Let's begin with the premise of that type of time travel, the one in which no matter what you try, the past cannot be changed, if you tried to kill your grandfather you wouldn't be able to as much as you tried it.

We can consider that this happens because the universe itself can prevent things by making physically possible things to happen, for example, I guess under some circumstances a pistol could get blocked and you wouldn't be able to shoot it when you are going to use it.

But there are things, that no matter what I think cannot be be physically avoided, if you were to send to the past a 300 kg rock some meters above the head of person so it hits his head, I don't think there's a possible way the universe could avoid that, even if the universe were to suddenly form a hurricane or divert a missile so it hits the rock and the person doesn't get hit, it's impossible to be on time for that.

What could explain that an event that's 100% assured to happen wouldn't happen without breaking any physical law?

Any type of scientific theory, even if not proved but possible, would be ok as an answer.

  • $\begingroup$ What excatly can be done via time travel then? $\endgroup$
    – Raditz_35
    Commented Jul 29, 2018 at 12:30
  • $\begingroup$ You could do anything you want as long as it wouldn't change the future. If it were to do it the universe would avoid it as much as possible, or maybe there are scientific theories that would make able to bypass 100% assured events. $\endgroup$ Commented Jul 29, 2018 at 12:32
  • $\begingroup$ Please name one thing that can be done that wouldn't change the "future". Or perhaps a time span after which the future is preserved after altering stuff. Is this just relative to the initial point from which you traveled? I think all of this is highly situational, each situation needs to be dealt with separately $\endgroup$
    – Raditz_35
    Commented Jul 29, 2018 at 12:34
  • 1
    $\begingroup$ about your last sentence, if I had a scientific theory of time travel, would I share it with you on this site or would I use it for my own good? $\endgroup$
    – L.Dutch
    Commented Jul 29, 2018 at 13:01
  • 2
    $\begingroup$ "What could explain that an event that's 100% assured to happen wouldn't happen without breaking any physical law?" You appear to have painted yourself into an insurmountable corner. $\endgroup$
    – Ash
    Commented Jul 29, 2018 at 19:40

9 Answers 9


There are two answers, neither of which is especially satisfying. The key point is that if there is only a single timeline in a single universe, then it must be self-consistent. That is, the past you experience when you go back in time is exactly the same past you remembered before you went back (ignoring the fact that we never have complete memories of the past and rarely have fully accurate memories of the past.) If the past you experience when you go back is different, then you're in a multiple-timeline world.

So there remain only two possibilities: First, that the actions you freely take are exactly the events which had already happened before you stepped into the time machine. Basically, that whatever universe this is, if time travel exists, the reverse causality of time travel is part of it and always was part of it and there is no inconsistency. There is no force guarding time and forcing consistency your actions are not constrained, but it necessarily turns out that what you freely choose is what actually happened. That's what having a single timeline means.

The second option is to note that here can't be any external force that keeps you from killing your grandfather. But what force? How does it work? How does it know what you ought to be doing? Magic? Anyway, just the action of that force would be enough to change history even if the bullet misses your grandfather. History is chaotic (in the mathematical sense), so any change no matter how small including the action of the Mysterious Force restraining you from shooting may have arbitrarily large consequences. ("All for want of a nail.") So the only way you can have time travel is for you to be able to change the past. In other words, time travel is inconsistent with an unchanging timeline. Pick one or the other because you can't have both.

So. Time Travel plus a single timeline. Either it just works out or it's impossible.

(If you are writing a story and need time travel and a single timeline, just do what other writers have done: Don't over-explain, just tap dance around it and go on with the story.)

  • $\begingroup$ A variation of your first possibility could be to make the time travelers immaterial, like ghosts. They are present, but only as spectators. Their matter can never touch matter from another time. The total energy/mass of the universe must be constant. $\endgroup$
    – Wolff
    Commented Jul 29, 2018 at 12:58
  • $\begingroup$ @Wolff Even that is difficult if you want to stay consistent with physical law: Just to see what's going in the past on requires that the time traveler must absorb some light which means that it is interacting with the past universe and thus changing it. OTOH, there are many fine stories that use exactly that premise and are no less entertaining because it's not correct. $\endgroup$
    – Mark Olson
    Commented Jul 29, 2018 at 13:04
  • $\begingroup$ Your right @Mark Olson, What if time travel was done using some kind of meditation? The time traveler gets into a deep trance and is then able to "enter" a person from the past and sense everything that person senses. It would be like "watching" a recording of the past, and you wouldn't be able to interact in any way. $\endgroup$
    – Wolff
    Commented Jul 29, 2018 at 13:24
  • $\begingroup$ You say "that here can't be any external force" and then ask "But what force? How does it work?". That doesn't make sense. $\endgroup$
    – RonJohn
    Commented Jul 29, 2018 at 17:42
  • $\begingroup$ So it turned out he wasn't your grandfather. Also there's no need for such a force, events will conspire such that the timeline is consistent no matter your intentions. You missed, the gun misfired, the round was a dud, you accidentally shot a guy who was trying to stab him who turned out to be a future version of you having another go. Whatever it was, you didn't kill him. $\endgroup$
    – Separatrix
    Commented Jul 31, 2018 at 7:25

