Before humanity develops actual bidirectional time-travel – assuming it’s possible at all – there will be years, probably decades or even centuries before that during which physics theories prove the general possibility of traveling through time. Philosophers will realize their duty to come up with the morale and ethics surrounding the topic (see section Sub-Questions below).

So, judging and extrapolating from the actual history and state of philosophy, what would future Temporal Ethics probably look like?

If you have read and understood “Multiple History model” or lax type 4 (“Quantum‐Forking” or “Multi‐Divergent”) you may skip the Theory section coming up next, although it deviates in certain details and to help with that, keywords have been emphasized.

Time-travel theory

Bidirectional time travel is not possible by our current understanding of physics. (Relativistic effects of very fast speeds are similar to a one-way trip to the future.) The laws of thermodynamics would probably be broken less, at least, if each travel worked by exchanging equal amounts of (random) mass-energy between origin and destination. Alas, we’re probably talking about a lot of energy.

Just to make it more interesting, I chose to use a time-machine that arbitrarily travels through time itself (like a Tardis or Delorean) and works autonomously. That means it’s neither a sending (and possibly retrieving) device located at the time of origin (as in Terminator) nor a pod-to-pod temporal teleporter (akin to a wormhole), although I think both are better justified than a time vessel. During time hops it can also travel through space to compensate for astronomic movements.

I assume a finite but exponentially growing number of time-lines similar but not equal to those explained by Doc Brown in the movie Back to the Future II. Time travel is almost instantaneous. Each time travel branches off a new time-line at the destination. My main difference to BTTF is, there are no ripple effects as time-lines exist absolutely.

As a (novel) corollary, there is always a new branch at the origin, too! Branches at first only differ in the presence of the time machine with its passengers and cargo. Their impact will make branches diverge more or less quickly. I also assume butterfly effects are a thing, i.e. little cause can have huge impact, but does not have to, i.e. any distinguishing relevance may just as well fade with time.

There is no direct hopping between parallel time-lines, which some may like to call alternate universes. Unlike some varieties of quantum theory where each non-deterministic state change causes the creation of a new branch, I explicitly restrict that to time travel (even if that’s just for the sake of simplicity). With time progressing forward there will be, by definition, more and more alternate universes.

Hops to the past are always along ones current time-line, up the tree. Traveling to the future is along the time-line where the time travelers have been removed from existence upon departure. (Marty couldn’t meet his future self or son.) If there are branch-offs between origin and future destination, i.e. there’s a local time-(sub)tree, travelers cannot choose to follow a certain path throughout, because they will reenter in each future branch of that tree at the same time, thereby bifurcating all of them!


It is certainly possible to meet multiple doppelgängers of oneself: You first travel back and meet your younger self. Then you travel back again to a point between first arrival and first meeting. Now there is three of you. Any of them may die without affecting the other ones. There is no grandfather paradox and there are no time loops.

Time travelers can go back and kill Hitler™ (or help the Nazis win WW2). That’s not changing their own history, however, only their new future. If they go back to the time they came from, they will hardly recognize it and nobody would recognize them for their old history.

There is a trick to travel to a future that has “yourself” in it and in all its history since your departure: First travel back an hour or a day. Convince your past self not to travel through time. Travel to their future. This will be very similar to what your future would have been. Even if you didn’t convince them and they go on to create a branch like you did by time-traveling, there would still be the other branch wherein the travel doesn’t happen and a “copy” of you will rematerialize in both.

One strange effect of this kind of branching is, you can never determine whether the time machine actually works. Imagine this: you enter the chamber, set the destination coordinates, hit “start” and … space-time branches. Much like Schrödinger’s cat, you’re still here in one branch (wondering if the machine is broken) and you went away in the other branch with no way ever to return to the original branch or influence its past and future history. You may of course live in a time-line that has had a time-traveler landing in who proved their story and thereby the possibility of time-travel, but that’s only verifying the concept, not each individual travel attempt.

In this world, egoist time travelers can create a time-line for themselves where they’re incredibly rich and powerful, e.g. by importing future technology. People like that wouldn’t care about others, including alternate themselves in other time-lines. They wouldn’t care about the number of time-lines either.

Altruist time travelers, however, could try to create time-lines with as little human suffering as possible, although they know it won’t affect their original time-line in any way and they cannot go back to a familiar world. They gave up home. Likely, they would become restless travelers for good, like Doctor Who. Some may deliberately settle in a time-line they like and expect to remain likable, others will accidentally strand somewhen.

If time-travelers continuously try to improve their temporal tree, the percentage of “better” worlds should increase over time, and hence the probability of being in one.


