Time Travelers Beware

By Shamus Posted Wednesday May 13, 2009

Filed under: Random 117 comments

I thought of this problem with time travel years ago, and I thought I was pretty clever at the time for noticing it. But the internet has shown me that many authors and sci-fi fans came up with this before I did: It’s impossible to make any slight change to history. The only possible changes are massive ones.

The timeline is a lot more fragile than is usually portrayed in sci-fi. Marty McFly didn’t need to worry about getting his parents back together. He irrevocably erased himself and all of his siblings from the timeline the moment he skateboarded into town and interacted with someone.

Imagine a man gets a great big armful of individual twenty-sided dice, and gives them a toss. They scatter in a particular pattern of numbers. Now, time-travel back to just before he rolls those dice and stop him for a five-second conversation. Will he roll all the same dice in the exact same pattern? Of course not. The ever-so-slight different movement and timing will have him roll a completely new arrangement.

Now apply that same thinking to an even more chaotic event: Conception. Anyone that you interact with in the past will be nudged off of their original rails and onto an imperceptibly new path. This change will be slight only until the moment when they are involved with the conception of a child. Even if the kid is conceived on the same day at roughly the same time (which isn’t a guarantee) it’s going to be a different kid than was rolled in the timeline before you began mucking about. That different kid will live their entire life, nudging everyone else off of their behavior rails, resulting in different children for everyone they meet, and so on.

Poor farmer Bob originally had a son Alan, but thanks to your meddling ends up having a daughter Alice instead. Later, the woman who originally married Alan will marry someone else, and have a completely different slate of kids. Moreover, Alice will likely end up “stealing” a husband from elsewhere in the gene pool, and she will also produce a completely new mix of kids who never appeared in the history you’re familiar with. When the wave of change hits the ruling class you’ll end up with different rulers and wars and different discoveries being made. Every single historical figure born more than a generation away from your arrival will be obliterated, and new ones will appear in their place.

Every time traveler now has unthinkable power. Their slightest action will remove billions of people from the present, and replace them different billions of people. Their every action is re-rolling their future, every moment.

If you time-travel back to (say) 1909, you don’t need to worry about erasing yourself from history by accidentally killing your (N-great) grandfather as he (say) got off the boat on Ellis Island. You’ll obliterate yourself even if you travel to San Fransisco instead of New York, and all you do is bump into someone on the street.

It’s hard to say how small a change you can make, and fiddling around with parts of the timeline becomes a study in chaos theory. What if I warp to an empty pasture in Wales in the mid-1650’s and swipe a single stone off the ground? Will that initiate a ripple effect and overwrite the timeline? What if I swipe a large stone from a wall around someone’s property? What if I bury an out-of-place stone from another part of the world? What if I swat someone’s livestock on the head when they aren’t looking and then poof back to my own time? It seems like you’ll end up with a binary outcome: Your change will either be be noticed and thus alter the behavior of a single human being (and eventually everyone) or it will go unnoticed and have no effect.

You could play around with this and make “sleeper” changes to the past. Locate some ruin that was disconnected from the rest of the world, and leave some change to confound archaeologists. Pompeii is an excellent candidate for this sort of time-traveling prank. Go there just before the eruption of Mount Vesuvius and hand out a few iPods or revolvers or something. Since the city was buried, your historical shenanigans will be concealed for 1,700 years, and the timeline will proceed unchanged in the outside world. From AD 79 to AD 1748 the outside world will proceed along the familiar path, but the moment a digger sticks his shovel into some fragment of plastic or precision-machined metal your changes will suddenly manifest and propagate.

This means pretty much any time travel ends up in a paradox. I dislike this paradox that emerges from any meaningful leap through time from a purely science-fiction point of view. It’s much more interesting to write (or, I suppose, read) about situations where you can travel through time. (The technology required to do so is usually hand-waved by authors and readers alike, but the repercussions of the technology are not. People are funny.)

One way around this would be to eliminate the possibility of a paradox. You could come up with a set of rules where changes to the future don’t have any causality link to the past. So, you can erase your own grandfather (and thus yourself) from the time line, but you won’t instantly vanish the moment you shoot the old boy dead on Ellis Island. You’ll still be standing there, but if you go forward in time you’ll find a different arrangement of people in place, one where your family doesn’t exist and nobody has ever heard of you. This seems to be a common approach to writing about time travel. (Although most stories allow the protagonist to “fix” the changes they made and set things “right” again, which isn’t possible if we take conception into account.)

If we really want to start playing around with scientific buzzwords and write some rules for time travel, we could devise a system where your “changes” propagate at the speed of “time”. (How much this makes sense depends on how much we want to lean towards magic or science for our time-travel ability.) So, if you go back and kill (say) Hitler in 1939 and then jump back to the present, everything will look the same. Your change will be creeping forward through time at the same rate you are, and if you want to see 2009 with your changes in place, you’ll need to wait 70 years. You can stay in 1939 and watch the new history unfold, or you can bugger off to 1 million BC and do your waiting there.

Not that this is “science”, but if you want to sell a setup like this to readers, then you might explain it like this:

Imagine your time machine is simply a device that will teleport you from A to B instantly. If you fire a laser beam at Alpha Centauri and then teleport over there, you’ll still have to wait for your beam to arrive. The time machine works the same way, except with time instead of distance.

This system suggests time is “layered”, and that one dimension of time is contained within another. String theory suggests that there are 11 dimensions, and so you could borrow from that to explain your time layers. If you go back and kill Hitler, you’ll need to travel in the 4th dimension. To see the result in 2009, you don’t just need to go into the future, you need to go into the future of the future. So you’ll need to travel in the 5th dimension. Or wait.

I’m sure a physicist would scoff at this for various reasons, but this explanation is probably good enough to sell the reader on time travel without them putting the book down.

Anyway, use caution if you’re going to be time-traveling. I’m having a pretty good run here and I’d really rather not be erased. Thanks.

EDIT: To be clear, I’m not arguing that time travel IS possible, I’m just musing about effects and rules that would be involved if it was.


From The Archives:

117 thoughts on “Time Travelers Beware

  1. Jonathan says:

    There are also a couple of multiverse theories:

    1) You can’t change your own past, but you can start a new set of timelines that branch off at the point of your arrival.
    2) You change the past, and alter the future to the point where you are not there any more, causing you to not change the past, so you are there. Repeated loops of you existing and not existing occur. Minor variations and ripple effect changes will slowly change things until eventually there’s a loop where you go back to the past, do your stuff, and don’t eliminate yourself from the future. At that point, the timeline carries on in a stable fashion.

  2. Legal Tender says:

    I suscribe to the following mechanics:

    It’s not that you can’t change history by travelling back in time but rather than the moment you are able to go back you are effectively splitting your self from your own time line.

    Say I go back and I somehow convince JFK not to take that ride. Things will change but I just will never be able to get back to my own timeline. I’m now locked in the one where he didn’t get shot and my timeline continues to exists without me. My loved ones would think I got lost and died somewhere and never hear from me again.

    I can stay in the JFK-didn’t-bite-it timeline or I can keep traveling back and forth but I will never be in a place where my parents met and had me or where everything is as I remember it.

    It’s a frightening prospect…and yet I would still jump at the chance if I could do it.

  3. Kel'Thuzad says:

    I would use time travel to…
    I dunno.
    Who invented the idea of time travel?

  4. Melf_Himself says:


    Yep, this does seem to be the way that game designers are approaching the problem these days!

    Nice parallel evolution of the idea :)

  5. JB says:

    If time travel will ever be possible, why don’t we have lots of tourists from the future arround?

    (Okay, it comes from a quote, but I’m to lazy to google it and find out exact wording and who said it)

  6. Caffiene says:

    Oh good, Im not the only one who puts way too much thought into these kinds of things…

    I do want to point out, though, that the theory of “throw a few iPods just before Vesuvius erupts and it wont be noticed for years” is rather optimistic. I see no reason why these “unnoticed” changes couldnt have just as large an effect as your example of pausing someone for 5 seconds.

    In the Vesuvius example – people may not consciously notice the iPods under all that ash, but they will still affect things. Say the ash they displace by being there causes a tiny stone to roll down the hill where it wouldnt have had the iPod not been there… That stone causes a bird to fly away 5 seconds earlier… And who knows where that bird might go! Who might see the bird pass by 5 seconds earlier and act 5 seconds earlier or later than they might have done in an alternate timeline?

    The classic example of chaos theory is the butterfly causing the hurricane – even the tiniest change in airflow from your very existence could change the course of history.

