I'm writing a story that features modified humans that have a very long lifespan/ practically immortality. Immortal characters in fiction aren't exactly rare, but I want to write something more realistic (which might be ironic considering the premise).

If we suppose a person wouldn't physically die given extremely long age, what would happen to the brain of the person? Could a person that has lived for thousands of years act as a part of society or would he have trouble with recalling memories and other functions, etc?

Edit: I'm assuming that diseases due to immortal nature are not a factor. Though I'm no expert in any of this, so nothing's to say there aren't some diseases that would affect the brain on the long run despite immense recovery capabilities I'd link with immortality. The immortals have stunning recovery capabilities, and are for example capable of regenerating bodily damage almost in an instant, though I'm not exactly sure what this would mean to the brain.

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    $\begingroup$ Welcome to WorldBuilding iio! An interesting premise that different media sometimes explore in different ways. Take typical very-very-long-lived elves for example - they are rarely depicted as differently acting as that would hurt the story. But long-lived vampires are often shown more as monsters then men, partly due to their long lives of brutality. Can we assume that there won't be any health problems, so that diseases that affect the brain are out of the line? If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Commented Mar 15, 2018 at 13:23
  • $\begingroup$ Hi, I'm assuming that diseases due to immortal nature are not a factor. Though I'm no expert in any of this, so nothing's to say there aren't some diseases that would affect the brain on the long run despite immense recovery capabilities I'd link with immortality. $\endgroup$
    – iio
    Commented Mar 15, 2018 at 13:26
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    $\begingroup$ This similar question might have a good answer. Quick summary: everyone will be very specialized and remember only the exciting parts of their life. $\endgroup$
    – Giter
    Commented Mar 15, 2018 at 13:28
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    $\begingroup$ To better work with this issue, we'd need to know the method of their immortality. Because if it's a lack of cell death but not cell immunity-from-harm, then eventually you'll have defective cells, leading to brain disease. If it's total immortality, then there's a question on if memories would even be able to settle in at some point, considering knowledge/memory involves some rerouting/changing of the structure. $\endgroup$
    – Carduus
    Commented Mar 15, 2018 at 13:29
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    $\begingroup$ Very interesting adaptation covering several issues of immortality: tardis.wikia.com/wiki/Ashildr Including, but not limited to: Memory loss, death of loved ones with the inability to change it, unability to die when feeling too empty to live anymore. I mean, image the sun exploding and you are drifting through space until the heat death of the universe. What would you do with your time? Assuming, you are not in constant agony because of the lack of oxygen. $\endgroup$
    – SK19
    Commented Mar 16, 2018 at 8:56

16 Answers 16


Neurons are a little bit odd, because they don't divide and therefore can't reproduce quite like most other cells.1 They can only be produced by neural stem cells, which are active largely in the early stages of an organism's life. In some adult animals, however, neural stem cells are active in the hippocampus, where they may be instrumental in forming new memories. Without the generation of new neurons here, there could be negative implications for memory storage.

You could argue, then, that immortality with a continuously functioning brain would require the presence of a large number of neural stem cells. However, it's not quite so simple. While therapeutic use of neural stem cells has been tried - and has, to some degree, succeeded - replacing large sections of the human brain over time would require extremely delicate procedures, which might have to be developed naturally. In other words, the brain might have to be designed such that this replacement is possible.

Without such replacement, then, immortality wouldn't be possible. A person would gradually lose neurons over time - even if they weren't stricken early with a neurodegenerative disease. Their brain cells would quite simply die. It might be like a slow-motion version of Parkinson's disease. Memory would begin to fade, as would other brain functions.

I've seen many estimates for how fast neurons die, and how fast certain types are made in the hippocampus. For the former, figures vary from a bit under 10,000 per day to several hundred thousand per day. For the latter, it seems that less than 1,000 neurons are produced by the hippocampus each day - and those usually for memory-related functions.

