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Far into the future, time travel is invented. Unfortunately, this is about 10 minutes before the universe is supposed to end, so everyone promptly gets into the time capsule and heads back to some convenient era to start anew. Since they have all of the knowledge they gathered throughout the lifespan of the universe, they aren't starting from nothing and they are able to advance yet further. Alas, all things must come to an end and at the end of the universe, everyone jumps into the time machine again and starts over again, this time with two universe-ages of development behind them.

But wait...didn't they already all go back the first time? Why doesn't the world fill up with time travelers?

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    $\begingroup$ Hilbert's Infinite Hotel Paradox. $\endgroup$
    – AlexP
    Commented Aug 26 at 18:19
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    $\begingroup$ "didn't they already all go back the first time? Why doesn't the world fill up with time travelers?" with any of the standard 'how time travel works' tropes it does .. you'll need to invent a new trope, or have them not just going back in time but back in time and to an alternate universe where humanity never evolved or died out (cf "Sliders"), or go back in time and move to previously never colonised planets or galaxies. $\endgroup$
    – Pelinore
    Commented Aug 26 at 21:05
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    $\begingroup$ why would they have all the knowledge, most peole barely know anything and no one human can know everything. $\endgroup$
    – John
    Commented Aug 26 at 21:08
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    $\begingroup$ @John I don't think that one's a valid concern, this isn't one or a handful of people going back with limited equipment and supplies, it's the whole society, with all their scientists and technical specialists, a long planned mass migration where they'll have all their scientific and technical manuals organised and boxed to go long with appropriate materials and tools to build anew when they get there. $\endgroup$
    – Pelinore
    Commented Aug 26 at 21:31
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    $\begingroup$ Are you asking us to brainstorm? How will you objectively choose a best answer? (a) Time travel causes sterility. (b) Nobody's foolish enough to jump back only a short time, so there's plenty of time for everything from disease to war to even out the population numbers. (c) There's an infinite number of habitable but uninhabited planets in the universe and they go to those. (d) People notice what's going on and kill the incoming time travelers. MY POINT: Qs leading to all answers having equal value are prohibited by the help center. I don't want to VTC. Can you edit to fix this? $\endgroup$
    – JBH
    Commented Aug 27 at 2:19

22 Answers 22

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Only one Time Machine actually works. The others were decoys built so people didn’t steal the inventor’s real one. Everyone else actually dies at the end of the universe.

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    $\begingroup$ It's a insidiously nice twist :D $\endgroup$
    – Antares
    Commented Aug 27 at 16:20
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    $\begingroup$ It still has the same paradox: the universe becomes old, one person goes back in time: we now have: n + 1 persons at some time. Universe grows old again, one person takes the time machine: n+2... etc etc $\endgroup$
    – paul23
    Commented Aug 28 at 11:04
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    $\begingroup$ @paul23 and all it takes is one iteration dying without passing on the keys to the machine to break the chain. The billions of machines is probably self-sustaining. The loner is not. $\endgroup$
    – SRM
    Commented Aug 28 at 12:30
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    $\begingroup$ @paul23: Not necessarily - as soon as another person shows up in a time machine at the time machine's usual jump, the original time traveller kills that person, since they think they stole their time machine. Or vice versa - either way, only one universe's worth of information comes back with a person to that timeframe. $\endgroup$
    – Alexander The 1st
    Commented Aug 29 at 21:41
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Time travel necessarily creates paradox or an explanation how to circumvent paradox. As the creator of the world, you will have to write the rules which fit your story ...

