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On earth, most animals live decades, and only a handful over 100 years. What has led to such a 'short' lifespan across the board?

Assuming there may be other carbon-based life forms similar to that of earth on other planets, would they also be limited to shorter lifespans, or is it possible to have a species that might live 100,000 or even a million years? Is there anything theoretical limit/reason that might prevent this? Let's say 'years' in earth years for convenience.

EDIT:

When I said life forms, I really meant 'animals'. In other words, multicellular, complex, macro organisms such as mammals.

** EDIT 2: **

Adding some more context. Initial description wasn't detailed enough, that's on me, but here goes. I'm thinking that in a universe that FTL travel isn't possible (no wormholes, hyperspace or what have you, you're just limited to travel through space good old style), the only real way for interstellar travel is generation ships, OR, if animals can live a long time. So, in that context, I mean 'animal' in the sense that a species that's not stationary like a plant but something that is intelligent and can move about and do space travel.

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    $\begingroup$ Plans are animals by your definition: they are multicellular, complex, and macro organisms. They are unlike mammals, but I'd expect any alien lifeform to be unlike mammals in the first place... so, would you accept plant-like lifeforms as answer? And if not, how would you define "animal" in an alien context? $\endgroup$ Commented May 17, 2023 at 13:07
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    $\begingroup$ Check out Wikipedia's "List of longest living organisms" en.wikipedia.org/wiki/List_of_longest-living_organisms $\endgroup$
    – Fred
    Commented May 17, 2023 at 14:42
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    $\begingroup$ We have one, it's called DNA-based life. It's mutated significantly from when it first arose, though. $\endgroup$ Commented May 17, 2023 at 15:20
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    $\begingroup$ There are some animal species that don't seem to have an old age problem. Granted, these species are usually hard to observe (most I've heard of are deep sea creatures), so this may not be the actual case, but my point is that mortality in a given species is not necessarily similar to humans. There may already be super old individuals on Earth, or at least the theoretical possibility for one. $\endgroup$
    – user458
    Commented May 17, 2023 at 15:50
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    $\begingroup$ With regards to scifi, "super old organism" is a trope that most readers I think are happy to take as a given. Throw in some perpetual hibernation that awakens with the right stimulus and its even more believable. $\endgroup$
    – user458
    Commented May 17, 2023 at 15:57

15 Answers 15

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A problem with such a life form: how would it come into existence? Evolution requires production of new generations with varying characteristics and selection of those characteristics by environmental pressures. Even if individuals are biologically immortal, they must be competing with their progeny. Such an extended lifespan would suggest that they're an evolutionary dead end, either no longer reproducing or unable to produce offspring more fit for the environment than they are.

You might get around this with a colonial species, where the colony has an identity separate from that of its individual members. A colony could maintain a distinct identity across so many generations that its members become a different species.

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  • $\begingroup$ What a great point !!!! In a sense, the insight in this answer is: "Such aliens would only have got that way, by modifying themselves technologically - for example modifying their own species cell mechanisms, rather than via evolution." Brilliant. $\endgroup$
    – Fattie
    Commented May 17, 2023 at 19:27
  • $\begingroup$ I love this, and this may help me circumvent the problem while being as scientifically accurate/plausible as possible. As I described in my 2nd edit, I'm looking for a species that can space travel, so this will help my issue without actually having to be a generational ship. $\endgroup$
    – Sach
    Commented May 19, 2023 at 15:10
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It's a plant

The entity you are looking for will in some way be akin to trees on earth. Redwoods and oaks are known to be able to get older than 200 years, and with the right circumstances, such an organism could grow for about a millennium or way more in perfect conditions. Plant-based lifeforms have a lot of benefits that non-plants don't have: they don't need to consume other lifeforms to feed themselves, and they only require soil, carbon-dioxide, and light.

If the metabolism of our tree is right, then it consists of hundreds of thousands of trees that are symbiotically linked and share - not just nutrients and sugar, but also possibly memories. Such a tree-hive might swap its constituting trees, but as a whole stay the same mental being!

Or... in other words: Welcome to Alpha Centauri, where mind worms want to merge you to the planet's mind.

