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

Relational Quantum Mechanics

“Facts are relative to the witness.”

There is no view from nowhere. This is the theory that stopped looking for one.

Before the date, you ask around. One friend says he's the sweetest guy she's ever met. Another says intense — kind of exhausting, actually. A third says: honestly? A mess. Then you meet him, and you get a fourth person. Yours.

So which one is he really? Wrong question. Nobody has ever met him-in-general — every version was earned face to face, each one is exactly who he is with that person, and there's no referee version standing behind them keeping the official file. He isn't hiding behind the versions. The versions are all there is.

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

There's no view from nowhere. Relational quantum mechanics says things don't have properties, full stop — they have properties *relative to whatever they've interacted with*. The cat is definitely asleep relative to the box. Relative to the hallway, there's no fact about the cat yet. Neither description is the "real" one, because there's no cosmic ledger keeping the official version. The universe isn't a warehouse of objects. It's a web of meetings.

What it fixes

The measurement problem dissolves — a "measurement" is just an interaction, and any interaction counts. Mushrooms qualify. No special observers, no collapse, no extra universes.

What it costs

When two people compare notes, their comparison is just another interaction — with its own relative facts. There's no bottom ledger. Ever. Whether that's the deepest insight on this page or a very elegant dodge is genuinely unsettled — and if you're honest, the fact that it's unsettled is exactly what you like about it.

The story

Physics has pulled this move before. Velocity feels like a property a thing just has — until relativity points out it's a relation: you have a velocity relative to something, and there's no absolute answer, and the sky doesn't fall. In 1996 Carlo Rovelli — a quantum-gravity physicist by trade — proposed that every physical property works like velocity. Not just speed: position, spin, being-alive-or-dead. All of it relational, all of it facts-for somebody, none of it facts, period.

Run the cat through it. Inside the box, the interaction has happened: relative to the box, the cat is definitely asleep. Relative to the hallway, no interaction has happened yet, so there is genuinely no fact about the cat — not a hidden fact, no fact. When the hallway opens the door, the hallway gets its own fact, and the accounts always turn out consistent when compared. No contradiction ever arrives, because a contradiction would need a master ledger where both entries sit side by side — and there is no master ledger.

What does "measurement" mean here? Nothing special: any interaction. A photon bouncing counts. A mushroom counts. Observers aren't privileged, consciousness is irrelevant, no collapse pulse rolls across the cosmos. The [[wavefunction]] isn't a thing that lives in the world; it's bookkeeping for one system's information about another. Quantum mechanics stops being a theory of objects and becomes a theory of meetings.

The cost is the regress. When two observers compare notes, the comparison is itself just another interaction, with its own relative facts — it's relations all the way down, and anyone looking for solid ground never finds it. Whether that's the deepest lesson quantum mechanics has been trying to teach us for a century, or a beautiful way of never answering the question, is genuinely unsettled. In 2019 it stopped being armchair-only: photonic [[wigners-friend]] experiments produced two observers with irreconcilable accounts — and quantum mechanics endorsed both.

The argument

“This is just relativism with equations.”: It's relativity with smaller things. You already accept that velocity has no absolute value and you don't call that relativism — you call it physics being honest about what's measurable.
“When we compare notes, we always agree. Doesn't that prove there's one shared reality?”: It proves the theory guarantees consistent meetings — which it does, every time. You've never once checked agreement outside an interaction, because there is no outside. Try to describe one.
“Facts that aren't facts for everyone aren't facts.”: Said the hallway to the box. Both of your ledgers are perfect. There was just never a third one.

The experiment

Photonic Wigner's-friend tests · 2019-2020

In 2019 this stopped being pure philosophy: a photonic version of the "Wigner's friend" thought experiment created two observers with genuinely conflicting accounts of the same event — and quantum mechanics sided with both of them. Facts that refuse to be absolute, demonstrated on a lab bench. You were saying this at parties years ago.

Proietti et al., and successors

The books

Helgoland by Carlo Rovelli — written with such warmth you'll briefly forget the missing ledger. Relative to you, it resolves everything.

Your neighbors

QBism — your closest kin — but they say only agents get a ledger; you let the mushrooms in.
Copenhagen — also dropped the view from nowhere — then closed the question you keep open.

← 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.