  • Whenever a time traveler shows up within the one timeline, it becomes impossible for a second time traveler to visit the same time and place. The mechanics of the time machine prevent it. Find some suitable technobabble to define the size of the exclusion zone.
    Variant, it takes ever more energy to bull into the timeline, but it is not impossible.
  • Time travel creates alternate universes whenever someone shows up in a timeline. The question would be if the number of timelines grows fast enough to prevent overfilling.
    You have one timeline. A person goes back. Now you have two timelines, one of them with an extra person. Average 1/2 extra persons. A second person goes back. Now you have three timelines, with two extra persons among them, average 2/3 extra person. That slows the filling, it does not stop it ...
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  • $\begingroup$ A little piece of technobabble that might be valuable if the OP so chooses: the time machines produce time-oriented gravitational waves, similar to GWs except that they cause temporal strain over space instead of spatial strain over time. The radiation is generated when a TT gateway is opened, and it creates a localized area of space and time where trying to navigate through the chaotic timestream with another time machine would be exceedingly difficult to the point of impracticality. When embedded in the timeline, the radiation seems to dissipate right away, but in reality it lingers… $\endgroup$
    – controlgroup
    Commented Aug 26 at 21:40
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    $\begingroup$ In option B you will have N extra people in N+1 timelines, or an average of N/(N+1): always less than one person. I would say that does effectively stop the "filling". But the problem now is not "where do all the people go", but "where do all the timelines go". $\endgroup$
    – Jacob is on Codidact
    Commented Aug 27 at 7:25
  • $\begingroup$ "Time travel creates alternate universes" if everyone travels back together it isn't helpful, not even a bit, you arrive in a xerox of the original timeline and when you go back from that one end up in a xerox of that so it's functionally the same as with no splitting. .. or are you suggesting that every individual who goes back ends up in their own individual timeline (pop back individually rather than all together)? > so 20 billion (or whatever) timelines each with just one person added to each? > that solves the issue for a very long time, and produces a whole bunch of new issues ;D $\endgroup$
    – Pelinore
    Commented Aug 27 at 13:13
  • $\begingroup$ @JacobisonCodidact, you have this timeline with one extra person, and that timeline becomes the source of another time travel, and timelines split from that have two extra persons, etc. $\endgroup$
    – o.m.
    Commented Aug 27 at 16:17
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    $\begingroup$ @Pelinore, the question would be how many people fit into one time machine. Ten, fifty, a thousand are no real problem. You would have to transport enough people in a single hop to make a difference. $\endgroup$
    – o.m.
    Commented Aug 27 at 16:19
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Eventually they figure out how to stop the Universe from Ending

If they have a perfect accrual and expansion of information and knowledge, then after enough cycles, they are able to stop the end of the universe (or at least pro-long it) - which then gives an infinite increase to the amount of time (and space...) they have available.

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    $\begingroup$ Or perhaps the other way round: After enough cycles, there are so many extra copies of everybody that their combined mass is the reason for the Univerese to collapse $\endgroup$
    – Hagen von Eitzen
    Commented Aug 27 at 16:52
  • $\begingroup$ @HagenvonEitzen ;D like a snake eating itself, each time they drag more mass (themselves) from the end of the universe back to the start of the universe it fractionally shortens the time between the beginning and the end for some reason they can't figure out ;) makes zero sense I'm aware of scientifically but nice allegory for, something. $\endgroup$
    – Pelinore
    Commented Aug 28 at 20:36
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So long as they time travel more than one generation, you have fewer problems, because the ancestors Can be the same people who made the initial jump. Ie:

Z -> A -> B -> .... -> Y -> Z

It does beg the question of genetics, and how descendants of Z had exactly the same genes as Z, but it's fairly stable, and most readers won't question this.

Or you can say that the time travel isn't real time travel - it creates a clone universe and runs it forwards until they need to arrive (or jumps to another identical one). As they are unlikely to be the top universe (or bottom universe), the universe they left will also have in it's history themselves arriving, and you have a stable enough time loop.

Stephen Baxter has the Xeelee do something like this (in his xeelee universe/series) to achieve incredible levels of tech development.

Where it gets interesting is if they jump back less than a generation, because then you obviously end up with the same person present at different age groups. If they jump back 10 years, then you may have the same person present at age 10, 20, 30, 40, and when they jump this time, now you have them at 60 too. I can't see any easy way out of this other than alternate/parallel universes, but maybe some other answerer can.

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    $\begingroup$ Props for the Xeelee reference. Yeah, thinking about this as essentially abandoning the old universe and starting over in one identical to the way it was when it was young seems like the simplest approach to OP's problem — assuming the goal is indeed to have such a "tech loop" in the first place. $\endgroup$
    – parasoup
    Commented Aug 26 at 19:37
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Mass nemisis.

