What are the best depictions of Time Travel?

[Q99]

Time travel, it's cool, but it's subject to a lot of handwaves.

Doctor Who just flat-out throws it's hands and says 'timey-wimey,' shifting the rules of how stuff works.

Back to the Future is pretty cool, but when you think about it, it does result in, "An effect can exist without a cause, but only for awhile, so you can have people without parents be around and then not."

What stories handle Time Travel the best, with least resorting to handwaviness and things moving back and forth on whether they can be done or not?

Bill & Ted's Excellent Adventure comes to mind- time travel is a thing, but not time alteration, there's one timeline with the characters moving around it.

Also, Schlock Mercenary's time-travel arc for the alternate consistent model of time travel. Without Paradoxal causality, we can't succeed

[The Romulan Republic]

Doctor Who occasionally uses it fairly well (as in "Blink", and to some extent "Waters of Mars"), and at other times exceedingly stupidly.

Terminator (the original film) does it very well in terms of having a consistent, coherent plot despite the continuity issues that tend to accompany time travel.

Gargoyles likewise takes the self-fulfilling loop route, which is my preferred method, narratively, for handling time travel, even if it does somewhat limit the author.

[bilateralrope]

I think that time travel stories can be broken down into three categories:
- The past can not be altered. Though it doesn't have to be what the characters thought it was in the first place. For example, Fry being his own grandfather in Futurama. He always was his own grandfather, it's just that nobody knew it until they went back in time.
- Not really time travel. It may look like time travel at first glance, but it's really travelling into a parallel universe, an illusion, etc.
- A story that that contradicts itself. For example, imagine someone going back in time to kill his younger self. If he succeeds, then he isn't around to go back in time, thus he can't possibly kill his younger self.

Only the first two types of time travel store hold up once you start thinking about them.

[Terralthra]

Primer is the best time travel story I've ever seen.

[Q99]

I think that time travel stories can be broken down into three categories:
- The past can not be altered. Though it doesn't have to be what the characters thought it was in the first place. For example, Fry being his own grandfather in Futurama. He always was his own grandfather, it's just that nobody knew it until they went back in time.
- Not really time travel. It may look like time travel at first glance, but it's really travelling into a parallel universe, an illusion, etc.
- A story that that contradicts itself. For example, imagine someone going back in time to kill his younger self. If he succeeds, then he isn't around to go back in time, thus he can't possibly kill his younger self.

Only the first two types of time travel store hold up once you start thinking about them.

The third works fine as long as you're fine with acausality. Someone goes back and kills their younger self. They don't need to go back in time to do it, because the event's happened. The killer version from the future can hang out and have pizzas, and margharita shooters. In cases like this, there's no going home for time travelers, they come back, change something, and that's it, it's changed. The only cause you need is "someone arrives in a time machine." To them, they have all the events that lead up to that, so who cares if nothing else does? Not the universe, that's for sure.

You can't have 'go back and change stuff' without acausality and have it work, to be sure. As soon as you set it so that even after someone goes back, they have to make sure certain events happen (i.e. you have to be not be killed by your future self so you can make the time machine to go back), well, when you think about it, setting things exactly as they were, making change both possible and required not to happen... that's when the contradictory begins and has no real solution. It is also, unfortunately, the model that so much time travel fiction decides to go with.

[bilateralrope]

The third works fine as long as you're fine with acausality.

Acausality sounds like a contradiction to me.

Sure, I've enjoyed some time travel stories that do involve altering the past. But I'm not going to pretend that they make any sense.

[Starglider]

The third works fine as long as you're fine with acausality.

Acausality sounds like a contradiction to me.

Not in mathematical or logical terms. It is merely an alternate construction of physical laws, in the same way that phenomena that violate the laws of thermodynamics aren't logically impossible or theoretically inconceivable, just unlikely to exist (based on our current understanding of the real universe).

[bilateralrope]

So what are some self-consistent physical laws that would allow someone to go back in time and successfully kill his younger self ?

[Q99]

Acausality sounds like a contradiction to me.

Did you read the Schlock comic link? Basically it's one person talking to another about exactly that ^^

If one can go back in time and change an event, and then the entire future they came from (thus automatically including where they traveled from)... that's actually just fine. Because the traveler into the past is *here*. They already had their cause. The cause of any further existence in the present/near future is the previous moment in the present. They don't need their traveled-from future to continue to exist at any moment past when they left it.