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    $\begingroup$ A quick search for "oldest living plant" on Google returns a wealth of records of various plants having lived several thousands of years. One such tree is nicknamed Methuselah, and is thought to be around 4,850 years old. $\endgroup$ Commented May 17, 2023 at 13:04
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    $\begingroup$ Just TBC, it's relatively common for trees to live some 2000+ years. $\endgroup$
    – Fattie
    Commented May 17, 2023 at 19:25
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    $\begingroup$ you mean welcome to pandora and its tree of life $\endgroup$
    – Sam B
    Commented May 18, 2023 at 12:09
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    $\begingroup$ +1 for the SMAC reference. I somehow scored a gig as a volunteer beta-tester on that game, which in turn scored me my own IMDB page. $\endgroup$
    – T.E.D.
    Commented May 18, 2023 at 14:52
  • $\begingroup$ Thank you! While the answer is pretty good, just not what I was looking for, and that's mostly on me because I didn't describe my situation in detail. But on the other hand that had led to some really good different answers so I'm not too upset. $\endgroup$
    – Sach
    Commented May 19, 2023 at 15:11
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In theory there is no reason why a very long life span should not be possible from a chemical perspective. It should be possible to create an entity that has sufficient repair mechanisms to repair any damage done by radiation or other forms for degradation.

However although theoretically possible such a repair mechanism would come at a high cost and would not in practice occur.

The primary issue being that living organisms are subject to death by accident, such as falling off a cliff, drowning, being eaten by a predator etc. This means the life expectancy of an organism is limited and genes that provide an advantage in youth but a disadvantage in old age would spread though the population. This is simply because there will be so many more younger organisms.

So in principle it should be possible to create a body that has sufficient repair mechanisms to live to a great age, but in practice these repair mechanisms are likely to be weeded out by evolution that will prefer the younger body that does not have the "expensive" repair mechanism overhead that can efficiently reproduce before death.

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  • $\begingroup$ What you say makes sense, but what if the repair mechanism also made you stronger (e.g. like healing muscles IRL) rather than just slowing down aging, and even better at reproducing. This would be positively selected by evolution $\endgroup$
    – komodosp
    Commented May 17, 2023 at 11:48
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    $\begingroup$ There is also the factor that such a long lived entity would be competing against their offspring in the long run. And eventually the environment will change in ways that other animals with shorter lifespans are able to adapt, while you are stuck with the original genetics. $\endgroup$
    – Brianorca
    Commented May 17, 2023 at 19:50
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    $\begingroup$ @Fattie why not? The cost is in additional enzymes, proteins and other cellular chemicals required for very robust repair and defence mechanisms. The food and energy that goes into those could be spent on other things like reproduction instead. It's the same reasoning when considering why we don't have stronger bones that never break , they come at a cost that is not worth it in evolutionary terms. $\endgroup$
    – Slarty
    Commented May 17, 2023 at 20:39
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    $\begingroup$ @komodosp let's say hypothetically that an older generation we'll call "boomers" is able to live much longer than previous generations, and then let's say that a younger generation "millenials" grows up alongside this older generation. In previous generations, the older generation didn't live as long, and so their resources passed down to the younger generations pretty directly. But in our hypothetical example the "boomers" would hold onto their resources, making much less available for the "millenials"... it's about limited resources, not longevity directly $\endgroup$
    – Blackhawk
    Commented May 18, 2023 at 20:14
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    $\begingroup$ For any animal here will always be a hard limit on maxumum population of those animals the environment can sustain, simply because of the availability of food and water and other things the animal needs to survive. There will be a point where if the older generations never die, there will be no more room for the younger generations. So such a long lived animal will not get the benefit of evolutionary mutations, because you need offspring for that. $\endgroup$ Commented May 19, 2023 at 12:00
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Age limits because of limits

Life, ageing and dying all revolve around limits.

First of all, any animal has a limited amount of adaptability to it's environment. This is important, as over time the environment changes. They can easily die off if they cannot adapt, or adapt quick enough to such changes. To reproduce (and die) is thus a great way to stay current with these changes.

Second is resources. We see that in nature a delicate balance emerges. There's fewer tigers than antelopes for this reason. The antelopes have less limits in food and foraging area than a tiger. If we couple the resource limit with needing to stay current with your environment, it makes sense to remove older less adapted creatures.