Everytime you go back in time you are increasing the mass and energy of the universe. If you keep doing that you start to break the universe. The evolution of the universe is constrained by the mass, and thus the rate of expansion, and it may influence fundimental factors of the universe. Now they could limit themselves early to prevent this from going all the way to creating a big crunch. But it could also be as simple as the universe changes just enough to make the method of time travel not work anymore after a few times or even after the first. Some fundimental constant is changed just enough that their more advanced technology doesn't work. The time machines don't work anymore, and a lot of he supporting technology does not wor either. Maybe even some of the people did not make the trip because my the time most of them arrived the incoming time machines failed. Sure they have billions of years to remake it, but how much more is going to fail next time, how diffrent will the universe be?

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Time travel does not add up, it resets.

The universe can't fill up with time-travelers, because the universe still ends 10 mins after everybody left.

Either at this point everybody is dead, if we assume there is only one universe but different "time lines" possible.

Or we assume each new timeline creates a new universe:

Then the whole universe replays for those who traveled back in time. But each of them has their own universe, so they cannot even meet themselves but only the past selves of their friends (well, their ancestors only).

This universe is also not guaranteed to develop like the one they used to know. So even knowing all events from their universe cannot help them predict anything in the new one (or only to a restricted degree).

At the end of their respective universe's timeline they do not travel back in time inside the same universe (where they have traveled to before), but into a new "clean" universe again (but back in time).

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You can't actually travel in time.

Using the multiverse theory, when you "travel back in time", you actually change dimensions to a similar one but is behind our timeline.

enter image description here

You could travel back in time to kill your grandfather but really you're killing the grandfather of your copy from the new dimension thus no paradox is created.

Now the reason why the world isn't overrun with time travellers is every time they jump, they all move to different dimensions. With a infinite number of dimensions, they will never encounter another time traveller nor can they ever meet their fellow time travellers from the original dimension again once they jump.

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Only the people go back. And incomplete knowledge is especially useless without the tools to exploit it.

Or, in other words: if you woke up tomorrow, 1000 years in the past, with nothing but your sleepwear, would you be able to build a computer? How well could you pass all your modern knowledge on to your children and grandchildren?

Your time-travellers scattered throughout history. Some arrived yesterday, some arrived last week, some arrived millennia ago. But very few of them know how to properly prove their knowledge from first principles, and even fewer know how to bootstrap the technology from near-scratch. For the most part they are, at best, regarded as "not quite all there". Further down the scale, words like "crackpot" start to be thrown around.

Ironically, those who went back further are better off from that perspective — modern science requires empirical evidence, or repeatable experiments. The Ancient Greeks just needed you to be suitably skilled at oration and debate.

In an era where signs of futuristic medical care are even more impressive, your time travellers may even be able to rally tribes together, and join forces to build great and advanced civilisation. But, evidently, something went wrong: Atlantis, Mu, Dvārakā, and Shangri-La are now only spoken of in myths and legends.

The end result? Knowledge is lost, corrupted, or decays. Society advances… and winds up exactly back where they started. They don't advance further, because they still have to spend so much time recovering what was lost.

So turns the wheel of time.

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    $\begingroup$ "if you woke up tomorrow, 1000 years in the past, with nothing but your sleepwear, would you be able to build a computer?" Yes. If someone were willing to pay for it. Just give me some carpentry tools and a box of marbles and feed me for a year or two. The bigger problem than proving the knowledge from first principles is the infrastructure. 1000 years ago .01mm tolerances were possible, but expensive. Today we use them everywhere just to make things more convenient. Even if you know exactly how it works it takes time to build the tools to build the tools to build the tools to build... $\endgroup$
    – Perkins
    Commented Aug 29 at 19:35
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Declining fertility rate and the geometric series

As you observed, with a universe of experience, your time travelers aren't starting from nothing. Their arrival will launch the worth along a path of technological advancement that vastly surpasses what humanity has seen before. It's well-known that the fertility rate decreases, for a variety of social reasons, as wealth and technology grow. So no doubt the time-travelled to world settles at a lower population than the one that preceded it. And in each iteration, the pattern repeats: technology climbs to an unimaginable high, and the population hits a new low.