Now, there are tens of billions of neurons to maybe one hundred billion neurons in the human brain, meaning at the current rate of replication, it would take millions of days to recreate that many neurons at adult production levels. Neurons die at a much greater rate than that. So you'd need one of two solutions:

  • A substantially lower rate of neuronal death (arguable your best bet).
  • A substantially higher rate of neuron production and the ability to integrate new neurons into the brain.

Without this . . . the brain would decay within a relatively short period of time, even without degenerative diseases.

  • $\begingroup$ There are 86 billion neurons in the human brain, though it's commonly quoted as 100 billion (which is incorrect). $\endgroup$
    – forest
    Commented Mar 16, 2018 at 8:05
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    $\begingroup$ 100,000,000,000 / 100,000 / 365 = 2739.7 - so if losing over 10 percent of your brain would mean that it is seriously degraded, then it would take around 2500 years for that to happen according to your numbers. Or maybe 910 years if you lose 300k a day. Or maybe 25,000 years if it is less than 10k a day. Just wanted to point that out. $\endgroup$
    – Mike Wise
    Commented Mar 16, 2018 at 8:39
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    $\begingroup$ Question: "Effects of immortality on human brain", answer: "Immortality is not possible". $\endgroup$
    – Exerion
    Commented Mar 16, 2018 at 13:39
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    $\begingroup$ @Exerion My answer was that either 1) The brain would degrade, or 2) It would continue function, and that this depends on the ability of the brain to produce new neurons and integrate them into the system. $\endgroup$
    – HDE 226868
    Commented Mar 16, 2018 at 13:42
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    $\begingroup$ @Forest I'd feel comfortable calling it 100 billion give-or-take-a-beer-or-two ;-) $\endgroup$
    – Cort Ammon
    Commented Mar 17, 2018 at 6:49

As I've grown older I've noticed my mind converging on ways of thinking. When I was young the possibilities were open but over time my mind has accumulated a sense of how the world is. This seems a lot like optimizing for your environment by putting blinders on.

Maybe that was advantageous in the past in the same way that eating as much salt and carbohydrates as possible was. The problem is that in our world things change, fast. Technology and society change almost every decade and the key to staying agile seems to be about continuously deconstructing what you have learned.

An immortal person would need a way to fight against this type of convergence or they would soon, relatively speaking, find themselves in a world that doesn't make sense to them. Actually, you can kind of see this today but if people were immortal I think the issue would be magnified and become much more serious.

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    $\begingroup$ Science advances one funeral at a time. -- Max Plank (the old observation that people can set themselves into ways that are very difficult to change) $\endgroup$
    – Real
    Commented Mar 15, 2018 at 23:31
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    $\begingroup$ "An immortal person would need a way to fight against this type of convergence or they would soon, relatively speaking, find themselves in a world that doesn't make sense to them." Reminds me of Tolkien's immortal Elves growing weary of the world and leaving Middle Earth. $\endgroup$
    – RobertF
    Commented Mar 16, 2018 at 1:59
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    $\begingroup$ This is due partly to neural plasticity decreasing over time. As more and more neuron dies, interconnection that were existing starts to break, isolating synaptic paths. Neurons that are understimulated are the first to go, hence the convergence. $\endgroup$
    – werfu
    Commented Mar 16, 2018 at 17:35

On brain : probably nothing.

For someone to be practically immortal it is necessary that their body does not degrade over time. That especially includes the brain. It further implies they can recover from inevitable environmental damage, which means they'd have to be functionally eternally young. For brain you can assume they have good memory and learning ability combined with maturity and experience of age.

Think a young person who acts much more mature than expected.

On consciousness : more or less unknowable.

People like this would have had lots of time to get to know themselves and develop their own personality. So they would have very strong individual personality and would be unique individuals.

Still such strong personalities would probably show as self-confidence and charisma. They might also appear mature and considerate, both pragmatic and principled. They might not be any of those but experience would duplicate the effects. Knowing what causes problems from experience makes you look considerate. Knowing what kinds of solutions will not work makes you look pragmatic. Knowing what compromises you will be happy with or not look much like having principles. And all that will make you look quite mature.

