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And now for something completely different.

Quantum immortality is a controversial speculation supported by a handful of quantum physicists and abused by many science fiction authors. In essence, it states that the many-worlds interpretation of quantum mechanics implies that conscious beings are immortal. In this post I’ll attempt to show that quantum immortality is completely useless, even if it really works as described.

You Are The Cat

The idea of quantum immortality was derived from the quantum suicide thought experiment, which is a close cousin of the famous “Schroedinger’s undead cat” experiment. Instead of a cat in a box + poison triggered by radioactive decay, the quantum suicide thought experiment involves a mad scientist and a lethal weapon (which I will refer to by the unscientific name “gun” from now on). This gun has been cleverly modified so that when one pulls the trigger, it will measure the spin direction of a passing photon and fire only if the direction is, say, “up”. This amounts to a 50% chance of the gun actually firing.

So the mad scientist points the gun at his head and pulls the trigger. Depending on the spin direction of a random photon, either the gun fires and kills the scientist, or it just goes “click” and leaves him unharmed and free to go on doing what he must because he can.

According to the Everett many-worlds interpretation, each run of this experiment will cause the Universe to branch into two alternate world-lines – one where the scientist lives, and one where he dies. However, the scientist will only experience the world where he survives, because he obviously won’t have any conscious experiences in the worlds where he’s dead. Even if the experiment is repeated an arbitrary number of times, the scientist will only experience the world(s) where he survives.

From the point of view of the surviving copy (or copies), he’ll be immortal.

That’s the essence of the quantum immortality argument. You can’t experience the world-lines where you die, so you can expect to experience only the one where you survive – indefinitely.

Who Wants To Live Forever?

As a matter of fact, I do. Unfortunately, quantum immortality is unlikely to help me achieve eternal life. In fact, the concept is pretty much useless, except as a quaint “what if?” brain-tickler. Lets look at that quantum suicide thought experiment again :

Gun stock photo credit: Fastfood @ sxc.hu

If the many-worlds interpretation is correct, there will indeed always be at least one world-branch that avoids the fatal outcome, one copy of you that survives. Even if you get thrown into an active volcano, there will be an Everett branch where Harry Potter swoops in at the last moment and carries you to safety on his magical flying broomstick (depending on your cultural preferences, this may or may not be worse than being burnt alive by magma). The proponents of quantum immortality will point at that lucky branch and clap their hands giddily.

But – and here’s the kicker – you don’t get to choose in which Everett branch you end up. Yes, there may well be a world-line or two where you live forever, but what are the chances that you – this particular version of you, reading this blog post – are in that world-line? It would be extremely generous to say they’re rather slim.

As for the “you can only experience the worlds where you survive” thing, it’s just plain fallacious. It’s like a sales-person asking you “Will you be paying by check, cash, or credit card?” when you haven’t even decided if you actually want to buy their product. The question intentionally leaves out a valid option – not buying anything – and tries to make you pick one of the options most beneficial to the sales-person.

Similarly, the idea that you can only experience the Everett branches where you are alive incorrectly implies that being dead is somehow not a valid option. Unfortunately, it is. Magic aside, there is no overriding reason why a conscious creature couldn’t just… die, and thus stop being conscious (though that might change sometime this century).

Even if Everett was right and there are numerous alternate worlds, and in some of them a version of you is subjectively quantum-immortal, that doesn’t change your chances of survival one bit.