We can cheat a little for longer lifespans. If for some reason the metabolic rate is much lower, we can increase the lifespan. Imagine a complex lifeform that needs to wait a month for the next piece of food. Or one powered by sunlight like an intelligent tree. As life would be slower on average to survive, so would the amount of time given to reproduce.

If we plug in the formula we can reverse to the point of longevity in complex animals.

They need to be highly adaptable, able to adapt their environments to a huge degree, or able to adapt themselves. That way any changes in environment or themselves can be overcome. This reduces the need for reproduction, which reduces the need for dying to make space.

They need to reproduce incredibly slowly, or basically not at all. As mentioned, spawning new generations makes that you hit limits in resources. Something has to die if such limit is reached. If you hit this limit much more slowly you can live much longer.

As an extreme example, imagine space faring humans. They go from planet to planet gathering plenty of resources. They create their own environment wherever they go, while they use DNA technologies to adapt their (partially) biological form for the rest. Adapting seamlessly from long 0g space flights to the high gravity of a super Earth. They have little need to reproduce, though they can over time. They could live thousands of years without hitting limits.

Conclusion

You need creatures that are able to adapt their environment and/or themselves to a high degree. If this is reached, the rest will follow over time. Less reproduction and less need for death to keep everything within limits. Or to quote a fictitious purple alien: "Perfectly balanced, as all things should be."

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This may also depend on what you count as an individuum.

Just as we outlive our indiviudal cells, an ant colony outlives its individual ants (Douglas Hofstadter once played in "Gödel Escher Bach" with the idea of an ant colony being an intelligent individuum whose opinions may diverge from the opinions of its constituents). And if we consider ourselves "cells" of the "individuum" homo sapiens, then the life span is already quite long. At the extreme, you may consider "Earth bound carbon based life" as "individuum"

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    $\begingroup$ +1, because I like honeybees. I love my hive, but do not love any individual bee. Maybe that's how God sees us? Individuals can suffer greatly, but humanity continues to thrive. $\endgroup$
    – user458
    Commented May 17, 2023 at 16:06
  • $\begingroup$ This is the same idea proposed in the top answer and I like this. This seems to be the only way I can get around the problem with being at least somewhat scientifically plausible. $\endgroup$
    – Sach
    Commented May 19, 2023 at 15:16
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Qualified yes

The primary thing that's keeping humans from living forever is actually a molecular clock. If we can re-bind our telomeres, then we could theoretically live forever, except ...

Molecular clutter

As a side effect of our chemical processes, a lot of clutter accumulates in our cells. The worst case of this is Kuru (laughing sickness), where a protein gets bent in the wrong way, then starts recruiting other proteins to do the same thing. Our systems don't know how to clean it out, so fingernail-like plaque slowly fills up your brain cavity.

There are a LOT of things that build up in our cells. Sci-fi usually deals with this via nanotech or teleportation, but that involves intervention.

A natural way to avoid this is to have a high rate of cell division, diluting this clutter faster than it can accumulate. This involves a very high metabolism, and requires that ALL of our living cells duplicate, including the brain and bones, and that we have some mechanism for flushing the worst of them.

Gene police

Your creature would need a gene police to make sure it stays the same creature. This is similar to how we currently eliminate most encoding errors, which would otherwise result in cancer. We get cancer because our genetic auditing isn't adequate, and sometimes the cells find loopholes.

Phenotype regeneration

Just because your cellular structure can survive indefinitely doesn't mean that your spine would stay straight, or that accumulated damage to your knees won't cause bone spurs, or that your skin won't wrinkle. You'd need some chemical mechanism to ensure that your cells stay in a comparatively static shape, beyond mere regeneration of limbs.

So, overall, there's a lot preventing it, but for story purposes, it's nothing compared to overlooking the laws of thermodynamics, and that's endemic to most fiction.

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Unlikely, simply because if you take away natural causes of death (aging, disease), you still have the unnatural ones (accidents etc.).

This blog post I found estimates that if you made humans immortal, for example, the average life expectancy would "only" be about 2000 years. Obviously there would be outliers living far longer, but it would be very unlikely to have a million year old.