It's mentally weird to think about, but once you allow for time travel to begin with, that means the universe almost certainly doesn't care about it being that way.

Sure, I've enjoyed some time travel stories that do involve altering the past. But I'm not going to pretend that they make any sense.

Most of the time they don't make sense because they try and have their cake and eat it too. They include stuff like, "Oh, but if you fix it then everything'll be fine and go back to normal..." or "Time will naturally try and fix minor errors (as if 'time' is an active force with goals)," or so on.

Going back in time and altering your past so you don't go back in time is just fine, it simply doesn't prevent the original time travel arrival, so you don't have to worry about time loops, fixing time, or any of that stuff. It's just really, really weird to our brains.


And this leads to the big thing about time travel- in none of the coherent models named is there really a 'fixing' of altered timelines. At best, if you have acausality, you can have something similar to the original but different by whatever small alterations the butterfly effect causes.

[Starglider]

So what are some self-consistent physical laws that would allow someone to go back in time and successfully kill his younger self ?

Bearing in mind that I am not a physicist and am merely summarising what I've read about the subject (although I have written quite a bit of causal reasoning software) there are three common constructions;

1) Branching timelines, where the cause of the arrival of a time traveller appearing in one universe is the departure of a time traveller in a different universe (from the same tree but further along the timeline). The causality is real and possibly still observable.

2) Single timeline, act of time travelling is equivalent to resetting state of the universe except bubble around time traveller to a state equivalent to past time t. This is practically equivalent to the idea of a mutable timeline being 'overwritten'; the proximal cause of the time traveller existing is the act of time travelling, the distal cause is real but no longer observable. When people think of 'metatime' where there is a sequence of timelines that is appended to by each time travel act, they are thinking of either (1) or (2) depending on whether the universe the traveller departs from exists after they depart and whether means exist to observe / travel to anything other than the 'most recently created' timeline.

3) Single timeline, arrival of time traveller is merely an unlikely event with probability determined by the distribution of future timelines involving a time traveller, but without any direct causality. As a theory this is constructable but rather intractable, and the practical implications are similar to (2). There is still a kind of 'virtual causality', but that is no stranger than the other wierd 'virtual' entites that you get in QM. This can be combined with conventional QM many-worlds to produce a model similar to (1) but more confusing.

[Adam Reynolds]

I am going to go with an example that isn't actually time travel: Person of Interest.

Through the extremely accurate predictive abilities of an ASI, it uses many of the same dilemmas as time travel. One episode is largely the same idea as the Edith Keiller dilemma from Star Trek: should you kill an innocent for the greater good. Another is Groundhog Day, in which a series of simulations have the characters interacting as if it were real. But unlike every other version of Groundhog Day, none of the characters remember what occurred in the various versions, which puts them all on the same playing field and makes the real version more interesting. It also ends the simulation before the heroes escape because it ran out of time, which also gives the finale a bit more drama.

One negative version that should be brought up is the timeline in which the person already traveled back in time yet failed to carry out the primary act they traveled back in time for in the first place. The third Harry Potter movie and the film Deja Vu both had this problem. Throughout both stories, characters saw evidence of the actions of their past selves, but also never saw evidence of the action they traveled back in time to do in the first place.

[SpottedKitty]

I've always been fond of Harry Harrison's Technicolor® Time Machine; it's a fun romp, even if it does jump up and down cheerfully all over causality. And of course any list has to include Heinlein's classics "By His Bootstraps" and "'— All You Zombies—'".

[Q99]

Primer is the best time travel story I've ever seen.

Oh yea, I hear that one sticks to it's model well.

I am going to go with an example that isn't actually time travel: Person of Interest.

Through the extremely accurate predictive abilities of an ASI, it uses many of the same dilemmas as time travel. One episode is largely the same idea as the Edith Keiller dilemma from Star Trek: should you kill an innocent for the greater good. Another is Groundhog Day, in which a series of simulations have the characters interacting as if it were real. But unlike every other version of Groundhog Day, none of the characters remember what occurred in the various versions, which puts them all on the same playing field and makes the real version more interesting. It also ends the simulation before the heroes escape because it ran out of time, which also gives the finale a bit more drama.