If we’re living in this scenario, there’re some conclusions to be made.

  • Since there is only a single uninflicted time-line, but possibly myriads inflicted ones already, it is highly unlikely that history as we know it is without intervention.
  • We have no actual proof or evidence of interventions and can therefore not confirm the possibility of time-travel.
  • Certain historic events and people are very likely prime targets for alteration by many altruist time-travelers. Having the holocaust in our time-line, for instance, can mean one of several things:
    • There were actually worse things to fix which we will never know about.
    • Keeping it is considered to have more positive than negative longterm effects. (Time-travelers can test that empirically to some extent.)
    • We’re in a less likely branch where it didn’t get fixed or where an egoist time-traveller profits from it.
    • Humanity in our futures never develops time-travel (and aliens are not interested in Earth) and there are no active travelers in our past.


  • Would Temporal Ethics develop in most time-lines with confirmed time-travel?
  • Is there a justification or even an imperative to build a time machine and travel with it if the present cannot be changed?
  • Would it become a moral imperative (good) to change the past for the better even though that won’t affect one’s own present?
  • Would it be considered bad to actively or passively create time-lines that are worse for humanity overall than the uninflicted one?
  • Would it be better justified to improve the present by importing knowledge from the future or by changing the past?

  • Building and running a time-machine is unlikely to be easy and cheap, so only already powerful future entities (like governments) can start a time-travel program. Are their motives more likely to be in line with the principles of Temporal Ethics than that of a random rogue individual?

More related questions

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    $\begingroup$ I can see that you put a lot of effort into asking this question, but I admit defeat. Half way through I simply couldn't do it anymore, and gave up on reading it. Consider condensing it to oh ... about a quarter. Just friendly advice assuming you want someone to actually read and answer it. $\endgroup$
    – AndreiROM
    Commented Jan 13, 2016 at 14:39
  • 3
    $\begingroup$ As it stands, this question feels more like a series of questions, ones that probably could not be answered without a book's length of text. For instance, I read at least eight questions here; get that down to one and this might be answerable. Otherwise, it will be closed as too broad. $\endgroup$ Commented Jan 13, 2016 at 14:44
  • $\begingroup$ @DaaaahWhoosh I have moved all sub-question into a separate section at the end. I hope that helps already. I have not shortened the Theory section as @ AndreiROM suggested, but made a note before it to explain when it can be skipped. $\endgroup$
    – Crissov
    Commented Jan 13, 2016 at 14:54
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    $\begingroup$ +1 Awesome Question! The background work in building this question is likely to be more valuable to future readers, than any answers we are likely to provide. Well Done! $\endgroup$ Commented Jan 13, 2016 at 16:24
  • $\begingroup$ Not enough for a full answer, but there is a possible reason to time-travel. Some religious groups may want to create the most time-lines possible, because it's kind of like creating new Earths (and thus, creating thousands of billions of lives) and the bible tells people to "Be fruitful and multiply" or something like that. $\endgroup$ Commented Jan 13, 2016 at 16:40

9 Answers 9


First off, I would suggest watching the anime "Stein's Gate" as it deals with a very similar type of time travel.

Ethics I would expect...

Time Travellers' society That said, most realities would likely only have a few time travellers as each time travel would reduce a universe's time travellers by one. (I suppose a time traveller's society could be "created" by travelling back to the big bang, not touching anything, and then travelling to the distant future, creating a future point that's largely unaffected by past events from a primordial past event. Group time-travel ethics could emerge from what would be this group. It would likely be proportionately small for whatever scope of people's they represent. (Only one per world in a galaxy would be a very large group indeed.) One thing that may happen is the group trying to maintain at least somewhat of a standard history (possibly automated?) so that they can still navigate time appropriately, which may give some semblance of more classical time-travel shinnannigans. But each time traveller would like have relative autonomy within their growing bactch of created-and-abandoned timelines.

At this point, I think a few ethics would emerge... Don't mess with other traveller's timeline areas unless asked, treat other travellers with respect. Basically, this is getting into MADD territory. Any time traveller would be able to easily screw up another traveller's timeline and get them forever lost and rather vengeful, at which point you'd be doing it to yourself as well since you can't actually remove them. They would only remove themselves by time-travelling away. Also means each timeline would constantly be depleting it's supply of time travelers asymptotically.

Look out for the safety of the universe This may seem dumb (seriously, take care of the universe?) but it makes sense. When your "world" has changed from being so limited to so unlimited, you have a vested interest in ensuring it's as unlimited as possible. If the future ends, and you hop into it accidentally, you're dead. Most time travellers would also be effectively immortal (go to the future where anti-aging is perfectly researched, get it, return to travelling). There would be no limit to the time they could explore and travel to other than the duration of reality itself. So, for one, stopping the heat-death of the universe would likely be a priority.

