The deep dive · one of eight answers to the same experiment
Spontaneous Collapse
“Put up or shut up.”
The only theory on this site an experiment can kill. It's proud of that.
One popcorn kernel with the patience of a saint: left to itself, it pops maybe once in a hundred million years. You could watch it your whole life and see nothing. Now the bag: a trillion trillion kernels — and in this bag there's a rule: when one pops, they all pop.
"Almost never, each" times a trillion trillion equals "instantly, somewhere." The bag can't stay quiet for a single heartbeat. One kernel gets to sit there undecided forever. A bagful is forced to commit before the previews end. Small things get to stay fuzzy. Big things pop.
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
Everyone else on this quiz is interpreting the equation. This theory *edits* it. Add one tiny ingredient: every particle, at random, very rarely, snaps to a definite position — about once per hundred million years per particle. A lone atom basically never snaps. A cat is a trillion trillion particles, so a cat snaps instantly, always, everywhere. Small things get to stay fuzzy; big things are forced to be definite. The measurement problem just... becomes ordinary physics.
What it fixes
Definite outcomes with no observers, no branching, no philosophy seminar. A mechanism you could point at.
What it costs
It predicts new physics that has never once been seen — and the experiments keep coming back with polite nothing, shrinking the territory where the snap could be hiding. This is the only theory on the quiz that can be killed by data. You're proud of that. You should be. It might still get killed.
The story
In 1986, in Trieste, Giancarlo Ghirardi, Alberto Rimini, and Tullio Weber got tired of the interpretation wars and did the rudest possible thing: they changed the equation. One added ingredient — every particle, at random, very rarely, spontaneously snaps to a definite position. How rarely? About once per hundred million years, per particle. For a lone atom, that's never. Nothing in any atomic experiment would show it.
The trick is [[entanglement]]. In a measuring device or a cat, particles are linked — snap one and the whole configuration snaps with it. A cat has around 10²⁷ particles, so "once per hundred million years, each" becomes "within a microsecond, somewhere" — and one snap localizes the lot. Small systems keep their quantum fuzz; big systems are forced definite by sheer headcount. The [[measurement-problem]] doesn't get interpreted away. It gets solved by a mechanism with two new constants of nature in it.
John Bell — hard to impress — wrote about GRW with real warmth: at last, he said, someone had replaced the vague word "measurement" with actual physics. And because it's actual physics, it's different physics: GRW makes predictions that differ, slightly, from standard quantum mechanics. Interference should fail for objects above a certain size. The snaps should very faintly heat things up, and should make matter emit a whisper of radiation that standard theory forbids.
So the hunt is on, and it's real. Giant-molecule interferometers keep pushing the size ceiling up without finding it. In 2020, a detector deep under the Gran Sasso mountain listened for the forbidden whisper of X-rays from a slab of germanium — and heard silence, wiping out a large slice of the theory's parameter space. Proposed space experiments would squeeze most of what's left. The theory now lives in a shrinking box, which its fans will tell you, with genuine pride, is exactly what an honest theory's box should do. One day the box closes empty — or something snaps in it, and every other page on this site gets rewritten.
The argument
- “You invented new physics nobody has ever observed.”
- Yes. That's called a prediction. The others on this site invented universes, cosmic conspiracies, or a permanent ban on asking — and none of it can ever be checked. Mine can. Keep shooting.
- “The null results keep coming. Your theory is dying.”
- My theory is being tested. Learn the difference and you'll understand why I sleep well.
- “The two new constants are suspiciously convenient — tuned so the snap hides between atoms and cats.”
- Convenient is another word for "where the data allows." The window's still open. It's just — measurably — closing. I'll be here for the verdict either way. Will you?
The experiment
Underground collapse hunts, and their null results · 2020
The snap should leave fingerprints — atoms glowing faintly for no reason. In 2020, a detector buried under an Italian mountain listened for that glow. Silence. A big slice of the theory's hiding room, gone. Your theory is being actively hunted right now, and you wouldn't have it any other way.
Donadi et al.; the parameter space keeps shrinking
The books
Beyond Weird by Philip Ball — rigorous, fair, allergic to hype. You'll feel seen, then audited.
Your neighbors
Pilot Wave — the other mechanism-lover — but their machinery hides in principle; yours glows in the dark, faintly, catchably.
Many-Worlds — your opposite number — they deleted the collapse; you made it real enough to shoot at.
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.
- amplitude
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A complex number whose squared magnitude gives a probability.
Amplitudes are what makes quantum mechanics quantum. Because they are complex, they can cancel — two ways of reaching the same outcome can add up to no chance of it happening at all. Classical probabilities can never do this.
See also: Born rule, interference
- Born rule
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The recipe that turns amplitudes into probabilities: probability equals the amplitude's magnitude squared.
Max Born added it in a footnote in 1926 and won a Nobel Prize for the footnote. Every interpretation has to reproduce it, and several struggle to explain why it holds rather than simply assuming it.
See also: amplitude
- superposition
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A state that is a combination of other states, with amplitudes attached.
Commonly mangled as "being in two places at once." More accurately: the system is in one perfectly definite state, which happens not to be a state of definite position (or spin, or whatever you are about to measure). The indefiniteness is relative to the question you ask.
See also: eigenstate, interference
- eigenstate
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A state with a definite value for some particular observable.
A state is only ever an eigenstate with respect to a specific question. Definite momentum means wildly indefinite position, and vice versa. There is no state that is definite about everything.
See also: superposition
- interference
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Amplitudes for different paths adding or cancelling, producing fringes.
The double slit is the canonical case: close one slit and the bright band at a given spot can appear, open both and it can vanish. Adding a second way for something to happen made it stop happening. No probability theory built on ordinary numbers does this.