Interesting, and yes. Time Travel-esque without requiring actual travel.

[Simon_Jester]

Time travel, it's cool, but it's subject to a lot of handwaves.

Doctor Who just flat-out throws it's hands and says 'timey-wimey,' shifting the rules of how stuff works.

Back to the Future is pretty cool, but when you think about it, it does result in, "An effect can exist without a cause, but only for awhile, so you can have people without parents be around and then not."

The idea that causality has inertia (but finite inertia) is interesting even if there's no logical reason to suppose that it is true. A thing that has no reason to exist continues existing, until its causal inertia is dispersed by friction with its external environment.

I think that time travel stories can be broken down into three categories...

- A story that that contradicts itself. For example, imagine someone going back in time to kill his younger self. If he succeeds, then he isn't around to go back in time, thus he can't possibly kill his younger self.

Only the first two types of time travel store hold up once you start thinking about them.

This type of story is fairly rare as far as I know, probably because most people figure a causality paradox results in the universe imploding or spinning around in circles around a closed timelike curve or something.

Acausality sounds like a contradiction to me.

Did you read the Schlock comic link? Basically it's one person talking to another about exactly that ^^

If one can go back in time and change an event, and then the entire future they came from (thus automatically including where they traveled from)... that's actually just fine. Because the traveler into the past is *here*. They already had their cause. The cause of any further existence in the present/near future is the previous moment in the present. They don't need their traveled-from future to continue to exist at any moment past when they left it.

It's mentally weird to think about, but once you allow for time travel to begin with, that means the universe almost certainly doesn't care about it being that way.

Basically yes. What it comes down to is that as soon as you arrive in "the past," you are no longer in YOUR past. You changed it. You're in some other past that may or may not happen to resemble your past.

Whether your future still exists or not depends on whether you believe in parallel universes. But if you travel into the past 100 years, then travel into the future, you don't reappear in "your" timeline even if it looks like you did.

There may be a you-shaped "hole" in the world for you to step into, if you manage to not change the past enough to prevent you from firing up your time machine in the present. There may be an alternate "you" who was born, grew up and did more or less the same things you did, and then stepped into a time machine and vanished just as you did.

But you're still filling another person's shoes by doing that.

So (for instance) in my interpretation of Back to the Future, Marty returns from 1955 to a 1985 that isn't his 1985. It just happens to look a lot like his 1985. Luckily for him, his new 1985 contained another Marty who was conceived at more or less the same time, looks more or less the same, and was living a rather happier life than he was... right up until he helped Doc experiment with a time machine and vanished into the past, just like 'our' Marty did.

So Marty, from a 1985 unaffected by his temporal meddling, reappears... In a 1985 that was thus affected. He takes the place of the Marty who grew up affected by the meddlings of Original!Marty. New!Marty is off somewhere else or doing something else in yet a third timeline, but he's not the protagonist so we don't know how that turned out.

Sure, I've enjoyed some time travel stories that do involve altering the past. But I'm not going to pretend that they make any sense.

Most of the time they don't make sense because they try and have their cake and eat it too. They include stuff like, "Oh, but if you fix it then everything'll be fine and go back to normal..." or "Time will naturally try and fix minor errors (as if 'time' is an active force with goals)," or so on.

The basic argument is a sort of negation of the butterfly effect- that MOST small changes to the timeline will be statistically insignificant. So as long as the original conditions are 'close enough,' it is probable that the final conditions will also be 'close enough.'

Pratchett portrayed this as "like a man swimming to shore from a shipwreck; the waves will break against the shore no matter what you do." Large historical phenomena are not sensitive to small changes, it is argued. They're only sensitive to changes that specifically alter specific things which are relevant.

And this leads to the big thing about time travel- in none of the coherent models named is there really a 'fixing' of altered timelines. At best, if you have acausality, you can have something similar to the original but different by whatever small alterations the butterfly effect causes.

You can also get 'fixing' in a parallel universe setting, one which preserves causality... but which does not allow you to return to your timeline of origin. If you return to your timeline of origin, you find that nobody actually changed anything. And you never showed up in the past in the first place, or accomplished nothing.

That's an amusing take on some kid's shows involving time travel and interaction with historical events (e.g. Peabody and Sherman shorts). You can go back and have wacky adventures in the past precisely because nothing actually changed; you are from a timeline where your own adventures never actually happened.

