Has anyone addressed the ethics of time travel?

Time travel looks like a gold mine for moral dilemmas, but I don't know of anyone who addresses these issues.

For example:

  • Am I morally obligated to go back in time and change a tragedy that already happened?
    • If so, how far back into the past is one obligated to "fix" things? Must we prevent the Black Plague? Do we stop the extinction of the Neanderthals?
  • If the only possible way to stop a serial killer were to kill him while he was younger, would it be right (or even mandatory) to kill him while he is still innocent? If so, what about during his childhood?
  • Is it okay to go back in time and preventing a child from being born? Is it any different from murder? Is it any different from birth control (or even not having children at the age of 13)?
    • Do we prevent the Black Plague (from the earlier bullet) if it will cause many of the current people to never have been born?
  • How would the ability to change the past affect punishment for crimes? If a crime can always be undone, how bad can it be?

I'm less interested in answers for these sample questions (although it would be interesting to hear various opinions) than I am in knowing if there is any serious material regarding the issue.

  • Just recently I read a science fiction story how Hitler was killed some time in 1916 - and the logical consequence was New York being destroyed by atomic bombs in 1960. Travelling back in time would be incredibly dangerous.
    – gnasher729
    Commented Aug 16, 2015 at 16:17
  • 1
    personally, i think the physics trumps the ethics. the only time travel we get to do is toward the future. at a rate of 1 second of travel per second, unless we have a black hole hanging out nearby, then you can hop in your spacecraft, take a short little trip around the black hole and find out that your grandchildren have died of old age. ain't no going back. Commented Aug 17, 2015 at 1:49
  • 3
    "Time keeps on slippin', slippin', slippin' into the future." Commented Aug 17, 2015 at 1:50
  • Also suppose you went back in time and averted the invention of the time machine ? Douglas Adams does a fantastic skit on this kind of thing in the Hitch Hiker's Guide to the galaxy (one of the 4 books in the trilogy - can't remeber which), and points out that the main issue isn't physics or even ethics, but the grammar. The problem is which tense to use when trying to describe soemthing you are going to do in the past to someone who will soon be born 50 years ago. ::passes out:: Commented Aug 17, 2015 at 14:47
  • 1
    Very interesting. Has anyone asked Michael J Fox? Commented Jul 11, 2019 at 7:25

9 Answers 9


Time travel and moral dilemmas are a common pairing in science fiction because they can take things so seriously. One of the difficulties with exploring implications of moral codes is that they always seem "right" within a narrow scope within which they were created. However, it is much harder to determine if they are right in a wider scope without challenging it.

Time travel has the advantage of being just barely on the edge of possibility (for instance, we've never seen it, but it can be defined in a General Relativity universe, if need be). It pushes the limit between "what we know" versus "what physically is," and often explores infinite looping implications on one's actions. These structures are very effective for challenging a moral code of conduct.

I don't know if it would be possible to find any synopses of time travel and morality issues, because each author has their own flavor of how to do it. The joy of science fiction is not everyone is expected to agree upon morals between their exotic worlds. It also helps that the many different flavors of time travel have very different implications. The moral implications of strange loops in Heinlein's books (All You Zombies is a personal favorite) are very different from the moral implications of HG Wells' The Time Machine.

  • 1
    "Time travel has the advantage of being just barely on the edge of possibility". Asserted with cautious but persuasive conviction. Commented Aug 17, 2015 at 15:57

I think you are looking at the entirely wrong moral problem. You seem to assume that with time travel into the past, you could make some predictable change to events. For example, take a serial killer who murdered ten people, shoot him before his first murder, and you have ten people alive and ten happy families.

But it's not that easy. It is practically impossible to predict the exact outcome of such an action. One of the saved murder victims could become a school bus driver and kill thirty school children by causing an accident. Saving the Neanderthals could directly lead to the extinction of all of mankind.

The moral problem is not whether it is moral to cause or not cause a predictable outcome; the moral problem is whether it is moral to meddle with things when the outcome is absolutely unpredictable. The problem isn't whether you should play god or not; the problem is whether you should use god-like powers when you haven't got a clue what you are doing.

  • 5
    How is that different from doing things in the present? If I try to stop a present serial killer, I don't know what the outcome may be. Commented Nov 29, 2015 at 6:53

Moral dilemmas of this sort are discussed by SF authors, for example Heinlein in his short story By his Bootstraps; and in the main stream of literature where its been influenced by such, ie Tom Stoppards The Real Inspector Hound; but quite amazingly not at all in the most famous time travel story, I mean that by H G Wells - The Time Machine - which is more in the tradition of Victorian high adventure, and earlier the epic.