One solution to this problem is something similar to the 'Mysterious Force' Mark Olson talks about, but without generating the problems he mentions.

The universe has to follow Novikov's self-consistency principle, which states that the probability of an event creating a paradox is 0. This enables the illusion of an invisible force while avoiding any contradictions it may create or explaining how does it work.

One way of seeing the principle is thinking that every decision (or any physical process) creates a timeline for each possible outcome, forming a branching tree. Every timeline that contains a paradox selfdestructs, deleting its branch and forcing you the other way. The timeline of your world is a path though this tree.

  • $\begingroup$ Welcome to Worldbuilding, Dadanian! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$
    – Gryphon
    Commented Jul 29, 2018 at 22:01

What you describe has been called "the mother of all paradoxes". This is an image from a 2012 Scientifc American issue entirely dedicated to the nature of time (sorry for the low quality):

Now you're thinking with portals!

No matter how hard you try to hit that person with that 300kg rock, you will miss, probably in a very Looney Tunes like style. You can call it plot armor, narrativium resistance, trying to change a fixed point in time, or the silent work of the time police...

If you haven't watched Gravity Falls, go do it now

... The fact remains that time will be self consistent and physics will find a way to prevent you from changing history.


My answer is going to go outside your box a bit so you can see a wider picture. But, before I begin, let me point out...

You're thinking that the past is independent of the present. That you, today, have the ability to make a decision independent of what has actually happened in the past. Well.... maybe you are... maybe you're not.... You need to decide which it is.

Theory #1: There is only one timeline.

The past has already happened. It has already accomodated whatever nonsense you try to do. The reason you can't kill your grandfather is that your grandfather has already survived whatever you try to do to kill him. From this perspective, you exist due to the already resolved causal consequeneces of your future actions. Do you remember that family story about Grandpa miraculously avoiding the only boulder that ever rolled off the mountain and into the road in recorded history? Yup. Cheers.

Theory #2: The timeline is constantly recreating itself.

Let's assume that every infinitely thin moment of time is independent of all other time such that whatever you do to kill your grandfather never affects you because your past is no longer malleable. When that boulder lands on your grandfather (ouch), it happened to someone who wasn't your grandfather because that instant in time is independent of your instant in time, forcing time from the perspective of your late grandfather to recreate itself into a parallel timeline that has nothing at all to do with you. Of course, this means that when you travel back yourself there is no longer a future for you to travel to because the infinitely thin moment you entered instantly recreates the timeline around you. This is known as a one-way trip.

Theory #3: The Conservation of Causality

Or, we can assume that time must instantly snap itself into the shape forced upon it by a changing past. This is the "kill grandpa, pop out of existence" theory. The cool thing about this one is the plausibility of the butterfly effect because what seems like a few meaningless lives (grandpa, dad, me) are actually lives interwoven into thousands of other lives that, themselves, are interwoven into thousands of other lives and the entire chain will be modified because the three of you aren't where you were to begin with. The parabolic hurricane from the butterfly's wings.

Theory #4:Time is an illusion and time travel will never be possible.

Finally, there is no timeline. There is only the infinitely thin moment of time RIGHT NOW! There is no future, only a predicted future. There is no past, only memories and records of things people did. There's nothing to go back to, and therefore there's no way to send a boulder back to off poor old grandad. Time is nothing more than a mathematical concept that gives structure to the chaos surrounding our moment-by-moment lives.

There are undoubtadly other ways of looking at the effects of time travel, but this will do for now.

In your case, you're kinda looking at Theory #1 and wondering, "why can't theory #1 be like theories #2 or #3?" Well.... because it's theory #1.

By the way, all time-travel theories are unproven and will remain so until someone discovers how to affect a moment in time that isn't right now. (Don, why is my glass of water boiling? I don't know Linda, but hey! Why don't we try to affect the past by causing a small space to heat up?) Without that ability, there's no way to actually prove anything about time travel — even mathematically. We can play games with the concept, but to quote the movie Volcano, "Certainty is a big word."

Which was a fun but somewhat lengthy way of saying, "there isn't a way to explain how an event 100% assured to happen didn't happen without breaking any laws."

Of course, an astute philosopher might add, "what laws?"


There is a Physical Concept Called Looped Time

If there are eleven dimensions of space-time, like string and M-theory posits, all but our familiar four (up/down, left/right, fwd/back, and time) are compactified: meaning they are closed in a loop so tight that it is so far impossible for us to recognize the extra dimensions are present in our daily life.

What makes the familiar four dimensions special and non-looped? Further thinking is that, maybe, even these dimensions are looped on some larger scale.

There are several practical examples of this principle: provided no extra energy, any orbital path is a loop. Velocity may increase and decrease, position may change, but everything will return to the same space and velocity when it comes around for the next orbit (barring some intervention). Similarly, if not interrupted by air, a pendulum follows a path of speeding up/descending, slowing down/ascending, and repeat.

The idea of wormholes presumes that time and space our curved and that negative energy bridges simply make it possible to travel between points on the curve by travelling through the middle.

In a system of completely looped time, everything would happen precisely once. At some turnaround point, it all rewinds (or coincidentally seems to return back to the same conditions as the start) and plays back again, and again, and again, but the participants may (very likely) be unaware of this. Maybe there is some small degree of freedom, or maybe everything is guaranteed to follow the one track until an intervening force.

So, assuming time is looped, and your protagonist has found a way out to arrive out-of-place at another spot. Essentially, the protagonist has created a larger loop: the moon orbits the earth, and they both jointly orbit the sun. Maybe your protagonist does come back and kill his grandfather successfully, disturbing the inner loop. Like the balance kinetic and potential energy in an orbit, the disturbance will try to restore itself (assuming total energy has not changed) on the next go-round. So, your protagonist kills his grandfather then the loop goes round again. At the same time, by the way, your protagonist is probably in his or her own loop as well. Provided enough energy (maybe a 300 lb rock can do it; maybe not), the whole orbit (looped time) can be nudged a bit and a new once-only story is being told.


The whole loop must be self-consistent.

Consider superconductivity: you have a material with electric resistance of zero. For complete, "true" superconductivity, this creates a paradox: any magnetic field entering the material would either need to violate Lenz's law, or create an electric current of infinite intensity.

So, true superconductors are impervious to magnetic fields. No magnetic field can penetrate, hence no paradox.

And you could think "yeah, but I can create magnetic fields of enormous strength. Whatever effect keeps the magnetic field outside the skin of a superconductor cannot reasonably overcome the power I can wield: that itself would be paradoxical".

So you throw your monster magnetic field against the teensy piece of true superconductor, and guess what - it stops being a superconductor.

Time travel could work in a similar way: you send something back in time, but to do so requires energy. If you send something in the past, you have already sent it in the past, and whatever it will do, it already did, and it has already been evaluated. The energy requirement increases with the size of the chronoclasm - and a true, irredeemable paradox would require infinite energy.

Even altering something that someone remembers differently would require impossible amounts of energy, because physically there is no difference between a grandfather's death and a neuron firing out of synch - they're both phenomena that couldn't have happened. And this drastically limits - probably destroys - any use of the time machine to alter the past, since you cannot alter the present in any way.

You might try and salvage ancient objects of art before they're destroyed, but after they were last seen. Or kidnap people out of crashing planes, replacing them with lifeless clones or replicas (both tricks have appeared in SF stories).

(On the other hand, it turns out that sending something outside the time machine's light cone is possible and free. So, just like Asimov's antigravity machine turned out to be useless for antigravity, but also a free energy generator, the time machine turns out to be useless as such - but it is also a FTL device. This also has been exploited in Roger McBride Allen's Chronicles of Solace).


Timeline and parallel universe theory states that for every potential existence there exists a reality that pertains to it. There is a reality where the rock just stops existing before it hits them.

Basically nothing is 100% certain but the chance of deviation is so small we often ignore it, but it is one of the factors of quantum science.

I'm not sure what situation you want to apply this to, but quantum science can be used to explain some pretty crazy stuff. Try looking into that.

