The deep dive · one of eight answers to the same experiment

Many-Worlds

“Everything happens.”

Take the math at its word and the price is everything else. The interpretation for people who trust the equation over their own feelings — all the way down.

The train doors are closing. In one cut she slips through; in the other she's half a second late and watches it pull away.

Ten years later she's in a kitchen in Tokyo, laughing: "If I'd missed that train, none of this happens."

Ten years later she's on a rooftop in Barcelona, laughing: "If I'd caught that train, none of this happens."

Nobody's lying. Nobody's dreaming. Both mornings happened, both lives got lived, and each of her is certain she's the only one telling the story. The doors didn't choose. The doors did both.

Now shrink it.

The experiment that started all the trouble

Fire tiny particles at a wall with two slits, one at a time, with a screen behind it. One at a time matters: there's only ever one particle in flight. Common sense says each one goes through one slit or the other, and you get two piles. You don't get two piles. You get stripes — the pattern waves make when they ripple through both slits and overlap. So you put a detector at the slits to catch which one each particle uses. The stripes vanish. Two piles.

Everything on this page is one theory's answer to a single question: what, in the name of God, is going on between the gun and the screen?

Drag to orbit — two fingers on a touchscreen. Particles leave one at a time and land one at a time. The middle is deliberately not drawn — what happens there is the whole argument, and the lens is this theory's answer.

The idea

One equation runs the whole quantum world, and nothing in it ever says "collapse." Many-Worlds just takes that seriously. When an experiment could go two ways, the equation says both happen — so both do. The universe branches. You branch with it. The you reading this sentence is one of the versions, and every result this quiz could have given you was given, somewhere.

What it fixes

Everything, using nothing. No collapse, no special role for "observers," not one symbol added to the math. It's the cheapest interpretation ever proposed — measured in assumptions.

What it costs

Measured in universes, it's the most expensive idea in the history of ideas. Also: if everything happens, what did "30% chance" ever mean? Everettians have answers. They have *several* answers. That's the problem.

The story

In 1957 a Princeton PhD student named Hugh Everett III asked the forbidden question in its simplest form: what happens if we just delete the collapse rule? Keep the equation — the [[unitary]], smoothly-evolving Schrödinger equation — and add nothing. His answer: the equation already describes a measurement perfectly well. The measuring device ends up correlated with both outcomes. So does the physicist reading it. The mathematics doesn't describe one result happening; it describes both, each with an observer who saw it cleanly. Everett's move was to say: believe that.

Copenhagen's old guard was not amused. Bohr's circle dismissed the thesis; Everett got the message, finished a heavily trimmed version, and walked out of physics into Pentagon war-gaming. He died in 1982, at fifty-one, decades before his idea became respectable. It got its name — "many worlds" — from Bryce DeWitt, who championed it in the 1970s when almost nobody else would.

The modern version is more careful than the science-fiction picture. Branches aren't parallel universes that "split" with a pop; they're parts of one wavefunction that [[decoherence]] has made permanently unable to interact — practically separate histories, all equally real, all embedded in the same math. Nothing collapses, nothing is added, observers aren't special: you're just one more physical system that ends up on every branch, and each version of you finds a single definite past in their memory.

The honest open wound is probability. If every outcome happens, what were you saying when you said "30% chance"? The Oxford Everettians — David Deutsch, David Wallace — have a sophisticated answer involving how a rational agent should bet across their own branches. Others have different answers. The critics' point isn't that there's no answer; it's that there are several, and they don't agree, and for a theory whose whole sales pitch is "we added nothing," needing this much philosophy at checkout stings.

The argument

“An infinity of unobservable universes is the least parsimonious idea ever proposed.”: Count rules, not universes. I have one postulate and it's the equation you already believe. You're the one adding a collapse mechanism nobody has ever written down properly.
“It's untestable — the other branches can't be observed even in principle.”: I'm not a separate theory; I'm quantum mechanics, taken literally. Every interference experiment that keeps working on bigger objects is a test I keep passing. Find a real collapse and I'm dead. People are looking. That's testable enough.
“If everything happens, choices don't matter.”: The version of you that chooses well still lives a different life than the one that doesn't — and the weights aren't decoration. You're not off the hook. You're on every hook.

The experiment

Interference of 25,000-dalton molecules · 2019

The "both at once" trick was supposed to be for tiny things. Then it worked on atoms. Then molecules. In 2019 it worked on molecules of around two thousand atoms each — big enough to almost see — and the wave behavior showed up right on schedule. Every year the trick works on something bigger, and nothing in the math says it stops before it reaches you.

Fein et al., Nature Physics

The books

Something Deeply Hidden by Sean Carroll — the case for taking the equation at its word, written by the person at the party you'd actually enjoy arguing with. Then David Wallace's The Emergent Multiverse when you're ready to go professional.

Your neighbors

Pilot Wave — your fellow wave-realist — kept the wave, added a particle, skipped the branching.
Superdeterminism — the other fully deterministic picture — one history instead of all of them.

← all theories

Glossary 24 terms

wavefunction

The mathematical object that assigns a complex number to every possible configuration of a system.

Written ψ. It is not a wave in space like a ripple on a pond — for two particles it lives in a six-dimensional configuration space, not in the room. Whether ψ is a real physical thing or a bookkeeping device for what you know is precisely what the interpretations argue about.