If we return to the good old up-down spin, then if you measure the spin of a particle to see if it’s up or down the universe would split into two new universe , one in which you found out that the spin was down and one where it was up.
This means that every different situation that can happen, happens but in different universes.
This creates something known as the Tegmark quantum suicide experiment. In it there’s two persons and they have one gun(sounds like two typical american 8 year olds to me) connected to a apparatus which measures the spin of a particle, if the particles spin is up then a shot is fired and if it’s down it doesn’t. Then one of puts the gun to the other ones head and pull the trigger. For the first couple of times the spin might be measured to be down and no shot is fired.
But sooner or later a shot will be fired. At that point the universe splits and two different outcomes happen:
- To the person pulling the trigger the spin was measured up and for the person which shot the other one, the other one of course dies.
- For the person which gets shoot the gun doesn’t ever fire, the spin will be always be measure down. So to him they all continue to live happily but their friendship might have been placed under strain.
This is because at each measurement the universe splits. And the person being in front of the barrel always continuse to live in one of the universes, since both spin outcomes happens.
It explains the interference patter by saying that it’s created, not by one particle in our universe, but many particles each in different universes interfering with each other. The problem is that for the interpretation to hold we can’t be able to communicate or travel into the parallel universes, they would only interfere indirectly with our universe via e.g. the double slit experiment, so we can’t detect parallel universes.