Universal Life-saving As per the previous mention, a time traveler could literally have all the time they want to live. If they're altruistic, why not save a copy of everyone and take them to an alternate future? Especially everyone you'd be sad about losing. Why not save everyone you want when you can? It kind of follows Hitchhiker's Wowbagger principle. If you're immortal and can travel through time, eventually you're going to get bored. People, in general, are the most interesting thing in the universe, so it seems that eventually one would desire to save them all just to watch how they would interact with each other. (How would Gandhi act if he had Hitler at his mercy? How would this changed Gandhi and the original Gandhi interact? How would your now-four types of Gandhis have a conversation with Martin Luther King Jr.? List goes on and on for "entertainment" value.)

Is there a justification or even an imperative to build a time machine and travel with it if the present cannot be changed?

Yes. Present can't be changed, but that says nothing about changing the future or saving those from the past. Just because you can't change the present doesn't mean you can't save everyone in it, just that they won't be in that timeline anymore. For those who would argue, "But it's not really them," the line split. They're no more-or-less legitimate than their doomed counterparts. And to quote one individual, "There's mostly dead, and then there's completely dead." Otherwise, imperative would be personal comfort or desires.

Would it become a moral imperative (good) to change the past for the better even though that won’t affect one’s own present?

Of course. After your first time travel, there would no longer be a "your own present," only your current present. So why not try to have the best one possible? A better present means likely a better future for you to experience. It'd be akin to upgrading your home. Except instead of a home, its your timeline.

Would it be considered bad to actively or passively create time-lines that are worse for humanity overall than the uninflicted one?

Unless there was that time-maintaining system I mentioned earlier, I don't see how someone could be stopped from doing it, but it'd likely be self-defeating eventually if it got too bad.

Would it be better justified to improve the present by importing knowledge from the future or by changing the past?

Mainly for one's own benefit or trying to save lives, I would expect.

Building and running a time-machine is unlikely to be easy and cheap, so only already powerful future entities (like governments) can start a time-travel program. Are their motives more likely to be in line with the principles of Temporal Ethics than that of a random rogue individual?

Countries, governments, etc. are traditionally be self-centered. They'll probably try to bring tech from the future, but with the 50% reliability, you'd probably eventually run out of volunteers since "noone knows what happens to the failures." So they would probably use it just for tech and knowledge. Otherwise, you'd probably have a high likelyhood of those using it abandoning their post the moment they realize they're out of reach, dropping the return rate even further. Especially when they get a historian's look at their own government, which almost is never really that positive in retrospect.

  • 1
    $\begingroup$ Also, thought... When time bifurcates, does it bifurcate in a Y or a X? There's no reason to assume bifurcation is single-direction. Each time you travel, you may create a new past as well as a new future. (This would destroy my "time travel society" method, and pretty much guarantee that nearly every time traveler is all alone in their timeline.) So time would look less like a branching tree, and more like a chainlink fence. Also there's the question of gravity, which may bleed into higher dimensions. Do similar near timelines merge? (Deja vu explanation?) If so, returning may be possible. $\endgroup$
    – lilHar
    Commented Jan 13, 2016 at 22:51
  • $\begingroup$ The "similar timelines merge" idea is very similar to what happens with quantum decoherence, so would be my preferred time travel explanation. $\endgroup$
    – sh4dow
    Commented Nov 17, 2022 at 17:00
  • $\begingroup$ @sh4dow Fair enough. I've always thought it made more sense, especially once you include the concept of gravity being weaker than magnetism due to it leaking out in the 4th spacial dimension. $\endgroup$
    – lilHar
    Commented Dec 1, 2022 at 6:09

The question is very broad (as most ethical considerations tend to be) but here's a quick try at general theory (akin to 'murder is bad, charities are good' in it's scope)

Temporal ethics would almost certainly evolve, and as with most incredibly powerful things that are hugely unpredictable and have the capacity to blow-back horrendously in the face of humanity, these ethics would be:



Unless its going to cure cancer/give us seemingly free energy/puppies for all..

People would be dead set against anyone travelling to the past. At all. Going to the past is a Bad idea, as it might mean you accidentally destroy X thing or make a timeline where Y is the president, and that's unacceptable. No amount of tinkering in the past is likely to help, so leave our history alone!

Conversely: Travellers from the future are already here. They've probably already messed up their times as badly as possible, so we should exploit their knowledge and technology to make our own lives (and by extension, their futures) better.