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    $\begingroup$ Immortality, some have suggested, would drastically change human behavior. Notably, one such likely change would be a much more intense aversion to any physical risk. If we gained a medical immortality, the goalposts would quickly move to physical invulnerability. My point, I think a lot of people would become shut-ins, taking the accident rate way down. Then in steps VR and computer-based immortality, but that's another can of worms. $\endgroup$
    – user458
    Commented May 17, 2023 at 16:02
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    $\begingroup$ I've thought about this for sci-fi plots. If humans could live biologically very long lives, we'd start living in an INCREDIBLY safer way. For example, cars today are spectacular safer than 50 years ago. BUT cars in such a society would just be absurdly, comically safe, ridiculously safe. $\endgroup$
    – Fattie
    Commented May 17, 2023 at 19:28
  • $\begingroup$ @Fattie that reminds me of Larry Niven's relevant short story "Safe at Any Speed" $\endgroup$
    – Brianorca
    Commented May 17, 2023 at 19:55
  • $\begingroup$ ah! he "beat me to it" eh :) $\endgroup$
    – Fattie
    Commented May 17, 2023 at 20:00
  • $\begingroup$ @fredsbend this is a pretty interesting idea. Gave me some food for thought. $\endgroup$
    – Sach
    Commented May 19, 2023 at 15:20
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Yes thousands, generally not millions of years.

enter image description here

This is a graph of humans if we didn't die of age of diseases. Accidents are common enough that they'll wipe out most people over 10000 years.

Animals are likely to be more at risk of death from accidents, predation and such over a long period of time, and bad luck from weather is likely to spike the death rate at times.

As such, even assuming they are very disease resistant and can regenerate perfectly, they're likely to die before 100,000 years.

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  • $\begingroup$ I was going to object to the graph, but then I realised you'd actually been a lot more clever than me! :) That exponential graph is simply what you get with a constant rate of accidental death. That said, accidents do have a bit of a bathtub curve - more when you're young and unable to predict them, and more when you're older and unable to react to them. But presumably if you took out aging, you'd lose the latter effect. $\endgroup$
    – Graham
    Commented May 19, 2023 at 7:43
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The question "what limits human aging" is an incredibly open question with all sorts of theories, ranging from telomere shortening to collective apoptosis.

Quote Wikipedia,

At present, researchers are only just beginning to understand the biological basis of ageing even in relatively simple and short-lived organisms such as yeast.[65] Less still is known of mammalian ageing, in part due to the much longer lives of even small mammals such as the mouse (around 3 years).

That said, yes.

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  • $\begingroup$ Thank you! I had something slightly different in mind, but I phrased the question wrong so that's on me. When I said 'life forms' I really meant 'animals' as mentioned in the first sentence. Essentially, can complex multicellular macro organisms such as mammals, birds, and reptiles (or their equivalent on other planets) live longer lives? $\endgroup$
    – Sach
    Commented May 16, 2023 at 22:19
  • $\begingroup$ @Sach - nobody knows, it's an active field of research $\endgroup$
    – Fattie
    Commented May 18, 2023 at 0:17
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Maybe not.

One reason suggested for limits to life spans of large animals is bacteria.

Consider that the DNA of a large animal is, very nearly, unchanged during its life. Thus, its immune system (or whatever it uses in place of one) changes minimally over its life.

Bacteria have a much shorter life span. While a human goes through one life, the bacteria around that individual goes through many thousands of generations. This means the bacteria evolves to better penetrate the immunes system over those thousands of generations.

Twenty thousand human generations is roughly 500K years. This is a substantial time for evolutionary purposes. It is getting us back close to the start of homo sapiens as a species.

The change in bacteria over a period of twenty thousand generations would be comparable. It is highly likely that an individuals with extended life spans would encounter bacteria that could simply brush aside their immune system.

With shorter life spans, it is an arms race. The large animal evolves along with the bacteria. Each generation of the large animal produces a re-shuffling of genes, and produces a whole new set of barriers for the bacteria to get over. Even if the large animal's immune system is not drastically better it is different. The bacteria that were busy adapting to Great Uncle Fred's bodily systems will suddenly encounter his niece little Frederica's systems. The protein coating on this is different, and the pH levels on that are different, and the temperature range of the other is different. So the bacteria can't get through. At least not right away.