    I… actually, I could go on rambling but Id more energy drinks than is healthy at 11pm. Nice post to incite some brain gymnastics, though.

    edit: Oh, on mechanics for timetravel – I quite like the description that time is simply how we perceive changes in state of matter. ie – time is just what we feel when the atoms in our brain move from one place to another. To reverse time, all you need to do is put all the atoms back where they were at that time. To “undo” changes due to time travel you just need to save the current state of all the atoms before you travel. Where you get the power and computation to do that I dont know, but theyre less paradoxes than simply lack of technology.

  7. MurrayHewitt says:


    I suscribe to the view that you can’t change the past. If you were to go back in time and do stuff, then that stuff must already have happened!

    To quote Lost: “Whatever happened, happened.”

  8. Mike Lemmer says:

    I also remember a Nova special on time travel mentioning the theory that a time traveling particle could influence its past particle so that it returns to its original course. They illustrated it using billiards:

    Transcript: http://www.pbs.org/wgbh/nova/transcripts/2612time.html
    (Search for “billiard” to find the relevant passage.)

    Snippet: “In order to be self-consistent, a ball emerging in the past must knock itself into the time machine so it can come out again along a trajectory that will knock itself back into the pocket. Novikov wrote mathematical equations for the possible outcomes of a ball hitting itself, and found that a self-consistent solution always exists. This implied that nature would not allow a paradox to arise.”

    You’re implying that due to chaos theory, any minute change could cause drastic consequences. Wouldn’t it also be possible that a consequence caused by said change could set off its own chain of events that would eventually nullify the drastic consequences? If any time traveler meddling was nullified by it, what would it say about our universe?

  9. Ding says:

    If it transpires that time and existence is one constant chain of events, this could be a different timeline to the one we had yesterday. We wouldn’t know, but maybe time is rewritten every day by clumsy time travelers. We could have come into existence with fully functioning memories five minutes ago as the result of some kind of time-line meddling.

    You may think you’re having a good run at the moment, but maybe you’ve only just started to exist.

    If that turns out to be happening on a regular basis, I want to know about it, because if I could stop existing at any moment, I want to go home from work early…

  10. Alrenous says:

    Any time travel at all will inevitably have an effect – greater distances just take longer to make changes humans appreciate.

    Simply being there will alter airflow patterns, which will eventually muck with weather slightly on some day. It’ll be warmer and someone will sleep a couple minutes longer and boom – everything’s different.

    Or slightly change the timing and location of a lightning strike, causing someone to look up when they wouldn’t have.

  11. Gahazakul says:

    I have the same outlook as MurrayHewitt stated earlier. This is what I like to call the “Puzzle Piece” time travel theory. It is used in the Hitch Hiker’s Guide series and in Futurama. The idea is the time line that you know exists due to the the pieces of history leading up to it. SO if there were any Time Travel shenanigans they would be included in the puzzle.

    Fry goes back in time and kills his grandfather on accident, but then has relations with his grandmother on accident and so his family tree keeps going and he is alive in the normal time line. It all fits together.

    If you are alive now, that means no matter what happens time travel wise you wind up alive. Killing Hitler would only give rise to Hiemler.

  12. Sho says:

    I recall a Futurama episode where they find an alternate universe inside a box where the only difference was that all the coin flip results have been reversed. Not quite the same as time travel (they tackle that more explicitly in other parts of the series), but it’s the same kind of “minute changes having significant (…well, largely cosmetic changes) differences” thing.

    But yes, I think I prefer the… theory-type thing.. where time travel involves hopping into parallel (or was it perpendicular?) dimensions. Mostly because I’m a simpleton Liberal Arts student and it’s the easiest way to handle paradoxes and all their techno-trappings.

  13. Strangeite says:

    I agree with Mike. I think you are misreading Chaos theory and assuming the “possibility” of drastic consequences will always result in large scale changes. Not every flap of a butterflies wings results in a hurricaine.

    If Scientific American is correct, the more widely believed model of the “universe” is that there is an infinite number of dimensions with each having an infinite number of different physical laws.

    So, it would be impossible to really “change” things because every possible circumstance (i.e. a dimension that is exactly like this one but you have one less atom or a dimension where LOTR is reality but Gimili is actually Shamus) already exists.

  14. I’m a fan of the Novikov self-consistency principle model (I think I got the right one) of time travel: a time traveler can’t change the past but rather fulfills the role that had always occurred. It just feels right, as the timeline would settle comfortably into a non-paradox, consistent state via some kind of negative feedback / process control mechanism.

    I like it a lot better when fiction takes this route, because it’s always more consistent and more clever. When it’s not done this way there tend to be inconsistencies in their model.

    My favorite example is probably Red vs Blue. SPOILERS FOLLOW! In the second season (right?) Church, who died in the first season and now appears as a ghost haunting a robot, goes back in time to the first season to try to correct the mistakes led to his death. It turns out that he was secretly the cause of every malfunction of the first season, and ultimately caused his own death. The past didn’t change as his time-traveling self was always the cause.

    He doesn’t understand the immutable nature of the past and so, at another historical event (I forget what), he runs into hundreds of copies of himself at the same time all trying to prevent this other event. All of them have failed in some way.

  15. Benjamin O says:

    I would argue that Shamus has it right…time travel has huge effects.

    From a strictly scientific standpoint, I’m going to just go ahead and say that I don’t buy it. Time travel isn’t possible. I don’t care what anyone says, it doesn’t work. The ONLY way time travel within THIS universe would work is if there is an outside frame of reference from which it is observable. Unless someone can supply direct evidence of that…

    BUT from a literary standpoint, it is fun. Time travel makes life interesting. My advice to anyone writing a time-travel plot: decide a priori what your rules are and stick to them. If you later realize you need a rule to change, make it change for the ENTIRE story. People will accept that within the strictures of your story that’s how things work, even if it isn’t really true for the real world. No matter how sciencey you get.

  16. Marauder says:

    As I alluded to in my post in the previous thread about black/white holes, super string theory allows for baby and parallel universes.

    Physics works very hard to avoid paradoxes, maintain causality, and otherwise keep from generating infinite loops and there are many theories as to how this is handled. A leading theory is that an incursion into the past would necessarily spawn a new universe/timeline as “a child process” where the previous timeline continues unaltered while the new thread continues and propagates any sort of changes that may have been created.

    All current plausible theories of backwards time travel preserve causality and while allowing matter and individuals to “travel backwards” also requires them to travel an equal or greater distance through space (ie. any method to travel backwards, say, five years, would put you at a MINIMUM distance of five light years, likely greatly more) thus eliminating any sort of impact either direct or via communication (propagates at light speed). Even quantum entanglement, which might though unlikely be able to provide for faster then light communication, would first require the entangled particles to be “slow boated” at less then C to the far distances, eliminating even that as a method to break the rules and send a message back to the past (such as “Mr. President, don’t go to Dallas”) because by the time you get the entangles particles, or get there yourself in order to send the message or otherwise impact the timeline as you knew it to be, the present (the time from where you originated) would have already past by the time you got there and you would find yourself at a LATER time then when you left.

    Best case scenario, you are able to travel through a wormhole (which are holes through both space AND time) backwards five years, and it places you at 5 lightyears, the theoretical closest place it could put you. You transmit a radio signal and/or able to at C back to Earth with some message or intent to change something. By the time you return, because you traveled at C, your local time would be at a standstill, but the local time on Earth would continue as normal and you find yourself/your radio message arriving at exactly the same time you originally left, thus preserving causality.

  17. NRowan says:

    Did you ever read the short story “A sound of thunder” by Ray Bradbury?

    It was about a guy going back in time and stepping on a butterfly.


    Just DON’T see the movie adaptation of it. It’s terrible.

  18. MattF says:

    Benjamin O: “The ONLY way time travel within THIS universe would work is if there is an outside frame of reference from which it is observable. Unless someone can supply direct evidence of that…”

    Do you mean that to witness time travel, you need a vantage point outside time?

    Witnessing space travel within this Universe doesn’t require leaving space. Why is time different?

    I don’t necessarily disagree — I’m just not sure that I follow your reasoning.

  19. Mr. J says:

    I believe there are tourists from the future around. When their camouflage devices malfunction, we catch a glimpse of them. We then call them ghosts.

  20. Sesoron says:

    I, for one, prefer that Novikov model, as supposedly used on Lost, as a device for fiction, though I consider it absolute rubbish if we’re to exclude the supernatural.