As a result any temporal manipulation would be from timelines where it had all gone so horrendously wrong that backwards time travel seemed to be the only viable alternative, and those in power should never allow time travel unless such a situation were the case.

Any more detailed than that and I'm afraid you start getting into some major philethimorical hot water, and I haven't deigned to come back from the future with the Time Travel 101 handbook yet.

  • $\begingroup$ Does that mean you're a time traveler from the future who is conducting time travel without the Time Travel 101 handbook!? $\endgroup$
    – Frostfyre
    Commented Jan 13, 2016 at 14:58
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    $\begingroup$ @frostfyre Nah, I was hoping that me from the future would be coming back with a spare copy I could borrow, but it seems that future me is just inconsiderate! $\endgroup$
    – Joe Bloggs
    Commented Jan 13, 2016 at 15:15
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    $\begingroup$ @JoeBloggs - I'm charging you with time travelling without a manual. The penalty is 10 years in prison. Oh, wait. $\endgroup$
    – AndreiROM
    Commented Jan 13, 2016 at 15:47
  • $\begingroup$ @AndreiROM: Give me the manual once I get out and I can solve this whole problem. Though by then I might be a bit jaded and just decide to let past me rot in jail.... $\endgroup$
    – Joe Bloggs
    Commented Jan 13, 2016 at 16:14
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    $\begingroup$ @AndreiRom: Sure they don't. But past me is going to rue every single day of them. I'll make sure of it. All I have to do is not give him the manual!! MUAHAHAH!!!! $\endgroup$
    – Joe Bloggs
    Commented Jan 13, 2016 at 16:22

Ethics are personal/irrelevant.

Due to the branching nature of the timelines, you only ever see a new branch. Nothing you do can ever affect the timeline you come from, since ethics can never be enforced, you can only screw up "your own" life (and of course the lives of the people in the timeline branch you just created), but you can never go home. As soon as you've actually travelled in time, all you can do is spend the rest of your life trying to undo the damage you've done, but each time you travel in time you just create a new branch for your new changes. While from your point of view it may feel like you're making things better, all you're doing is creating more and more divergent timelines.

Other people from the prime timeline can't even learn from your mistakes, there's no time travel in the prime timeline. Every time you actually travel in time you're creating a branch, the prime timeline can never be affected by time travel.

But I want to make the world a better place!

Well you can't. So there.

Not only can you not make the world a better place, if you try then all the suffering is now your responsibility. While there may have been similar people in your source branch who were suffering, you have just created new people, who didn't exist before, and made them suffer. This is your responsibility, all of it. You created them and it's your fault they're suffering. Not just on this planet, not just the ones you know about, but on every inhabited world across the entire universe in this timeline that you have in your selfish desires created. It's all your fault. All the unimaginable suffering across the entire universe, your fault.

The only ethical thing to do with time travel is to destroy the machine along with any evidence it ever existed and any documentation relating to the required technologies.

But "horrible thing" happened to someone I love and I want to go back and fix it!

Again, you can't. You might be able to prevent it for a copy of your loved one but not for the actual person (now here's a good one for the hardcore philosophy). Once you push that button you can never see your loved ones again, only pale copies with different experiences, effectively different people. The people you knew have been left behind. To live out their lives (without you).

  • $\begingroup$ Exactly. Is it good to increase the number and proportion of “good” timelines? $\endgroup$
    – Crissov
    Commented Jan 13, 2016 at 19:25
  • $\begingroup$ @Crissov if we go by Darwin's theory, the ultimate good is to maximize the amount of timelines with abudant time travel :-D $\endgroup$ Commented Jan 13, 2016 at 22:21
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    $\begingroup$ Reminds me of youtu.be/vBkBS4O3yvY $\endgroup$ Commented Jan 14, 2016 at 5:36
  • $\begingroup$ @Crissov, there's no such thing as a good timeline. For every happy person there's still an inordinate amount of suffering. To ask that question is to know that time travel is bad. Think of the people who are suffering, for every iteration you create to try to make the world a better place, you're creating copies of everyone and dooming many to live out their lives in abject misery. The human mind cannot comprehend the amount of misery generated for every newly created timeline. $\endgroup$
    – Separatrix
    Commented Jan 14, 2016 at 8:48
  • $\begingroup$ @DavidMulder, this is much the same theory of time travel, but without the plot twist $\endgroup$
    – Separatrix
    Commented Jan 14, 2016 at 11:16


One school of thought would certainly be LEAVE IT ALONE, as already mentioned, but we all know that won't happen. Humans are far too curious / greedy / rebellious / suspicious to follow that stricture. Someone will eventually build a prototype no matter how illegal the process becomes in any given society, and just knowing that fact will be enough to motivate everyone to try to do it first.

