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Suppose humanity lives on for the next trillions upon trillions of years. One day we'll get to a point where no useful work can be done and hence no life can exist: the heat death of the universe.

But somewhere along the way we invent time travel and are able to travel back in time. Is it theoretically possible for humanity to survive by traveling back in time every time we get close to the heat death of the universe assuming we "escape" in time? If so, how will that work out? Could it be done indefinitely?

I might have misunderstood Worldbuilding. Let's say it's theoretically possible, and humans do end up going back in time indefinitely. How would that pan out?

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    $\begingroup$ Hello Arnold, welcome on Worldbuilding. A question like "theoretically possible" sounds like something that should be on a science site like Physics. Are you asking about physics as we know it now, or some fictional setting? If so, the writer is making the rules, and we can help to make them consistent and interesting ... $\endgroup$
    – o.m.
    Aug 21, 2015 at 5:31
  • $\begingroup$ Are you suggesting that the future us can send a "fix" into past when our universe is less than a second old and change some of the parameters so that instead of an open universe we have a flat universe meaning time nevers run out? If not then ditto. $\endgroup$
    – user6760
    Aug 21, 2015 at 8:57
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    $\begingroup$ Was that question inspired by Doctor Who ? ;) $\endgroup$
    – Mystra007
    Aug 21, 2015 at 13:33
  • $\begingroup$ This sounds like the perfect question for @L.Dutch, considering he lives in a restaurant at the end of the universe :P $\endgroup$
    – Liam Morris
    Apr 22, 2019 at 6:57
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    $\begingroup$ "Assuming they end up doing this indefinitely, how would it turn out?" ... Well, they did it indefinitely, so it turned out that it worked....right? $\endgroup$
    – Muuski
    Apr 22, 2019 at 20:05

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No. It is not possible according to the laws of physics.

Instead of "is this possible", perhaps you should ask how to design a fictional universe in which such a plot is possible and (presumably, or you wouldn't bother to ask) acceptable to knowledgeable "hard" SF readers.

One common time travel moodel used in literature is where time travel leads to a different clonee universe whose future is causally disconnected from the original.

An interesting story would be for the race to misinterpret which kind of time travel model their universe admits, and end up going back in a fixed-history block universe, not a mutable copy.

However, such an advanced elder race would not be "like us", so it would be difficult to make a story approchable without meeting up with "us" to tellmthe story from our point of view.

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    $\begingroup$ Reminds me of a Lovecraft short story told from the perspective of someone who was replaced by a time-traveling alien species whom, when the end times of that species arises, simply switch bodies with beings of another younger civilization. This is implied to have been happening already with our species. $\endgroup$
    – Neil
    Aug 21, 2015 at 6:35
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I would say if such a thing were possible then the universe at this time would be filled with life. If even one race made it to the end and came back and kept repeating that, after a 100 times there would be at least 100 different races (assuming only one race became that intelligent to begin with). Over a trillion years, humans would likely be a hundred different races, and each one that came back would also turn into hundreds (thousands? millions? billions?) more each one spreading out into the galaxy. It would be teaming with life.

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  • $\begingroup$ Not if its the first iteration. $\endgroup$
    – Malkev
    Feb 5, 2019 at 10:16
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    $\begingroup$ There is no 'first iteration' in this situation. $\endgroup$
    – bowlturner
    Feb 5, 2019 at 14:17
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    $\begingroup$ +1. I suspect this would also imply that the universe would be hastened towards its heat death by civilizations harvesting sources of usable energy and converting it all to heat. $\endgroup$
    – Qami
    Mar 30, 2022 at 17:45
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If this was possible you would face another problem. In theory you could do this an infinite number of times but then the entire universe would be crowded with humans.

Each time they go back in time, the previous iterations of humans having done the same thing would also be there.

They cannot fight, because killing a previous copy of yourself would also kill you.

So they have to coexist peacefully but at some point there would just not be enough resources for the newcomers.

So it's a loose loose situation.

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The only way this would work is if the time travel is such that what they do when they go back already happened; it isn't sustainable if the time travelers can change history.

Humanity at the end of the universe is theoretically a very large population, saturating the available space on inhabitable planets in the universe. Even if they could send themselves back to all the other planets besides earth (and assuming they were originally uninhabited), this would only work once. When you get to the end of time the second round, there's nowhere to go.

If the group becomes extinct before humanity emerges as a species, it works. If they integrate themselves into their own history (as in this Star Trek episode), it works. But either way, it only works once.

If you are dead set on making it possible, then humanity must have the capability to create a brand new universe each time they hit the reset button. If they don't, even if they can create new planets out of thin air and limit the number that goes back each time, they will eventually run out of room. Space is immensely vast, but ultimately finite. But at this point, time travel is arbitrary in making things work. They don't need to travel back in time unless they arbitrarily want all the universes running in sync together. And if they can create new universes to live in, they will do it well before the one they're living in runs out.

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As some other answers point out, the most interesting part of the question is how you handle (avoiding) meeting with current time civilizations and overpopulating because of infinite iterations.

Civilizations from an ending universe would look really different from their own specie millennia ago and wouldn't be able to integrate into their society. Also, the limited resources would mean a smaller community. As an elder civilization that has seen the end of the universe they would be likely fatalists and if found out may force their own destruction to avoid interfering in the regular timeline.

With this setup, I would consider them living in small worlds scattered in distant stars and purposely hiding by avoiding galactic groups or clusters using stellar engines to move their whole solar system. I am supposing that if they have time travel and stellar engines, they can move their whole planet/system and would be able to "steal" suns to replenish. Ideally, they should steal the suns close to a Black Hole to avoid affecting the timeline (as objects in a black hole lose their information according to current theories)

This would allow them to handle nearly infinite recursions, but at the same time, harnessing mass from black holes (or avoiding it to get sucked in) would slowly reduce the time to heat death, so in infinite iterations, they would "eventually" end up meeting their demise.

Also want to point out that it would be OK to meet with older or newer recursions that have crossed the heat death horizon as they would simply part ways knowing better not to interact.

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