    I’ve noticed a somewhat depressing truth to the multiple-universes-theory: say something awful happens in the time-traveler’s reality that he then goes back to undo. If we assume he can travel backward in time but he cannot jump to the future, or that jumping to the future would only take him through the same line of universes without the waiting, that means there’s still a number of universes out there where the awful thing still happened, and those people have to suffer with it! Aaron Diaz refers to the moment of the time-traveler’s arrival in the past as “splitting the timeline”; the traveler the abandons his old timeline in favor of the one he improves.

  21. Rob Conley says:

    For a free ranging view of time travel I recommend David Gerrold’s the Man whole folded himself.


    Makes use of the Many Worlds hypothesis of quantum mechanics.

  22. Krellen says:

    My favourite version of time travel is what I’ll call “ripples in the pond”: a time traveller is like a pebble (or rock) thrown into the pond of time; his changes will cause waves and ripples, but always, inevitably, the pond calms and everything is returned to normal. Unless the rock is large enough to disperse the pond altogether (in other words, you travel back in time to destroy the universe), than no matter what change you make, history will eventually “right” itself and return to the path you’re expecting.

    So you can change minor details of history, but not the overall march. So if you, say, kept JFK from being assassinated in Dallas, then he would simply be assassinated somewhere else, or he would be impeached, or he would lose a primary race to LBJ, thus setting the country back on the same track anyway.

  23. nine says:

    Another explanation of time travel: although you can make changes, rather than magnifying over time these changes actually shrink until eventually the time line is identical to the original time line.

  24. froogger says:

    It’s fascinating to ponder timetravel, but for some reason it seems impossible for me to travel backwards in time. Even with 11 dimensions and parallell realities it just doesn’t feel right. Perhaps it’s the same limitations of my mind that makes me fail to grasp eternity and the size of the universe, but it only seems feasible to travel forwards. I sure don’t have the knowledge to strengthen this hypothesis, it’s just a hunch.

  25. Deoxy says:

    If you can really “change” the past (such that it overwrites the future, or what have you), then I prefer the theory (don’t remember the name) that states that, if such a thing is possible, it will NEVER happen, simply because, if it happens, someone will change the past… and that will keep happening until you get a “timeline” (for lack of a better word) where the time-travel capability is never discovered.

    Marauder: the problem with the “must also travel through space” thing is that such travel can be BENT – yes, you travel through space, but (with the help of some strong gravity, for just one example) you end up where you started again (go in a big circle). So that doesn’t prevent anything. Sorry. :-(

    Indeed, unless things have changed since I last checked, there were several theories of time travel that did NOT explicitly preserve causuality… and many of the theorists were hoping that some other mechanism would do so.

    One great problem I find with a lot of time travel sorts of things is the issue of good old 3-dimensional SPACE – that is, if I were to travel back in time 30 seconds, I would LAND ON MYSELF, as I was sitting right here 30 seconds ago.

    Any kind of time travel runs this sort of possibility, though “out in space”, you’re a bit less likely to hit someone. Of course, depending on which theory of time travel you are using, it can be an INHERENT problem (moving one mouth of a worm-hole around at light speed while not moving the other, for instance – EVERYONE who went through would come out at the same moment in the same place – OUCH!).

    Fun topic. Well, fun if you are the mood for complicated, solution-less discussion of great complexity. heh. :-)

    Krellan: the problem with that theory is the scale of the ripples – all of human history would fit between two of the waves. Sure, things would return to “normal”… humanity will eventually cease to exist or leave any record, and things will be “normal” again, but the mechanisms for that, even if there are any, certainly wouldn’t be fine enough to correct things in the human life time. See the enjoyable but entirely-unrelated-to-the-book movie “The Time Machine” to see an example of how RIDICULOUSLY contrived that becomes almost immediately. The main character tries to save his fiancee several times, and each time, something manages to kill her anyway… the last time he tries, a PIANO FALLS ON HER HEAD. Yeah.

  26. asterismW says:

    Don’t forget to put up instructions in your time machine.

  27. Krellen says:

    Deoxy: I set the scale. You can’t just state as fact that the ripples are bigger than human history.

  28. perry says:

    nice discussion going on here! but consider this. i say that time travel will never be possible. let’s consider that i invent a time machine tomorrow. others will come to know of it eventually and more machines will be built. now at least one person will go back in time to some instant. since any person can do this many, many times and there are many many people in the world (not only in the present but the future too), time travel will be known to people of every time period. someone could go to 1997 and tell people there. he could also go to 1297. since we know it to be true that there is no documented case of time travel in history, it means that time travel will NEVER be discovered. so what do you all say? any flaws in my thinking?

  29. Lupis42 says:

    @Mike: It sounds like you’re talking about a http://en.wikipedia.org/wiki/Closed_timelike_curve.

    @Perry: There is always the possibility that backwards time travel is impossible, but forwards time travel is possible. This also inherently resolves all paradoxes. It is also possible that time travel is possible, but only back as far as the point that the time machine was created. And of course, it could simply be so impractical that it has only ever been practiced by people who succeeded in not being noticed.

  30. elias says:

    Greg Dean posted this today. Have at it.

  31. wererogue says:

    I’ve been really excited about Achron for a while. Watch the videos. It looks fun.

  32. MadTinkerer says:

    Three things:

    1) My great-uncle actually coined the term “time travel paradox” in his short story “Paradox” back in the early 1930s.

    2) Both he and Shamus are under-estimating what Chaos Theory actually says about time travel.

    Accroding to Chaos Theory, if you can go back in time and it’s possible to change anything, you will irrevocably change the weather patterns of the entire world the instant you displace the atmosphere with your presence. Take a breath and you’ll probably destroy/undestroy a city by altering the course of a hurricane some amount of time later. After that incident, any other specific consequences of what you do become insignificant.

    3) Time is most likely unchangable. If you travel back in time, you always had travelled back in time and what you did is what happened anyway. I forget which specific theory this is called, but it’s what makes the most sense. It also means that the future is set in stone because the past isn’t really any different from the present or future.

    Given the whole concept of space and time being part of the same thing, this really just makes the most sense. It takes the “fun” out of time travel stories a bit (I feel sorry for my great-uncle), and means that “destiny” is an undeniable hard fact (I feel sorry for the entire scientific community), but it makes the most sense.

  33. Dev Null says:

    it's going to be a different kid than was rolled in the timeline before you began mucking about

    When gamers reproduce…

    You can’t change _anything_ when you time travel back into the past – anything you do already happened. Go back in time and murder your grandfather as a young boy? Well then he obviously wasn’t _really_ your grandfather, because he was already dead…

  34. Kat says:

    Shamus, have you read Connie Willis’s time travel novel, To Say Nothing of the Dog? It seems to use that Novikov principle other commenters have mentioned, and the Timeline itself is self-correcting (and seems to have some kind of guiding intelligence). And it is great fun!

  35. Groboclown says:

    I’ve had an idea for a short story about time travel running around in my head for about 15 years now. Essentially, a guy goes back in time to try to stop the JFK assassination. However, when he gets there, all he finds are people from the future either trying to stop the assassination, cause the assassination, or just observe it.

    Personally, I think that everyone else is a time traveler come to watch the past. Which only tells me that the future is really, really boring.

  36. Tango says:

    It’s not uncommon for stories to treat temporal changes as something that takes time to propogate across time and space.

  37. Factoid says:

    There’s always the good old predestination theory: No matter what you change in the past, it won’t change the future because those changes have already been factored into the present.

    Thus if you go back in time and try to kill hitler, you will inevitably fail, and your actions will be remembered as Operation Valkyrie or something.

    It is self-defeating to use predestination when writing time travel fiction, though, because it just ruins every story. Still, sometimes authors make a head-nod to it, or incorporate one aspect of predestination, usually with some kind of loophole to get out of it.

  38. K says:

    I realized this when I thought about how much of a chance encounter my last girlfriend was. I was slightly sick and did not know if I really wanted to leave the house, it was basically a coinflip. I did, met her and literally spend a year with her. That year would have been incredibly different otherwise.

  39. Vladius says:

    Awesome post. Your “conception” argument is very interesting; I’m writing some stuff right now and that’s one of the dynamics. Time travel also changes everyone else’s life, not just your own. Minute things will be off because of the “freeze frame” of when you left.

  40. Nixorbo says:

    I subscribe to the Terry Pratchett theory of time travel – namely that history has a lot of inertia. You can change the details but you can’t change the big picture (paraphrasing).