In fact, ethically speaking, it's probably an easier sell with this version of reality than many other sci fi implementations, since hey, not only are the ill effects not going to happen to US, but the miscreant who's doing the tampering won't be our problem anymore either! Hooray! Go right ahead folks, have a great time. Don't let this reality hit you in the ass on your way out.

This is the single biggest trick of this entire question. Ethics are, basically, a codified social contract. Right and wrong conduct is entirely relative to the society in which the conduct is executed. So the problem is that the time traveler does not belong to the society he's influencing. By definition he's an outsider, who has spun up his own personal version of reality in order to make his own utopia/dystopia/heaven/hell/playground/laboratory. Ethics, per se, don't even apply to this circumstance.

It's worse than that.

But let's assume, for a second, that they do apply. That some society, preparing for time travel, has spent years and years wanging on about the rights and wrongs of time travel, the solemn responsibilities of travelers, and the dire effects of influencing uninflicted timelines. And let's assume, for the sake of argument, that these ethics are inflicted universally across the entire population of time travelers, just to make the point.

Per studies cited in this New York Times article, our ethical self is a wonderful, noble construct that we access consistently in hypothetical circumstances, but the percent of us who call up that same construct when thinking they're unobserved and actually making those decisions in the real world is very, very small. Ten percent was the stated percent for trivial decisions; weighty decisions are an undefined (and cannot reasonably be extrapolated), but assuming equivalency with trivial decisions, this implies that nine out of ten time travelers will screw ethics and do what they want.

In an uninflicted timeline, there is no proof of alteration, so not only can we not tell whether our timeline has been altered, we cannot tell by whom or why, and a time traveler knows this. The decisionmaker is always unobserved. So even if the traveler does identify himself as belonging to the society in question, his ethical self will only be tapped, on average, nine out of ten time(line)s.

Amorality / Personal Aesthetics

So, in the end, we're left with personal motivation as ethic, and not much else. We can posit and posture as much as we like, but all the philosophical training and conditioning in the world will be unable to alter the basic truth that if nobody's watching, a human will flip a coin until it comes up heads. Once the tech is viable it's basically a done deal.

This doesn't, actually, answer your primary question in full, but that's kind of intentional. The problems of relying on the ethical self in unobserved circumstances are well-known even without the Times article's specific data. I strongly suspect a seasoned philosopher, or board of philosophers, charged with creating a system of ethics for time travel in this setup, would smile, shake their heads, and ask how much you were paying, because they'd know deep down it's all just blather when the rubber meets the road.


Agree with Joe Bloggs answer. But for fun let's investigate Temporal Ethics:

Would Temporal Ethics develop in most time-lines with confirmed time-travel?

Almost certainly. It's certainly an easier topic than regular Ethics. for simplicities sake let's define a minimal ethics framework. You have a moral choice, a moral action, a goal state, and a moral function. A choice can be moral but the action could be unsuccessful in actuating the choice. Likewise if observed individually the morality of an action or choice could change (local, "kill one", versus global, "save many", properties). You have a goal that you're optimizing for (most number of people with most happiness, etc.). You also have a guiding function that let's you determine morality (religion, common opinion, game theoretic, random, etc.) in relation to the goal. Much of ethics will deal with what is good and bad and try to precisely define the function. Regular ethics will likewise try to define the goal state as it provides a reference for building the metric that is the guiding function.

But there's good news! Temporal Ethics need only concern itself with the temporal aspects. That is, we don't need to define what is good, that's for regular ethics, we just need to define how our temporal aspects relate to the classical problem. The entirety of Temporal Ethics in actuality would be concerned with pinning down what time-travel is and how it effects the current time-line, as well as determining the dangers and weighing them in risk analysis, since regular ethics covers the other questions. As luck would have it we have WOG on the physics of the matter so we can bypass the quibbling part of Temporal Ethics and get down to what the "Final Conclusions" would be.

Is there a justification or even an imperative to build a time machine and travel with it if the present cannot be changed?

No. Based on your premise, you cannot affect your own time-line whatsoever, if you could (destabilize the universe or whatever) then you could communicate and the main time-line would be self-reinforcing OR paradoxical. So, since we know there is no effects, there is no downside to time-travel. Each time-traveler basically makes his own universe when he travels. He populates a universe with initial conditions based off of however he acted to change the time-line and cause the split. He basically sets the goal to optimize for, he sets the goal. With two caveats: the moral functions that the people develop may not coincide with the "spawner's" goal. And there may be a "super-goal" either by being carried forward from the root of the tree or from an "extra-universal thing". If you keep traveling until you get to a universe that's altruistic/egotistic enough for you then you're really just tweaking your initial conditions until you get it right for what you want. Each attempt abandons the universes you were in prior.