[Ziggy Stardust]

Primer is the best time travel story I've ever seen.

Seconded. It is a must-see for anyone interested in a unique and intelligent perspective on time travel. Nothing else comes even remotely close, in my opinion.

[Q99]

The idea that causality has inertia (but finite inertia) is interesting even if there's no logical reason to suppose that it is true. A thing that has no reason to exist continues existing, until its causal inertia is dispersed by friction with its external environment.

Hm, imagine if this was, say, some specific kind of time particle, that was created by causes-having-effects, and something that is missing a cause will use up these particles at variable rates, and you could actually measure the decrease in the particles...

There may be a you-shaped "hole" in the world for you to step into, if you manage to not change the past enough to prevent you from firing up your time machine in the present. There may be an alternate "you" who was born, grew up and did more or less the same things you did, and then stepped into a time machine and vanished just as you did.

But you're still filling another person's shoes by doing that.

Idea I haven't seen, but want to-

Someone comes back after altering the past, meets their alternate.. and then uses mental technology to transfer their memories over/merge with that one, so the resulting person is effectively both versions in one.

The basic argument is a sort of negation of the butterfly effect- that MOST small changes to the timeline will be statistically insignificant. So as long as the original conditions are 'close enough,' it is probable that the final conditions will also be 'close enough.'

When I hear that, I think about all the stuff that'll change if I go back in time two centuries, to an empty space, stay there for five minutes without anyone seeing me, and leave.

First, the patterns of air going through where I am is changed. Both my by presence, and my breathing. This small change in the atmosphere will persist because the air molecules will bump into new ones from this different position which'll bump into others etc., it could have a really long time before it hits anything important, but eventually a slight change in breeze will cause a somewhat larger difference, and things'll chain reaction from there. Different plants get pollinated, leading to plants existing in different positions affecting the flow of wind for the entirety of their existences, and etc.. Eventually you'll hit whether some animals live or die, where buildings are built, where people live or die, and etc. etc., it'll just keep growing.

Second, future microbes. I'm leaving bacteria and viruses from my time in the past. There's a constant 'red queen's race' of biology going on, their setups don't remain the same. Where I'm standing, the bacteria war has some combatants ahead of the curve by a few centuries. And we don't think about how the conflicts of the bacteria realm go, but given two centuries to spread new DNA/RNA sequences in the past.... well, I bet something is gonna be different! Odds are different people are going to die of disease because I stood there for five minutes breathing.


As soon as you have one person die/live who wouldn't otherwise... well, everyone who would've met/not met them has a different future (include entirely different sets of kids, genetically speaking). Everyone who meets one of *those* people on a different course, will have a different course, etc..

Change just spreads, and it doesn't stop.

[Simon_Jester]

The idea that causality has inertia (but finite inertia) is interesting even if there's no logical reason to suppose that it is true. A thing that has no reason to exist continues existing, until its causal inertia is dispersed by friction with its external environment.

Hm, imagine if this was, say, some specific kind of time particle, that was created by causes-having-effects, and something that is missing a cause will use up these particles at variable rates, and you could actually measure the decrease in the particles...

I'd prefer not to bring up particles in this context. There are a lot of things in nature that are not particles- momentum and energy both come to mind. Temporal inertia, if it were a thing... should be a thing more like those. A property of matter, not a type of matter.

The basic argument is a sort of negation of the butterfly effect- that MOST small changes to the timeline will be statistically insignificant. So as long as the original conditions are 'close enough,' it is probable that the final conditions will also be 'close enough.'

When I hear that, I think about all the stuff that'll change if I go back in time two centuries, to an empty space, stay there for five minutes without anyone seeing me, and leave.

First, the patterns of air going through where I am is changed. Both my by presence, and my breathing. This small change in the atmosphere will persist because the air molecules will bump into new ones from this different position which'll bump into others etc., it could have a really long time before it hits anything important, but eventually a slight change in breeze will cause a somewhat larger difference, and things'll chain reaction from there. Different plants get pollinated, leading to plants existing in different positions affecting the flow of wind for the entirety of their existences, and etc.. Eventually you'll hit whether some animals live or die, where buildings are built, where people live or die, and etc. etc., it'll just keep growing.

Here's the catch.

