Undergrad Many worlds and interference

[JuneSpring25]

Hello,

Looking for some help on this one!

As I understand it, most many world interpretations explain the interference after the slit experiment as weak interference between seperate universes / branches before they decohere (I know not everyone finds it helpful to use the term 'universes' but it's probably helpful shorthand here).

I understand decoherence prevents further interection between the two branches after the slit experiment on a macroscopic level - eg the different observers will not detect the particles in the other world because the interaction from particles in the macroscopic structures (such as an observer or measuring device) in their own branch / universe will counteract any slight intereference from the other branches / universes.

However, why don't we see the effects of weak interference from other universes in other sensitive expeirments, why just the aftermath of the slit experiment?

I understand there is no (or less) inteference when the electron is measured before entering the slit and I think MWI interprets this as being because the electron is no longer so well alligned with the other universes after being measured.

But surely there must be an astonishing number of universes by now and many of them will overlap significantly. If the unmeasured electron leaving the slit can be jostled by electrons in other universes enough to create an interference pattern, I would have thought we would see an interference pattern for the electron that has been measured before leaving the slit, simply because there must be electrons flying in all direct within other branches / universes? There could be billions of other universes / branches where the slit experiment is taking place in a slightly different order so that the elctrons allign or in the case of a photon, there is multiple universes with a light source where the experimental equipment is in our universe so the photon would be bombarded with photons from other branches / universes. I would have thought these other branches / unvierses would have a small effect of the trajectory of the particle on some detectable level.

I have the same question with the Many Interacting Worlds Theory cited here: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.4.041013

I'm not a scientist, please explain this to me like I'm twelve years old!!

 

[PeterDonis]

most many world interpretations explain the interference after the slit experiment as weak interference between seperate universes / branches before they decohere

No, they don't. There is no branching before decoherence occurs, and decoherence does not occur at the slits. It only occurs at the detector screen.

 

[JuneSpring25]

No, they don't. There is no branching before decoherence occurs, and decoherence does not occur at the slits. It only occurs at the detector screen.

Ah, thank you. So how do MWI explain the interference pattern?

 

[PeterDonis]

how do MWI explain the interference pattern?

As the standard evolution of the wave function prior to decoherence. All interpretations explain it that way.

Where interpretations differ is on what happens at the detector screen. In an interpretation that says measurements have single results, there is just one interference pattern on the detector screen, the one that is observed.

In the MWI, however, there are multiple decohered branches of the wave function ("worlds"), each one with an interference pattern that, while it looks the same at a high level, resolves into a slightly different set of impact points for the particles in the experiment (photons if the experiment is done using light). That is because, as I said, the decoherence happens at the detector screen, so that is where the different worlds branch.

 

[Fra]

As I understand it, most many world interpretations explain the interference after the slit experiment as weak interference between seperate universes
...
I'm not a scientist, please explain this to me like I'm twelve years old!!

Explanation means you explain something in terms of something else using some inference rules; that you accept as more natural than what is to be explained.

But we have difference preferenses for the natural starting points.

I suppose one can call initial state + hamiltoninan evolution => final state and explanation, but the quantum state space and its hamiltoninan, still is unexplained.

As per my preferences of explanatory value none of the mainstream interpretations "explain" neither the structure of non-commutative features of quantum states nore its dynamical flow rules in terms of samething that is more satisfactory. For me this is at heart of the level of Feymanns quote "nobody understands quantum mechanics".

The problem with understanding all this in terms of classical wave interferences, as the heuristics of the wave particle duality, is that these waves are not "real" in the classical sense at least not without getting into new and bigger problems. So it REALLY does not "explain" anything IMO, it more a heuristic or suggestive value only.

/Fredrik