# Many worlds and entanglement

1. Sep 30, 2015

### Erik Ayer

How does many worlds reconcile with entanglement? Is it just that in one world the particles were up/down, and in another world they were down/up?

Last edited by a moderator: Sep 30, 2015
2. Sep 30, 2015

### Staff: Mentor

That's pretty much it, but be aware that it's not just entanglement that's handled this way, but all superpositions.

Prepare a spin-1/2 particle in the spin-up state along the vertical axis (this state is a superposition of horizontal spin left and horizontal spin right), pass it through a Stern-Gerlach device oriented along the vertical axis and the particle is still in the spin-up state. Pass a similarly prepared particle through a Stern-Gerlach device oriented horizontally and you have a 50% probability of getting spin-left and a 50% probability of getting spin-right, as you'd expect in from the superposition.

The MWI crowd would say that in the latter case the world split into a two worlds, one in which we measured spin-left and the other in which we measured spin-right, and we found ourselves in one or the other. The collapse interpretation crowd would say that there's only one world and the superposed state collapsed into either spin-left or spin-right when the measurement was made.

You'll notice that, although the words they use to describe it are different, both have a "measurement problem".