Suppose time-travel is common. There is someone who I want to eliminate for nefarious purposes of my own.

If I go back in time and kill that person, then surely it's murder, but what if I go back and stop them from being conceived - say by preventing their parents from meeting. When I return to the present they no longer exist. Have I killed them?

What if I went back in time and killed the first humans, would I have committed genocide? Who would be there to complain?

How can I resolve this problem for a time-travelling community? Is there any way of legislating and/or policing this? Is there any proposed model of time travel that would allow law enforcement?


There are several models for dealing with TT paradoxes. Please ignore the paradox aspect as much as possible and just deal with legislation and policing. You can simply assume that is possible to go back in time and interfere in this way. Choose your own model, as this will be part of the solution I'm looking for.

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    $\begingroup$ Ending life is different from stopping life from being created in the first place.... Is it murder to not have children? Destiny doesn't exist and time is not linear cause of the chaos theory.... Just going back in time might change totally the future and you'd have to do nothing to make them not meet, your presence will be enough to cause a fluctuation that will result in a butterfly effect changing the world forever. Maybe you will never be born yourself if you go in the past. But then again apllying real world laws to this might be useless. $\endgroup$
    – user81643
    Jan 3, 2021 at 22:31
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    $\begingroup$ I'm pretty sure that if there is time travel, there are going to be very strict rules against tampering with the past. Assuming legislation has caught up to the technology. $\endgroup$
    – VLAZ
    Jan 4, 2021 at 0:38
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    $\begingroup$ What's murder? Some countries consider assisted suicide murder, others don't. Some consider suicide "self-murder," other's don't. Some consider abortion murder, other's don't. Is the time-traveling community capable of noticing timeline changes? Or does the chaos of self-repairing timelines reign? VTC Needs Details: (a) Rules of time travel in your world (especially the rules that resolve, or not, paradox). (b) Legal fundamentals of your world. (c) Philosophical/religious bias of your world. After all, what happens if the person you erased invented time travel? $\endgroup$
    – JBH
    Jan 4, 2021 at 3:46
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    $\begingroup$ Depend which pictures you adopt, some suggest there are many more outcomes(timeliness) branching from one action(event). Other think time travel is utter nonsense at least going backward to visit the past. See one's modus operandi, it can be murder, homicide, or mischief. $\endgroup$
    – user6760
    Jan 4, 2021 at 4:46
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    $\begingroup$ I really fail to see how this is "opinion based". It's pretty clear: you can't murder someone if they don't actually exist yet. $\endgroup$
    – elemtilas
    Jan 15, 2021 at 3:18

8 Answers 8


You don't change anything, you just put yourself in the timeline branch where their parents didn't meet and remove yourself from the one where they did.

If time travel is common, I.e. there aren't any laws restricting it's use then the consequences have to be personal. The way time travel works in your world is just that you transport yourself back along the tree in time and your actions just decide which branch you end up on. You haven't killed that person because in your original branch of time they still exist, you don't though. This also protects you from paradoxes - another massive reason why time travel wouldn't be common.

  • $\begingroup$ That really depends if time in his world works in a multiverse sense or it works in a realistic way, and paradoxes are part of our reality. $\endgroup$
    – user81643
    Jan 4, 2021 at 14:10
  • $\begingroup$ @user81643 I'm saying that for time travel to be common and not heavily restricted to only those with special clearance and checks then the consequences of time travel must be minimal. The only way this can be is that time splits off in this way and the time traveler can't effect the history of the timeline they start off in. $\endgroup$ Jan 4, 2021 at 14:22
  • $\begingroup$ To split off to another universe, one first needs to create another universe... No only is it hard to find the energy to travel to another existing universe, to create another identical to yours but in a different time.... You'd pretty much consume 100% of all the energy present in your own universe and be left with 0 energy to actually travel to that other universe. To create a full universe, you must first empy another universe. Unless more universes already exist, which in case you are not preventing paradoxes from happening, you are just moving a paradox from one universe to the other. $\endgroup$
    – user81643
    Jan 4, 2021 at 14:46
  • $\begingroup$ @user81643 Debating energy transfer and the branching off of universes only works if you have the details of how time travel works as a frame for the questions. As it is we have nothing other than that time travel is 'common'. If time travel had the ability to manipulate the reality you left do you think it would be common? I concluded it wouldn't and this is the explanation I could think of that holds with the details provided. $\endgroup$ Jan 4, 2021 at 16:33
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    $\begingroup$ it always bothered be with this model, that there must already be a version of you in the other branch. what happens with them? $\endgroup$
    – ths
    Dec 2, 2021 at 11:29

There's no current law in the world which could charge you with murder directly for that.