An additional bacterial consideration is co-evolution of less severe versions of bacteria and viruses. When a new pathogen arises it is sometimes very lethal. Only a small number of hosts live. But this means that the more lethal forms of the pathogen wind up getting shut out by killing their hosts. So, with shorter life spans, hosts that can better fight off the pathogen come to be more common. And pathogens that don't kill their hosts become more common. This only works with life spans that are not too long. If the population of hosts does not reproduce, it simply gets used up, reduced with each passing plague.

So, for animals, life spans drastically longer than 100 years are a big challenge. You might get a few hundred years. But thousands is difficult indeed.

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Well, yes and no.

There is at least one complex animal on Earth that may well be immortal: Turritopsis dohrnii, aka the immortal jellyfish. They achieve this by being able to return to earlier stages of life, effectively winding back their biological clock under significant stress or old age. They revert to a phase analogous to a gamete cell and begin their life cycle over from there.

This reversion to a prior developmental stage isn't entirely unique, and there are other longevity mechanisms - such as DNA repair and increaded telomerase production - that we've observed in various animals from microscopic hydrae to some lobsters and even certain types of whale, all of which enjoy greatly extended lifespans compared to other similar animals. But "greatly extended" seems to have limits measured in centuries rather than aeons.

What has led to such a 'short' lifespan across the board?

Well, that's a big question with lots of possible answers depending on which mechanism and species we look at and very few answers that we can definitely say are true. There are some trends we can look at though.

Long-lived species tend to be juvenile - i.e. incapable of breeding - for longer periods of time compared to short-lived ones. Replacing individuals lost to predation, accident or age is generally a more difficult process.

Not only that, but long generational delays - the time between birth and sexual maturity - means that any significant change in the environment results in much lower survival probabilities for the species. Where short-lived species can adapt via mutation and population diversity, long-lived species would have to be capable of extreme individual adaptation to remain viable.

And finally - on the reproductive viability list at least - long-lived species cannot expand to take full advantage of short-term resource blooms in their environment. Where short-lived species like mice and locusts can multiply their numbers and produce viable adults within a couple of months (for locusts) or 3-6 months (for mice), humans take well over a decade to reach first reproductive function. If we have a massive increase in resources it'll take us decades to expand to utilize it... by which time the billions of mice bred during that time will likely have consumed the resources.

And that's just one factor.

Assuming there may be other carbon-based life forms similar to that of earth on other planets, would they also be limited to shorter lifespans, or is it possible to have a species that might live 100,000 or even a million years?

Odds are good that we're going to find vastly more short-lived examples of alien life than long-lived... and the chances of a non-engineered carbon-based life form with a natural life span exceeding 1,000 years seem fairly slim to me.

Why?

Well, we're talking about evolutionary processes here, so you'd need to find a significant survival advantage to such a life-span... and evolution appears to favor short-lived life. The vast majority of Earth's fauna have lifespans better measured in months or even days than years. Bacteria have life spans measured in hours, plankton in a small number of days, bees in weeks, ants can survive for maybe 2 years... and so on. Very few species have natural lifespans of more than a century, and only a handful of animal species (like, maybe 2 or 3) routinely live for more than 500 years.

So is it theoretically possible for a naturally-occurring quasi-immortal to arise from natural evolutionary processes? I guess so. But is it likely? Nope.


Oh, but wait... who says that it has to be natural? I present to you: The Bandersnatchi. These beasties were created as a weapon in an ancient war against the Thrint Slavers several thousand years ago and are both exceedingly long-lived and genetically locked, incapable of change. They reproduce by budding, creating clonal copies of the original parent with no genetic differentiation: every Bandersnatch is genetically identical to every other.

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  • $\begingroup$ The Bandersnatchi got my attention! $\endgroup$
    – Sach
    Commented May 19, 2023 at 15:26
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Yes

Fundamentally, living things live as long as they need to in order to maximise their fitness. On Earth this is usually very short, but can last a few hundred years. What drives this is rate of input of energy (and possibly other elements), body size, and the ability to successfully reproduce later in life.

What you need, then, for an alien lifeform to be exceptionally long lived is for it to be a large creature living on a world with low energy input - for example a world further from the star.

