This isn’t serious. It’s making assumptions I wouldn’t make seriously, and I’m no quantum physicist anyway so I don’t know how correct my physics are even given that. Still, I thought this was interesting.
Suppose the many-worlds interpretation of quantum mechanics is true, and every time a random quantum event happens that could have happened many ways, all of those ways do happen and each gets its own universe. The universe splits into many all the time. Some people who should know (better) believe this.
Suppose you are a human in a universe in such a multiverse. That you will die one day is overwhelmingly likely, so that the way we usually think about it is that it’s inevitable. Quantum events are not deterministic, so that they could only happen one way, but they are statistical, so that immensely large groups of them such as we always see in practice (since they happen to elementary particles so tiny that everything is made up of a huge number of them) always happen the same way in practice. Eggs can break, increasing entropy, but they don’t jump back together again, even though technically there’s no absolute law against it. Similarly, people die under certain circumstances as their bodies break down, but they don’t do the reverse by having their bodies suddenly jump back into good shape due to a lot of random quantum events all happening to aid that.
However, we have the infinite number of worlds. If every possibility happens in some universe, then we can expect that everything will happen in some portion of the universes. At first, this doesn’t seem to help: it will still be in a tiny minority of the universes, so we’re always going to end up in one where it didn’t happen.
However, what if we’re talking about the universes in which you die versus those in which you don’t die? Then you will only continue to exist in those in which you live.
From the many-worlds interpretation, you know that your future is not one, but many. There will be many of you. Presumably it makes better sense to identify you as an individual as only one of the many copies of you that currently exist, since each has no contact with the others. Right now, you are one. What happens to parallel universe versions of you is of no use to you. Still, all the people branching off from you as your universe branches in the future, all of these are equally much your future, because how could you say which one is and which isn’t? Your continuing to live in the future can only mean that many people branching off from you now will live.
Apparently it was Epicurus who said, “Death does not concern us, because as long as we exist, death is not here. And when it does come, we no longer exist.” We can now take this a step further. There’s no meaningful future for you in those universes in which you no longer exist. But since those are not the only ones, there is a future for you in those in which you do live. Out of all the possible futures, only those will be selected for you in which you continue to exist. Death need not concern us because when it comes, we will be elsewhere rather than nowhere.
Thus, though you may die in an enormous majority of all universes, the only ones that count for you are the ones in which you don’t. Other things that may happen but are astronomically improbable you won’t see — only such a small minority of all of your counterparts will. But this one, surviving forever? It’s automatically selected for you, because you are selected to continue by it.
You can also take heart in knowing that even if you have got unlucky and are about to die, you are still about to be saved by some quantum miracle. Even if your heart was torn out and your dying is now extremely probable, you will only continue to exist in those worlds where something miraculous just happens to happen, like a new heart forming. Thus, there is no branch of your history doomed to extinction.
There are negative sides to this immortality, though. Since other people are not selected in those universes in which you continue to live, you will see others die with the same overwhelming probability as usual. You will thus be a lonely immortal in the way you might imagine if you just imagined being immortal without all this multiverse babble.
I’m also afraid that it may be overwhelmingly likely that all this will lead to your living a weird half-life dependent on constant miracles. The thing is that any time that you are saved by an improbable miracle when you should have died already — and most of your futures will, so you can count on it — you will more likely be saved in some way that doesn’t restore you to normal health, which would require a bigger change, but rather keeps you clinging to life for just a moment more. Then you are again overwhelmingly likely to die very soon, and only those of you will survive who get another miracle of improbability, and most of them will again only get a temporary respite. Since you can expect to always go the way that is overwhelmingly likely unless it’s a way that kills you so that you just don’t have a future in that universe, you can expect to just go on experiencing this cycle indefinitely. You’ll probably be in a lot of pain, although this may depend on how violently you were dying.
Ironically, at the same time, the multiverse will presumably eventually have at least equally many branches in which you are miraculously restored to more improbably better health and go on living healthily. This is because though those miracles are rarer, they can spawn more branches in which you continue to survive. However, you will not reach these branches, because they all branch out of the more improbable branches you know you won’t be going down in the first place. It’s just that a minority of your futures will have disproportionately many offspring with you still alive. Since you’re stuck on one (past) path and will always go down the more overwhelmingly likely one — in this case one with a smaller miracle rather than a bigger one — you will not get to these branches. Your can’t jump to adjacent branches once they’ve split from yours.
Of course, given an infinity of time, you will experience a bigger miracle too, because anything will happen if you wait an infinity (or close enough). Given enough time, and this time much less, you may also find yourself cured by advanced science by people taking pity on the eternally suffering freak. (Of course, each of them will have counterparts in a similar state, but not in the same universe, since universes with two such anomalies are so much rarer still you won’t end up in one.) However, that will only be after a long period of suffering.
So perhaps it’s just as well that this is pretty much nonsense — though it’s logical nonsense, to a point.