If time travelling became common, obviously they'd need laws and mechanisms against this, or else you could create havoc. Largely, they'd probably, restrict time travel greatly, for fear of the butterfly effect, and you'd have to jump through some hoops just to get a guided tour of the past.

For that specific case, they'd probably call it, "Conspiracy to Eliminate in the First Degree," or such, since it's a premeditated plan to eliminate someone from history for personal gain.

And for killing Adam, they'd clearly have some kind of shield or guards to protect humanity's existence, as that's their existence, and likely they would shoot any timetraveller on sight for trespassing in a heavily restricted zone.

Of course, if you wiped out humanity, that would be a paradox.

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    $\begingroup$ How would you detect if this had happened? Maybe that's a whole other question. You could conceivably both eliminate a person from the timeline while also preventing yourself from planning it. $\endgroup$
    – Carlos
    Jan 4, 2021 at 13:02
  • $\begingroup$ @Carlos Yeah, that would probably require a new question, and it'd have to be entirely speculative. Supposedly, if you can travel through time to specific points, you should be able to sense the "flow" of time, and distortions in it. You can also just keep track of where people go, and ask them why they went where and what they did. The bigger question is how paradoxes are prevented. $\endgroup$
    – Johnny
    Jan 4, 2021 at 13:07

If time travel is common, then governments will want to regulate it, and that means passing laws which criminalize things like changing the past, and forming a Time Enforcement Commission which hires Timecops to enforce those laws.

So... yes, it'll be illegal.

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    $\begingroup$ Nah, you just go back further in time and eliminate the person who drafted the law $\endgroup$
    – Separatrix
    Jan 4, 2021 at 9:21
  • $\begingroup$ @Separatrix that's what a criminal would do. More importantly, #1 it's not just one person who drafts a bill, and #2 enough people will want it that others would write a similar law. $\endgroup$
    – RonJohn
    Jan 4, 2021 at 13:51

A philosophy question if I ever heard one. I suspect you could get an entire book out of this subject if you really sunk your teeth into it. I'm not going to do that here, so I'll just outline what I see as both primary cases for, and against, this being murder.

For: Deliberate action to eliminate life

You undertake a sequence of actions, knowing they will have the effect of depriving this individual of their life. Furthermore, the actions are deliberately aimed at causing this eventuality and indeed serve essentially no other purpose. You want this person gone, and you cause them to be gone.

This second part is important, as for example burning things to generate electricity kills thousands of people every year. Nobody considers this murder. Likewise, even though the deaths-to-miles-driven ratio (.49 : 100M miles) and the size of Comcast's fleet 37000 vehicles mean, on average, ~ .5 to 1.5 fatalities per year (depending how much mileage goes on the average vehicle in a year) - nobody considers that murder, either.

Against: You can't kill something that never existed

It's difficult to reconcile the above with the idea that arranging things so that two people don't have a child really shouldn't be considered murder.

Having children is a choice. If you break up with someone because they want children and you don't, you haven't murdered anybody. If you choose to remain celibate, you aren't murdering your children, you simply haven't got any in the first place.

So let's change the plot a little. Suppose instead of preventing the parents from meeting, you walk up to the two of them on their first date (before they have a chance to hit it off) and pay them twenty thousand dollars each to walk away and agree never to see each other again.

If that makes you a murderer, would it not also make them murderers? I'm having a hard time seeing a meaningful distinction here.


To be murder it must be a deliberate act.

To be a deliberate act you're walking down the cause and effect path, ultimately your aim is to eliminate the cause of your effect and the paradox rules prevent your action.

Hence you can never go back in time to prevent Hitler. Hitler's actions are the cause, if you eliminate Hitler then there's no cause for you to prevent, so you never attempt it, a variant on the Grandfather Paradox. There are various literary options for handling it, but there can never be a legal option that is sustainable.

Several options have been suggested:

  • The Timecop option: Prevent the legal system that prevents you from acting from ever being set up. Hence there can never be legal repercussions for your actions. The fundamental physics here (apart from some random plot point) being very similar to:

  • Back to the Future option: You succeed but the effect is inevitably worse than the cause. The "you" that acted is isolated from effects, though may no longer exist in that timeline. You then spend the rest of the plot trying to undo the damage.

  • The branching timeline is the most popular. You create a new branch in which your memories don't align with anything that happened. Hitler never existed, possibly the local version of you doesn't either (I wouldn't) but "you" would as you're from a different timeline. You have committed no crime as the person you prevent never existed here. You can never go home.

