[bobm_db]

Something I don't quite understand about quantum mechanics and the MWI... At the point the atom passes through the diffraction grating, it has a quantum superposition. According to MWI, reality has "branched", right?

Then the two realities interfere with each other to form the probabilistic pattern at the detector. So, according to MWI, reality has "merged" again?

If we accept the mulitiverse idea of MWI, doesn't that mean at some point particles aren't able to marge with their branched-reality versions?

[evanb]

The "at some point" doesn't have to do with time. It has to do with the rest of the wavefunction of the universe. If the atom becomes entangled with everything else (in fact, a single quantum-mechanical spin suffices) the two branches become orthogonal and the interference terms from 'merging' vanish. We say the wavefunction 'decohered' when it becomes too complicated to see how to reverse this effect---as more and more particles get involved it becomes hopeless.