    “Shoot the dictator and prevent the war? But the dictator is merely the tip of the whole festering boil of social pus from which dictators emerge; shoot one, and there’ll be another one along in a minute. Shoot him too? Why not shoot everyone and invade Poland?”

  41. Jazmeister says:

    You could travel to the beginning of time and re-roll humanity. You could also stay there and teach them how to live in an egalitarian society and have EyePoddes in 50 years time.

  42. Martin says:

    Regarding Chaos Theory: It’s easy to think of history like a fractal, say the Mandelbrot set. There are parts of the MS where the tiniest change causes large effects, and there are parts of MS where it is solid and large changes in co-ordinates cause no change.

    Re Time Travel in General: There are dozens of models, some more fun than others. I’ve always liked Niven’s story where some space-archeologists point out the their government that past civilizations that seemed on the verge of completing time-travel projects (which were large and unsubtle) always collapsed before completing said projects, often from quite unlikely circumstances. They theorized that the Universe had an “immune system” preventing time travel. Said government then *faked* a time travel project so their enemies would try to keep up with a real time travel project and get destroyed.

    Alternately, there’s the view of history where some highly jaded yet nigh-omnipotent being at The End Of Time uses time travel to send itself back with highly altered consciousness to various parts of history to have “interesting and authentic experiences.” This being is, in fact, everybody.

  43. Twistyarm says:

    Shamus’ theory brings up interesting consequences when applied to existing fiction where time travel works effectively: are there higher powers at work guiding the timeline in an intelligent manner in that universe? (Well, yes…the writers…) Shows like Quantum Leap give a definite yes to the idea of God or Father Time adjusting things, as do most comic books and possibly Star Trek (The Q have to do something when they're bored).

  44. Claire says:

    String theory and M-theory postulate extra, tiny space-like dimensions… what is needed for timeline changes (in the absence of multiverse theory, or perhaps as a supplement to it) is an additional, unbounded (or at least, usably-large) time-like dimension. Just take the idea that space and time are alike, but temper it with the wisdom that they are in some ways un-alike. For instance, we can accelerate in space quite easily (up to a point), but accelerating/deccelerating in time (or teleporting in time, as much time travel mythology apparently indicates) is a neat trick. However, because time is dimensional (like space), we can conceive of a timeline as a sort of “block” (look up block time or 4-dimensionalism.) Gods trying to mess you up. (Seriously, though, my father has made the claim, in all earnestness, that the Tower of Babel was probably a particle accelerator.) Certain rules bear on how objects relate to one another in spacetime… if X is moving at 3 m/s away from Y at time t, then at time t+1 second, assuming no force acts on X or Y, X must be 3 meters further from Y than it was at time t. If the universe is deterministic (as it seems it must be for block-time to be true) then the laws of physics (the “real” ones, not human approximations… yes, huge metaphysical realist bias here…) and the state of the world at a time entail both the history and past of that world. (that is, you can predict, or post-dict, at least in principle, any event in the timeline.)

    This isn’t so weird… consider a spacetime block as a like heap of slag. There’s a certain way slag piles up and becomes a heap, and this depends on physical laws and where it’s placed… but if you drop 20 million bits of slag from above a point, you’re going to get a roughly conical heap. Now, making changes in the particular shape of that heap requires, at the very least, time. But to make changes in a spacetime block, we can’t use time. We’re trying to make changes TO time. So, we need hypertime, and background time-like dimension in which block universes exist.

  45. Duffy says:

    I will admit that Time Travel is one of my favorite debate topics and a plot mechanic I loath now.

    *Disclaimer: This theory is based on my limited knowledge of astro-physics, such that we and the universe are spinning through space and since the universe is expanding we are spinning away from the center point.

    My main complaint, which Caffeine brought up, is the state of time. The past does not exist, nor does the future, they are states of matter at specific locations, at specific times (time being a measurement we use and not an intrinsic property or force) within the universe. Once the states change (very fast rate), that particular arrangement is gone and thus that moment in time is gone. You can’t travel back in time to say 1776 because all that’s there (most likely) is empty space. The universal arrangement that was the signing of the Declaration of Independence in 1776 no longer exists.

    The only way to travel “back” would be to reassert the entire Universe’s state back to match that moment, which I am pretty sure is impossible (would require massive amounts of energy, data on the state and the changes needed to get to it, and teleportaion/matter altering technology), but doing so wouldn’t actually be time travel, it would be popping the universe into that State in the current timeline; wiping out the physical existence of everything the universe has done up to that point. However the actual dates and progression of time(from the Universe’s point of view) have not changed.

    Since you can’t time travel anyways, I don’t have to worry about causality. Now to hand wave for a moment, if you could magically somehow do it, there is no logical reason the self-consistency plan would work (since you can’t time travel anyways) therefore I would posit that Shamus is right, any change no matter how small has a chance of significantly changing the future, and probably negating your existence. Hell, you could bump into a person and immediately change from yourself into a completely different time traveling person of the opposite gender who wouldn’t even notice anything had happened. The person you bumped into wouldn’t even notice since from his point of view it was always a woman. It’s as possible as any other change.

    That’s something that always irked me, in almost any time travel system that allowed causality, changes should propagate thoroughly, no one, yourself included, would ever notice the change.

    I won’t bother attacking multiverse ideas since I lack any psuedo-science to back it up aside from I just don’t think it makes any sense.

    Done ranting now.

  46. Zel says:

    If you want a pretty interesting movie about time travel, I’d recommend Primer. All the pseudo-technical talk at the beginning can be a turn-off, but once the time travels start happening the events becomes very hard to follow. By the end, I was completely confused and had to watch it again and read an explanation to finally get what happened.

    I think time travel is possible in both directions, but with severe limitations. You can witness the past of your particular location, if you can somehow go faster than light or take a shortcut through space that light cannot. You can witness the past of all other locations, how far in the past depends on your relative distance to them. But you cannot interact with the past or alter it in any way.

    The future is the opposite, you can interact with it, by doing something in the present, but you can only guess what will happen, and hope you’ll witness it. The probability it will happen depends on what you know and what’s true of what you know. Sometimes this probability is close to 1 (100% sure), but you’re never safe from anyone or anything altering your prediction by his own actions. The future is anything but set in stone, as the smallest thing can change it radically (see examples from everyone, they apply to present actions on the future). You just can’t know what it will be like for sure.

  47. Claire says:

    Oh god, the particle accelerate thing is obviously misplaced in my message above… I was talking to someone struggling with physics, lulz.

  48. Suraj says:

    Like Nixorbo I think the time travel theories in Discworld are the best. You should read Thief of Time in order to understand it best.

  49. Kellandros says:

    I’m trying to remember where, but there was an article about a genius working on a different view of time and space.

    Physics relies on time and the speed of light as the two great constants. Every other term/measurement can be derived from those two. Relativity, however shows that time is not exactly constant.

    His theory/argument was that time is derived from velocity, instead of the reverse. There are no objects in the universe that are not in motion; the best you can do is be still within a given frame of reference- and then everything has the same starting velocity relative to the larger frame of reference.

    This would make time travel pretty much impossible though. But I did always find this idea interesting and simpler than requiring string theory.

  50. Skyy says:

    I highly advise you read Brian Greene’s “The Fabric of the Cosmos” (and its precursor, “The Elegant Universe”). Both tackle quantum mechanics, relativity, and spacetime in ways that the average intelligent but scientifically (specifically physics) ignorant reader can comprehend. “Fabric” has a large section devoted specifically to the possibility of time travel and these so-called “paradoxes”, and outlines exactly why they are illusions of our “common sense” perception of reality.

    I’d recommend the same to anyone else attempting to muck about with pseudo-theories about how time travel would “really work”.

    1. Shamus says:

      I did see Primer, and I did get lost near the end. I got what he was doing technologically, but not WHY. Then I got lost on the minutia. I intended to go and read a synopsis and try to sort it out, but never got around to it.

  51. Yar Kramer says:

    @NRowan: The thing about A Sound of Thunder is that it relies a bit too much on that hovering walkway-thing as a plot device. Apart from anything else, what did they do when they set it up? Second of all, stepping on a butterfly changes modern-day politics for the worse, but shooting great big dinosaurs has no effect whatsoever? Third, in the story, politics is all that changed. The change was started millions of years before humanity even existed. We’re supposed to believe that humanity STILL EXISTS (essentially, Shamus’s conception-argument applied to evolution itself), let alone has roughly the same people?

