According to Everett, the wave equation never collapses: it always applies. But not just to the particle; it applies to the combined system of particle and observing apparatus. This means an experimental observation "puts your mind into two states at once … so that at the end of it, there will be two different versions of you. Each subjectively feeling just as real as the other, but completely unaware of each other's existence." It's as if the universe repeatedly splits in two, resulting in innumerable copies of you in parallel worlds
To the objection we don't feel such a truth, Tegmark counters that we don't feel like we're travelling in space at thousands of kilometres a second, but we are !!