You can argue that infinitesimal changes will have consequences that grow larger and larger until they snowball out of control- which is basically the argument you've just advanced.

But you can equally well argue that most infinitesimal changes will have consequences that turn out not to matter. For instance, consider a snowstorm. Changing the shape of a few individual snowflakes might cause some tiny, micrometer-scale differences in air currents. But there is no guarantee that changing those air currents will actually change anything else. Not all systems are sensitive to tiny perturbations.

If a certain mouse dies, that was going to be eaten by an owl the next day, that doesn't mean the owl will go hungry. It may just mean they find something else to eat, and the overall population of mice remains more or less the same, eventually returning to exactly the same level it was before (because some baby mouse who would otherwise die for lack of resources to support them instead lives).

If a man embarrassingly trips and falls in the street after slipping on a banana peel, he may have some bruises that exact comment, and his life may be slightly altered in the short run, but there is no guarantee he'll live differently in the long run. He may well end up doing more or less the same things he would have done anyway.

The effects of a small change MAY spiral out of all proportion. On the other hand, they may also "damp out" into insignificance. Both positions are reasonable. In my opinion, the truth lies in a combination. The precise consequences of a small change may be unpredictable (for want of a nail... the kingdom was lost). But other small changes occurring in less sensitive conditions may have no net effect (a random guy's horseshoe loses a nail, he shrugs and gets it replaced, everybody's fine and life goes on).

Second, future microbes. I'm leaving bacteria and viruses from my time in the past. There's a constant 'red queen's race' of biology going on, their setups don't remain the same. Where I'm standing, the bacteria war has some combatants ahead of the curve by a few centuries. And we don't think about how the conflicts of the bacteria realm go, but given two centuries to spread new DNA/RNA sequences in the past.... well, I bet something is gonna be different! Odds are different people are going to die of disease because I stood there for five minutes breathing.

Now see, that's a stronger argument, which underlines the point that not all changes are equally consequential. Releasing a few strategically placed smallpox virii in the past is a small change (very little physical matter) with huge consequences (precisely because smallpox virii can reproduce and cause epidemics).

On the other hand, if poorly placed, those viruses may not infect anyone before they die, in which case they wind up have no effect on the past whatsoever.

As soon as you have one person die/live who wouldn't otherwise... well, everyone who would've met/not met them has a different future (include entirely different sets of kids, genetically speaking). Everyone who meets one of *those* people on a different course, will have a different course, etc..

Change just spreads, and it doesn't stop.[/quote]

[Simon_Jester]

I had a response to your last two paragraphs that didn't get included properly, here it is:

As soon as you have one person die/live who wouldn't otherwise... well, everyone who would've met/not met them has a different future (include entirely different sets of kids, genetically speaking). Everyone who meets one of *those* people on a different course, will have a different course, etc..

See, that's the thing. Whether the air molecules in a closed room move differently doesn't automatically constitute life and death for another person. Your germs might, but then again they might not; our immune systems haven't improved vastly in the past few hundred years, and if the worst communicable disease you're carrying is something like the common cold, the odds are that the worst anyone will suffer as a result of your appearance is a few days' inconvenience. Not death, not necessarily enough to even change their life or even be remembered a few years later.

Change just spreads, and it doesn't stop.

My thesis here is that change CAN spread and cause massive effects... it doesn't have to.

In other words, while every cause in some way interacts with every effect, not every cause is a necessary condition for every effect.

One consequence of the butterfly effect is that if the flap of a butterfly's wings can trigger the formation of a hurricane, then it logically follows that ALL butterfly wings influence the hurricane. There is no single privileged butterfly that automatically gains the power to make this happen or not happen, and no guarantee that the butterfly you just stepped on was that privileged butterfly.

If so, then the difference between having one billion butterflies flap their wings, and having 999,999,999 butterflies do the same, may not actually make a measurable difference to the hurricane.

Moreover, there are physical and social processes that are not easily stopped or changed by the individual actions of one person. A butterfly's wings might conceivably alter the course of a hurricane- but they cannot prevent winter from turning into spring.

...

Now, there may be specific instances where a small change DOES have far-reaching consequences. Give Lee Harvey Oswald a severe stomach bug on a certain day in 1963, and the assassination of Kennedy fails to happen, history changes. But giving a random person a similar stomach bug at a random time might well have NO significant effect, because their being out of commission for a particular week simply doesn't matter very much in the grand scheme of things. They miss some work, but then they do that work at a later time, they have different but comparable conversations and experiences. Life goes on.