    As for “history with inertia”, that’s an interesting point. Shooting Hitler won’t simply erase the circumstances that led to World War II. (I once had an idea for a story in which two characters discuss time travel, one of them mentions giving Hitler art lessons so that he’d get into that art academy he was rejected from, and the other one says “Yeah, in this alternate history, Hitler never goes into politics but instead becomes the world’s greatest artist, hailed as a modern Da Vinci, but is tragically killed in the bombing of London.” “What?” “Germany goes on to win World War II, because they aren’t being led by such a damn fool this time around.”) You’d have to go back far enough that the changes would also erase World War I.

    As for the 2002 Time Machine movie, someone should make a montage of all the scenes leading up to the woman’s death, and dub “Might is Right but Tight” with the caption “GAME OVER – Press R to reload” over the deaths.

  52. Veylon says:

    That book “Millenium” had a framework where, if changes were made in history, it took time back in the future.

    Also, it’s funny that I, too, had an idea about people going back to assassinate Hitler. Only, someone else had already done that (killed him in his cradle), and they get picked up by the police after spending too much time wandering around asking questions that don’t make any sense and all their future equipment gets confiscated. I don’t know where things go after that, but…

  53. Clint says:

    As long as we’re on the subject of time travel, WikiHistory is an amusing short story about a future where time travel is commonplace. It doesn’t follow the chaos-theory guidelines that Shamus mentioned in the article, but it’s a fun read nevertheless.

  54. Daemian Lucifer says:

    The butterfly effect is an excelent movie about chaos theory showing exactly that:Even the smallest change can alter the entire universe in ways you cannot imagine.

    The theory of parallel universes is the most plausable one when dealing with time travel,and it eliminates all the paradoxes.This way,you are never altering your own timeline,but some very similar one where a different you is being affected.

  55. Gray Ghost says:

    If you’re in the mood for interesting speculations on this topic, I’d recommend Larry Niven’s essay “The Theory and Practice of Time Travel;” Niven is a sci-fi writer with a good grasp of science and a good sense of humor to boot — it’s been awhile since I read it, but I remember it as being quite interesting, and along the lines of what you’re doing here.

    The same volume has a bunch of other similar Niven essays, my favorite being “Man of Steel, Woman of Kleenex,” which details the incredible lengths Superman would have to take to procreate with Lois Lane — quite hilarious.

  56. Chuk says:

    You can time travel to the future already, I’m doing it right now. It takes me about sixty minutes to travel one hour ahead, though.

    But you could do it faster if you travel at relativistic velocities. There are astronauts who are slightly younger than people who were born at the exact same time as they were.

    Of course, you can never come back, so it’s not like time travel is usually portrayed in stories.

    I’m glad someone brought up the fact that the Earth is constantly rotating and revolving (not to mention that the solar system and the galaxy are moving too), so that if you could travel in only time, you’d end up in airless space. Unless gravity affects time travel somehow.

    I think it was Larry Niven who first wrote about the problem with time travel changing the past. If you have a universe where time travel changes the past, the past keeps getting changed, until one of the changes makes time travel impossible, and then the universe stays like that. So the most likely state for the universe to be in is one in which time travel that can change the past is impossible.

  57. steve? says:

    I still think it is possible to make small, yet noticed changes in the timeline. If a time travelling dog were to run across my field of vison (and upon leaving it promptly went back to the future without anyone else seeing it) I don’t think it would change any of my actions.

    I know that it would minutely change air currents and bend some blades of grass, but those effects are so small that it would probably take thousands of years for those to matter in the timeline.

    Any change in the timeline needs to have an associated length of time with it. If I went 5 minutes into the past and shot the queen of England and came back I wouldn’t notice a change. However if I shot the queen of England 50 years ago, then I’m sure I would notice some significant changes upon arriving back to the present.

    All actions in the past have some space/time propagation rate, to find a minor change due to time travel you just need to time it so that the amount of time you go back is the “elbow” of the effects vs time curve.

  58. Robyrt says:

    You’re giving the butterfly effect too much credence here, Shamus. We don’t know that history is an inherently unstable system. For all we know, it could already be at a stable point, and no amount of time travel will perturb it from its course. Of course, the first time you time travel, you’ll find out. :-P

  59. rbtroj says:

    I often have to discourage my wife from traveling through time by insisting that it will undo the genesis of our children.

    Of course, the way I describe it to her is usually more colorful. Like, “if you throw off by even one second the time I blew my wad in you, then you’ve just murdered our child who has yet to be conceived. However, feel free to travel back in time AFTER the children were born and have sex with me more. A lot more.”

  60. Xak says:

    Don’t know who here are fans of interactive fiction, but check out a short story/game called Shrapnel.


    That’s a list of all the IF games that this guy has done, but Shrapnel is among my favorites. I can’t tell you too much about what happens since it’ll ruin it all, but it involves a lot of time travel.

    Oh, and if you die the first time around, don’t get too frustrated. ;)

  61. Magnus says:

    I’d subscribe to the “only forwards” theories, mostly because it feels right (very scientific!).

    But to ignore all the phsyics posturing for a moment, if you were able to go back in time Hollywood style, surely you would also be taking the vast amount of microorganisms, that use your body as home, back also. Anything you touch, breathe on, could have transfer of organisms. Very War of the Worlds, but from a slightly different perspective.

  62. General Karthos says:

    Hmmm… so not sure if anyone has mentioned this.

    I was thinking along these lines… actually have been for a while.

    You take a trip back to 1909. Whatever you do back then has already been done. It has already happened. So not only can you NOT change history, but your trip back in time was ABSOLUTELY NEEDED for the world to continue as it was supposed to.

    Of course, this means that up until you complete the time travel trip back, you have no free will… so let’s think about that for a while.

  63. Josh W says:

    I enjoy talking physics and astrophysics. What about this though…

    Perhaps Time Travel/Time Altering is impossible simply because the concept of “time” is meaningless. What if the human concept of “time” is simply the attempt of the brain to quantify an infinite concept that has no finite paralell? Ultimately, if you cannot get to a beginning, and cannot arrive at an end, then the concept of “time” is really irrelavent except as a “local” event management tool. In physics, we talk about multiple dimensions but honestly, is there really a way to describe “travel in the n-th dimension” in a way that our finite minds understand?

    I am reminded of a book I read once called “Flatland” by Edwin A. Abbot. Although it was presented in the context of a commentary on Victorian society, the mathematical and “physical” principles behind the story were profound to say the least.

    Not saying that the above argument is even a valid one, but although I am fascinated by the discussion of multiple dimensions and space-time theory, I have a hard time saying that that is absolutely the right path to go down for a workable theory…

  64. Lupis42 says:

    I see two flaws in the application of Chaos to actions performed in the past. The first, and simplest, is that what seems like a relatively large change can actually have an unexpectedly small effect. Say for a moment that you went back in time and arranged for Hitler to be accepted into Art School. It might turn out that another little brown-haired man with a toothbrush mustache named Adolf Hitleman goes on to achieve almost exactly the same things. The people and events closest to the change would probably be the most affected, but if the net effect of any change on the next event in the causal path is less than the magnitude of the initial event, you have causality shock absorbers instead of ripples.

    What’s more, the majority of the ripples will propagate very slowly. If you go back in time 3 months, and blow up Alpha Centauri, then return to your start time, nothing that you can perceive will have changed.

    (Anathem has a discussion on a related topic, called causal domains. I won’t reproduce it in full here, but imagine that every event projects a cone forward in spacetime. The cone is the “causal domain” of the event, and includes any point in spacetime that a photon, travelling at the speed of light, could reach.)

  65. Lupis42 says:

    @General Karthos:
    Free will is not inherently removed simply because what you do is determined, you just have to look at free will as having a different chronology than you.
    When you travel back in time, you become an observer of your own actions and decisions. Those decisions have already been made by you, at the time you have traveled back to. Your consciousness, however, is still causally in the future, perceiving the results of choices it has already made, but was unaware of.
    Imagine that you watch a home video of actions that you took yesterday, but no longer remember. You no longer have free will in regards to those decisions, they are already made, but you *did* have free will at the time that you made them.

  66. Cuthalion says:

    “Who says he didn’t invent the thing?”

  67. Shishberg says:

    “Me too” at Alrenous and MadTinkerer about the weather thing. After all, a time travelling human is bigger than a butterfly. And it might not even have to affect someone to the level of sleeping in or looking up or anything – not that I know much about the details, but I’m guessing that any change in temperature would affect a guy’s metabolism enough to differently shuffle his, uh… essence.

