Max the Scientist puts his cat Schrodinger into a box. This box will kill the cat if an atom fissions, a “quantum” event which occurs one half of the time.
The Many Worlds interpretation of this experiment says that, when Max runs the machine, the world splits into two distinct universes: one in which the particle fissions and Schrodinger dies, one in which the particle stays whole and so does the cat. Each universe also contains a Max the Scientist. When the Max’s look inside the box, they discover whether they are Max-One, who is in the universe with a live cat, or Max-Two, with a dead one.
As bizarre as this sounds, it is a very plausible way of translating quantum mechanical theory into the physical world with which we are familiar. But it has some strange consequences:
Max climbs into the box and triggers the experiment. The universe splits into two: one with a live Max, one with a dead Max. But the thing about being dead is that you are not around to notice the fact. Subjectively, Max only perceives the universe in which he still lives.
The universe is just this experiment writ large. Every event that could possibly kill you is a result of quantum interactions which divide the universe into worlds in which you are alive and worlds in which you are dead. Subjectively, you will only experience those universes where you continue to live. Subjectively, you will never die. You have “quantum immortality.”
This is great. But because I am an anal law student, I am going to point out that just because the universe splits (presuming that it actually does) every time there are multiple options does not necessitate that every time your death is an option, that your lack of death will also be an option. Ergo, you live longer or shorter in various realities, but since there is a zero percent probability that you will live to be a thousand, there can be no possible realities in which you live that long.