A lot of leaning on metaphor there. By what measure is going thru the
left slit only a "small" difference from going thru the right slit?
Brent
On 8/8/2025 12:47 PM, John Clark wrote:
*I had this conversation with Mr. GPT5:*
*Me: With regards to quantum erasure and the delayed choice
experimentit seems to me that in Many Worlds a split happens when
there is a difference; and normally the universes will never coalesce
again because it's astronomically unlikely they will ever become
identical again, but if the difference between 2 worlds is very small,
like the only difference being that an electron went through the left
slit rather than the right, and you don't wait so long that other
effects might change them, then a skilled experimenter might be able
to coax those worlds to coalesce back into just one world. And that
could be a way of visualizing things. Does this make any sense?*
*
*
*And the following is what Mr. GPT5 said to me:*
*================*
*
*
Yes — that makes *a lot* of sense, and you’ve actually hit on one of
the deep intuitions behind how Many Worlds plus decoherence works.
------------------------------------------------------------------------
*1. In MWI, branching is difference*
Exactly: in Everett’s picture, a “split” between worlds happens when
two parts of the wavefunction become /orthogonal/ — meaning they’re
different enough that they can’t interfere.
In real life, this orthogonality happens because the paths get
entangled with a /lot/ of environmental degrees of freedom (air
molecules, photons, detectors…). That’s decoherence.
Once the states are orthogonal, they evolve separately, like two
parallel novels whose plotlines never intersect again.
*2. Why they almost never recombine*
Normally, once a difference leaks into the environment, it spreads
like gossip — trillions of particles get correlated with the outcome.
The odds of those trillions of correlations just /happening/ to come
back into perfect agreement is astronomically small — essentially
never in the age of the universe.
That’s why in our everyday world, branching is for all practical
purposes irreversible.
------------------------------------------------------------------------
*3. Your “small difference” case*
If the *only* difference between two branches is something microscopic
— like /“electron took left slit”/ vs /“electron took right slit”/ —
and you’ve kept the rest of the universe perfectly isolated from that
fact, then:
*
The two branches aren’t fully “macroscopically different.”
*
The experimenter still has coherent control over the differences.
*
By cleverly manipulating the system (like in a quantum eraser),
they can bring the two states back into /exactly the same overall
condition/ — which means they’re no longer separate worlds, but
one again.
That’s exactly the kind of controlled “world merging” a quantum eraser
performs. It’s only possible before decoherence has spread the
difference to the environment.
------------------------------------------------------------------------
*4. How to visualize it*
Think of the world as a giant branching tree:
*
Every branching point is when quantum states differ in some way.
*
Big branches with tangled, permanent differences never touch again.
*
Tiny twigs that split but haven’t drifted far apart can, with
care, be grafted back together.
The quantum eraser is like a *world-surgeon* who snips two twigs
before they grow into separate trees and splices them back into one.
------------------------------------------------------------------------
*5. The moral*
Your description — “worlds split when there is a difference, and can
only re-coalesce if the difference is small and hasn’t spread” — is a
perfect Many Worlds visualization of the quantum eraser.
It captures both why /most/ splits are permanent and why some can be
undone in delicate lab conditions.
===========
*John K Clark See what's on my new list at Extropolis
<https://groups.google.com/g/extropolis>*
vxz
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