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I have some bad news: you might live forever
[The bulk of this is a retelling of and musing on a paper by David Lewis, but without any of the actual good philosophy--in essence, it's a less artful way of presenting the same ideas. For some good philosophy, read the paper.]
Please, let me ruin your day.
Consider the following scenario: you're about to enter into a Star Trek teletransporter that will disintegrate your body here on Earth and put you together in the same configuration on Mars. You step onto the platform, hear the operator press the button to begin the process, and so close your eye in anticipation. Then...nothing. You open your eyes, still on Earth. The machine, it turns out, has malfunctioned and failed to disintegrate you. Nevertheless, as you can see from the familiar face smiling back at you from Mars, it worked well enough to put together a copy of you there. Which one of those people is you?
This question is, in one sense, unimportant. Some people think that both people are you. That may be right, but I happen to think that from your point of view (regardless of who you actually are) the question will seem absurd. You are the person who has your subjective, first-person view and that's just the person who has this point of view. No other person has, or, indeed, can have this point of view, regardless of how much he looks like you, what they say, what kind of history or memories they have. So, while the question of who's the real you might pose a problem from the third person perspective of the embarrassed teletransporter technician, it will never be a problem for you.
The same point, of course, applies to whatever or whoever your doppleganger is on Mars. For him, there likewise isn't any question about who the real him is--it's the person who has his point of view, i.e. him. And the same point applies to you who are reading this.
The same thing will be true if tomorrow you woke up with an entirely new body. If, for example, you were cursed to inhabit the body of your teenage daughter this Friday, on waking up and taking your usual first-person perspective, the question of who you are would never arise. You might think to yourself "my god, I'm in my daughter's body!" but you would think "my god, I am my daughter!" That's just a mistake you could never make--you could never mistake yourself for somebody else from your own perspective. You are necessarily the one who has this perspective and this perspective can't be somebody else's.
I also believe the same thing will be true if you woke up with all of your memories wiped clean (well, at least those memories that don't involve remembering how mirrors work or how to speak or whatever. Play along!). In that case you might in some sense be wondering who you were, but you would still know that whatever the answer to that question is, you're still now the person who is looking out from these eyes and who's trying to piece things together. Subjectively speaking, there's no deep question about your identity.
Back to the teletransporter. Suppose that instead of malfunctioning in the way that it did, a different mistake had happened. Once again, you remain unharmed on Earth. However, when you peek at the screen broadcasting from Mars, you hear screams of horror and see something that looks like your lifeless body, crumpled on their receiving pad. Did you die? The answer is, of course, no. You are where your first-person perspective is and that's here on Earth where you are very much alive. Furthermore, if you were to die, you couldn't have any first-person perspective. Necessarily, you can't experience being dead since do be dead is to have no experience whatsoever (maybe if you have a soul you could still have the perspective from your heavenly ethereal body, but that would still be living a kind of life--an afterlife--and there, too, you would know it by taking this first person perspective). To dust off the old adage: wherever death is you are not, and wherever you are death isn't. Whew--close one.
Your trials aren't over just yet. Imagine yourself as the victim of a Schroedinger inspired act of cruelty. You're put in a cell with a bottle of a deadly nerve agent that is hooked up to a photon detector, which, if triggered by a photon will smash the bottle, release the gas, and kill you. A photon is set to be fired into the detector, but in its way is set up a diagonal half-silvered mirror. In one scenario, there are no collapses (see the Lewis article for the full explanation of what those are), the photon goes into a superposition, hits the mirror and bounces away, the photon detector goes into a superposition of untriggered, the bottle into a superposition of not-smashed, and you go into a superposition of remaining alive. In a different scenario, there are collapses and, well, things don't turn out as well for you.
You realize all this. What does it look like from your perspective? What should you expect? Well, if there are no collapses, then you should follow the intensity rule: "expect branches according to their intensities" where branches are the resolutions of the quantum indeterminacy and intensity is a measurement of the likelihood of different events occurring (I'm doing my best here--I don't know anything about quantum mechanics). In this case, you should expect equally to be alive as well as dead. But, as we just saw, you shouldn't expect to be dead! You can't have that experience at all since that's not an experience you can ever have! So your expectations should be adjusted and you should expect to be alive. In fact, no matter how many times your captor repeats this process you should expect to remain alive. As long as there are no collapses, you'll always survive your torture.