  • Inevitable failure: the Fixed Timeline. You can't succeed. Any attempt you make can only fail as he must exist to cause you to try. This is related to:

  • The self fulfilling prophecy. Rather than preventing the situation, you end up being the cause of it. What you remember happening must be what happens, no matter your actions.

  • The paradox option. You do succeed, removing your cause and hence your action. So you don't attempt and the cause once again exists etc etc, timeline flickers between two states. It's clearly not sustainable.

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    $\begingroup$ "if you eliminate Hitler then there's no cause for you to prevent so you never attempt it" Not true I think. You attempt (and succeed) the first time but when you return to the present you don't attempt it the second time. Either you have a memory of having done it already so you can check that it worked, or if you have no memory of doing it, you simply won't need to do it a second time. $\endgroup$ Jan 4, 2021 at 13:15
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    $\begingroup$ @chasly-supportsMonica, you don't do it a second time, it's the cause for you having done it the first time. That's how the paradox works. The effect of you doing it is to remove the cause for you to do it in the first place. It's a variant on the grandfather paradox $\endgroup$
    – Separatrix
    Jan 4, 2021 at 13:18

It is impossible to do time travel in the way that you describe without rampant manslaughter. Unless there is some deity-like "corrective force" or other mystical phenomenon that keeps time intact or pushes events back to some deterministic time-flow, it is impossible to interact with people in the past without butterflying away countless lives and bringing new ones in existence.

A hypothetical example:

You travel back in time and you're standing on a street corner. Some guy walks past you and turns his head to briefly look at you (something he wouldn't have done if you weren't there). This brief shifting of his gait shuffles the 'potential babies' he has stored. Later, after having sex with his wife, a different sperm cell 'wins the race' and a different person is born.

Now, just from one guy turning his head where he hadn't before, you've potentially killed someone, and introduced a whole new person into time. This small change similarly ripples through time, potentially leaving the time traveler with an unrecognizable future.

  • $\begingroup$ This <insert up arrow>. Either time can't be changed or time is changed just by travelling in time. $\endgroup$
    – Thorne
    Jan 6, 2021 at 3:22

Suppose you go back and stop the first child being born (would not need to do much, probably just need to delay the parents from having sex for a few minutes so a different sperm meets the egg). So now there might be a "new" child, different to the old one.

If someone else came back though to try and "fix things" and restore the timeline, have they killed the "new" child? The new child stops existing so under the original logic then that is also murder, but then they never existed in the first timeline so relative to that timeline, its wouldn't be murder? Can you murder something that never existed in the now reverted first timeline?

I suspect any reasonable government would either ban all time travel or if they didn't ban it, someone would go back in time to make sure it got banned to stop all these headaches everyone would get trying to think about this.


This depends on your culture's reproductive mores; yet the paradoxes of time travel overshadow it all.

There is a wide continuum of political positions possible in these matters, of which yours is at one extreme.

  • Paterfamilias: the head of the family has the right to control or dispose of those under his dominion at any age.
  • Infanticide: parents can (or must) decide when children are too weak to thrive.
  • First breath: no restrictions on abortion at any stage
  • Viability: restrictions on abortion if the child can survive outside the womb
  • Fetal consciousness: abortion prohibited if pain or higher thought is involved
  • Fetal heartbeat: basic excitability is measured
  • Implantation: IUDs permitted, abortion prohibited
  • Conception: contraception permitted, IUDs prohibited
  • Comstock Law (etc.): contraception prohibited, but reproductive timing or choice permitted
  • Subjugation: no substantive decision making about reproduction tolerated within the social framework

The position your scenario suggests is near the bottom end of this. The "potential to create human life" is held sacred; by this argument the presence of a chaperone at a social dance might be tantamount to murder. In practice such a culture likely would not tolerate such a dance in order to limit its effects.

I should say that in practice, I doubt your beliefs starting from time travel would get you to this point. Rather than deciding it's a crime to keep parents from meeting, the society will decide it is a crime to have your life rewritten, even if you end up losing the devoted wife, becoming a drummer in a garage band, and fathering children in seven states. Given a past causality based model of time travel where it is possible to "change the future", societies will decide that doing so is essentially the murder of a universe - all the people they could have been.

On the other hand, since any universe in which this kind of time travel will ever be developed is one that will be retroactively altered so that it never was developed (save by the arrival of past travellers), they don't actually need this ethics in the universe they live in. But when the travellers from those nonexistent pasts arrive, the receiving society's notions of justice may be cruel - and may account for why time travel is never developed in their timeline.


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