When universes are in phase they are coherent, you can move back and forward through them. But we have decoupled from them and vibrate at a frequency that no longer interferes with them. It’s like a radio tuning into different stations – we can listen to radio 1, but not radio 2 or radio Moscow at the same time, even though these frequencies are all there in our room. It’s because your radio is decoherent! It does not vibrate in unison with these other frequencies unless tuned in. It’s the same with quantum physics. We consist of atoms – and our atoms vibrate. But they do not vibrate in unison with these other universes – we have decoupled from them – we have decohered from them. We cannot enter these parallel universes because we are not vibrating in unison with them.
Notes taken from a talk by Michio Kaku – theoretical physicist