You can think of this in terms of branching timelines. On one branch (from a third-person perspective) you die, and on the other one you live. The more times the torturous experiment is repeated, the fewer branches there are in which your survive. If there is collapse between the different branches, then, sadly, you will die. However, if there's no collapse, then there are at least some branches in which you survive. And given that you will always be where your perspective is, and that there is no perspective in which you are dead, there is also no branch in which you will be dead. Thus, you should expect to always survive this experiment.
The same is not true for your torturer. For him, there are plenty of branches in which you die, but this is fine since we're not asking what he should expect, but only what you should.
Nevertheless, we can ask what he should expect with respect to him and the answer will be very much the same as with you. That is, if there's no collapse, he will never find himself on any branch in which he's dead, though, again, many of those branches involve your deaths.
This point generalizes. One doesn't need to be in a torture chamber to risk death and it's clear that there are many events that could occur that could result in death. These can be chemical, biological, or mechanic. Regardless, from your perspective, however, you shouldn't expect to ever find yourself on any of those (though, of course, I should expect it for you).
So, if there's no collapse, you should expect to live forever. If there is, however, you should definitely not expect this. So, is there collapse? How would we know? Well, here's the rub: if there is collapse, then you won't be able to get evidence of it since, as Lewis says, "its prediction is that in all probability you will die soon." And once you're dead your death can't be used as evidence of any kind for you since you won't be the kind of thing that will be able to assess evidence--you'll be a corpse. Notice, also, that the deaths of others won't give you any kind of evidence as to whether or not there's collapse. The fact that someone (in fact, everyone!) else has died is compatible with you continuing to live in a non-collapse universe on a branch where someone else has died. The only way you could get evidence that there is collapse is if you died. But then, to repeat, that won't be any evidence for you.
Should you believe that there's collapse? Well, you are, of course, free to try dying, though if you're successful it won't matter to you anymore and it won't make a difference as to whether anyone else should believe that there's no collapse. If you survive, however, you have stronger evidence and the more improbable your survival, the stronger the evidence. As Lewis advises us:
If someday you find that you have survived a remarkably long sequence of dangers, the no-collapse hypothesis will then deserve your belief more than it ever did before.
So, the longer you find yourself alive, the more you should believe that there's no collapse. Furthermore, you'll never find yourself not alive (wherever you are, there you are), and, given that you've made it this far...
BUT YOU SAID THIS WAS BAD NEWS! WHY DOESN'T THIS SHOW THAT I'M GOING TO LIVE FOREVER?
If there are no collapses, then you really will live forever. But that doesn't mean you won't age, grow infirm, diseased, and incapable. Furthermore, while in your timeline you will always survive regardless of what happens to you, from your perspective nobody else will. Everyone else around you will age and die as you, the oldest person alive, will, in all likelihood, continue to survive in constant agony. Even if you tried to kill yourself, you would necessarily find yourself in the timeline in which the gun doesn't go off, the poison doesn't work, and the rope slips. Even worse! You might very well find yourself in the timeline in which the gun does go off, but only maims or; the poison does work, but only makes you horribly ill; and the rope doesn't slip, but only paralyzes you! And this applies to every single person!
A silver lining and an addendum:
First, if there's no collapse you might be able to live a less horrible life in the far future in which horrible infirmities and aging can be reversed though, say, tremendous scientific advances. You would still inhabit a world in which everyone else dies and you continue to live forever, so that's a bummer, but it's better than the alternative! Given that in every timeline you'll necessarily be the person who has lived the longest, you'll probably get some special preferential treatment in testing!
And note, if everyone believes that there's no collapse sufficiently strongly, everyone can get to work on making these scientific advances. True, from each person's perspectives, everyone around them will die, but collectively, we might be able to make it so that in each person's timeline science is advanced enough to keep the misery at bay.
So, really, the worst is getting through the time that you're alive now until the science gets really good. Furthermore, once you become convinced that there's no collapse, you should definitely volunteer to be tested on since you'll speed up the process and will know that you'll survive (in whatever miserable state that is) regardless. That might make waiting through that horrible middle phase go much faster.
Chin up, fellow immortal! It's only a matter of surviving hell for a couple of hundred years. Then it's just permanent solitude until the heat death of the universe!
Finally, let me add that I know I've butchered the quantum mechanics stuff and that I'm not doing justice to Lewis. Relax. This is just another thing you'll survive.