  68. NBSRDan says:

    Before we can even start thinking about the exponentially growing changes caused merely by the displacement of air required to bring a human back in time alive, we must make sure our time machine is also the most sophisticated spacial teleporter in the universe. Even if the sun is in an exact, fixed position in the universe, which is doubtful, a safe time machine would still only be able to travel in increments of exactly 1 year.

    There are two common ways to get around the butterfly effect that you didn’t mention:
    The first is the static model (e.g. Gargoyles; Harry Potter), in which the results of every change made in the past are visible before and after time travel.
    The second is the branching out model (e.g. Dragon Ball Z), in which each backward time movement creates a parallel universe where the changes have taken place, while preserving the original universe of the time traveler.

  69. Graffy says:

    Ya know what’s funny? A pair of PhD’s from MIT wrote an experimental game on this same idea – think CnC with time travel. You destroy an enemy’s base in the past and the repercussions move forward at linear speed. A friend of mine is producing the sound track. Ya know what’s funnier? EA wants to buy the rights.

  70. Lupis42 says:

    Usually, as far as I can work out, time travel (as described in fiction) is either A)Inherently a superset of space travel, or B)Stationary with respect to the time machine, that is to say, wherever the time machine was when you come through it, that’s where you are, or C)Stationary with respect to the film set, meaning whatever film set you were on at one end, that’s the set you’ll be on at the other.

  71. Time travelers don’t change history. They replace it.

  72. Pinky says:

    I was going to bring up Achron, but it seems that several people have already done so. Oh well.

  73. Graffy76 says:

    Achron. That looks like it. Suppose it’s fairly obvious I didn’t bother looking at the other posts, eh?

  74. Derek K. says:

    Martin has hit on it – Chaos Theory generally turns in to “What Ian Malcolm discussed in Jurassic Park.”

    There are two big pieces to Chaos Theory – Sensitive Dependence on Initial Conditions, and Strange Attractors. SDIC is what Malcolm talks about – minute changes have a big effect – it’s usually called the Butterfly effect.

    The other part, though, is Strange Attractors. In a general sense, that basically says that minute changes *can* have large effects. But sometimes, they’ll end up right where you would have started from. It’s amusing that graphs of strange attractors often end up looking like butterflies (well, amusing to me….).

    But basically, it’s saying that sometimes, lots of places lead you to the same place, and sometimes, a tiny change moves you to hugely different places.

    An example I heard that is not at all related, but an interesting model:

    Imagine a trampoline. There’s a big weight in the middle. You put a marble on the edge, and it rolls to the middle. You move the marble an inch out, and it’s not in the depression, so it rolls where ever. But you move the marble all the way across the trampoline to the other side, and it rolls to the middle again. The path was different, but the end result was the same. That’s a Strange Attractor (it’s not, but it is).

    I’m also hand-waving a lot, and probably making people that really know what I’m talking about annoyed. ;)

  75. General Karthos says:

    @ Lupis42: That’s an interesting way of looking at it. It’s perfectly valid, I think, but when you’ve got your own personal time line that doesn’t coincide with the world’s time line, an action that is in your own future is in the past for the world, meaning that to an observer who is not a time traveller, you have already acted.

    Perhaps he can even tell you what you did. Or hand you a sealed envelope telling you what you did. No matter how “unpredictable” you try to be, since you have “already” done it, you cannot change your actions.

    That’s what I meant by not having free will. Maybe you -think- you have free will, but apparently you don’t. Unless you somehow dimensionally slide into a parallel universe when you travel through time. (When we travel through three dimensions, we unintentionally and uncontrollably move through the fourth, which is time.) Maybe when we move through four dimensions we unintentionally and uncontrollably travel through a fifth dimension? (Leaving aside for the time being the fact that the 7 dimensions beyond time are too small to be observed and have very little affect on us beyond the sub-atomic level.)

    Now going into the future is nice and easy. Take a quick trip close to the event horizon of a black hole, or travel at (or very, VERY close to) the speed of light, and you will have -effectively- traveled into the future.

    There’s just no way back. Unless you travel into a future where they have invented time machines and you can steal one. (Or buy one.)

  76. Sittem says:

    For years, I always thought that time travel would never be figured out, based on the idea that if at any point in the future it WERE figured out, someone would have gone back far enough that it would exist NOW.

    Then I heard a long interview with a scientist who has been working on time travel for forty years or so (I think it was a segment on This American Life). What he’s worked out is that time travel should be possible, but only from the point of its invention onward. That is, if I invent a time machine in 2010, I can use it to travel as far forward in time as I want, but I’ll never be able to travel back to before the moment I first turned it on, in 2010.

    It made a lot of sense to me. The amazing film Primer used a similar model of time travel, but unlike the movie, I didn’t need to listen to the interview eight times before I fully understood it.

  77. Avatar says:

    Not all chaotic changes will “take”, though.

    A butterfly flaps its wings and a storm forms halfway across the world. Okay, possibly, but not -necessarily-. It’s entirely possible that the effects would be lost in the chaos of the system, especially if they’re small. If you time-travel to 1935 St. Louis, fart, and go home, it’s hardly inevitable that every person born in St. Louis that year will be radically different. Sure, you MIGHT change something (and because you can’t reliably predict such things, time travel into the past without some kind of forking universe is dangerous as all hell), but you might not, as well. You could probably stomp around the city all day, leave a little garbage lying around, say hello to people you meet, and still not have any kind of lasting effect on the world.

    Heck, you could even save someone’s life, and so long as they didn’t become “important”, or affect someone who was, it probably wouldn’t do much to affect the time stream as a whole – at any rate, not enough to prevent you from existing and causing paradox.

    Of course, this rules out most of the interesting things you could do with time travel. No talking to Lincoln, no killing Hitler, and for goodness’ sake stay the hell away from your grandparents.

    (All this assumes that the past is mutable to start with. An alternative possibility is that changing the past is impossible, because the changes you made -were made already-. Real questions of predestination there, though; by the same logic, if you met a time traveler from the future, it would be impossible to prevent his future from coming about…)

  78. Corsair says:

    Yeah, Avatar, but what Shamus is saying is that the slightest permutation causes drastic differences in history. You nudge someone, and you will alter the flow of their time including when they procreate, and when dealing with conception, the slightest change results in massive differences. You might not notice significant change in a year, but you would in 25 years, because this guy has bumped a lot of other people, who bumped more people, who bumped Queen Elizabeth, Jimmy Carter, and the Pope.

  79. HeadHunter says:

    I don’t believe that chaos theory and the Butterfly Effect are quite as powerful as people seem to think – but that’s academic and it’s hard to change opinions on theories.

    One thing I do think you’ve overlooked in your reasoning, Shamus – while every action or decision creates multiple possible universes, not all paths *diverge* forever. Some choices made later along a timeline might cause it to *converge* with another once more.

    Don’t think of alternate timelines as rays emitting from a point, rather, think of it as an ever-expanding wedge.

  80. ngthagg says:

    I apologize if anyone has posted this already–I haven’t read through all the comments.

    http://www.youtube.com/watch?v=aCQx9U6awFw is a brief explanation of higher dimensions that is pretty easy to follow.

    One of the ideas that this video presents, as it relates to time travel, is that all possible time lines already exist. We happen to be experiencing one right now, but the other ones, including those which feature time travel, also exist.

    But this still raises an interesting question for me: suppose you had a machine capable of traveling in the sixth dimension. Obviously, you could travel to certain timelines and do things like burying iPods at Pompeii. That timeline already exists, and you are just visiting it. But could you travel to a timeline which features no time traveling? It seems to me the answer must be no. A 6D traveling machine must be inherently limited: you can only travel to a timeline which features you traveling to it.

  81. deiseach says:

    General Karthos hits upon what annoys me about time travel as a conceit, even a fictional one – it negates free will. Either the events that happened when the time traveller travelled were meant to happen, in which case existence is one great big piece and we are all but pawns in some grander plan, or there are multiple / infinite universes (infiniverses?) in which every action is played out, and if every action that could possibly occur does occur somewhere out there then there is no thought attached to your actions because it’s going to happen.

    Man, that’s a lot of commas.

  82. empty_other says:

    If anyone of you have looked a bit about time traveling around the net, you probably have read this. But i repost it for those who havent..:

    And i have this poster hanging on our door to the WC at home: http://www.joeydevilla.com/2009/04/11/time-travel-cheat-sheet/
    … you know, just in case anyone found the hidden time-travel button on my electric razor.

