Hello David,

David Barrett-Lennard wrote:

Please note that my understanding of QM is rather lame...  Doesn't MWI
require some interaction between branches in order to explain things
like interference patterns in the two slit experiment?  What does this
mean for the concept of "identity"?

- David


There is a technical difference between interference and interaction.

Interaction refers to two or more particles influencing each other through the exchange of force. Only particles within the same universe (within the broader multiverse) may interact with each other in this way.

These particles are represented by wavefunctions in quantum mechanics, which have wavy properties like amplitude and wavelength, and so can exhibit interference just like waves on a pond. Also just like waves on a pond, particle wavefunctions can pass through each other, even annihilating completely in some places, without interacting (i.e. without exchanging force).

Typically in single-particle experiments like Young's double slits, there is no interaction, and the interference arises from the sum of all the different trajectories (or worlds if you like) that the particle may have taken.

In experiments involing two or more particles, frequently every possible path of each particle and every possible interaction must be considered as a separate world. Interference then takes place between these possible worlds, and must be taken into account in order to correctly make statistical predictions of how the particle system will behave.

So in answer to your question, no, the MWI does not require interaction between branches to explain interference. Indeed interaction (exchange of force) is prohibited by the linearity of the Schroedinger Wave Equation (SWE), which indicates that its different possible solutions (universes) should move through each other as easily as ripples through a pond. We can only see the interference when we're not interacting with the rippling system. Once we do, the rippling system expands to include us within its folds. From that point on, there are multiple versions of us, each experiencing a different ripple, completely unable to interact with the other versions of ourselves moving through us all the time.

Hope this helps,

Matt.

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When God plays dice with the Universe, He throws every number at once...

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