Imagine if you will, a simple mathematical model. Say your original world has 8 billion people at 10 'til doomsday. They all travel back, and the new world they create will be twice as wealthy, but half as populous. So by the time the clock winds down again, only 4 billion people are around to travel back in time. And so on, and so.

Now we can use the happy mathematical fact that the geometric series converges. We have

16B + 8B + 4B + 2B + 1B + 500M + ... = 32B

So at no iteration will more than 32 billion people will be present when the cycle restarts.

Of course, a less happy mathematic fact concerns the last whole number in that sequence. After only 34 iterations, well... your story better come to a satisfying ending!

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It can only be done once

Time travel is impossible in our universe, at least how we know it. That means the system how to travel is up to you.

In our current understanding we see some rules that can't be broken. The speed of light can't be matched, no energy can be created or destroyed, etc. We can use this for time travel.

You can only time travel once, or only a few times. It takes a certain resource of the universe that can't just be replenished. At the end of the universe it'll be gone at one point, stopping the cycle of more travellers. It can be as simple as a 'binary' element of the universe flipping. Imagine the Higgs Boson having no spin, and after the travel it turns right, preventing further travel.

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Uniqueness. At a subatomic level. The particles that make up the travellers disappear from wherever they were in the universe when the travellers arrive and translocate to their bodies, this is messy, the universe will be rocked by disasters large and small. As a result whichever repeat is set latest in the history of the universe largely wipes out anyone who came back to a point in time before they did. If repeat cycles manage to recur to spite the chaos it is possible that the displaced particles could reduce/remove the ability of the universe to form life supporting structures, like stars, unlikely though.

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Only consciousness can be sent back

This version of time travel needs a host body in the past to jump into.

This obviously has it's own paradoxical timeline issues but that will depend on the rules of your universe. Is continuity preserved if society eventually makes time travel no matter your actions?

Does your future population numbers mean some people are fighting for spots in the past? Especially after you've gone back a few times, is there a governing body that limits total lifetimes? Or is your population controlled to not exceed the total human population across time?

The show 'Travelers' also has an interesting take on the ethical issues of this method where they use historical medical records to try and jump into bodies moments before they should have died and then prevent it.

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  • $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$
    – Ash
    Commented Aug 27 at 10:32
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    $\begingroup$ @Ash thanks. I look at it differently. I think the writer of the question is a bit contradictory in that state. They also mention "a convenient era to start anew", which suggests it wasn't around the big bang. I think the 'two universe-ages of development" is more of a convenient reference. They could just as well go to the past where science 'started', with the first homo sapiens or other alien creature an acceptable fit for conscious transfer, compared to obliteration. Their technology might be so advanced they can join our current society, being equal to just after discovering fire. $\endgroup$
    – Trioxidane
    Commented Aug 27 at 11:32
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    $\begingroup$ @Trioxidane That is a satisfyingly Lovecraftian solution that does sit well with the inherent contradictions of the question. $\endgroup$
    – Ash
    Commented Aug 27 at 11:35
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    $\begingroup$ @Ash I appreciate your comment but want to be clear that none of the questions I posed were asking for clarification, my answer was meant to be taken as is and those were open questions intended as considerations on the concept that might help develop the idea in their setting. Trioxidane has helpfully addressed the other point with the same interpretation as me; it says two universes ages of 'development' which I took to mean the window of human existence and doesn't specify when they would go back to. Do you still feel it doesn't answer the question as written? If so, how could I improve it? $\endgroup$
    – Jamie
    Commented Aug 28 at 13:14
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    $\begingroup$ @Ash, "The Shadow Out of Time" of Lovecraft does come to mind as a direct usage of "consciousness time travel", with some "cousciousness swapping" on top. $\endgroup$
    – Raphaël Duchaîne
    Commented Aug 28 at 14:07
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Mass conservation

In order to go back in time, you must move an equal amount of mass to the present. Eventually, you will not have enough "old mass" left to send more travelers

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"Local" time travel only

Note: I'm stealing the basis of this answer largely from John Scalzi's "Old Man's War" series, where it was used to explain FTL travel rather than time travel, but still...