This tradition, despite its intrinsic interest, is unlikely to appeal to practitioners of moral philosophy; as it has no bearing on actual moral problems; or even possible ones; given that the scientific consensus is that time travel is an impossibility now, and in the near or far future.

Still, its not an impossibility with enough ingenuity; for example Parfitt uses literary techniques from SF (cloning) to discuss a particular problem in the philosophy of mind (its unity and coherence over time and space).

  • 1
    Time-travel "its not an impossibility with enough ingenuity" - really? Commented Aug 17, 2015 at 15:56
  • @degnan: admittedly it does read like that, but its the following sentence that's its modifying not the previous; ie its not impossible that SF techniques are used in moral philosophy with enough ingenuity. Commented Aug 17, 2015 at 16:03
  • Oh, sorry. Yes SF is used like that, to better effect with a believable framework. Commented Aug 17, 2015 at 17:06

Ethically, changing historical events without being able to predict the results is clearly immoral. At it's core, this is risking the existence of every person in "this future" for a (potentially) better one. Without their consent. With no real ability to predict the long-term outcome.

Another way to look at it is (to risk referencing TVTropes), "killing one man who did evil doesn't get rid of the circumstances and structure that put him in the position to do evil in the first place." You go back in time to kill Hitler, and when you return to the present you find the Soviet Union started World War II... this time, with nukes!

  • I affect events all the time. Not on a large scale, but I do things and abstain from others without being able to predict the results. Suppose I did have the power to change things on a large scale. I'd risk the existence of people in the future. I'm not getting why changing the past is wrong and changing the present isn't. Commented Sep 6, 2018 at 18:02
  • @DavidThornley Personally, I would say the difference is knowledge. If you're changing history, you're gambling the present state of the universe with an unknown quantity. Unless the current state of existence is in dramatic peril, I would find it difficult to justify spinning the roulette wheel. On the other hand, your current actions are measured against the potential for present/future harm, as you likely cannot know for certain their outcome. Commented Sep 6, 2018 at 21:32
  • tl;dr Changing the past is ethically wrong for the same reason as performing medical experiments on unknowing people. You are irrevocably eliminating/changing the lives of other sentient beings without their consent. It is also ethically wrong to do those things in the present as well, we just usually don't get to know the outcome so clearly. Commented Sep 6, 2018 at 21:36
  • When I got a vasectomy, I irrevocably eliminated the life of one or more possible sentient beings. In raising my child, I irrevocably changed the life of a sentient being without his consent. In order to see these are ethically wrong, I'd have to see a darn good argument. In any case, you seem to be saying that actions are ethically wrong if I know what I'm doing too much and right if I don't know what I'm doing, or else that there are no ethically right choices. Commented Sep 7, 2018 at 18:03
  • Potential sentient =/= sentient. Also, ethics is very much about knowledge. In fact, often knowledge is all that differentiates an ethical action from an unethical one. As an overly simple example, whether you know the person knocking at your door is both armed and intends you harm makes all the difference if you plan on shooting them through the door. Commented Sep 9, 2018 at 7:37

In line with robert bristow-johnson's comment, the only time travel is at a local rate of 1 second per second. That goes for everywhere, because across the board everything only happens at one moment, (even if the local rates are different due to different gravity etc.) So time-travel in the science-fiction sense is a logical fallacy, and time-travel ethics would be a nonsense.

  • 2
    1. That is not proven to be true; 2. Even if time-travel is completely theoretical, it can be used as a crutch to help consider ideas of what is ethical. Commented Aug 17, 2015 at 12:36
  • 1
    I'm saying it is not even theoretical; it's illogical, and therefore can't be used for ethical speculation. Commented Aug 17, 2015 at 13:54
  • 2
    This is totally contrary to the notion of relativity, wherein simultaneity is impossible, because when observers are in different inertial frames, each observer sees the other's time as slowed. The part in parentheses pretends to account for this difference, but it would imply there is a privileged frame immune from relativity, that does not seem slowed to all the rest of us, but sees all of us slowed. Ehen you look into it, this notion is logically inconsistent.
    – user9166
    Commented Aug 17, 2015 at 14:18
  • 1
    Speeds aside, each observer stands in the Now. That's the main point. Their clocks may tick at different speeds, but everything is still all happening at one moment everywhere. There is just the Now. Commented Aug 17, 2015 at 14:26
  • 1
    It isn't inconsistent that their clocks should move at different speeds. In one place gravity may be denser and all particles movements are slowed down. Measured time may be slower but the universal moment is still the one moment that everything exists in. Commented Aug 17, 2015 at 14:33

This is a really great question, that was difficult to answer.

I would make the arugment that lack of action could be unethical. For example, if a child is starving and you have plenty of food. By not helping the child and allowing them to die of natural causes would be unethical.

