and throw it into the other end.
5. Release cloned Michael Crichtons into the Jurassic.
6. Organise a mutual infanticide pact.
7. Prevent this suggestion ever being made.
8. Persuade Lewis Carroll to write "The Time Machine".
9. Abduct your grandchildren and bring them up as your own kids.
10. Plant a suicide note in JFK's pocket.
11. Develop the temporal equivalent of waterskiing behind a speeding Tardis.
12. Swap Adolf Hitler and Charlie Chaplin at birth.
13. Genetically engineer yourself for maximum skill at genetic engineering.
14. Go to Hiroshima, 06-Aug-45, and run amok with a chainsaw.
15. Establish a Time Patrol Corps to prevent such frivolous tamperings with history (in 1897).
16. Find whoever coined the phrase "Time Paradox" and hand them their own skull.  Repeatedly.
17. Simplify the controls on the average VCR remote by substituting chronoscopy for television.
18. Steal Schrödinger's cat.
19. Sell ten-year-old yoghurt without contravening its "best before" date.
20. Park your Time Machine on the Turin Shroud and travel backwards to see where it really came from.

(If you think that list was one item too long, maybe you'd prefer Heinlein's or Clarke's, in my retrofuturology section...)