Time travel jumps the person travelling into a parallel universe: you can't select which parallel universe you'll end up in but it will be one that is very close in detail to the one you start from - as timelines/universes diverge with each quantum "choice" the ones that most closely match the one you're in stay closer than the ones that diverge, so the closest universes are those where you wouldn't notice any physical differences at all; it could be the spin of a single subatomic particle that divides them. The moment you jump into a close universe, it becomes massively different from the one you started from. Forget particle spin, you suddenly have a completely different amount of mass (and therefore energy) in the universe you arrive in. This knocks it way out from being local to the starting universe, making it hugely, almost impossibly, unlikely that anyone else jumping in time will land in the same parallel universe as you. Thus if seven billion people jump, only those who jump in groups will end up together; anyone who goes in another jump will end up in a different universe completely. Add to that a maximum number of people who can jump together (energy constraints, too much matter risks "smearing" the travelers all over the destination as a thin paste, material constraint imposing a maximum size for the transmission booth, whatever) and you have a limit to the numbers built in to the process.

You could play with unlikeliness of arriving in the "same" universe if you want to have some people who really shouldn't end up together actually doing so.

Thus you cannot end up with any world filled up with time travelers and each one gets their own universe to live in and advance, every time they travel. Maybe - just maybe - one of them will eventually find a way to prevent their universe ending. Maybe none of them ever will, but they will have an infinite number of universes to try their ideas out on.

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The self retconning universe:

Disclaimer: From the formulation of your question I think you want a single timeline where the "last generation" jumps back to the stone age and when time has come the new "last generation" makes again a jump.

The self retconning universe is a marvelous thing. It keeps itself consistent... sorta. Alrik, Bertha and Carl (and 20 billion other people) jump back into the jurassic age and quickly rebuild their modern society before the human has even emerged through evoloution. By the time the first humans evolve and want to beginn their hunting and gathering the great-great-...-grand-children of Alrik, Bertha and Carl (and all the others) populate the earth. Alrik, Bertha and Carl themselves however are long dead.

Then Alriks decendent Adam drives through africa with his cyber jeep and runs over a proto human. Wiping out Alriks and therefor Adams own line of ancestry. This would lead to the well known grandfather paradox and wipe out the universe.

Or it would, if the universe didn't have some tricks up its sleeve. Luckily it does. The universe retcons itself. Alrik did not exist. Neither the Alrik in the future, shortly before the end of the universe, nor the Alrik that traveled back into the jurrasic age and had children with Bertha vanishes both from the future and the past. Adam however stays. The universe can't retcon someone away while they are still here or people remember them.

By the time the universe again comes to an end all the people that jumped into the past last time have been retconned away. And when the new last generation jumps back into the jurrasic age all they find are some dinosaurs... and queen elizabeth ofcourse.

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Time machines can't exist because having time machines creates an unstable timeline.

When someone invents a time machine, they can travel back and forth. More time machines are created and more people travel back and forth. The time line fluctuates.

Eventually, someone travels back to before time machines were invented and does something that causes time machines to not be invented. Now the timeline is stable again.

So every time a time machine is invented, the timeline fluctuates and eventually settles into a new timeline with no time machines.

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The population won't just keep infinitely growing, resources, which are finite are needed to support larger and larger populations so it doesn't really matter that you add people to some time period, the better and better technology would support more and more people eventually, but it's still gonna hit a limit.

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  • $\begingroup$ As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Commented Aug 28 at 3:52
  • $\begingroup$ @Community hi, can you explain what is unclear about my answer and I will attempt to further explain $\endgroup$
    – David
    Commented Aug 28 at 4:01
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Not everyone jumps back to the same time. Maybe it's random to when you jump or maybe everyone just picks a time they're intrigued by.

Either way, the knowledge they hold is scattered. Individuals might have some knowledge that could advance the science of the time a little, but not enough to make a huge impact. But the vast majority of people won't be able to contribute anything at all.

Then there's also the matter of trying to teach the locals what they know. Maybe their technology and understanding is so advanced, the locals simply can't understand it without the millennia of science that led to that knowledge.