With the assumption listed above, I would support the notion of using time travel to change the future for the greater good.

Now this becomes a very high risk behavior because potentially an action could have a ripple effect that could result in something as severe as nuclear warfare or mabye never leading to the scientific discovery of time travel. Also it is unclear have time travel would impact reality or if there is multiple demensions.

At the end of the day, I think it would have to depend on the type of change being proposed, the known risks, and how far back in time the travel is taking place.


There is a great confusion here stemming from language.

You use the words "past" and "already happened". But as soon as you invent time travel, those words don't mean the same thing anymore. If you can change it, then how did it already happen? If you change it and it does not happen, did it happen because it happened before you changed it, or did it never happen?

Our ethics likewise assume a linear, single-strand chain of events. One thing or another thing could happen, which one is better? This follows our understanding of time as being linear and single-stranded, i.e. there is one timeline.

Does this still hold true with time travel? Paradoxes are a main topic of SciFi literature, because you cannot have time travel and not create paradoxes. Any theory of how time reacts to time travellers needs to resolve paradoxes, such as postulating that minor events are smoothed out (e.g. the air molecules that time traveller displaces return to their "ordinary" position instead of causing a chain reaction). This creates a ton of physics problems (e.g. which force returns them to their position?) and is generally ignored in SciFi.

The best treatment of time I've seen so far is the Doctor Who approach - time is wobbly wibbly and unpredictable, and stop worrying so much about it because you can't predict the long run anyway, but you can do the right thing in the time and place you find yourself in.

From this flows a clear answer to the ethical question:

Assuming time travel is possible, you have to abandon your linear, single-causal thinking. Your ethical system just broke down completely, because you can no longer think like that. Between "Bob killed 10 people" and "I go into the past to kill Bob" and "I killed Bob, so Bob cannot kill someone in the future" there is no longer a simple chain of causes. Whatever paradox-resolution is found to be true, it will by necessity negate your logic. For example, if the above causal chain were true, the first statement becomes false at the conclusion of the third one, invalidating the whole chain.

You cannot ask the ethical question without resolving the physical first, namely how this all works. Time travel is not a small thing to add to the equation, it's an entirely new equation and will require an entirely new ethics.


Ethics depends on context. The Ethics of Time travel into the past is dependent on how physics of reality, (or beyond reality), manifests these disruptions. Is Time Immortal, eternal or is it finite? Is the Universe made up of Alternities- where each possible probability variation of each unit of spacetime flowing into its own branch of existing reality? Would such bifurcation operate only forward in time, or backward as well? Does going back into the past extinguish the original future? Are there Artifacts? Temporal clones or things that should not exist because their first cause has been made un-existent? Etc…. The Ethics of time travel needs to have variant reality interpretations for each ethical standard considered, much like psychology of the 20th century which needed multiple model interpretations because of both the limitations of model parameters and because of the sheer unknowing. Past Time Info/Travel is a wicked problem- Is it an omnipotent power? Does it need an omniscient awareness? Given F() & G() are both the total temporal Ethical sum of different universes; is it possible then that all F(good1,bad1) equals all G(good2,bad2)? Ethical sum constancy would seem probable if we begin to work in infinities, which an infinite time would suggests? Would ethics then merely be an illusion of the finite in an infinite world? If our understanding of ethics is present-consciousness-based and assumes cause and effect; and interprets 'ethics' as a relationship of 'intent-action-and-consequence- then is there some new ingredient(s) that need to be added to this formula, or are these ethical reference tools recursively scale-able to this new ethical dimension of reverse time? My guess is that this might deserve a whole school of philosophical thought and should be instrumental in helping to clarify previously intractable ethical problems of the past.

  • Can you add paragraphs?
    – virmaior
    Commented Jan 31, 2018 at 14:25

I now believe that there are laws in time. If you try to change something, other events occur to generate the same outcome.

Turns out that this is consistent with Novikov's arguments; in fact, I discussed this via Email to some folks a while back. It became obvious that wormholes are not just a mathematical oddity but exist in real space/time, using exotic matter analog generated by an antimatter interaction with electromagnetism that has yet to be discovered, as it is comparatively low energy, and incidentally responsible for ball lightning! See work relating to pair production in thunderstorms.

I am writing a paper on my ideas in the hopes that others can one day fill in the blanks and make sending a signal back in time possible.

As it happens have some circumstantial evidence that broadcasting a signal at certain key frequencies at a region of imminent air breakdown might open the bridge, but this is pure conjecture.

  • 1
    I don't see how this answers the question about the ethics of time travel. Perhaps that these ethical questions don't matter because other events will balance any change the time traveler might effect? Specific references to Novikov or others would be useful. Commented Sep 6, 2018 at 11:08

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .