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I was watching a video on Black Holes and the speaker said that it would take a septenvigintillion years to completely evaporate. That's a 1 with 84 zeroes after it.

Life extension technology will likely never allow a human to live that long but lets just say hypothetically; it could. You live this long and survive due to cybernetic and biotechnological upgrades. Would the human mind dissolve into insanity by being around this long and would our ability to form memories be depleted?

Basically I'm interested in,

1.) How memory will work at these timescales.

2.) How the personality and mental state of the person will be?

This is extremely theoretical but to summarize. My question is "How long could the human mind last before deteriorating. What is the approximate theoretical limit?"

I’m looking for hypothetical but hard, realistic science answers.

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    $\begingroup$ You absolutely do not want to use the hard-science tag, which requires real-world documentation, not speculation. Since there are no known cases of biologically immortal humans, this question is unanswerable with that tag. (Hard-science is the next level up from science-based, so they are mutually exclusive tags.) $\endgroup$
    – Jedediah
    Commented Dec 16, 2022 at 20:44
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    $\begingroup$ If the body is biologically immortal why wouldn't the brain be? The brain will delete old memories to make room for new (well not explicitly "delete," but older memories become weaker over time and newer ones take their place). Have you never come across something you did that you don't remember? The brain only keeps around stuff you're constantly recalling, so after a zillion years you'd still know who you were and probably how old you are, but you won't be able to recall specific memories clearly if you haven't been constantly recalling them before. $\endgroup$
    – stix
    Commented Dec 16, 2022 at 23:18
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    $\begingroup$ You might also want to choose between "brain" which is an organ of the physical body and "mind / psyche / etc" which something else entirely. $\endgroup$
    – elemtilas
    Commented Dec 16, 2022 at 23:48
  • $\begingroup$ Yeah that’s a good point. Like if ghosts exist; they don’t have brains but they have minds. $\endgroup$
    – user73829
    Commented Dec 16, 2022 at 23:55
  • $\begingroup$ brains start deteriorating the day you are born and by min-maxing menthal and physical health on can probalby reach a healthy mental state to the grave at 120 years of age, but still more degraded that when they were young. For the average guy, expect their brain to be fried by the age of 60. $\endgroup$
    – user100394
    Commented Jan 8, 2023 at 21:38

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It would seem that there is some debate about the maximum limit of a human life span 115-120 years might seem a reasonable estimate today. https://time.com/4835763/how-long-can-humans-live/

Given the complexity of the brain and the degenerative issues it encounters it is not unreasonable to assume that the life time of the brain is likely to be similar to the lifetime of the body.

So for an ordinary human brain the chances of a vastly extended life time seem remote at the moment. Perhaps some form of hibernation might help, but this is just a vague possibility for the time being and I suspect that the question really refers to conscious lifetime.

Regardless of life extending technology there are other matters that must be considered. There are approximately 1.5 x10^14 synapses in the human brain. It must take at least one synapse to record one item of data of memory (and probably a lot more) so this must be an absolute upper bound for memory

https://psychology.stackexchange.com/questions/7967/how-many-synapses-in-the-average-human-brain

10^14 is 10^70 times smaller than 10^84 so it is clearly impossible for an ordinary human brain to have any meaningful memory over such a vast expanse of time. The only option is to suspend the brains function, preserve it and revive it later. That said any preservation over such a time period would encounter many technical difficulties (to say the very least).

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    $\begingroup$ If there are S synapses, and every synapse is either active or inactive, that's still 2^S possible states for the brain to be in, and 2^(10^14) is vastly larger than 10^84. I don't think we've reached the state where we can say that any given memory is exactly determined by any small set of synapses in isolation. $\endgroup$
    – chepner
    Commented Dec 17, 2022 at 15:04
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    $\begingroup$ You would need a bit of information to hold anything meaningful. A lamp that's permanently on doesn't tell you much. A lamp that could be off or on conveys one bit of information. You are right about synapses working in groups but memories can't be reduced below one bit of information. $\endgroup$
    – Slarty
    Commented Dec 17, 2022 at 16:21
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    $\begingroup$ Yes, but it's a leap at this point to assume that any particular set of synapses represents a bit of memory. We don't know how memories are stored, only that forming and recalling a memory involves synaptic activity. $\endgroup$
    – chepner
    Commented Dec 17, 2022 at 17:19
  • $\begingroup$ You're framing a synapse as a binary connection that can only transmit "on" or "off", which is inaccurate. They're incredibly complicated connections and you could spend a lot of time just reading about their connections and signals. I don't know how many "bits" you could equate a synapse to but A) that's not how memories are stored and B) it is way more than 1 bit of information going on there. $\endgroup$
    – JamieB
    Commented Dec 18, 2022 at 4:00
  • $\begingroup$ You may be right to some extent. We don't fully understand how memories are formed. but it doesn't really matter for the point in hand. Just supposing that each synapse could hold 10^14 memories (regardless of bits) that leaves 56 orders of magnitude to be accounted for (assuming something is remembered just once a year). $\endgroup$
    – Slarty
    Commented Dec 18, 2022 at 14:12
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I know you specified "hard, realistic science answers" but I'm going to take a jab anyway, because 1) I wonder about this a lot, actually and 2) I don't think there exists a hard, realistic science answer. Obviously, we can't test it, and I don't think we have sufficient understanding of how memory works to speculate what would happen over a million years.

With that, the answer is "whatever you want it to be for your world".