Also, depending on the time people jump to, they might get welcomed, but they might also end up being shunned or even prosecuted. Imagine a time traveler from the far future ending up in the middle ages. They simply won't be able to communicate, unless they specifically learned to speak the languages of the time. So at best they'll live a relatively simple live away from everyone else. They certainly won't hold any important positions.

But the most important part, even if the above aren't concerns, is that with only a couple of people entering any given time, they're not going to have a big impact on the overall human population. Fertile men and women might cause a couple of extra children to get born, but I doubt it's enough to have a noticeable impact in the long term.

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Because our Big Bang theory is wrong, and you can go back infinitely.

The assumption history will suddenly be filled with time travellers assumes that history is finite (ie. the Big Bang occurred).

What if the universe isn't expanding, perhaps we got our measurements wrong, and our current theories of the universe are incorrect.

For instance:

  • galaxies aren't actually moving away from each other, only light becomes redshifted over time by some other fundamental interaction
  • Or Dark Energy does not exist universally, but instead is localised to our current observable universe (part of a larger unobservable universe), thus bringing our mistaken error in assuming the universe expands and the Big Bang occurred (ie, violation of our Cosmological Principle)
  • Or our CMB detectors are wrong/misunderstood and we are observing some other yet-unknown phenomenon. As this was the primary confirmation of the Big Bang, if our interpretation is incorrect then this may put in doubt the finiteness of the Universe.

If any of the above is true, then as time travellers launch themselves further back, they discover they can indeed travel as far back (perhaps trillions, or quadrillions, of years or more). Perhaps to far more prosperous, wondrous or exciting eras, or even where replicas of themselves or family and friends occur. It would be easy to be 'lost' in a sea of infinite time.

History would then be seemingly devoid of time travellers, as they become lost in their history of infinite time.

Note: It is worth noting that as well as being lost in infinite time, there is a possible solution to the Fermi paradox of being lost in infinite space. For instance, if someone invents a drive that instantly transports you to any location in the universe, at no cost, then most civilisations would simply 'dissipate' relatively suddenly and cause the end of their civilisations, where everyone becomes 'lost' in a sea of infinite space.

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Human nature to fck things up.

Once traveler causes paradox conditions the timeline collapses and the particular future where he came from ceases to exist. This makes travelling very rare and dangerous. Only stable future is the one where only few are allowed to do this.

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Your mind goes back to a past "you"

This method of time travel is similar to the one found in "days of future past" where the x-person Kitty Pryde sends Wolverine's mind back in time to his past self to change the future. If a machine could zap your brain back in time to a younger you, you would have your whole life to live again. Obviously issues would be raised with younger people not being able to go as far back as older people, but by having the same body you had when you first inhabited that time, you do not add any extra people to the timeline, only knowledge of the future.

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The self-consistent model of time travel: if you travel back in time, you've always been there. https://en.wikipedia.org/wiki/Novikov_self-consistency_principle

For example, this is what happens in Harry Potter when we realize that mysterious lucky things that happened were actually due to a hidden time traveller.

So, traveling back in time does not change history. It gives these travelers some extra time to enjoy the rest of their mortal lives, but they eventually die.

Self-consistently, their impact on history has already been baked in.

On knowledge: maybe some were able to live second lives as geniuses and share their knowledge in a bootstrap paradox kind of way. But likely, most of their knowledge is lost because it is not believed, or decays over the course of 100-10,000 years, or their knowledge was already incomplete or incomprehensible. So in the end, the technological progress is baked in.

On descendants: if any were successful in leaving behind descendants, those descendants had already existed.

Some might choose to only jump back a few moments in time, or short enough to try to take another trip, but if their 2nd copy was not already here with the rest of us, something must have gone wrong. Maybe they were teleported to a hostile part of the dying, cold Universe. Or maybe they really did succeed! But in that case, there are N copies of some people and always have been, and that N+1th person fails.

The Universe is too old and too hostile for a civilization to really try to make use of a Universe-age of knowledge. Civilizations collapse and knowledge is lost on much shorter timescales. The time travelers might become the Ancients or maybe even the Big Bads, but their impact on history comes to an end in a self-consistent way.

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