They'd generally be calm and stable since they have typically found out the ways of thinking that will work for them. That might come off as being methodical and efficient. Maybe to the point of stubbornness or dogmatism even.


We know little enough about how the brain works, how aging works, and how the two interact that the answer could be anything you want. Mainly because modifying humans to make them immortal would mean modifying their brains - as others pointed out, we lose neurons throughout our lives and they don't regenerate much. Any kind of immortality would involve some way of compensating for this, which means changing pretty basic aspects of how the brain works.

On memories, this neuroscience talk on Youtube points out an interesting difference between young people and older ones:


It seems younger people have stronger episodic memories, i.e. memories of specific things, and when asked to recall them they bring a wealth of details to the description. Older people have weaker episodic memories and rely more on semantic memories - when asked to recall something they will bring less detail and talk more about their general knowledge of the world.

This is one example of how memory could evolve over time for someone immortal; specific memories would blur into a general sense of what the world is like. But again: the brain would have to work differently for immortality to work. So maybe memory could evolve differently over their lives. There is no theoretical reason why immortals should have worse memory than non-immortals. From a data perspective, if there is a maximum amount of memories a brain can contain, all that means is that the longer an immortal lived, the smaller the proportion of their life they would remember. But that's already true now I think; when I was a child I remembered a lot more about my childhood than I do now, but I also have a lot of memories of my teens and adulthood that I didn't have then. I see no theoretical reason why a thousand-year-old should have fewer memories than a short-lived person.

There is also the question of people becoming more rigid or set in their ways as they grow older, but again: is that a consciousness thing, or a brain thing? That is, one can make an argument that as you live longer and accumulate more data about how the world is, you draw conclusions about it and it will be harder to change your mind on those conclusions (it will take at least as much data as that you have already gathered) than is the case with someone who has a lot less data. I.e., we can suppose that getting more rigid in old age is inherent to intelligence itself.

But then again, maybe it isn't; maybe it's a consequence of our brain's development over our lifetimes. And maybe it's a bit of both. And brain development would have to be different for us to be immortal; and if it's different, maybe one difference could be to maintain flexibility and openness regardless of age.

The same is true of our subjective sense of how fast time flies. Is it inevitable that any conscious being will sense time as a proportion to how much time they've already lived (if that's how our time sense actually works), or is it a property of our brain? Unknown.


Depending on the way they live and use their brain there will be vast differences.

To take the example I mentioned in a comment: look at elves first. Elves are generally depicted as a very social race and while they may act weird when compared to humans or dwarves, there are not so many differences between comparatively long-lived elves and younger ones. If your immortal humans were living in a society that is ruled by strict law and not really inclined to brutality then everything will be fine.

They will forget a lot of things as humans normally do, but they will make sure that they will remember important stuff, just like we for example take pictures or write a diary. The normal everyday activities will of course stay in their mind perfectly fine, but there will be more lost if they don't regularly remind themselves of the past. Looking through old books they have written will be a far more normal part of their lives than it is for us - it's important to not forget those that may have died for example, but whom you don't want to forget. Elves are often shown as magically inclined with advanced medical care, which makes them suitable candidates for "immortals", though they are a bit farther from science-based. Looking just at how their society works though is still a valid approach for judging your immortals.

If you look at vampires on the other hand you have some depictions that are very close to what you want. Most of the time vampires are depicted as humans that are somehow transformed - be it via magic or a virus - into a vampire. Such a vampire is normally living in solitude - humans are your meals and other vampires would steal your meals or make your meals aware of you, which makes drinking the important parts of them difficult. They are living alone and they are living a brutal life. There is regular violence against what was formerly their own race, there is regular violence trying to protect your free-range humans from other vampires.