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I can not comment, but multicellular, complex macro includes many more beings than mammals, which are a rather short-lived kind of animal. Also, we would possibly not have "animals" on other planets, as this is only a small branch of life which is defined biologically or how close those species are together/what evolutionary history they have in common. Out of the three kingdoms of Bacteria, Eucariota and Archea, animals are one small subset of the Eucaryota. We are closer to fungi than they are to plants -- and both are already rather complex kinds of life. So what is your meaning of "animals"? We could easily imagine plant or fungi-like organisms acting on their own behalf, and especially fungi can grow very old. Also consider the immortal suberite, which is a complex animal here on earth that lives millions of years -- but is far from the behaviour of a mammal. If you are thinking about the organism's interaction with the environment, also think about the timescales of this interaction. It could be that we are just too short-lived to perceive the agenda of fungi ;). So those two cents on behalf of the hypothetical exo-biology; for worldbuilding, we could clearly imagine everything, but if it were animal in the taxonomical sense, they have a common ancestor with the animals we know.

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  • $\begingroup$ I see no evidence that suberite are immortal; the most I can find is an uncited Wikipedia statement on Demosponge that "many species [have] long life span[s] (500–1,000 years)", which is consistent with the other lifespans previously mentioned. $\endgroup$
    – prosfilaes
    Commented May 19, 2023 at 0:27
  • $\begingroup$ Thanks, thats a great comment, maybe I have mixed something up. I will look for some hints on its ageing. $\endgroup$ Commented May 22, 2023 at 7:29
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Well, during such long lifespan, even if the creature manages to avoid predators, accidents, diseases, starvation, etc., radioactivity, high-energy radiation, viruses, random copy errors, chemicals, etc. all contribute to corrupt the DNA (or similar replicator) bit-by-bit until the accumulated damage eventually take the toll.

Even if the creatures can live for such long periods, evolution would be much slower than it already is on Earth, making it difficult, if not impossible, for the species to adapt to environment changes.

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Yes it is entirely possible. This can be a result of Einstein's Special Theory of Relativity. Basically it states that time slows down for an object moving at relativistic velocities, or for an object in a massive gravitational potential.

So for a planet near a black hole (and by near I mean outside the event horizon), or for a planet moving at relativistic velocities compared to Earth (in the first case, the gravitational pull is massive, and in the second case, the planet is moving at relativistic speeds compared to Earth), time slows down for that planet compared to Earth.

This is also true for our Solar System, where for Mercury (which is closer to the Sun and hence the gravitational force is more for Mercury compared to Earth) time passes slightly slower on Mercury than on Earth.

Another example is for satellites moving at high velocities in Earth's atmosphere. The relative velocity of us humans present on Earth is less when compared to the satellites moving at very high speeds. Hence for a human present on the satellites (the ISS for example) time passes slightly slower compared to humans present on Earth (although the difference is very minute because the velocity of the satellite is not relativistic).

This can be easily extended to a planet which is just outside the event horizon of a super massive black hole. The gravitational pull of the black hole is much more than what us humans experience on earth from the Sun. Hence time will pass very slowly for any civilisation that might be present on such planets. (Although the only caveat is that the life present on such planets should have evolved to withstand high amount of radiation.)

Another possible scenario could for a solar system in which the planets are moving at relativistic velocities compared to earth. Hence for life present on such planets, time would pass much slowly.

In both the above cases, the biological/chemical processes/ageing for life present on such planets would proceed much more slowly and hence when compared to us humans, they would outlive us. Maybe for thousands of years or hundreds of thousands. It all depends on how massive the gravitational pull is or how fast the planet if moving.

This is a proven theory and experiments have been done to calculate what the time dilation would be if an object is moving at relativistic velocities. The only thing I am unsure about is whether time dilation also affects the ageing/biological processes of carbon based life forms. I think it will slow down the process but someone can correct me.

Anyways this is the only way I think a civilisation can outlive our life time. Forget any other civilisation, even if some humans escape our solar system at relativistic speeds and come back after some years, we would have long gone and they would come back to Earth where our off springs would greet them.

Cheers!

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  • $\begingroup$ Too much bolded text makes it quite hard to read. $\endgroup$ Commented May 17, 2023 at 12:27
  • $\begingroup$ You are confusing subjective time with changes in external time as relativistic time dilation produces. $\endgroup$
    – Boba Fit
    Commented May 17, 2023 at 17:15

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