so... quantum immortality is weird. but if we take it seriously, we get a curious result:
there could be large slices of probability space - results of quantum interactions - that we never witness, because they explode so fiercely they destroy the universe, and we're not around any more to see it.
yes, crazy. but still possible.
so the things you can witness are not the things that actually happen. they're the things that happen and leave a you around to notice.
(this has a name. observation selection. i didn't invent it. i just want to point it somewhere new.)
now. the double slit.
if we're in an ensemble of universes, and the photons merely fan out after the slit, then when we measure the pattern it should not have stripes. naively it should be simple addition. so what simple mechanism could explain those dark regions?
think about what a dark stripe actually is. it's a spot that gets less with two slits open than it got with one. you give the particle a second way to arrive, and fewer of them show up.
that's nuts.
because ordinary probability cannot do that. probability only adds. two ways to happen makes a thing more likely, never less. you cannot pile up positive chances and carve out a hole.
so a dark stripe isn't a probability. it's a subtraction. as Aaronson said, quantum mechanics is "probability theory with minus signs."
which is the question i actually care about, plainly: how can you get probability to work like complex numbers - to subtract - out of normal probability, which only adds?
here's my answer.
observation selection does it. selection removes, and removing makes deficits. take a pile of ordinary positive outcomes, every one a plain "this happened," and cull some of them. the spot you culled now has less than it started with. less than the sum of its parts. and if the culling is keyed to the two slits, a spot can end up with fewer survivors than one slit alone would have left.
darker with two slits than with one. out of nothing but positive pieces and a rule about which ones get removed.
that's your minus sign. it isn't in the ingredients. it falls out of who's left.
so a dark stripe isn't a place the photon avoids. the reason it's dark is that the observer who saw a photon land there no longer exists. you read the screen from inside a branch that survived - so your screen has holes exactly where the survivors aren't. the holes look like cancellation. they're really just absence.