GPT5 explains quantum eraser and the delayed choice experiments in terms of Many Worlds
John Clark
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:
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The two branches aren’t fully “macroscopically different.”
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The experimenter still has coherent control over the differences.
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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:
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Every branching point is when quantum states differ in some way.
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Big branches with tangled, permanent differences never touch again.
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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.
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John K Clark See what's on my new list at Extropolis
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Will Steinberg
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John Clark
> You should really set some memories to prevent sycophancy. Starting its response with “Yes — that makes a lot of sense” is a bad sign. If you want access to full intelligence, it’s best to set as many restrictions on sycophancy as is possible.
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Will Steinberg
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