This leads old vampires to hate other people, vampires and basically everything else - their brains wouldn't physically be altered, but their psyche would transform over the centuries in order to be able to keep up and stay alive. An important part is that they can still perfectly blend in - that's necessary to lure your victims in the next dark alley after all - but they would never really form a society.

There are sometimes depictions where vampires band together and rule over the land. I am thinking about an Anime right now but will have to look up the name later. Humans are treated as lifestock and a new vampire socitey arises.

This perfectly shows that the style of living would influence your immortals. If they have a society that welcomes them the way they are and embraces immortality then they will live a perfectly normal happy life with some minor differences, such as more elaborate diaries and reading to remind you of the past. As the brain wouldn't deteriorate from natural diseases there isn't really anything you would need to fear, except for the loss of some abilities you haven't trained in a few centuries - forgetting how to ride a bike would actually be possible. If your race on the other hand lives in solitutde or undercover the individuals would slowly distance themselves and become less humane - they need to stay up-to-date to blend in and act like normal people, but in reality they are predators only looking for their next meal and survival who would surely forget about their past lives in relatively short order of maybe a few centuries after being transformed. They would remember that they once were human, but nothing more.


I am quite fond of books by a Russian writer, Alexander Lazarevich, who among other novels, explored technologies for achieving practical immortality and their implications on individuals and society, in "Wish Generator" and somewhat in "Nanotech network" as one of the aspects : http://technocosm.narod.ru/e/wg_e.htm

Being a sci-fi writer, he leans toward longevity of the conscience - not necessarily confined to the original biological body it was born in. Not necessarily to one body at all (e.g. spawn a copy of yourself - and both are fully yourself, no paradox and conflict of clone/origin - to travel the universe (perhaps in a body that can live in the cold and void or dive into stars), then sync back the impressions of your copies whenever their ways cross again. The technologies and biological basis for what he proposes as initial stages of modularizing the body (and eventually replacing, as it ages) as a carrier of consciousness are already available or will probably emerge in foreseeable decades.

Regarding your question, one of his outcomes was that true immortality on scale of many (all) humans can cause them to become very responsible about what they do. There is no more place for "I can do whatever, those who come after me will sort it out" - I will live to be hit by the consequences; even if you're immortal, living forever in a wasteland with no way out is not a good prospect...

This will probably boost technologies and space travel as well: "If the Sun burns out in a billion years and swallows the Solar system, this is my personal existential threat, and a problem I have to solve." Quite possibly, the immortal humanity will have to invent ways to travel the galaxy and take the planets (and stars) with them as big comfortable spaceships to fit them all.

It can impact the work of memory as well: it is not infinite, so the trick is to remember what is needed, what matters, and to be able to lose the cruft over time. "The perfect memory is not one that remembers everything collected over millennia, but one that is usable and where you can still find the needed memories in reasonable time."


Fundamentally, if there's way to make someone live for hundreds or thousands of years (or longer), their brain is going to have to function on a biological level, or the person will die. Whatever you do to keep the rest of the cells and systems in the body from breaking around the hundred year mark, you would presumably be able to do for the brain and nervous system as well. Having said that, the big issue you're likely to encounter in the brains of immortal (or long-lived) biological beings for which there isn't a plausible solution is the problem of neural plasticity, specifically, what's called activity-dependent plasticity (or "learning", if you prefer less technical terminology).

Because of the way neural networks function at a basic level, they become more rigid over time. Memories and experiences and knowledge are stored in the brain, as are the connections and pathways between them. This has the effect of giving more weight ("a stronger signal") to existing experiences over new ones. (Because there are more old experiences than new ones, of course, and this effect grows stronger over time.) Since neural networks function on a strongest-signal-wins basis, this becomes a problem over time. Past experiences have more weight than new ones, making it harder to integrate new experiences ("learning"). There's many an adage along those lines, such as "can't teach and old dog new tricks" or whatever stereotypical pejorative you want to throw in about old people being cranky and stuck in their ways. It's rooted in the way neural networks function, preferring a large body of historical experience over a smaller number of new experiences.

