Here is an explanation of Alexander Franklin's paper, "Incoherent? No, Just Decoherent," translated into simple, everyday language with creative analogies.
The Big Picture: The "Many Worlds" Problem
Imagine you are watching a movie. In the standard version of quantum mechanics (the rules that govern tiny particles), the movie has a glitch: the main character seems to be in two places at once until someone looks at them, at which point they "snap" into one place. This is called the "measurement problem."
The Everett Interpretation (or "Many Worlds") says there is no glitch and no snapping. Instead, the universe is like a massive tree. Every time a quantum event happens, the tree branches. In one branch, the character is in the kitchen; in another, they are in the garden. Both are real.
The Problem: Critics (like Baker, Dawid, and Thébault) say this idea is "incoherent" or circular. They argue:
"You can't say the other branches are 'real' but ignore them unless you have a rule to say they are 'unlikely.' But to say they are 'unlikely,' you need a rule for probability. But to get that rule, you need to assume the branches already exist. It's a circle!"
Franklin's Solution: He says, "Stop worrying about probability for a second. Let's look at how these worlds actually separate." He argues that the "Many Worlds" don't need a probability rule to exist; they just need a physical process called Decoherence.
The Core Concept: "Screening Off" (The Noise-Canceling Headphones)
Franklin uses a specific definition of "emergence" (how big things come from small things). He says something "emerges" when it becomes independent of the tiny details underneath it.
The Analogy: The Bouncy Ball
Imagine a bouncy ball.
- The Micro Level: The ball is made of trillions of atoms. Each atom is jiggling, vibrating, and bouncing around chaotically.
- The Macro Level: The ball as a whole bounces up and down.
Franklin argues that once the ball hits the ground, the exact position of every single atom doesn't matter anymore for predicting where the ball will go next. The "jiggling" of the atoms is screened off. The ball's bounce follows its own simple rules (gravity, elasticity) that ignore the chaos of the atoms. The ball has "emerged" as its own thing.
Applying this to the Universe:
In the quantum world, the "jiggling" is interference. This is when different versions of reality (branches) try to talk to each other, creating a messy superposition.
- Decoherence is the process where the environment (air, light, dust) interacts with the system and acts like a giant noise-canceling headphone.
- It cancels out the "interference" between the branches.
- Once the interference is canceled, the branches stop talking to each other. They become independent.
- Because they are independent, they can be treated as separate, real "worlds" with their own rules (classical physics), even though they are all made of the same quantum stuff.
The Case Study: Hyperion, the Wobbly Moon
To prove this isn't just philosophy, Franklin looks at a real object: Hyperion, a moon of Saturn.
- The Situation: Hyperion is shaped like a potato. It tumbles chaotically in space.
- The Quantum Prediction: If Hyperion were isolated in a vacuum with no outside interference, quantum mechanics says it would eventually become a "superposition" of tumbling in every possible direction at once. It would be a fuzzy, wobbly mess.
- The Reality: We see Hyperion tumbling in a specific, chaotic way. It looks like a normal potato moon.
- The Reason: Hyperion is constantly bombarded by sunlight (photons). This interaction causes decoherence.
- The Result: The sunlight "screens off" the quantum weirdness. The interference terms (the parts that would make it wobble everywhere at once) become so small they effectively vanish. The moon is forced into a single, classical, chaotic path.
Why this matters: This proves that "worlds" (or in this case, a single, stable reality) can emerge from quantum mechanics without needing to calculate probabilities first. The physics of the interaction (the sunlight) does the work of separating the worlds.
Addressing the Critics: The "Circularity" Trap
The critics say: "You can't ignore the tiny interference terms unless you say they are 'improbable.' But you can't say they are improbable without the Born Rule (a probability formula). And you can't prove the Born Rule without assuming the worlds exist. It's a circle!"
Franklin breaks the circle with a simple shift in perspective:
It's not about "Improbability," it's about "Irrelevance."
Imagine you are trying to predict the path of a hurricane. You don't need to know the position of every single water molecule. You don't ignore them because they are "unlikely" to be there; you ignore them because they are dynamically irrelevant. Their tiny movements don't change the path of the storm.The "Born Rule" is a Tool, not a Prerequisite.
Franklin argues that in the context of decoherence, the math we use (which looks like the probability rule) is actually just a measure of dynamical weight. It tells us which parts of the wavefunction are strong enough to drive the future, and which are too weak to matter.- We don't need to say "This branch is unlikely to happen."
- We just say "This branch is too weak to affect the outcome."
The "Hyperion" Proof.
Since we can observe Hyperion's orbit and see that it follows classical chaos (not quantum fuzziness), we have empirical evidence that decoherence works. We don't need to assume the probability rules to see that the "other branches" have been silenced by the environment. The evidence comes from the observation of the moon, not from a philosophical calculation of odds.
The "Primitive Ontology" Objection (Maudlin's Worry)
Another critic, Maudlin, asks: "If the universe is just a giant wavefunction, where are the actual objects? Where are the chairs and moons? You need 'stuff' to make a world."
Franklin's Reply:
Think of a Whirlpool in a river.
- Is the whirlpool "real"? Yes.
- Is it a new kind of "stuff" added to the water? No. It's just the water moving in a specific pattern.
- Does the whirlpool have its own rules? Yes (it spins, it pulls things in).
Franklin argues that objects (like Hyperion) are like whirlpools. They are stable patterns in the quantum wavefunction. They don't need to be "fundamental stuff" added on top. They emerge because the pattern is stable and independent (screened off) from the rest of the water. We don't need to find "quantum atoms" to explain the moon; the moon is the pattern that has emerged.
Summary: The Takeaway
- The Multiverse is Real (but Emergent): The "Many Worlds" aren't magic; they are like distinct patterns in a complex system, similar to how a whirlpool is a distinct pattern in a river.
- Decoherence is the Mechanism: Interaction with the environment (like sunlight hitting a moon) acts like a filter. It silences the "noise" (interference) between different versions of reality, allowing them to become separate, independent worlds.
- No Circular Logic Needed: We don't need to assume probability rules to prove these worlds exist. We just need to observe that the "noise" is silenced, leaving behind a stable, classical reality (like Hyperion's orbit).
- The Verdict: The Everett interpretation is not incoherent. It is a respectable scientific theory where "worlds" emerge naturally from the math, just as "temperature" or "fluidity" emerges from the motion of atoms.
In short: The universe isn't a messy quantum soup where everything is connected. It's a soup where the ingredients have settled into distinct layers. We live in one of those layers, and the physics of "decoherence" explains exactly how we got there without needing to solve the mystery of probability first.