  83. Simplex says:

    Shamus, Few years ago I had this revelation about conception and time travel. I tried sharing it with friends, but they did not see how profound it was.

    As far as time travel and geekiness is concerned, I think you will like this site:

    It analyses time travel in movies like Terminator, Back to the Future, etc.

  84. Moridin says:

    Interestingly, while current theories of physics seem to allow time travel, they allow it only to the point where the time machine was done. There is one problem with the butterfly effect: You’re assuming indeterminism. There is no reason to do so.

    Nitpick: String theory actually calls for 10 dimensions. The M-theory calls for 11.

  85. Gasoline says:

    Didn’t read all the posts (lack of time), so maybe I am repeating a thought already mentioned.

    Who says that you would be able to change the past (and wit that the future) at all? Everything you would do while travelling back in time, would already have happened in your present. So you would not change the timeline, just fullfill history.

  86. Zaxares says:

    I’m a proponent of the “Constant Now” theory of time travel. Put basically, going back into the past to attempt to change it would make utterly no difference, because it’s already been done. Planning on going back to assassinate Hitler? You may as well not bother. The fact that history shows Hitler still managed to carry out all his misdeeds means that either you failed in your attempt, or you called it off altogether. While there may eventually be technology that could take you into the past (or into the future), such technology has no effect on the timeline, because a timeline does not exist. There is no past. There is no future. There is only the present moment.

  87. Mayhem says:

    Two very interesting works on time travel and resultant effects are

    Where people from the future try to change their history by preventing Columbus reaching America only to discover that people from *another* future had specifically sent him there to prevent their own disastrous history from happening.

    The other is

    Where the mechanism for communicating with the past is to send a message by manipulating tachyons (which are particles that travel faster than the speed of light) and pointing them at a specific area of space which is where the earth *was* some 30 years earlier and when a scientist was known to be conducting the first research on tachyons.

    Both stories result in the destruction of the future from which the events came, but both involve a large amount of trying to avoid the various paradoxes involved.

  88. MOM says:

    How similar this conversation is to a conversation about competing Christian doctrines of Calvinism (total predestination) and armenianism ( absolute free will of man)

  89. Larryboy114 says:

    So as a physics grad student and a long-time sci-fi nerd, time travel is one of my favorite subjects. The vast majority of physicists believe it simply isn’t possible and that if it is, only due to the many-worlds interpretation of Quantum Mechanics. The general view is that we time flows in only one direction. My response is that if we exist in three spatial dimensions symmetric about zero, why is there only one time dimension that travels in only one direction? Why not 3 time dimensions also symmetric about zero? (See Heinlein’s Number of the Beast: http://en.wikipedia.org/wiki/The_Number_of_the_Beast_(novel))

    Also, food for thought, no one has debunked this guy yet: http://www.physorg.com/news63371210.html
    He claims he can create an experiment to prove time travel using Einstein’s equations. His work has actually been refereed and published.

  90. Mephane says:

    One thing many people also forget about time travel: If it is ever possible, always do it in open space, as far away from any stars, planets or whatever as possible.

    If you are using a time machine here on Earth and go back 1 day, you will die a horrific death in space, as Earth has since moved on. Depending on the time of day where you are, travelling only a few seconds backwards or forwards could cause you fall to your death or be stuck in the rock underground (although I think merging two pieces of matter this way would cause them to explode because the electrical forces that prevent solid matter from going through each other would immediately come into effect.

    I find this worse than all the casuality paradoxa. Because this is an issue that we actually know how it would end, but yet authors pretty much always forget about it. If there’s any time travel ever, you want to do it in space ships. Anything else is like a lottery where not winning the jackpot would kill you, like, really.

    (This also produces more paradoxa, since positioning is always relative, where would you actually end up if you travel through time but stay in the same spot? Earth moves, the sun moves, the whole galaxy moves, the local galaxy cluster moves and so on, and there is no absolute position, only always relative to other things. So if you stay in the same spot during your time travelling – in relation to what?)

  91. HeadHunter says:

    To clarify a little on my earlier concept:

    Two people are standing at a street corner. One goes North, the other goes East. Shamus’ theory is that those two paths would diverge, never to rejoin again…

    …until the first guy makes a right turn at the next two corners and winds up at the same place as the second fellow.

  92. Mephane says:

    My response is that if we exist in three spatial dimensions symmetric about zero, why is there only one time dimension that travels in only one direction? Why not 3 time dimensions also symmetric about zero?

    I tend to believe that the analogy “time = a 4th dimension in addition to 3 space dimensions” is simply wrong. Time is of significant different quality than space. It’s like the ether – scientists once believe something they called “ether” to fill everything and that electromagnetic waves would be waves in that ether similarly to waves on water. They were finally proven wrong, however, that this ether does not exist, and that electromagnetic waves can exist without such a “medium”. I think one day, scientists will find out that time does not exist, and is only an abstract model to better observe and understand physical processes. Just like there is no centrifugal force, it only seems so, it’s actually just inertia. Yet we treat it as if it existed, because it helps understanding some processes better.

  93. Strangeite says:

    LarryBoy114: I was waiting for someone to bring up Number of the Beast.

  94. Moridin says:

    @Mephane: You’re assuming an absolute frame of reference.

  95. Carra says:

    I also figured this out quite some time ago. Just by going back into time you’re changing things. It’s know as the butterfly effect as has been said in previous comments. Maybe by going back in time you change the windpatterns which could cause a storm. Probable? No. Possible? Yes. Just by going back into time you can change huge things.

    If people see you things go wild quickly. They’re seeing you, are having different thoughts, are doing different things. And yes, maybe by having to evade you on the street, another sperm wins the battle in the evening.

    An alternative to time travel is used in the books I’m reading now (Ender’s series by Orson Scott carrd). Just send someone into space near lightspeed. The moment he gets back a lot of time will have passed for those who remained behind.

    And yeah, chaos theory does remind me of Jurassic Park :)

  96. ngthagg says:

    Great time-travel comic: http://www.pvponline.com/2009/05/13/waste-of-time/

    Again, I apologize if someone has already linked this. I’m still too lazy to read all the comments.

  97. Dennis Brennan says:

    A few things that may not have been mentioned yet:

    * A Quran, Arabic language books and appropriate maps: wherever in Europe I end up, Cairo or Baghdad is going to be more pleasant.
    * Microfiche copies of appropriate books and technical manuals, and a lens. Things like recipes for gunpowder and concrete may be of interest. (The Egyptians and, later and more famously, the Romans, used concrete, but hte formula was then lost until the 1700s).
    * A sturdy plastic tarp
    * Flea repellent. Bubonic plague is going to show up in the mid-1300s.
    * A map of major gold deposits, such as the ones in Styria and Wales, and the silver mines in Frieberg, Saxony (which weren’t discovered until the 12th century). I might not be able to mine them myself, but I might be able to suck up to the local monarch by pointing out where they are.
    * A nice mirror

  98. Shishberg says:


    Re-convergence is only likely if there are a very small number of degrees of freedom, e.g. 2 in your analogy (since they’re walking in 2-dimensional space). Turn the degrees of freedom up to several gazillion and you have almost no chance of them meeting again.

    A better analogy would be this: you shuffle a deck of cards and it changes the order. Then you shuffle it again, and it puts it back in exactly the same order you started with. Sure, it’s technically possible. But only just. And it gets less likely very quickly the more cards there are in the deck.

    (Yes, this gets more complicated when we throw in strange attractors. But I stand by my last comment.)

  99. Derek K. says:

    To expand the bump concept:

    I bumped Bob, which slowed him down 3 seconds, adjusting his whole life.

    Except that Bob reaches the corner, where the “Don’t Walk” sign has been lit for 10 seconds, and turns to “Walk” 2 seconds later. So Bob is now reset to where he would have been.

  100. PAK says:

    I’ve tried to read through all the comments and didn’t see anyone else mention this. Contrary to someone’s (Factoid, I believe) assertion that storytellers never tell compelling time travel stories involving a fixed and immutable timeline, I would argue that “The Time Traveler’s Wife” is the best treatment of the material I have ever read. Not actually science fiction (it’s more magical realism), it takes the implications of time travel far more seriously than most works that think they’re sci-fi. And it’s a really compelling piece of literature, too.

  101. Zock says:

    I think you are putting too much emphasis on individuals. For example, if you look at the history, many of the new and revolutionary ideas were thought by a group or people, or multiple persons at the same time in different places with little or no knowledge of each other.

