You may call parts of this fantasy rather than science fiction. Sorry for the blurry genre boundaries – Michael
People don’t like to die. That seems to be the basis of one of the oldest attested tropes: immortality, or at least far-longer-than-normal lifespans. Like The Hero With A Thousand Faces, this one shows up in very old cuneiform tablets from 2600 BCE or so. There may be some underlying neuro-psychological reason for this trope being common, but I cannot speak to that.
Instead, let’s talk about how immortality is presented in some more recent works and how those would play out if things were handled more consistently.
We mentioned Heinlein’s Methuselah’s Children before – it is the namer for The Masquerade. The Howard families there aren’t immortal, at least not at first. They are simply long-lived, due to genetic factors. But biotechnology develops to the point that the longest-lived Howards born in the early 1900s live effectively forever. Heinlein was born in 1907. I deduce Author Appeal.
To take a more extreme example, consider the Highlander franchise – the first movie only. Scattered around the world, born at various times over the last several thousand years, are a population of immortals. They appear like normal people until they “die” – or rather experience something that would kill a normal person. They then stop aging, which is convenient for explaining why some immortals are portrayed by old actors and some by younger ones. There aren’t any immortal children, so this must be a Puberty Superpower. The immortals are also nigh-indestructable, with nearly perfect memories for thousands of years of life, and only decapitation will kill them. Clearly, they have very unusual brains.
We’ve previously mentioned a common problem to both of these: why is there a masquerade? The Howards didn’t need to keep their life expectancies secret, and should have been discovered far sooner. The Highlander immortals run around having sword fights to the death for no adequately explained reason. Why didn’t one of them set up as a philosopher-king or an unkillable gladiator or a legendary drinker two thousand years ago? The real answer is meta-fictional: the authors didn’t want to consider alternate history, at least not for the past.
But what forms of immortality are more or less plausible in reality?
Extending human life expectancy, both for people alive now and in later generations, is obviously possible. After all, in many places, life expectancy is now about twice what it once was. There are genetic contributions to longevity, so a selective breeding program like that of the Howard’s isn’t implausible. But the program in Methuselah’s Children is implausibly rapid. The requirement for being inducted into the Howards is having had four centenarian grandparents. That’s not a strong enough selection criterion to increase life expectancy to 150 years over a half-dozen generations. Longevity is determined more by environment and luck than by genetics, so many people with good genes would not qualify and many with bad genes would. And longevity probably wouldn’t present as an overall slowing of normal aging. It would be more like “you are less likely to get rapidly-progressing fatal cancers” or “your hormonal balance is a bit different than normal and you probably won’t get osteoporosis”.
But massively increased human life expectancy could be plausible, provided it either developed over enough generations or from very sophisticated biotechnology. The limiting case might be “biological immortality” – a cell or larger organism can persist indefinitely without passing through a reproductive phase that would count as a new generation. For example, hydras, the jellyfish Turritopsis nutricula, and perhaps even lobsters appear to have an indefinite lifespan with no senescene. This is not “true” immortality, of course: the animals die by injury, disease, predation, or out-competition for resources.
Why isn’t biological immortality more common? One model is the Red Queen hypothesis, which says that a population of organisms needs constant changes in genetics and overall biochemistry to survive in an environment while competing against each other (this also gets invoked to explain the evolution of sex). A population of short-lived organisms that can adapt to take advantage of the environment and changes in it will often have an advantage over a longer-lived population that doesn’t adapt so quickly. So while biological immortality is possible, it is not likely to naturally appear in that many species, especially not in humans – none of our ancestors for the last several hundred million years have had that skill.
Technological immortality is another possibility. Should someone create strong/Turing-test-passing AI, it would be possible to just keep copying the AI program to new substrate – subject to all of the usual considerations when porting software to new hardware. But, to the great disappointment of people like Ray Kurzweil, human brains can’t be uploaded into computers with any approximation of continuity of consciousness.
Implausible and Impossible
The immortals in Highlander have problems, mystic “Quickening” superpowers aside. Connor MacLeod is thrown overboard and sinks to the bottom of a lake. He walks ashore, after spending a long time underwater. Brain damage aside (the water in the lake was relatively cold), where are his muscles getting the oxygen for that exercise? And when he is surviving being stabbed repeatedly or shot by a submachine gun, how is his body patching everything back together? He has to have some Wolverine-like healing factor. Which means Wolverine and the Highlander immortals have the same problem: food. It takes a lot of energy, protein, fat, and calcium phosphate to rebuilt an arm or a shattered rib cage. Why aren’t they constantly swallowing protein shakes and bone meal? And why isn’t there a huge temperature increase as all of that energy rapidly cascades down into heat?
A related question: McCloud dies if his head is cut off, but what if he loses a toe or an arm or a leg? A bunch of other works have explored possible answers to that one. And what happens if a disease appears that the healing factor doesn’t protect against? Etc., Etc. There is really no way to make a blanket healing factor like that plausible, at least not without imposing very steep associated costs.
Even more durable immortals, like Wolverine or vampires or some zombies, are right out – they aren’t completely consistent because there is no way to do so without immediately violating some of the laws of physics.