I think current research on neural plasticity pegs 75 as something of a dividing line, but I'm not sure how much of that is due to biological aging, and how much is due to fundamental issues with neural networks in general, but it seems like over a long enough time span, the way neural networks weight new things against their body of existing experience is bound to be a problem.

The other issue is that the brain doesn't really scale to the type of time scales you're taking about. Can you recall many memories from 10 years ago? 20? I wouldn't say that's a big problem, as people get along just fine with only the vaguest memories of what they experienced years ago, but it will be a more pronounced effect the longer you live. In respect to this question, it means a person who's lived for thousands of years would only have reliable memories of a fraction of a percent of their lifespan, at least with their natural wetware. (There's definitely a sci-fi angle for memory storage implants in there, but the natural state of only being able to recall a tiny fraction of one's life seems weird and unsettling.)

So I would suggest that the biggest problem an immortal person would face is that they'd be very limited in what they're able to learn and adapt to after a certain age. They might live 10,000 years and gather new memories, but otherwise, they'll probably be mentally the same person as they were at 75 (or 100 or whatever).


Going from what we know of Artificial Neural Networks..

The key here is the concept that a given neural network has a finite information storage capacity. The exact value does not matter, but what is important is that for an immortal being with a finite (and presumably unchanging) number of neurons, the amount of information store-able is finite.

In practice, that means that older memories and concepts will be progressively overwritten by more recent and more frequently accessed ones. In order to be immortal in a practical sense, the above concerns of neural die-off and plasticity would also be addressed (you would just keep the values at age 25, for example). The probable mental effect would be that memories of strong events and concepts would persist for hundreds if not thousands of years, but many intervening events would be lost. Indeed, a 2000-year-old may read documented evidence of what they did as a 1000-year old and have effectively no recollection at all. They might as well be a different person entirely.


The recent discovery of Telomeres (2009) which preserve genetic information each time a chromosome splits would have implications for the genetic information of an immortal being. If we had no Telomeres, our chromosomes - which hold a cells genetic information - would get smaller and smaller each time they split, eventually leading to nothingness. Telomeres are replenished when we're younger, but As we age, our Telomeres shorten causing the cells to lose genetic information and change. A Telomere works like a password-cracking program in theory, which generates thousands of random numbers and letters per second. So if a chromosome messed up and got smaller, the Telomere would activate and guess each letter in the ACTG strand until the cell started working again.

This process isn't perfect, so each time a cell splits there's a roughly 1 in 1000 chance that it mutates, resulting in a different cell, which %99 of the time undergoes apoptosis and dies. But that 0.001% of the time, that cell stays there either because it happens to be overlooked, or it happens to be benign, meaning in an immortal being, his genetic information may slowly change over 1000s of years, meaning his brain cells might start to change, so in said immortal being you may see his personality slowly change, eventually resulting in a completely different person.

As far as how this person would fare in society, it would be interesting to see him go from being a a perfectly behaved, law abiding citizen, never a bad thought in his head, to complete lying scumbag capable of manipulating emotions purely for his own selfish gain.

Without taking away from HDE's answer: The Neural Stem Cells would still be capable of being damaged and changing. The brain has two main types of neurons, grey matter, used for fast thinking, and white matter, used for slow thinking. With the knowledge that Neural Stem Cells divide and differentiate into these types of cells, it's easy to see how a person could over time become slow in some areas, and fast in others, leading to total personality change.


The downsides to immortality:

Boredom. After a couple of centuries, existence could feel like "I have a thousand channels and nothing to watch." Personally, when asked whether I would rather be a vampire or zombie, I pick zombie. Reason: zombies have a shelf life. Eventually you lose enough parts, you're gone. As an immortal vampire I envision developing a death wish, growing more and more reckless -- or thrill seeking -- until I eventually died of misadventure.