    What if the reason why some people seem to rise above others is only because they happen to be in the right place at the same time? If they wouldn’t have been there, someone else would have taken their place. It’s a bit like a play – you can change the actors without affecting the script.

  102. Vesa says:

    In terms of chaos theory, what you’re wondering about is the Lyapunov exponent of history. If the Lyapunov exponent is above 1, then slight changes at some time will lead to very large changes at some time later – the greater the exponent, the more quickly it happens as changes amplify. If the exponent is less or negative, changes dissipate and you get a “ripples in the pond” effect.

    The fact that weather is very sensitive to initial conditions suggests the Lyapunov exponent of history is above one (self-consistency principle notwithstanding). Of course, that still leaves the question: How much above one?

  103. Burning says:

    My conclusion from reading this thread: theories of time travel tell you more about the person espousing the theory than they do about the nature of reality.

  104. Techni says:

    I agree with Legal Tender. If you go back in time, you have created a new time line. You could kill your grandfather before you were born if you felt like it, it would not erase you. But when you go back to the future, it will be one where you never existed.

    No chance for paradox.

  105. Captain 420 says:

    Travelling back in time is impossible, because people are stupid. If it were possible, some idiot from the future would go back in time and fuck everything up for everybody for all history. That hasn’t happened, so, ipso facto, backwards time travel is impossible.

  106. Sitte says:

    Captain 420: see my comment (#79)

    Basically, you’re exactly where I was for most of my life. Now, instead of “backwards time travel is impossible” I say “UNBOUNDED backwards time travel is impossible.”

    When invention day comes, TS will indeed HTF.

  107. Hankroyd says:

    “Pompeii is an excellent candidate for this sort of time-traveling prank. Go there just before the eruption of Mount Vesuvius and hand out a few iPods or revolvers or something. Since the city was buried, your historical shenanigans will be concealed for 1,700 years, and the timeline will proceed unchanged in the outside world.”

    I guess you suffer a little of the ‘Marty McFly Syndrom’ too. :)
    There is no such thing as a ‘non altering’ time travel.

    Conservation of matter is here for this :
    Let’s say you appear at pompeii half an hour before the eruption. During five minutes, you give stuff and then go back. Then lava goes up, everyone dies.
    You have altered something very important : The atmosphere, either by moving or suppressing molecules in the atmosphere.
    There will be little change at first, but in the end, maybe a rain will be delayed for 5 minutes, or a lightning will fall 200 meters where it was supposed to fall … and this little changes will make more changes that will affect everyone’s life in the end.
    By doing your joke for futur archaeologist, you created a new set of alternate futur … don’t bother hoping to existate.

    That’s why, if a person is about to throw 10D20, you can delay him 5 seconds to alter the result, and you could go back a minute before the roll, in the next room to take a beer unoticed by everyone and alter the dice result : (because your intervention will alter the ‘position/speed’ of all molecules in the atmosphere, and that will be enough to randomize the roll again.

    There is no such thing as a ‘slight’ change, in time travel. Only changes that goes bigger as the time passes.

  108. Jim says:

    I believe timetraveling to the past will not change the present or future as we know and assume to be, respectively. If I were to time travel to the past, in the future, then I and everyone and also everything and possibly the technology needed to timetravel exists this way(now, in the present) because of the change I made in the past.
    (that is, given that I can’t return to the original present or timetravel again)

  109. Daniel B says:

    ” From AD 79 to AD 1748 the outside world will proceed along the familiar path, but the moment a digger sticks his shovel into some fragment of plastic or precision-machined metal your changes will suddenly manifest and propagate.”

    Wrong. They won’t have the technology to propogate them.

    Let’s narrow the gap to more recent – let’s say a dude from the 1950s somehow ends up with a 2011 laptop. He could figure out how to use the interface without too much trouble, and use the available programs. But he couldn’t make money off of it. His computer technology is based on punch cards and vacuum tubes. The transistors are MAGIC to him. Even if he figures out the principles behind them, the world won’t have the ability to create transistors of their own until several other chains of technology all coalesce.

  110. Mephane says:

    I know I am late to this thread, but while browsing your archive for something else I stumbled upon this and just had to answer.

    Personally, I made a thought experiment to think of time a bit different than is typically done: a movement through the 4th dimension, yes, at constant speed, away from a common origin. Obviously, that origin is the “Big Bang”. In other words, at the moment of the Big Bang, the universe had the size 0, and 0 time has passed. In this model, expansion of the universe and traversal of time are actually the same thing. One obvious candidate for the speed of time – which is the same as the speed of expansion – is the speed of light, yet I am unsure about this one. Let’s just call this the “speed of time” and don’t care about its value for now.

    The universe basically has the form of a hypersphere, and the surface is our three-dimensional space. Now, that means that in order to go back through time, you’d have to leave it’s surface, i.e. our 3D universe, and move through the 4th dimension towards the point of origin. Now here comes the real trick:

    The space inside the hypersphere is not empty, but instead, all past events are still there. Any state the universe has ever been in is (almost) immutably fixed, and progression of time always creates new states right above the surface of the hypersphere-universe. If you’d want to interact with the universe in the past, you’d have to move synchronously again, away from the origin, at the very speed of time. This means you can interact with a given past state of the universe and actually inflict changes to it (that’s why I said almost immutable). These changes themselves would propagate forward, i.e. away from the origin, at the speed of time. Because the outmost surface, what we would call the “present”, also moves outward at the speed of time, no changes would ever reach the present, but an irreversibly disconnected past has been created, crawling forward and basically overwriting the states coming after it.

    If two time travellers went back to modify the past, they would have to resynchronize with the speed of time at precisely the same distance from the origin, which is probably impossible anyway. This means the two time travellers (in seperate time machines) might land in the same spot just a nanosecond after another, and both would change the state of the universe at the point in time where they are, yet none would ever notice the other. The one in the position closer to the origin would ultimately overwrite all changes made by the other one, who on the other hand will always have the lead in the “race” forward in time.

    Well, this model also implies that you cannot travel into the future past the actual “present” surface of the universe. However, that could actually be in the distant future, and we may be living one of the many past waves overwriting each other. Parallel universes could exist in the same place, seperated by a unmeasurably small time offset.

    It is possible that no matter how fine your time resolution, going one step forward or backward might send you to a totally different universe. The past where you wanted to shoot your grandfather could already have been replaced a million times and might at the “moment” feature an Earth ruled by giant robot dinosaurs, because someone in the distant future thought it might be funny to try to make it happen.

    And then on top of that, maybe it is impossible to move away from the origin at a speed faster than time at all, which means that a) if you go back through time, you can only go forward again at the normal speed of time, living through the very changes you just did, and b) it is impossible to determine whether you are living in the actual present Maybe the present even lies an infinite distance away from the origin (which would mean no new states are created at the outer surface, because that doesn’t even exist, only an infinite number of states that are being overwritten again and again).

    Then, as the universe actually lies motionless and just has waves of state changes moving away from the origin, the origin itself, i.e. the Big Bang, never actually happened in the sense that there was a time when it all started, because it might always just have been, and just the path from there ever changed. So from a normal, 3D perspective, a specific time ago the universe started in a single point, but from a 4D perspective, there is no time at all, but only waves constantly moving away from the center.

    I am absolutely sure this model has so many holes that it wouldn’t hold a minute against anyone trying to dissect it, but at some point I had this idea and found it intriguing, as it could merge a couple of models and ideas while even solving some of the old paradoxa, and trading others for new ones.*

    *For example: the very notion of a state of the universe and subsequent changes of that same state would require a time-construct outside the proposed 4-dimensional model, like a hyper-time on a even higher level. Then, what if all that has been said applies to that one as well, leading to infitely layered time-dimensions all wrapping our feeble three dimensions of space.

    1. Shamus says:

      I’ve played around with a similar model where changes have to propagate forward. If you leap back to your own time, you don’t see them. If you want to see what a change in 1990 looks like in 2010, you have to wait 20 years, regardless of “where” you wait.

  111. Windona says:

    Personaly I’ve always been a fan of ‘alternate reality’ time travel; where you exist in Earth A, and when you go back in time the changes you create result in Earth B, and you can end up accidentally going into the future of Earth B because you took the wrong fork in the road.

  112. Talby says:

    Maybe someone has suggested this already, and I know this is an old post, but; Shamus, have you considered turning this premise into a story? Either a short story or a big honkin’ novel?

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