[Crossroads Inc.]

When it comes to time travel, asking a question such as "What is the best depictions?"
For me... The answer is simply "What ever form is best for the story being told"

Because truthful, since we are talking purely fictional here, Time Travel is typical a plot device for telling a story. Your best version, is what ever form you want for your story...

If we want to actually postulate upon "Real" methods of time travel and how they may effect reality as we know it...Well.

The ever going discussion about just "what happens if you try and change history". Well, notions such as "History finds a way" or "History has 'momentum'" or the whole thing about "self fulfilling prophecy" and the like... Well, all such devices are usually predicated upon Story telling. If you "Really" had the ability to time travel, what does the universe care how things happen? The universe cares that Physics happen and follows its rules... I seriously doubt it cares wither Hitler gets assassinated or not...

I mean, I have no allusions about history as we know it getting "fucked" by any number of things.

Long ago I wrote up a rather detailed list of changes to history to create a world in which Russia became a constitutional Monarchy, and the Soviet Union never existed... Now, I am pretty sure that with such a change, 90% of anyone was born from 1930 and after that would never exist. But hey, oh well, new stuff will happen.

[Q99]

Hah!

Back to the Future comic has a new issue announced wherein Marty realizes his memories don't mesh with the past that happened in this timeline

Convenient timing.

Now, there may be specific instances where a small change DOES have far-reaching consequences. Give Lee Harvey Oswald a severe stomach bug on a certain day in 1963, and the assassination of Kennedy fails to happen, history changes. But giving a random person a similar stomach bug at a random time might well have NO significant effect, because their being out of commission for a particular week simply doesn't matter very much in the grand scheme of things. They miss some work, but then they do that work at a later time, they have different but comparable conversations and experiences. Life goes on.

Oh, it really does.

Consider sperm. Which sperm reaches an egg determines the genetic makeup of the person, the gender, etc.. And either partner moving slightly different will change who reaches first.

Everyone that person meets- be it it talk to for a minute, bump into, not cut off in traffic, etc., is now going to have a different set of children than they would otherwise. Daughters become sons and vice-versa. A kid got a different height than they would otherwise, or different haircolor (causing them to get made fun of when they wouldn't or vice-versa), a different disposition (which causes different relations with everyone in their school class- different friendships and rivalries form).

And once one such change is made... well, one person has a different sex. They get married to a different person than the other version- that means both their original spouse is gonna be with someone else, and their new spouse is not with who they were with the first version. Which in turn will cause changes in *their* romantic partners.

If I find the least-impactful person in a small town, and give them a stomach bug for a day, then within five years I expect there to be multiple children notably different than the pre-time travel change versions. Within twenty years, you've got people who wouldn't have existed otherwise spending a decade changing things and having kids that wouldn't exist.

Lee Harvey Oswalt is *immediately noticeable*, but one person having a single different kid is important and will cause a cascade of changes that cannot help but cause big changes with time, because people are not interchangeable, and inevitably those 'one random person replaced with another random person, causing a dozen different random people to do different stuff' changes will of course also cause differences in the more precise immediately noticeable events.

Life goes on, and that's why significant change is inevitable. Changes persist, even spread, until they hit something obvious.

Long ago I wrote up a rather detailed list of changes to history to create a world in which Russia became a constitutional Monarchy, and the Soviet Union never existed... Now, I am pretty sure that with such a change, 90% of anyone was born from 1930 and after that would never exist. But hey, oh well, new stuff will happen.

Sounds pretty cool, and yea, you'd have a different set of people! But handwaving for familiarity reasons and to make it more enjoyable is fun.

[Simon_Jester]

Oh, it really does.

Consider sperm. Which sperm reaches an egg determines the genetic makeup of the person, the gender, etc.. And either partner moving slightly different will change who reaches first.

How firmly do we know that? It's not like we can do controlled experiments on how the same umpty billion sperm cells perform in repeatedly trying to fertilize the same ovum by repeatedly doing IVF on the same ovum in a test tube over and over.

It's plausible but I honestly don't think it's a certainty.

