Now, this is a problem that popped into my head a while back, and I was curious about other people's opinions, so, here I am.

It's about immortality, right? As a concept, I've usually just taken it for granted most of the time, since it seems pretty straightforward. In a nutshell, I reckon it means you can't die.

But how do you know that? And more importantly, how do you know you're not immortal?

Because immortality means 'living forever', the only way to definitively and physically verify (that my little brain can think of) that you will, in fact, live forever, would be to sit down and observe whatever it is you are observing live forever. Which is a bloody long time, since eternity is one of those things that just keeps on going. Conversely, if you die of unnatural causes, by disease or being chucked out a window, how do you know you wouldn't, with 100% certainty, have lived forever?

Even more interesting to me is that there have been theories (which I unfortunately can't verify, since I am but a humble Design Engineering student, not a super-smart physicist) that the universe/time will, eventually, end. If those theories are true or if you define them to be true for whatever MLP fanfic you're writing, then wouldn't it be impossible to truly verify immortality since before you're done checking that, say, Celestia will live for all eternity, the universe gave up on you?

So, I guess my question is, what is everyone else's opinion on checking how immortality works and defining it? Have you given thought to this kind of thing? What happens when time tends towards infinity? Am I overthinking this? What does immortality mean and just how long is eternity? How would any being function for such a vast amount of time? All the questions!

I think, precisely because of the infinity problem, that you never really can. That's why I consider "functionally immortal" or "within delta of unaging" to be close enough for me to use the term. Though, I also don't use "immortal" to mean "absolutely unable to die", though I suppose there really should be a word for that concept. "Deathless" or "undying", maybe. An immortal being can still be killed, though it might be very difficult, depending on why they're immortal, but if they aren't, they'll live indefinitely, probably.

A merely indefinitely-regenerating being, like the Doctor, or the Mane 6 modulo Twilight in Tales of the Tutelary Spirits, should probably qualify too, though they do technically die.

5112076 But what qualifies as 'close enough'? 1000 years before signs of aging? 100 000? How old is 'old enough'?

Maybe there could be other ways of quantifying it. I think some animals were said to be 'biologically immortal', but my problem there is that, really, if you wait long enough, would a system in their body fail? Would a creature without a body not have this problem?

What does immortality mean

Well, there's clinical immortality, which means that you don't senesce like mortals; you never age, your body is able to maintain life processes indefinitely. Regular folks can't do this; partly because there's a natural aging process, and partly because bodies simply wear out.

A clinically immortal being can still be killed; they just won't ever die of old age. I imagine that this is probably the kind of immortality that alicorns have.

Clinical immortality can be achieved in various ways; most commonly, just by having the right biology for it, or by having good enough regenerative powers.

But there's also the more extreme kind of immortality, which is far more semantic: you just can't die, even if the conditions to bring about death are present. This is much more of a comic book superpower kind of thing, and it's the one that tends to result in unhappiness.

and just how long is eternity?

Such a question has no meaning; the length of eternity cannot be expressed as a number, so the notion of length is meaningless in eternity. It just never ends.

5112134 Yeah, but with clinical immortality... is there a way of proving that you can regenerate properly indefinitely, forever and ever and ever? Even stars die eventually. Some peeps believe even time is finite, although I am definitely not smart enough to explain that concept properly.

Does eternity only last as long as time exists? Or does it go beyond that?

What happens when time tends towards infinity?

As far as science can tell, our universe will eventually dissipate. All things in it will have turned to basic particles and energy; it will be in a state of maximal disorder, with no possibility of reorganization. The only things that can happen beyond this point are meaningless thermal fluctuations.

However, unbounded time does allow for some interesting statistical phenomena. With infinite time, infinitely improbable things become possible, including, some have suggested, the spontaneous formation of a new universe.

5112153

is there a way of proving that you can regenerate properly indefinitely, forever and ever and ever?

I think it has been established, biologically, that there's no known reason why lifeforms have to die. There are some species described as being functionally immortal.

Stars aren't alive, so the concept of death doesn't apply to them in the same way that it applies to a lifeform. (I mean really, a star's just a really long explosion. )

5112153
Getting into more biology (and I'm by no means an authority, I just know lots of random junk), the answer to

is there a way of proving that you can regenerate properly indefinitely, forever and ever and ever?

is yes, sort of.

There's a species of jellyfish that is able to revert back to an "infancy" stage of its life cycle when threatened (i.e. food supply dwindles, as opposed to predators). It effectively allows it to go back through other stages of life and "return" to adulthood. Now, assuming you're familiar with how aging works in humans and other animals, the reason we age is because over time, certain aspects of our cellular biology start to break down: DNA can't be replicated properly (i.e. mutations that stop the cell from functioning, or cause cancer) or at all (our chromosomes have small structures at the ends that keep them together, much like the plastic or metal ends of shoe laces; when these break down after too many replications, the chromosomes lose their structure altogether and can't work), cell walls and structures lose cohesion, certain types of cells that don't replicate quickly begin to break down faster than they're replaced, etc. With that jellyfish, it's able to avoid a lot of those processes.

Now, if you're able to show that a creature can, over a long enough period of its life cycle, can avoid that destructive aging process through whatever means, it's not entirely necessary to have to observe it for "eternity" to verify the claim. You can make an educated assumption that "OK, over x years, we haven't seen any evidence of destructive aging processes, so-and-so is able to mitigate or reverse the processes in such a way that we can say they're not aging and will continue to live as long as they're able to find food, etc."

5112153

Does eternity only last as long as time exists? Or does it go beyond that?

Eternity is unbounded time, so either the question doesn't make sense, or the answer's no.

My pastor likes to explain eternity like this. A bird flies down, picks up a single grain of sand, and flies it to the farthest reaches of the universe. It does this, again and again, until there isn't a single grain of sand left anywhere. At that point, eternity will have just begun.

5112176 Yeah, but they decay and stuff. Then blow up or something. I dunno, star stuff.

5112205 Anyhoo, the biological immortality thing is interesting. I'll have to read up on that.

5112251 Interesting, that's the second time I've seen a metaphor like that. The version I heard was like so:

"Far, far away, on a planet orbiting a no-account F-type star somewhere out in a galaxy nobody will ever notice, is a mountain, 100 kilometers high, made of diamond. Every million years a tiny little bird comes to that mountain to sharpen its beak. When, at long last, the bird has worn the mountain away to nought but dust, then, the very first Planck Time of eternity will have passed."

It was a weird fusion of science and mysticism, and sorta puts infinity in perspective...

5112334 There was nearly that same one in a recent episode of doctor who.