Peter Lewis is a Professor of Philosophy at the University of Miami. He graciously provided the following answers as part of the accompanying article on quantum immortality.
In Young's experiment, why doesn't the box qualify as an observer? Isn't it observing the photons and recording their passage through the slits as well?
I assume we’re talking about the context of the many-worlds theory here (seeing as we’re talking about quantum immortality). So what counts as an observation (or a measurement) in the many-worlds theory? Just when the state of one system (the observing system) becomes correlated with the state of another (the observed system). Nothing in the two-slit apparatus becomes correlated with which slit each photon passes through. If it did – if there were a photon detector attached to one slit – then the interference pattern at the screen would go away.
How different would our world be if quantum immortality is true, or if it was widely believed to be true?
I don’t think much would change. If David Lewis’s description of quantum immortality is right, then risky behavior is still risky, it’s just that the nature of the risk has to be described somewhat differently. You know there’s no possibility of subjective death – you will survive through a living successor after the risky event. But the chances are you won’t emerge unscathed – for most kinds of risks, most of your living successors will be maimed in some way. And your friends and relatives will probably see you die. So to cook up a case where qunatum immortality makes a difference in behavior, we have to think of a case where you don’t care about the physical state of your living successors, or about the grief of your fiends and relatives. That’s hard to do.