Would it become a moral imperative (good) to change the past for the better even though that won’t affect one’s own present?

No. There are two ways of looking at this: "Existential Probability" and "Existential Certainty". In the probability you want to increase the number of universes that are "good" so that the many copies of other folks might somehow live a "better" life, for some definition of "better" :). Now the problem is that you already have a main/prime time-line that is "good"/"bad". IF your initial jump does not cause perfect "good" you're screwed statistically. If you create a "bad" universe you'll have to jump again to try to do better. If you create one with the same level of "good" as the main universe you've achieved none of your goal. You have actually made it worse because each jump further increases the value of N, where N is the number of universes. Your goal as an altruistic time-traveler is to increase the average "good". Even a perfectly neutral jump diminishes the returns from future efforts. Likewise one "bad" jump could take a long time to repair. Since your first travel "determines" the average for a fairly significant amount of time you would NOT travel unless you were extremely certain of the results or desperate in your own time-line. Which brings us firmly into the risk-analysis territory of "Temporal Ethics". The other option to look at the morality is "Existential Certainty". Rather than discussing odds you discuss "empirical facts". YOU exist. You cannot effect another you. When you decide to travel, the new you is created on a separate time-line and the old one stays home unaffected. Since you are isolated you exists entirely within your current universe. Which may or may not have one time-traveler. You are certain of your existence. You are also certain that everything, including yourself, is existing in your universe and the probabilistic "good" matters not one lick, you have certainty, you can measure things and they will be unaffected by anything having to do with time-travel. You only can have zero time-travelers present if you're in the main universe. Otherwise there was certainly one time-traveler in your universe. Likewise you are certain that no time-travelers will ever enter your universe because the only one allowed always existed since the beginning. In fact, you might owe your existence to said person. The time-traveler leaving could cause an exiting effect on your universe (related to what is coming: We have no way of determining the effect of you leaving on the universe prior to your arrival in said universe). Since you could potentially independently time-travel you can be assured that your traveler leaving will not cause your universe to disappear. (Unless there's restrictions on time-travel outside of the main time-line which makes "Probabilistic Existence" increasingly more of a risky prospect). If you have all theses certainties then it doesn't matter if you travel. Everything in the new universe will have these same certainties and they will be permanent on entry. You can only create, never destroy. If existence is defined as "bad" in your new universe then they can easily destroy themselves, or in a pinch you can lend a hand. Otherwise they get what they have for "good" and "bad", they can change it themselves by either ordinary means or time-travel to a new universe with different "good"/"bad" ratios. Crucially, they cannot affect any other "good" or "bad" in any pre-existing universe. So as long as you existed before the created universe then "good" and "bad" mean nothing to you as far as time-traveling. And if you existed after then you just have to set about fixing whatever you find wrong. (Note that I don't consider that we should consider the feelings of the created universe. They don't matter. Simply because the probabilities are spread across the gamut of possibilities for "good" and "evil" and we have no objective way to deterministically quantify the amounts for all of time in the created universe or our own. So since you can't consider it: you shouldn't consider it. The best you could do is plot risk analysis for you unknown unknowns vs your known unknowns, which is ridiculous in this context since you can't even qualify your known knowns with a known amount of error.). Basically it boils down to "Don't do it unless you have a really good reason" vs "Do what you want". The only reason you need to consider the first one is if the laws of time-travel change off of the main time-line. Since WOG says they are a constant set of rules that really boils down to "Do what you want". In-universe though, the risk analysis will boil down to "Don't do it unless you have a really good reason" modified by our certainty in how the physics of time-travel works across time-lines.

Would it be considered bad to actively or passively create time-lines that are worse for humanity overall than the uninflicted one?

Depending on whether you look at it as probability or not we are once again divided on the pervasive thought. As far as actively creating them (versus "passively" as a result of going where you want...) you would probably only actively create time-lines if "existence" was valued highly by your moral function AND/OR the multi-verse was interpreted as "Probabilistic" as a whole. In the scenario where "existence" was was valued high enough by your moral function to lead to creation then the "quality of existence" would probably be inconsequential (although it could easily be factored in). If we're under a probabilistic scenario then it is definitely immoral. We of course know whether it is a probabilistic scenario or not, but in-universe they don't. So they will do as all risk analysis does and assume the worst. As such, creation of sub-par universes on purpose would definitely be seen as the height of immorality (you just slighted an entire universe!) accidental creation of sub-par would be unethical but could be fudge-factored via the "existence" part of the moral function into ethical.