Dismay at the immaturity of non-immortals. If the entire society is immortal, dismay at character attributes that you deem unproductive -- and that would build stress like political schisms do. But more so. These would be mental-health issues.

Population issues. Even with mortality, birth control, and pollution to slow us down, we traditionally have outbreaks of disease and genocide that cull the human population, but only for a bit. The pressure of ever increasing immortal numbers on the existing territory would be a psychological factor. Unless the birth rate slows, or science + culture decrease it.


Meditation. Pro-level meditation techniques would help keep the mind fresh, and the attitude positive.

Letting go of history / memories. Keeping the basic principles, and ever building on them, would be the path to sanity. Records could be kept -- the way we all bookmark the Egyptian pyramids, and emergence of modern science, and the major wars. Although they would grow less important, and more quaint, as they receded into the past.

The same way, when a teenager was trying to convince me I had approved a questionable activity, a friend said, "You don't have to remember any given conversation. You just know what your answer was, is, and would be," you would be less dependent on lists of facts, and more on accretions of your best practices. It would become indistinguishable from acting on instinct.

  • $\begingroup$ Welcome to WorldBuilding Mary! Nice answer. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Commented Mar 21, 2018 at 8:41

The answer depends on the traits of our immortality. I will assume that it means that our bodies are capable of preserving ideal homeostasis indefinitely. Specifically, I will assume that:

  • Damaged neurons can repair through anything but an information-theoretic death.
  • No toxic substances accumulate over time that reduce our mental capacity.
  • No biological glitches occur when our uptime lifespan becomes excessive.
  • All other properties of our brains remain unchanged.

With this in mind, from a purely neurophysiological point of view, the answer would that we would quickly hit the point where we are not taking in any new information. Memories that we consider permanent may degrade over time (do you think you'll remember yourself as having the same name 600 billion years from now?), and new memories will be formed. An immortal human will become far more wise than any mortal counterparts as knowledge gathered throughout their unlimited lifetime makes their understanding of the world around them more accurate (but not necessarily more complete, as our memory capacity is limited). Eventually however, a point will come where having a greater knowledge will result in us requiring a higher memory capacity than we currently have, at which case we will hit a plateau. This is assuming of course that we have not used our gathered knowledge to enhance our own mental capacities beyond what they currently are.

From a psychological point of view, we will not be much worse off. We will not be eternally bored unless there is nothing to do for eternity. Any emotion we have cannot be amplified beyond our ability to experience it, so at any given moment, even an eternity of hell will be no worse than a decade in hell. We will be emotionally stable most of the time, but the long lifespans means we are more likely to run into situations that cause long-lasting PTSD (how many horrible wars, famines, and tragedies do you think you'd live through if you were born in the 1600s and were still alive today?).

Over time, the culture of a human society drifts. This occurs both within and between generations. The elderly today would rarely agree with the opinions and views of their youth. Immortality can thus be thought of nothing more than a society which does not need generations. Languages will change, our moral framework will be vastly different in 100 years than it is now, even if we are the same person.

Overall, we would be more wise and would hold more knowledge, up to a limit. Our culture will drift over time. Other than this, we will remain relatively unchanged.

  • $\begingroup$ PTSD: Either you would get used to it, or you would avoid it. e.g. you would learn what the signs of coming conflict were, and be somewhere else. $\endgroup$ Commented Mar 21, 2018 at 15:33

Cracked did a piece on this very topic.

The only brain-related issue they discuss is the idea that your perceived flow of time would speed up so much you'd go insane. As you get older, and things are not new and exciting, you pay less attention to things that you're used to. So as you get older, there's less things you're spending time looking at, which is why days seem to pass more quickly. If you're old enough to have literally Seen It All, time would fly past at a rate reserved for trees.

Clicky-link - Why Immortality would be worse than death


Your skull has a finite size. Even with a future enhanced human, with perhaps a cranium engineered to be somewhat larger, the space is limited.

