The classic example is killing Hitler, with there being variations from assassinating him in the 1930s to actually killing him as a baby (unsettling though that idea is).
I find that people usually project out the major historical changes this would cause while assuming the same people would mostly continue to be born unless there was some clear major effect preventing this. I disagree. I think if you went back and assassinated Hitler after he became a well-known world leader, and then went back to the present*, they would find that virtually no one born ten months or more after the assassination would be the same person. Sure, many of them in that initial period would be born around the same time to the same parents, arising from the same maternal egg their alt-universe counterparts did; but to be the exact person you are requires your parents to have a very specific life course right up to the moment of intercourse, and for the man to ejaculate at that exact moment in that exact position. To read about such a dramatic event would change that at least slightly for almost everyone fairly immediately, I believe.
If the assassination were achieved in infancy, and undertaken quietly and in an unobtrusive way people marked up to natural causes, you might find when coming back to the present that there were genetically identical individuals born months or maybe years afterward. But decades? I doubt that very much, as changes ripple quietly outward.
*To avoid paradoxes including the possibility the time traveler/assassin would have their own birth negated, we’ll assume that they go back to the 21st century, but in an alternate universe instantly created the moment they arrived in the past.
This is the sort of question that I love, even though it can have no possible practical purpose.
Assuming that the timeline can be changed by a time traveller (which you do) then there is no reason for the assassin to actually kill Hitler. The mere presence of the assassin would displace air molecules, and cause a butterfly effect that would propagate across the world at the speed of sound.
How significant that effect would be is another question entirely, and I can’t estimate that, though it is possible someone else might be able to do the math.
[from the trivia desk] The “butterfly effect” was a term coined by Edward Lorenz after his experiments on a Royal McBee LGP-30 computer — an antique computer with drum as its main memory that has been mentioned in another current thread.
Lorenz discovered the phenomenon when his weather simulator yielded completely different results when started with what Lorenz thought were the same initial conditions. At first he thought that the LGP-30 hardware was making arithmetic errors. He later determined that small differences in the initial conditions were soon amplified, and caused a wholly different path to be taken. In his famous paper (pdf) he concluded
Lorenz is, of course, not credited with the discovery of chaos theory. That distinction is reserved for Henri Poincaré, the top mathematician of the late 19th century.
It depends on how deterministic the universe is. In an extreme case, if we assume the presence of true randomness and/or free will, just moving the molecules would in effect create a new universe, which will move forward in a manner completely different from the “previous” one. Every coin flip or dice roll could have a different result, every decision could be different.
I don’t believe in the idea of a Clockwork Universe. If a time traveler were to go back to a particular date, I think it has the effect of restarting the timeline from that date, and all events that follow may be different based on natural randomness of action.
For example, if the timeline were restarted before a game of Monopoly, would every subsequent throw of the dice be identical from the first playing? I see no reason why it should be. If it was, then one should be able to predict every throw of that game from the initial conditions, since that is exactly where we are resetting the timeline.
Some things may stay the same, if you restart the timeline in 1975, Microsoft might still wind up a big deal, but there’s little chance for Google since the two founders would have been babes, and it’s unlikely that they would come together many years later to create the same company as the first time around.
Well, we know that the weather can be completely different after two weeks, from just a butterfly’s perturbation. If we assume that a time traveler causes a larger perturbation than a butterfly, then it’ll take no more than two weeks from the time traveler’s arrival for the weather to be completely different.
Now think about all of the things that the weather can affect. We don’t know exactly how chaotic human interactions are, but we do know that weather is one of the inputs to human interaction, so the whole system must be at least as chaotic as the weather.
In dynamical systems theory, one can speak of the Lyapunov exponent or Lyapunov time which describes how rapidly nearby trajectories diverge. But you will have to formally define your system to pin the time down even within a few orders of magnitude. (What Chronos said.)
if you believe , with Robert Forward, that any apparent changes you make are bound up in the system (that is, they aren’t really changes, but were always part of the time-stream of reality), then there’s no delay at all, no matter how you measure it.