  • 1
    $\begingroup$ are you trying to answer or to comment? $\endgroup$
    – L.Dutch
    Commented Jul 29, 2018 at 13:06
  • $\begingroup$ Answer, they were asking if there is a scientific theory that could explain how something extremely likely wouldn't happen. Some areas of Quantum Physics can be applied here. $\endgroup$
    – ZoneWolf
    Commented Jul 29, 2018 at 13:17
  • $\begingroup$ So please put that into the answer. Just a "try looking into" is more a comment $\endgroup$
    – L.Dutch
    Commented Jul 29, 2018 at 13:20
  • $\begingroup$ This Answer doesn't answer the question. $\endgroup$
    – RonJohn
    Commented Jul 30, 2018 at 1:46
  • $\begingroup$ @RonJohn yes it does- "What could explain that an event that's 100% assured to happen wouldn't happen without breaking any physical law?" Quantum Physics $\endgroup$
    – ZoneWolf
    Commented Jul 30, 2018 at 6:25

Let's keep it simple.

One timeline.

You go back and change something. You then go back to your time but nothing changed. Why? Your change altered the parameters required for your trip into the past which means you took no trip into the past meaning you effected no change.

A clearer example: You go back and kill your mother before she gives birth to you.

You now don't exist to go back and kill your mother.

She lives and gives birth to you - timeline unaltered.

A bit like editing a document but you don't have the right permission so no matter how many times you hit save, only the original document is possible.

  • 1
    $\begingroup$ I like this answer. Time is ultimately recursive and the recursivity is always and instantly resolved. Of course, it begs the question, what if I go back and kill some random person? Oooohhh.... $\endgroup$
    – JBH
    Commented Aug 1, 2018 at 15:10
  • $\begingroup$ @JBH Doesn't that event then count as part of the timeline? You always go back and kill that person. Where's the evidence that they were supposed to live, there is nothing to compare it to. Your example does remind me of a tv show where someone is told if they push a button on a device then some stranger will die and they'll become rich. Later, after they've pushed the button a person shows up to given them their money and take the device away - to be given to someone they don't know. $\endgroup$
    – Daniel
    Commented Aug 2, 2018 at 21:46
  • $\begingroup$ What makes the example of a stranger interesting is that people's lives are far more interconnected than most people think. It would be the burden of the storyteller to explain this. But, if I go back in time to kill some random South American native living a basically stone-age life, who's to say their future didn't include inspiring a young person who grew up to design an influential component of the time machine? As the recursivity works iteself out, the alternate timeline where the native was killed is vacated in favor of the correct resolution. I like the idea. $\endgroup$
    – JBH
    Commented Aug 2, 2018 at 21:58
  • $\begingroup$ What's potentially mind-bending is the idea that this would give time travelers license to murder without apparent reprecussion as the timeline will always resolve itself and the murder (and its immediate effects, such as torture) would only exist until the traveller returns. It leads to an interesting question... How would that recursive effect propagate? What secondary effects may exist? Does the killer even remember killing sinc the recursion must correct the traveller as well? How would you even prove you travelled after the fact? $\endgroup$
    – JBH
    Commented Aug 2, 2018 at 22:01
  • $\begingroup$ There's a movie in this... it's just a question of figuring out how the killer gets his comeuppance. How does time, so to speak, get its revenge? $\endgroup$
    – JBH
    Commented Aug 2, 2018 at 22:02

Answer: No body knows.

If by "timeline" you mean a time-continuum which does not branch, then the question becomes: is the path the future will take predetermined?

In other words, is the state of the universe deterministic with respect to time?

If it is, no you can't.

if it isn't, sure ya can!

A lil' Quantum Mechanics & some background on this very present debate:

Determinism (a school of thought which believes the future is laid out for us, but is also more permissive than your timeline assumption in that it allows for "multiverses," aka for branching in the "line.") has seen a resurgence in recent years, since the advent of the time-dependent Schrödinger Equation.
Very loosely, it says that you can determine the future-state of an atom (or arbitrarily-large collection of atoms...such as the entire universe) using $t$ as the only independent variable:

$i\hbar \frac {\partial}{\partial t} |\psi(t)\rangle = H(t) |\psi(t) \rangle.$

I won't go into too many specifics, but notice that:

  • $i$ and $\hbar$ are constants (aka they don't vary and so aren't variables.)

  • $\frac {\partial}{\partial t}$ means derive by $t$

  • $H(t)$ is the Hamiltonian and operates only on $t$

  • $\psi (t)$ is the wavefunction and again, operates only on time.

The fact this equation operates only on time leads to a logical correlate: there are not, nor can be, any other influencing variables in the TDSE.

Unfortunately for deterministic physicists and philosophers (specifically those philosophers concerned with metaphysics,) this equation only works when there is no observer. This idea and equation "collapses" when you actually try to observe the state of any atom.


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