Would it be better justified to improve the present by importing knowledge from the future or by changing the past?

*Only justified in a probabilistic context and only when risk vs benefit is carefully weighed.*

As for a rough risk analysis: the only bad scenario is a "Probabilistic Existence" with a "bad" universe resulting from the travel. As long as the the probability of a "bad" universe is non-zero it is likely to happen at some point if you travel. You can't control universes outside your own. Sadly, N could be growing exponentially and your universe could have absolutely no say in it outside of the possibility of you making the initial choice not to time-travel. If you're not the main universe, you're likely screwed from influencing the relative "good" since you can realistically be expected to only ever be able to affect 100/N percent of the total "good"/"bad" ratio, where N is fixed at whatever it was when you decide to time-travel. This is because, since if you came into existence from time-travel it stands to reason that other time-travel events may have occurred and any time-travel choice you make may be duplicated for each branch in the tree.) If the chance of branches increasing is decently high then the "Probabilistic Existence" devolves into "Certain Existence" because, out of the likely-constantly-increasing choices going on among the branches, you will have almost no determining effect on the outcome. So just like "Certain Existence" this case assigns no moral value to time-travel, "Just do what you want".

Building and running a time-machine is unlikely to be easy and cheap, so only already powerful future entities (like governments) can start a time-travel program. Are their motives more likely to be in line with the principles of Temporal Ethics than that of a random rogue individual?

Probably they will be ethical. Since it all boils down to risk analysis anyway, which they are certainly going to apply from an economic standpoint at the very least. We can be relieved as outsiders since we can know that their choice is inconsequential anyways. As in-universe people we can be relieved since we know they are going to be more sensible than a rogue mad doctor.

And that about covers Temporal Ethics. The entire discipline can be said to be composed of Temporal Physics, Ethics, and Risk Analysis. The main question can be said to be whether Temporal Physics changes between time-lines and if it's possible to influence the current time-line. A universe that has not answered these questions satisfactorily will argue philosophically about paradoxes, certainty vs probability, etc. and frame them as ethical questions affecting the lives of entire universes. If enough is known these questions will fade and all we'll have left are the unanswerables: Is existence intrinsically good, etc. Which could only hope to be answered by some complete description of everything. Temporal Ethics will thus consist mostly of pointless philosophical debates of both sides of the coin on the various issues. Mainly because the physics cannot be trusted unconditionally, and when it is trusted it raises questions that cannot be answered. (Like does it remain unethical for you to choose a "goal" that is unethical from your current-universe's frame of reference if it isn't in the new universe? Does an absolute frame of reference for morals exist (god, etc.)? And so on... basically the really big questions of standard ethics are the only ones that matter for temporal ethics.)

  • $\begingroup$ Interestingly, time travel for the purpose of gathering information in a Probabilistic Existence is highly unethical since by definition you don't know enough about where you are going, and thus can't assure a "good" universe! If used to make a wholly better universe it may be a case of "Kill one to save Many". $\endgroup$
    – Black
    Commented Jan 13, 2016 at 19:35

To make sure I've read this correctly, let me sum up:

  • When travelling to the past, a traveler creates a brand new branch from his previous timeline upon arrival.
  • When travelling to the future, the traveler immediately creates a branch, then slides along it until reaching the destination time.

In either case, the time traveler is powerless to affect his original timeline (where "original", in this case, means the timeline he left). However, the time traveler cannot ever return to the 'trunks'. There is no merging, which means there is never any repercussion to the time traveler. From the time traveler's perspective, when he goes to the past, makes a change, then returns to the present, he is in his own time. If he tells a friend "see you in 20 minutes", travels 20 minutes into the future, his friend will be waiting for him; the time traveler will not realize that he is in a different universe.

This has an interesting effect. The time traveler will always be the only time traveler in the universe; any other same-branch time travelers will leave the universe by starting a new branch, and any different-branch time travelers will never be able to rejoin that branch. This means that the traveler will only be able to see his own actions. It may be that he never learns that he is creating branching timelines at all.

The following experiment may reveal branching timelines: the traveler goes back 10 minutes in time. He finds himself about to use his time machine, and tells his double to instead return one day in the past. He sets his time machine to one day as well, and the two leave at approximately the same time. The traveler will arrive one day in the past on branch 1; the double, having left a split second later, will arrive in branch 2. The two will never meet.

That means that:

  • A time traveler may well never realize that he is actually creating branching timelines at all. He will continue using his original ethical model, unaware.

