Go further · Chapter 6 · 6b · A step deeper
What do QBism, relational, consistent histories, ensemble and superdeterminism say?
The short answer (tap to open)
Beyond the six main interpretations, five more have serious followings. QBism treats the wavefunction as a person's betting odds. Relational quantum mechanics says facts are relative to each system. Consistent histories assigns odds to whole histories. The ensemble view says the theory only describes many runs. Superdeterminism says the measurement settings and the particles share hidden links.
What are the three questions again?
- Does one outcome really happen, or do they all?
- If one happens, what picks it: real chance, or detail we can't see?
- Is the wavefunction a real thing, a summary of something deeper, or our knowledge?
We'll measure the same qubit as chapter 6: 70% "up", 30% "down".
What are the five more answers?
1 · 1 of 5
What does QBism say?
QBism
Christopher Fuchs, Rüdiger Schack, Carlton Caves · 2000s
"The wavefunction is your betting odds, updated by experience."
Where it came from
Christopher Fuchs, Rüdiger Schack and Carlton Caves developed it in the early 2000s, first as "quantum Bayesianism", now QBism. It builds on the idea that probability measures a person's degree of belief.
The idea
The wavefunction isn't a description of the world. It's a user's personal tool for setting expectations about their own future experiences.
Quantum theory, in this view, is a rulebook for making consistent bets about what you'll experience when you act on the world.
Measuring our 70/30 qubit, step by step
- You assign 70% to "up" and 30% to "down". These are your expectations, not a property of the qubit.
- You act on the qubit by measuring it, and you experience "up".
- You update your expectations, just as you'd update your odds after seeing a coin land. Nothing in the world collapsed.
Where the 70% comes from
The odds are your own beliefs. QBists rewrite the Born rule as a consistency condition: it tells you how your beliefs about different experiments must fit together.
What supporters like
- The measurement problem and "spooky action" disappear, because nothing physical jumps.
- It takes the experience of the person doing the experiment seriously.
Main objections
- It seems to make physics about agents and their beliefs rather than about the world.
- Critics ask what the world is like when nobody is betting, and say QBism says too little about that.
Where it stands today
A small but philosophically influential group. In the 2025 Nature survey it sits inside the 17% who chose epistemic or information-based views.
2 · 2 of 5
What does relational quantum mechanics say?
Relational
Carlo Rovelli · 1996
"Facts are real, but relative: a result exists for the system that interacted, not for everyone at once."
Where it came from
The Italian physicist Carlo Rovelli proposed it in 1996. He works on quantum gravity, and he took a cue from Einstein's relativity, where speed and time have no single answer independent of who measures them. His 2020 book Helgoland brought it to a wide audience.
The idea
There's no single, observer-independent state of the world. A quantum state describes how one system relates to another.
When two systems interact, a definite fact comes into being, but only relative to those systems. "Observer" here means any physical system at all, from a detector to a molecule. Nothing about it needs a mind.
Measuring our 70/30 qubit, step by step
- Relative to the detector, before they interact, the qubit has 70/30 odds.
- The qubit and the detector interact. Relative to the detector, the result is now a fact: "up".
- Relative to you, who haven't yet interacted with the detector, the qubit and detector are still a combined, undecided pair. When you look, "up" becomes a fact for you too, and it matches.
Where the 70% comes from
Relational quantum mechanics keeps the Born rule as given. It gives the odds of facts that will become real relative to the system doing the measuring.
What supporters like
- No collapse, no special observers, no extra worlds and no hidden detail.
- It treats quantum theory the way relativity treats time: the strangeness comes from assuming one absolute viewpoint.
Main objections
- If facts are relative, why do different observers always end up agreeing? Rovelli and Emily Adlam added a rule in 2022 to guarantee it, which critics see as a patch.
- Some worry it slides towards "nothing is real except from somewhere".
Where it stands today
A small but growing group, boosted by Rovelli's books. In a small poll at a 2011 foundations conference, published in 2013, 6% chose it.
The qubit: Not yet. The detector: Not yet. You: Not yet.
What the toy shows, in words
- Qubit and detector interact. Both now have the fact if either already had it, or if one of them is the qubit.
- You look at the detector. Both now have the fact if either already had it, or if one of them is the qubit.
- You measure the qubit yourself. Both now have the fact if either already had it, or if one of them is the qubit.