Time travel changes take observable “time” to propagate through , well, time in movies like the Back to the Future series (with those fading in-and-out newspaper headlines and markings on tombstones) or throughout the film* A Sound of Thunder in order to build dramatic tension, and to show the time travelers who screwed up the timeline of their own reality that they have to get to work and fix it. It’s not because the filmmakers have some Grand Theory of Time Travel that posits how ripples in the time stream propagate through time.
*but not in Ray Bradbury’s original story. In his story, as in most science fiction of the time, effects of time travel are “instantaneous”.
I’m not disagreeing with you, but I feel compelled to point out that the point of the “butterfly effect”/chaos theory is that, even if the universe were completely deterministic, a seemingly minor change could result in a very different universe.
I agree with almost all of this, but I’m not sure you can assume that any “butterfly” change will neutralize Hitler, at any stage. I’m not sure just popping in and breathing a few breaths or whatever at the Nuremberg rallies will keep him from doing some bad stuff. I would be sure such a “pop-in” would make 2018 quite different from what it is in our timeline, but the 1930s and 1940s might be fairly similar.
If we assume the presence of true randomness and/or free will, then you would create a new universe which would move forward in a manner completely different from the “previous” one simply by starting again from a prior point, even if no molecules at all were moved. (This is basically the “many worlds” interpretation of quantum theory.)
We know it? The butterfly effect is a whimsical metaphor to describe an aspect of chaos theory, but it emerged from mathematical models. Do scientists think that the flap of a butterfly’s wings can literally cause a significant change in the weather at a distant location weeks later?
As much as I enjoy a good time travel story, where the changes are just a little off, like BTTF, the reality would most likely be 100% change. Go back to 1920 and kill Hitler, and 2018 would be unrecognizable. Not only would millions live that died before, millions would not be born who were now born. No one living on your street now would be there, most likely even the buildings wouldn’t be the same. Human nature being what it is, you could probably learn to assimilate, but you’d stick out. You’d ask someone where the Circle K is and they’d look at you funny, and you’d see job ads for “mystellogist” and “urban seamripper” and have no clue.
The state of technology might be far ahead of us currently, or far behind. There might be electric cars with vacuum tube radios. There might be no nuclear weapons, but Europe might be at war this very day.
In TNG’s Tapestry, where Q does a do-over on Picard’s life, and shows him the kind of change this thread is about, I wept* for all the billions of people born in the alternate timeline who simple never were once Q reset the timeline. What happened to all those people? They had entire lives in the years between the events. They lived, loved and died during those years. Did their lives have meaning after Q “never-wered” them out of existence again? Will no one remember them?
*figuratively speaking, of course
Well, a couple of things. First off, my own opinion is that if you were go to back in time to kill Hitler you’d basically be splitting off from our timeline and essentially creating a new universe or timeline. Coming back to this one would basically be exactly the same as when you left. Going back to this time in the new timeline would be substantially different.
I agree that many people born around that same time would have different lives…how could they not? Either there would be no WWII or it would be substantially different, with even the combatants being different (perhaps the USSR and some others against the UK and France with the US being neutral or something along those lines). People who died in our timeline might live in the new one, and vice versa. By the time we are talking about 70+ years later the world would be completely different…really, unrecognizable to the time traveler if he moved forward in the new timeline.
Here’s a chilling thought - what if we’re those people? Maybe *this *timeline is a mistake created by bumbling or malicious time travelers, and one day a hero will jump back and delete us all from existence.
Replace “weeks” with “centuries” and you’d have no doubt of the validity, right? Until we estimate the Lyapunov exponent DPRK refers to, we are just (to paraphrase Lord Beaverbrook) haggling over the price.
I think the OP’s point is that they’d be different people. If your conception had been timed just a little bit differently, it would have involved different sperm and/or eggs, and thus a different you.
I have doubt of the validity of this applied to large complex physical systems like weather (not mathematical models of weather). But I am not a chaos theory expert or even a mathematician.