See also: amplitude, decoherence
- the measurement problem
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Unitary evolution never produces a single definite outcome, yet we only ever see one.
The Schrödinger equation is linear and deterministic, so a measuring device interacting with a superposition should end up in a superposition of readings. It doesn't — you see one number. Every interpretation on this quiz is, at bottom, a different answer to this one problem.
See also: unitary, collapse, decoherence
- collapse
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The postulated jump from a superposition to a single definite outcome on measurement.
In textbook quantum mechanics it is simply an extra rule bolted alongside the Schrödinger equation, with no account of when it applies or what counts as a measurement. Interpretations either explain it, deny it happens, or make it a real physical process with its own dynamics.
See also: the measurement problem, spontaneous collapse
- decoherence
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Interaction with the environment rapidly destroying interference between branches.
Real and experimentally confirmed, and it explains why you never see a superposed cat. But it does not by itself solve the measurement problem, however often it is claimed to: it explains why the branches stop interfering, not why you end up in exactly one of them.
See also: the measurement problem, interference
- unitary
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Evolution that is reversible and preserves total probability — the Schrödinger equation.
Unitary dynamics never destroys information and never singles out an outcome. An interpretation that insists dynamics is always unitary (Many-Worlds) must therefore explain single outcomes some other way; one that admits non-unitary collapse must say when and why.
See also: the measurement problem, collapse
- ontic
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About what exists, independently of anyone's knowledge.
An ontic reading of the wavefunction says ψ is a real physical thing, as much a part of the furniture of the world as a field. Pilot Wave and spontaneous collapse are ontic about ψ.
See also: epistemic, the PBR theorem
- epistemic
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About what someone knows, rather than about the world itself.
An epistemic reading says ψ encodes an observer's information, so "collapse" is just updating your beliefs — no more mysterious than a probability changing when you look at a card. QBism takes this furthest. The PBR theorem is the main obstacle in its path.
See also: ontic, the PBR theorem, QBism
- locality
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Nothing here is influenced by a choice made far away, faster than light could carry the news.
Give this up and you can keep definite pre-existing properties. Keep it and you must give up something else. This is the central trade in the whole subject.
See also: Bell's theorem, entanglement
- entanglement
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A joint state of two systems that cannot be written as one state for each.
The pair has a definite state while neither member does. Measuring one instantly fixes what the other will give — but you cannot use it to send a message, because each side alone sees nothing but noise until the results are compared.
See also: Bell's theorem, locality
- Bell's theorem
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No theory that is both local and assigns pre-existing values can reproduce quantum predictions.
John Bell showed in 1964 that such theories obey an inequality that quantum mechanics violates. Experiments — Aspect, then loophole-free tests in 2015, Nobel Prize in 2022 — come down on quantum mechanics' side. It is the closest thing here to a settled result, and it is what forces every interpretation to give something up.
See also: locality, hidden variables, superdeterminism
- contextuality
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An outcome can depend on what else you chose to measure alongside it.
The Kochen–Specker theorem (1967) shows you cannot consistently assign definite values to all observables at once, independently of context. Bell rules out locality plus definite values; this rules out context-independence as well, even setting distance aside.
See also: Bell's theorem, hidden variables
- the PBR theorem
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Under modest assumptions, the wavefunction cannot be merely information about a deeper real state.
Pusey, Barrett and Rudolph (2012) showed that if systems have real underlying states and independently prepared systems are independent, then ψ must be ontic. Escaping it means denying one of those assumptions — which is exactly what QBism does, by denying there is an underlying state to be ignorant of.
- no-go theorem
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A proof that a whole class of theories cannot reproduce quantum mechanics.
Bell, Kochen–Specker, PBR and Frauchiger–Renner are the big ones. Together they have closed every exit marked "common sense." This is why no interpretation is sane: sanity was ruled out, and all that remains is choosing which strangeness you prefer.
See also: Bell's theorem, contextuality, the PBR theorem, Wigner's friend
- Wigner's friend
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A thought experiment where an observer is themselves in superposition, as seen by someone outside.
The friend, inside a sealed lab, sees a definite outcome. Wigner, outside, describes the whole lab — friend included — as superposed. Frauchiger and Renner sharpened this in 2018 into an outright contradiction: you cannot keep universal unitarity, single outcomes and observer agreement all at once.
See also: the measurement problem, no-go theorem
- pilot wave
-
Particles always have definite positions, guided by a real physical wave.
De Broglie proposed it in 1927, Bohm rediscovered it in 1952. It is deterministic, it has no measurement problem, and the double slit becomes an ordinary mechanism. The price is explicit nonlocality — the guiding equation depends instantly on the whole configuration.
See also: hidden variables, locality
- QBism
-
Quantum states are an agent's personal degrees of belief, not descriptions of the world.
Short for Quantum Bayesianism. A wavefunction is your betting position; collapse is you updating on experience. It dissolves the measurement problem at the cost of denying that quantum mechanics describes anything observer-independent at all.
See also: epistemic, the PBR theorem
- spontaneous collapse
-
Collapse is a real physical process that happens at random, all by itself.
Ghirardi, Rimini and Weber (1986) added a tiny random collapse term to the dynamics. One particle collapses about once every hundred million years; a cat, containing about 10²⁷ of them, collapses immediately. Uniquely among these, it is a different theory from quantum mechanics and experiments are closing in on it.
- superdeterminism
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Denying that experimenters' choices are independent of the system being measured.
Bell's theorem quietly assumes the settings you choose are uncorrelated with the hidden variables. Drop that assumption and locality survives. The cost is that the correlation must have been arranged at the beginning of the universe, and that no experiment can ever be a fair test of anything.
See also: Bell's theorem, hidden variables