# How does MWI deal with the destruction of interference?

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How does MWI deal with the destruction of interference in the double slit experiment when a detector is placed at the slits? Since the wave function never collapses, and the universe doesn't actually split in MWI, how does the interference go away? Does the measuring device at the slits reshape the wave function in a way that makes it impossible to interfere with itself?

PeterDonis
Mentor
2020 Award
How does MWI deal with the destruction of interference in the double slit experiment when a detector is placed at the slits?
In the MWI, the wave function branches when the particle traversing the experiment reaches the detector. In one branch, the detector fires, and in the other, it doesn't. The state of the particle, the detector, and everything else in the universe in each branch is the appropriate one for the detector result (fire/not fire) for that branch.

how does the interference go away?
It goes away in each branch because the state of everything in each branch has to be consistent with the detector result for that branch.

Does the measuring device at the slits reshape the wave function in a way that makes it impossible to interfere with itself?
In the MWI, the particle by itself doesn't have a wave function. Only the entire system does. The presence of the detector at the slits changes the way the wave function of the entire system evolves, yes, because it changes the interactions that are present.

In the MWI, the wave function branches when the particle traversing the experiment reaches the detector. In one branch, the detector fires, and in the other, it doesn't. The state of the particle, the detector, and everything else in the universe in each branch is the appropriate one for the detector result (fire/not fire) for that branch.

It goes away in each branch because the state of everything in each branch has to be consistent with the detector result for that branch.

In the MWI, the particle by itself doesn't have a wave function. Only the entire system does. The presence of the detector at the slits changes the way the wave function of the entire system evolves, yes, because it changes the interactions that are present.
But in MWI, the universe doesn't actually branch/split off into new universes, that's a misconception. The universe already exists in a superposition, where all possible universes already exist.

PeterDonis
Mentor
2020 Award
in MWI, the universe doesn't actually branch/split off into new universes, that's a misconception
If that terminology bothers you, just say the wave function of the overall system branches. The physics is the same either way.

The universe already exists in a superposition, where all possible universes already exist.
If you're going to use this terminology, the set of "all possible universes" changes with time: it is different after the particle has passed the detectors at the slits because it now includes branches for the different possible measurement results at the detectors. So you could say the set of "all possible universes" is what branches. You're still just describing the same physics in different words.

If that terminology bothers you, just say the wave function of the overall system branches. The physics is the same either way.

If you're going to use this terminology, the set of "all possible universes" changes with time: it is different after the particle has passed the detectors at the slits because it now includes branches for the different possible measurement results at the detectors. So you could say the set of "all possible universes" is what branches. You're still just describing the same physics in different words.
Oh, I think I miss understood you then. Are you saying that when a measurement is made the universe splits and creates new universes?

PeterDonis
Mentor
2020 Award
Are you saying that when a measurement is made the universe splits and creates new universes?
No. I'm saying that the various descriptions you have suggested are not different possible ways things could be; they're just different ordinary language descriptions of the same physics, the same way things are (according to the MWI).