Given the finite space, there is only so much information this human can store in it's head. Even if somehow they have re-engineered the brain to be more efficient in this respect, in terms of both bits per volume and compressing more data per bit, an immortal human will at some point begin to exhaust the available space or permanently forget things, in terms of both memory and learned skills.

Let's further hypothesize this human has a cybernetic implant, helping the brain overcome these limits to store information in another location. As the amount of information grows, searching or locating this information will become more difficult. Just getting a computer address location of memory data, collating that data, and recalling relevant information will begin to become an unwieldy process.

The point is, I have to think that memory would be effected after a few centuries. Maybe sooner.

This human would either become very forgetful, or would have a much slower thought process in order to evaluate available data, or would appear absent-minded, where it could remember and use a piece of information in one moment, forget all about it not too long afterwards, and then seem to remember it again later on. If a human like this ever became truly OLD, think about pushing all these effects to their extremes. It would possibly resemble forms of dementia or alzheimers.

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    $\begingroup$ The argument that the skull is finite alone doesn't imply that its limit will be reached before the heat death of the universe. $\endgroup$
    – SK19
    Commented Mar 16, 2018 at 20:10


First of all, if you eliminate degenerative diseases as a cause of death, you still have accidents. You can play with standard actuarial tables and you get an average lifespan of about 1700 years.

However you end up with a long tail. Like radioactive decay, people have a half life.

I'm not convinced. A better model would be a mix of radioactive substances. The short lived ones decay rapidly. Consider the difference in attitude between a 14 year old boy skiing and his 60 year old grandfather. Us old farts no longer believe in our own immortality, and we tend to be more cautious. This would greatly extend lifespan.

As to the memory issue that comes up in other answers. Suppose that we have a well integrated chip memory. With retraining, then much of our brain becomes indexes into the chip memory.

This could be a part of the background of your world. People get older, but they get less nimble mentally, and unless they work at it suffer from 'hardening of the neurons' where original thought or even decent decision making becomes increasingly difficult, to the point that some individuals are little more than human flesh automatons. Preventing this takes the same sort of energy and concentration that people spend now trying to look young in their 70s.

And there can be considerable quackery connected to these efforts. (Never eat these 5 things and stay sharp) You can have fun if some techniques (Zen meditation, daily treatment with flux capacitors, magic beans....) actually work.

Take another step. "Use it or lose it" Suppose stem cell research pays off and your brain is constantly rewiring. You remember well what happened in the last couple of decades, and anything older than that fades. You see this now: Try this as an exercise: Make a chart with a logarithmic scale: This year, 1 year ago. 2 years ago. 4 years ago. 8 years ago. 16 years ago. etc.

Now quick as you can jot down incidents. and put them in about the right time frame.

Routine stuff melts together much more than other things. I love the crime dramas, "What were you doing on the evening of June 7 2003" For me, the answer to that is, "How the F*** would I know. I don't remember what I had for supper last night."

Suppose that if you don't consciously work at remembering things, you forget. If your life revolves around work, you become an idiot savant about work, to the exclusion of all else. Most people over, say 200 years become less and less human in their outlook, and more and more like specialized AIs

One way people try to postpone this: They have a personal camera they wear glued to their forehead. It records their life. An AI processes the daily stream, weeds out the cruft, and indexes the rest. In addition for things that mattered (physio status of emotional indicators: pulse, sweating, respiration) it talks it over with you. Part of your day is spent reviewing events from previous years.


Lois Bujold addresses the organic and the chip memory idea in her Vor universe novel Memory.

Heinlein visits it in Time Enough for Love with the mention of the Libby technique of 'Cascading memory'


If you assume or want to adhere to relatively realistic biology of the brain (and leverage our ignorance of a lot of things related to it), there are a couple of factors to deal with (and things you can work with):

Cell counts and cell death:

We're assuming the source of immortality effectively defeats cell death from age-related causes (e.g. telomeres being restored via telomerase) and that ongoing radioactive exposure and other environmental factors do NOT in fact result in cancers (the saying is if you live long enough, you will get cancer, no matter what).

