GPT5 on Everett's Many Worlds

[John Clark]

I asked GPT5, which just came out yesterday, the following question:

"How would Everett's Many World's idea explain the way the two slit experiment works and the key role that "which way" information plays in it?"

And this is what he she or it said: 

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Okay- let's put on our Everett hat and walk through the two-slit experiment step by step, with the “many worlds” interpretation (MWI) in mind.

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1. What Everett says about quantum stuff

In Everett’s picture, the wavefunction is real and never collapses.
Instead of “the particle chooses a path when you measure,” all possible outcomes actually happen — but in different, non-communicating branches of reality.

The key: the Schrödinger equation rules everything, all the time. No special “measurement” magic, just physics.

2. Two-slit without which-way info

Imagine you have a photon heading toward two slits.

• Before the slits:
  The photon’s wavefunction spreads so it has an amplitude to go through slit A and an amplitude to go through slit B.
  In MWI terms, there’s still just one “world,” but in that world the photon is in a superposition of both paths.

• After the slits:
  These two path components overlap and interfere, producing the interference pattern on the detection screen.
  Since there’s no device or environmental disturbance storing “which-way” info, the two parts of the wavefunction can still affect each other.

In MWI language: there is no branching here — both possibilities are still part of one combined world and can interfere.

3. Two-slit with which-way info

Now add a detector at the slits that becomes correlated with the photon’s path.

• If the photon goes through slit A → detector state changes to “A detected.”

• If through slit B → detector state changes to “B detected.”

Everett’s take:
This doesn’t collapse the photon’s wavefunction — instead, the photon + detector become entangled.
The total wavefunction now has two components:

Branch 1: Photon through A  + Detector says “A”
Branch 2: Photon through B  + Detector says “B”

These are different worlds: in one, the detector reads “A”; in the other, “B.”
Because the detector states are physically different and don’t overlap, the photon’s A-path wave and B-path wave can no longer interfere — they “live” in separate worlds.

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4. Why which-way info kills the interference

From the MWI perspective:

• No which-way info: The two paths are in the same world and can interfere.

• Which-way info exists (even if nobody looks at it): The environment has branched into distinct versions, each containing only one path. Interference requires the waves to be in the same branch, so once branching occurs, interference is gone.

This matches decoherence: the environment records path info, which makes branches orthogonal (no overlap → no interference).

5. The “Everett moral”

In Many Worlds, nothing ever really collapses. What we call “measurement” is just branching into non-communicating worlds.
The two-slit experiment is then:

• No measurement → no branching → interference visible.

• Measurement → branching → no interference between branches.

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Me: "Thanks GPT, you are the man! Or rather you are the AI.

GPT5: "Haha — I’ll take “the AI” as a compliment! Glad I could help make Everett’s worlds a bit clearer."

John K Clark

[Lawrence Crowell]

You have a dichotomy on the phenomenological level:

No measurement → no branching → interference visible.

Measurement → branching → no interference between branches (or not visible to the observer).

For one thing, without a measurement there is no observed interference. The interference is only inferred through many measurements of nearly identically prepared systems. 

If one holds to Kantian metaphysics, where there is a "thing in itself" or noumena and phenomena or "thing as we observe it," then one might argue that on the level of the noumena there is no change. Freeman Dyson compared this to a frog's eye perspective and the bird's eye perspective, where we are the frogs looking through a glass darkly and the bird is maybe to be compared to God. However, if the noumena is unobservable then this is metaphysical wordsmithing. If there is actually no interference between branches then this is a change that is much the same as what occurs with the collapse. The density matrix is reduced to a diagonal set of probabilities and by some unknowable reason nature produces only one of the probable outcomes. If this is only so on the phenomenological level, the noumena is then a sort of abstraction. Husserl eliminated the noumena and rejected this sort of dualism. 

I think that all of these ideas of quantum interpretations are just metaphysics disguised as physics.

LC

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[John Clark]

On Fri, Aug 8, 2025 at 9:54 AM Lawrence Crowell <goldenfield...@gmail.com> wrote:

> I think that all of these ideas of quantum interpretations are just metaphysics disguised as physics.

I suppose an argument could've been made that Many Worlds was an idea that was of philosophical interest but not scientific interest if it weren't for the fact that Many Worlds allows you to form a mental picture of what's responsible for some very bizarre (but not paradoxical) quantum behavior, and we know from history that forming a mental picture can and has made somebody a better scientist. 

Also, science and philosophy are related. Science is for answering questions but philosophy is for when you don't even know what the right questions to ask are. For example, at one time astronomy was considered to be part of philosophy, that was back when we thought one of the most important questions you could ask was "Why are there only seven planets, Mercury, Venus, Mars , Jupiter, Saturn, the Moon and the Sun?"  Today we know that is not an important question or even a question that needs an answer.

John K Clark

[Lawrence Crowell]

Other quantum interpretations do the same. I communicate with Ruth Kasner and she is devoted to the transactional interpretation. She will argue ardently hard as nails over it. I have also talked with Chris Fuchs over QuBism; the same thing. While these may provide a sort of classical-like picture of things, they are still really more metaphysics than physics. 

LC

 

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