What it comes down to is the core assumption that the outcome of all 'random' events is automatically reset by any change, however slight, in the initial conditions anywhere in the world. That not only is every individual cause part of a group that is collectively responsible for a given effect, but that altering a single microscopic cause will be sufficient to flip a meaningful fraction of the effects.

This assumption is credible, but it is not proven.

And repeatedly coming up with 'new' arguments for causal mechanisms by which 'random' events can have far reaching consequences (e.g. different sperm cells fertilizing the same ovum, or different ova ripening at different times and being available for fertilization in a different month) doesn't really change anything.

[Terralthra]

One sperm doesn't fertilize an egg, in humans. A whole bunch reach it and cooperatively use their acrosomes to help following sperm penetrate the zona pellucida.

[Q99]

How firmly do we know that? It's not like we can do controlled experiments on how the same umpty billion sperm cells perform in repeatedly trying to fertilize the same ovum by repeatedly doing IVF on the same ovum in a test tube over and over.

It's plausible but I honestly don't think it's a certainty.

I'd say 99.99%+ certainty.

The sperm cells contain different stands of DNA, each unique baring incredible coincidence. Half of them, of course, include flat different gender information, but even aside from that, there's about a 50% chance of any specific DNA segment being in a specific sperm.

Even the most obvious and largest unit, entire chromosomes, you've got 46 different ones from each parent, with 23 contained in a reproductive cell. Flip a coin 23 times. Unless the results match exactly (same number of heads and tails made in the exact order), you've got a different person.

There's 8.38 million different combinations a single person's chromosomes can take. Given the same ovum (and I'm not sure that eggs will always be deployed in the same order anyway), that's a rather long shot.

Only it's more complex than that and there's other factors to up the differences even more, not all of which we know, we just know it's that much for starters before getting into transcription errors (there's always a handful of genes altered by chance in the process itself) or other factors. So, less than one in 8.4 million, but that's your baseline best-case scenario.

What it comes down to is the core assumption that the outcome of all 'random' events is automatically reset by any change, however slight, in the initial conditions anywhere in the world. That not only is every individual cause part of a group that is collectively responsible for a given effect, but that altering a single microscopic cause will be sufficient to flip a meaningful fraction of the effects.

That is some impressive large-scope high-precision manipulation when you put it that way, some sort of cosmic anti-chance reset mechanism...?

The initial alteration here is macroscopic, a person arriving and standing around, it's just that it'll affect events that we known are easily flipped on the microscopic level that have significant macroscopic consequences.

And repeatedly coming up with 'new' arguments for causal mechanisms by which 'random' events can have far reaching consequences (e.g. different sperm cells fertilizing the same ovum, or different ova ripening at different times and being available for fertilization in a different month) doesn't really change anything.

I'm just mentioning one of the most obvious, easiest to do, regularly occurring, rapidly causes large scale change ones.

If you don't have some active directed manipulation of chance itself on a scale that kinda boggles the mind in order to purposefully cancel out every change, it seems rather inevitable.

One sperm doesn't fertilize an egg, in humans. A whole bunch reach it and cooperatively use their acrosomes to help following sperm penetrate the zona pellucida.

Right, I'm simplifying.

[Zwinmar]

This Book I found to be pretty good.

[Simon_Jester]

[snip genetics argument]

It's not that I'm denying the truth of the stuff you just said, it's that you seem to be entirely missing my point.

Suppose I stand in a locked room for five minutes, then vanish. I have set up certain perturbations. The question is not "would perturbing the fertilization of ova result in different people being born?" That's the question you're attempting to answer in massive detail, but it's not a question that addresses my argument.

The question is, "By standing here, have I perturbed the fertilization of any ova?" Is my presence in the room sufficient to cause the effect in question? How much change does it take to result in a different sperm reaching the egg or a different egg leaving the ovary? There is no way to test this in reality. We cannot assume "a couple of air molecules hit people at a different angle" or some such is sufficient cause to make this happen.

A sufficiently large cause will result in a large enough effect that amplification and the process we call "the butterfly effect" come into play. But a sufficiently small cause is just as likely to be swamped by all the other (unchanged) events taking place. As I mentioned, you cannot assume that changing the course of one butterfly will change the course of a hurricane, just because the hurricane is somehow linked to the butterfly in a way that creates more than literally zero connection between them.