  • If the time traveler manages to discover that he is creating branching timelines, he will also realize that he can have no effect on previous branches; regardless of state, good or bad, those timelines are lost. That leaves him with two choices:

    • Continue in his current ethical model, making the timelines better (or worse) as he can, or
    • Stop time travelling altogether, knowing that any suffering he attempts to fix with time travel will double any previous suffering onto two branches.

It boils down to a time traveler either sticking to his original ethics, or ceasing time travel altogether.

  • $\begingroup$ You read correctly, except that each travel always creates two branches: one at the origin and one at the destination. A traveler to the past won’t care about the two branches he leaves behind. $\endgroup$
    – Crissov
    Commented Jan 13, 2016 at 19:19
  • $\begingroup$ This model of time travel is why the show Continuum was very weird. Enjoyable, but weird. As it was fundamentally impossible to fix the problems created by time travel with time travel. They managed by creating a wormhole that linked a future with a past so that alterations actually affected both branches. And some deus ex machina. $\endgroup$ Commented Jan 13, 2016 at 19:20

One of the problems with describing ethics is that even the normal kind is still a very moving target for us. Last I learned, being ethical was being in a constant, sustainable intermediate state between excess and deficiency in all things (I think that's from Plato, but I don't know which dialogue). The problem is, this doesn't really help: how are we supposed to know what the intermediate state is for anything? If you're eating cookies, is one cookie intermediate, or is it deficient?

From this, I propose that the main obstacle between humans and a perfectly ethical society is a lack of knowledge of how to be ethical. So the best thing we can do is attain knowledge- note that if this is not true, attaining knowledge will help us discover that it is not true, which I think you can agree would be beneficial. So by proof by contradiction the best thing humans can do is attain more knowledge on the nature of ethics and how to be ethical.

Now, let's say there are two ways to attain knowledge: by experimenting yourself, or by learning the results of other people's experiments. The first approach is dangerous, as experimentation can cause negative results, but the result of learning is knowledge, which we've agreed is good. So for time travelers, the most ethical option is to travel to the place with the most to learn from, which I would think would be the future.

Once in the future, the travelers should attain as much knowledge as they can about the nature of ethics and how to be ethical. Since at the end of this process they should know more about ethics than me, I don't know what they would do next, but I presume they would then travel into the past in order to create a timeline of near-perfect ethics.

One thing I would also consider is the uses of branching timelines. If you travel back in time and create a perfectly ethical human history, all humanity in the multiverse will still be tainted by this un-ethical original timeline. However, if you were to create nearly infinite (or truly infinite) perfectly ethical timelines, the stains of this timeline would become mathematically nonexistent. So if you have the ability to branch your perfectly ethical society infinitely, you should probably do so.

But like I said above, I don't know what it is to be perfectly ethical, so the only thing I've proven here is that time travelers should use their power to get smarter. Once smarter, they should use their knowledge to be ethical.


Any study of morality will depend on how you value a time-tree. You suggest they want to make as many timelines as possible "good." However, you also state that they want to be in a "good" timeline, which is a very contrasting goal. In that case, you only need to make sure any downstream trees are good, and you can ignore any tree you didn't take. This is good, because an "evil" opponent could decimate your metric simply by engaging in billions of time travel activities just to split the timelines.

I think the issue you may be facing is that your phrasing for the ethics presumes a uniform distribution across all of the timelines, when that is not actually a reality that any time traveler faces.

In theory, the only altruistic possibility is one where you actively seek to prevent anyone else from acquiring time travel. once you're sure you're the only one, you learn what you can about how to make the world better, and jump. That prevents that particular branch from growing exponentially. Thus, if you make this world better than it, you've made a statistical improvement.


The question is too long to read, but I was inspired just by seeing the title and from the many time-travel stories I've seen.

A branching timeline is like a revision control repository (like git or svn) or "persistent" data structure in Closure. You can change things on a branch, experiment, do what you want, and it has no effect on the main dev branch. That's why we have branches. Changes are not destructive as the old state still exists and can be revisited: that means no "change" is real in some sense, and changing one way is not special as a diffwrent way can be done in another branch.

If branches are heavy-duty and there is a specific dev branch (like working in svn) then you might get something like Shadows in Amber, with ours being considered real and others have little or no concern for us no matter what happens there.

If people feel that every state has multiple successors but none is privileged, they might still get a feeling that nothing really matters because the states exist in a platonic realm and you don't cause bad things to happen but just visit such a state that already has a platonic realism.

If something is bad in their home timeline, what's the point in doing anything about it since it only affects one possible branch? If travelling back to kill Hitler you don't help anyone since that original still exists. If living without travel but knowing about this nature, what's the point of accomplishment if another branch can still be made or people can travel back and redo the history leading up to it?


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