“For want of a nail…”
The key is “For want ofa nail…” Not just any nail, but a specific nail.
If your plan was to alter a timeline by removing that nail, you would have to know just which nail to target. Just any random one of thousands or more wouldn’t likely make any difference at all.
So just like there are key people in history, there are presumably thousands of objects in lynchpin positions. But again, which are they?
The paying attention part is certainly not what’s important. That was just meant as a descriptor of how small the change is. Nobody’s going to alter the position of their feet or the timing of their steps based on movement of a single small pebble on the path. In that sense, it’s even less significant than tying your shoe is, because the choice of whether to stop and tie your shoe does affect the timing of other events.
Again, I can see arguments for either a positive-feedback universe in which infinitesimal changes nevertheless add up over time, and I can also see arguments for a negative-feedback universe in which a change that is small enough can enter the random background noise of the system without making any changes.
I am on the side of saying that it doesn’t take much change at all to produce profound effects in the future and those effects grow the more times passes after it. Some of the obvious reasons have already been given. It only takes the smallest of changes to influence what children will be born in the future because conception is a chaotic event that depends on very tiny probabilities of a given sperm meeting an egg. Simply waving ‘hi’ to any man (or any interaction at all) constantly changes the timeline on which children they eventually produce even if it is years after the event. Those children also influence both history directly and indirectly including what children are born in the future from then on so future world history is constantly being influenced by seemingly unimportant events.
In other words, science fiction writers tend to go way overboard in their estimation of what it would take for a time traveler to severely disrupt a given timeline. You don’t have to go back and kill Hitler as a baby or his mother for example. All you have to do is ask his father a brief question a few days before the conception or influence him in any other tiny way. The chances are outstanding that will be enough to make another sperm meet his mother’s egg or maybe none at all.
That is only one example. There are countless others as well even down to the microbial level so moving a single pebble could well be enough to permanently change world history even without assuming any unusual events. It could (and I think will) alter the environment enough so that a larger events build on top of it until it cascades across the entire world. There is experimental evidence for that. It is what chaos theory is all about.
The interesting question (to me at least) is how isolated and small a change you can make without necessarily affecting larger systems in major ways. I think any action on the Earth’s surface will inevitably produce cascading changes that eventually affect every major system on Earth but what if you somehow moved a pebble on Mars or Pluto without anyone else knowing about it? That may take a while to propagate across the solar system if it ever did.
Gawd, I love that story! “That way lies roast rump of skunk and madness.”
And someone else leads the “FAZI” party to domination over Germany and starts a major war against Italy, bringing in the rest of Europe and the world, ending with one A-Bomb dropped on Shimonoseki.
And even if that isn’t true…Hitler is one of those sensitive cases. Go back and interrupt the conception of Werther Gaines, who drove a supply truck in the Battle of the Bulge: how much is that going to alter history?
You have to loosen the nail of the King’s horse’s shoe. Unhorsing Pedro Gonzales, way back in the fourteenth rank of the battle, who only followed the momentum, never led it, isn’t going to reverse the outcome of Pavia.
RealityChuck got it right in post #2. There is no way to know.
As long as there have been stories, there have been stories of regret and a desire to change past events. If it is possible to change the past, than an intelligent species WILL figure out how to do so, unless it goes extinct first.
So we may not be on an unstable timeline, just the road to extinction, well, better to have existed, I suppose.
I see weather as the positive-feedback chaotic system that would largely change the world. Very small changes will add up fairly quickly, until it seems the weather pattern only a month out, and certainly a year out, would be entirely different. (not, I am talking weather pattern, as in the current placement of clouds and precipitation and stuff, not climate)
Many battles have been won, avoided or delayed because of weather. D-day in Normandy would have been the 5th of June, rather than the 6th, had there not been bad weather, would the invasion taking place a day later altered the outcome of the war? Maybe not. But I’m sure it would have altered the casualty lists on both sides substantially, creating a new population in this universe, and negating one from the previous.
A different person driving that truck may have caused different people to be in different places … some die with him there who would not have otherwise and the pther way around. That battle … maybe the same outcome. But one of those people somehow was someone who had contact however indirectly with the parent of someone of some more major - and thus no Steve Jobs, or no FaceBook or Twitter, or alternatively different people with other major impact are consequently born into that timeline as consequence of a host of people all having the timing of their lives altered. Maybe the battle lasts a bit longer and the news comes out a bit later thus across the world exactly when people stopped to read and discuss it was different and which egg and sperm then met later is different as well.
It does not need to be the King’s horse to alter the course of what happens to everyone in that battle even if all the same people live and die. Each one will move in a slightly different path, bump into a host of other people cascading little changes that result in different lives occurring, having never occured, or ending at a different time, magnifying over time. The path of the world would be very different. Maybe the shape would still look the same (or not), maybe the end result would be congruent (or not), again that depends on what sort of chaotic system human history is.
If the change is sufficient to change the weather, it’s sufficient to change the time of conception of thousands of people by a minute or two creating a new cast of characters. While economic fundamentals are interesting to disentangle when interpreting history, personality and happenstance also play substantial roles. After the weather is changed (not climate) I’m not seeing a lot of strong attractor basins.
A pebble change may or may not affect the weather. A change in travel plans by a single human for a day might: I don’t have the meteorological background to assess the Lorenz equations.
Didn’t read intervening posts, but there are two main theories about time travel: actions are new, or actions are old.
The “actions are new” theory is that whatever the time traveler does is new to the timeline, e.g. Back to the Future. However, this does not cause the timeline of the time traveler to cease to exist, it simply creates a new timeline and the new timeline coexists with the old one.
The “actions are old” theory is that whatever the time traveler does they have already done, e.g. Terminator 1. Whatever the time traveler does has already occurred. In the Time Machine version, time has consciousness so that no matter what the time traveler does, events will end up in the same result.
Therefore, in the OP’s post, it doesn’t matter. The time traveler’s actions will not affect the present.
I’ll say that moving a pebble is irrelevant.
The mere fact that you go back in time and cause events to “replay” is enough to massively change the outcome.
I don’t believe in strict determinism. Whatever state the world is in today does not completely define future events. I don’t believe the events of the next 1,000 years, 100 years, 1 year or even 1 day is preset based on the starting conditions. For the timeline to remain unchanged, this must be the case, because there is no force outside of the timeline ensuring that it follows along the original path. There is enough uncertainty in position and momentum, and enough particles for that uncertainty to act upon to ensure that no two trips down the timeline will be the same.