# MWI universal wave function question?

In MWI, there is only one wave function in the universe ( the universal wave function). So say if I detected a particle and its wave function decoheres, since the universal wave function never collapses are there other parts of the universe that can still interfere? Is this a stupid question?

In MWI, there is only one wave function in the universe ( the universal wave function). So say if I detected a particle and its wave function decoheres, since the universal wave function never collapses are there other parts of the universe that can still interfere? Is this a stupid question?

In the MWI, information spreads locally. So if system A in region 1 decoheres, system B in region 2 may still undergo interference.

More broadly, parallel universes are an approximation in the MWI that breaks down in interference experiments and sometimes also for macroscopic systems. See 'The Beginning of Infinity' and 'The Fabric of Reality' by David Deutsch for popular accounts and for more technical accounts see

https://arxiv.org/abs/quant-ph/0107144

https://arxiv.org/abs/quant-ph/0104033

• Ok, so according to MWI, say if I had an entangled pair of photons. I detect the position of photon 1 and it decoheres into different position states. What happens to photon 2? Does that photon also decohere as well or no?
– user86072
Commented Mar 1, 2017 at 16:09
• If photons 1 and 2 have entangled positions, then they can't exhibit interference wrt position. However, if you measure photon 1's position and it is separated from photon 2, that measurement has no effect on photon 2. So photon 2 is not decohered by anything you do to photon 1. However, photon 1 may have locally inaccessible information about photon 2 that could in principle be returned to photon 2 which would allow photon 2 to undergo interference wrt position. Once you have measured photon 1 you can no longer do that experiment. Technical acccount arxiv.org/abs/quant-ph/9906007 Commented Mar 2, 2017 at 8:41
• What do you mean by entangled positions? What if they are shot off into different directions?
– user86072
Commented Mar 2, 2017 at 16:53