Originally posted by me on io9.com in response to this article:

See, to me, quantum immortality looked pretty neat at first. Oh hey, if there’s always a chance, no matter how small, that I won’t die—then I won’t. Ever. Cool, right?

Except that you slowly realize that in this scenario, your consciousness (and by extension, the consciousness of every other being… ever?) continues on indefinitely. No matter how old and impossibly frail your body grows, you remain alive. Universes peel off around you in droves like the layers of an onion. Your friends and loved ones all die as the probability of them surviving decreases (because the probability that you and any other given person survive forever together is just slightly less than the one that allows you to survive alone, even though that possibility must also exist somewhere in the multiverse) but because you are always on the track where you survive (because otherwise you cease to exist)… you do.

Somewhere out there in an infinite multiverse of infinite probabilities, you are celebrated as the oldest person alive. Eventually, everyone who remembers that dies, and yet you continue on. Eternally. Until at last, you are the single living creature to witness the heat death of your universe, billions of years hence, somehow, some way, still alive—because in infinity, this possibility must exist.

And this has happened for every other conscious being which has ever existed or will ever exist in the multiverse.

And that’s how the many worlds interpretation becomes existential horror.