Research topic: "neuroplasticity". But as long as you're granting someone immortality, you might as well grant them lifelong neuroplasticity -- that is, the ability to continually rewire the brain. As best as I understand the science, the problem is we run into brain degeneration (dementia, alzheimers, strokes, etc) long before we "run out of memory", if that's even possible. So any form of useful immortality must imply immunity to all wasting diseases. You might still have to answer, "what happens if their air gets cut off for 15 minutes." Can they recover? Are they Wolverine? Do their memories come back?

But really I think the Worldbuilding answer is "don't overthink it". I have certainly read stories that had entities that lived for hundreds or thousands of years with no ill impact what-so-ever and I think the lack of science in this area makes just about anything plausible.

If you want a mad scientist that invents an infinite regeneration potion and now it's a billion years in the future and he's still around, you can make him as sane or insane as you'd like and I don't think anyone can say "that's wrong".

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It's simply not possible to give a hard science answer.

The oldest living people today are a little over 100 years old. So we have no hard scientific facts about life past 100 or so.

If you are going to speculate that future technology may allow a human body to live for millions or 10^80 power years, presumably the same technology could be applied to the brain to also keep the brain alive and functioning. As no one today has any real idea how to extend human life beyond 120 or so years -- there are some theories but they're basically speculation with little or no experimental evidence -- it's just impossible to say how it would work.

Presumably if you speculate that human life can be extended indefinitely, then degenerative diseases would have to be cured. You could cure Alzheimers, etc., so that wouldn't be an issue.

One might plausibly ask if there is some limit to the memory capacity of the human brain. Of course people forget things, but there's no real evidence that we forget because we had to delete old memories to make room for new ones. Maybe that's how it works, and maybe not. No one really knows. Frankly I'd say probably not, as children forget things as readily as adults. If the observed reality was that people remembered everything that happened to them until they turned some age, say 50, and at that point they started forgetting one year's worth of memories for every additional year they lived, then a plausible explanation would be that they had filled up their memory and old memories had to be deleted to make room for new ones. But that's just not how it works in real life.

As the human brain is obviously finite, presumably memory capacity is finite, but just saying "finite" is a long way from saying how much. Could a health person hold 200 years worth of memories? 1000? 10,000? We just don't know.

Would people go insane from such a long life? I don't see why. There's no evidence that living to 100 makes someone go insane. Why would living to 1000 or 1 million? If one was locked in a small room for thousands of years, maybe so. But assuming that one could lead an active life, I don't know of any reason to think long life itself would drive people insane. Of course we have no experimental evidence of someone living to be a million years old. Maybe trying to extrapolate from 100 to 1 million I'm just missing a small trend that would grow very big over time. It's pretty much impossible to say.

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  • $\begingroup$ It depends on how insanity works exactly. If the chance of going inside is any particular percentage per year, then clearly that chance is increased the longer one lives. For the type of insanity caused by very traumatic experiences, that would apply - the chance of having such an experience would increase with years lived. $\endgroup$
    – Erwin Bolwidt
    Commented Dec 17, 2022 at 7:59
  • $\begingroup$ "Could a health person hold 200 years worth of memories? 1000? 10,000? " — I'm also wondering if there is really a finite span of time. Maybe they can remember millions or billions of years in the past provided it is a significant memory. Maybe they forgot what they ate last Tuesday, but they can still remember their first kiss 11.3x10^29 years ago. $\endgroup$
    – Greg Burghardt
    Commented Dec 17, 2022 at 14:17
  • $\begingroup$ @GregBurghardt Sure. That's how memory works. I don't remember what I had for breakfast 30 days ago, and I don't care. I don't remember my son's telephone number, and it doesn't matter because it's in my cell phone. I do remember my mother reading me a story about horses when I was 5 years old. Human memory is not like computer memory. A computer remembers things until you explicitly tell it to forget them, when they're gone forever. (Barring a malfunction.) Humans forget things in much less predictable ways. And then sometimes remember them again. $\endgroup$
    – Jay
    Commented Dec 17, 2022 at 18:17
  • $\begingroup$ @ErwinBolwidt "depends on how insanity works" Is there some totally random possibility that one will go insane in any given time period? I doubt it. I think there has to be a specific cause. Do I know that for a certainty? Absolutely not. $\endgroup$
    – Jay
    Commented Dec 17, 2022 at 18:20
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I think there is a pretty hard science answer for the most part...

Suppose whatever you have that is called an intelligence is within some device composed of ordinary matter. I am excluding 'the immortal soul', and 'beings composed of pure energy' and stuff like that.

In this long view there are no stable elements. Everything that can split into smaller parts will do so. A black hole has a huge kinetic barrier, so it will take a long time for particles to tunnel out, but it should happen eventually if there is time enough. The smaller, intelligent device will evaporate long before the black hole ever does.

Could something live within a black hole? There has been some speculative science about the nature of singularity. Thermodynamics does not like reducing stuff to a single point, as that destroys entropy. So, people have postulated things like 'holographic universes' on the singularity surface. This is a perfectly valid thing to postulate when we can't get into a black hole and poke it with a stick. I don't think it means all the particles that made it are somehow still knocking about on the surface. But if you wanted to write about a very old intelligence, it could live inside the black hole and be worried about the decay. That's not hard science, but it might be hard science adjacent.

The only other solution is time travel - get from now to then without passing through all the years between.

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Given that the brain is actually part of the body, the brain would last as long as the body would. Which, if the body is immortal, would be basically forever.

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