Your neuron count will largely stay the same once you reach adulthood (barring cell death due to various factors) except for a pool of stem cells that you're left with to expand those mental horizons. Even so, there's a limited pool (if not a very large one) of neurons to maintain everything from skills to memory and your personality.

Synaptic Plasticity and Memory:

With that limited pool, that means that memories, over the years, will likely fade as new ones are made. Neurons can adjust and change connections between themselves to strengthen memories and to change pathways as we live. While most of us, if we get old enough, will likely lose memories and functions due to decay or disease, an immortal would likely not. Instead, they'd just continue to make new memories (and perhaps learn new skills while enhancing existing ones).

I would argue, just like most adults don't remember the details of every day of their childhood (just the salient/memorable moments), an immortal will have memories that fade over time, something that might be a saving grace because traumas might have a chance of fading.

Life Experience:

We're all ultimately the culmination not just of our experiences but of the wisdom derived from those experiences. Our presence of mind (namely stemming from our interests in whatever the subject matter might be) defines just how much of those experiences we learn from and retain. If you aren't interested in a subject, you'll rarely retain much except those random facts that somehow got stored likely because something else was tied to it (e.g. environmental memory). And assuming we are focussed and healthy, we can generally accumulate a lot of knowledge...more than we often feel we should be able to.

When you talk to a fan who can quote stats across decades of a sport or someone who absolutely loves a topic and knows everything there is to know about it, remember that they also remember large portions of their lives, friends, family, people they hated, foods they loved, places they loved, people they admire, and so on. We store vast amounts of information in these squishy brains. :)

I think as you go on, say for a few hundred years or a thousand, you'd find that yes, you'd forget a name, or a face, you'll forget a song or the smell of a certain flower. There might be a mix of sadness associated with that (imagine how someone with dementia or Alzheimer's feels) and wonder (consider just how much knowledge you'll still have at your fingertips though). I could see someone like that, having learned that memories do fade even if it takes 100 years, taking on the same habits mortals take on (keeping photos, mementos, etc.). Meanwhile, there's something to be said about the confidence that stems from having a broad set of knowledge. I figure through just the sheer fact that they have the time for it to happen they'll accumulate a lot of knowledge. They may not be experts but they could piece together associations or bring ideas to a conversation or discussion that most people wouldn't think of simply because they don't have the breadth of knowledge someone like this would have.

Just one other thing to consider--something touched upon by some stories involving immortals: there is going to be some emotional scarring that results from seeing all of your family then all of your friends (including newly acquired ones) eventually getting old and dying. It can lead to defense mechanisms (like distancing themselves from mortals) which can definitely affect how they behave as the years go on. Plus, if they're trying to hide from society (e.g. Highlander), that creates a whole slew of stresses but also changes in behavior outside of the norm.

The other thing, which is sort of related to the latter part about their interactions with society, is the vampire grampa syndrome. In vampire stories, you often see super old vampires having trouble integrating with modern society. They were just fine with styles from 100 years ago. It's supposedly why they'd take a new victim to help them get up to speed with society and trends. Of course, they also tend to be separated and sequestered from general society so maybe not much of a concern?

Hope this helps.


Given a brain case that does not get any bigger, the brain inside it will have a limit to its capacity. Assuming all cells eventually die, and the only way to keep a brain from shriveling to nothing is if cell death was balanced by replacement (we'll assume an artificially-enhanced natural process), it's reasonable to expect that old connections will eventually disappear as new and different connections are formed. One might extrapolate this to:

  • Things not actively remembered, or knowledge/skills not actively used/practiced will eventually be lost
  • New information/skills can always be learned, maybe less efficiently over time, maybe not - if neuron death is offset by increased rate of replacement, the efficiency of learning new things might be maintained
  • That which defines "who we are", defined by the sum of our experiences, would be stable over the short/medium term, but would be plastic over the very long term.

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