Every system with a fact has the same one. They always agree.
3 · 3 of 5
What do consistent histories say?
Consistent histories
Robert Griffiths 1984 · Roland Omnès · Murray Gell-Mann and James Hartle
"Quantum theory gives odds for whole histories, as long as you ask about them in a consistent way."
Where it came from
Robert Griffiths proposed it in 1984. Roland Omnès developed it, and Nobel laureate Murray Gell-Mann and James Hartle built their own version, "decoherent histories", around 1990, aiming to apply quantum theory to the whole universe.
The idea
Instead of asking about one moment, ask about whole histories: sequences of events, such as "the qubit was up, then the detector read up". Quantum theory gives each history a probability.
There's a catch. You can only combine histories that don't interfere with each other, a "consistent" set. Questions that mix incompatible framings get no answer, rather than a contradictory one. There's no collapse and no special role for measurement: a measurement is just one more event in a history.
Measuring our 70/30 qubit, step by step
- Choose a consistent framing: history A is "qubit up, detector reads up", history B is "qubit down, detector reads down".
- Quantum theory gives history A a probability of 70% and history B 30%.
- One history happens. The detector's record is part of it, and nothing had to collapse.
Where the 70% comes from
The standard Born rule, extended from single moments to whole sequences of events.
What supporters like
- No collapse and no observers, so it works even for the whole universe, where there's no one outside to measure.
- It dissolves many paradoxes by showing they mix framings that can't be combined.
Main objections
- There are many consistent framings, and they can't all be combined. Which one describes what really happened? Critics say the answer "it depends on the framing" dodges the question.
- It can feel like accounting rather than a picture of reality.
Where it stands today
A small group with distinguished founders. Its ideas about decoherence and histories influenced quantum cosmology and the Many-Worlds debate. The 2025 Nature survey counted it with Many-Worlds, in one 15% option.
Consistent or not?
Can be combined
Can't be combined
Upper path then A, with lower path then A, and no record of the path
The answers, in words
- Upper path then A, with lower path then A, and no record of the path: Can't be combined. No record, no consistent story of which path. That's chapter 3 and 4 again, in history form.
- Upper path then A, with lower path then A, with a path tag: Can be combined. The tag records the path, so the two histories leave different records and fit together.
- The photon hits A, with the photon hits B: Can be combined. Hitting A and hitting B leave different records, so they're a consistent pair of alternatives.
- Upper path then B, with lower path then B, and no record of the path: Can't be combined. Like the first card: with no record of the path, there's no consistent story of which path it took.
4 · 4 of 5
What does the ensemble view say?
Ensemble
Albert Einstein · Leslie Ballentine 1970
"The wavefunction describes many identical runs, not a single one."
Where it came from
Albert Einstein held a version of it from the 1920s: quantum theory works, but it describes collections of systems, not individual ones. The Canadian physicist Leslie Ballentine set it out in full in a 1970 review.
The idea
A wavefunction describes an ensemble: an imagined large collection of systems all prepared the same way. "70% up" means 70% of that collection will read up.
About a single run, the theory says nothing beyond the odds. The ensemble view treats that silence as honest, not as a gap to fill with worlds or collapses.
Measuring our 70/30 qubit, step by step
- Picture a thousand qubits, all prepared the same way.
- Measure each one.
- About 700 read up and 300 read down. For any one qubit, the theory only ever gave odds.
Where the 70% comes from
From the definition. Probabilities are frequencies in the ensemble. The view describes the Born rule rather than explaining it.
What supporters like
- It's modest: no collapse, no extra worlds, no hidden detail.
- It matches what labs actually test, which is always statistics over many runs.
Main objections
- Modern labs routinely track a single ion or a single photon over time. Doesn't the theory describe that one system?
- If each member of the collection has definite properties underneath, Bell's theorem (chapter 5) bites; if not, the puzzle about single runs remains. Critics say it postpones the question rather than answering it.
Where it stands today
Few physicists name it as their interpretation, but many work this way in practice. A small 2025 poll asked which view of quantum states people prefer. The largest group, 41%, chose "purely statistical".
5 · 5 of 5
What does superdeterminism say?
Superdeterminism
Gerard 't Hooft · Sabine Hossenfelder and Tim Palmer
"The particles and the measurement settings share hidden links from the past, so nothing needs dice."
Where it came from
John Bell himself named the loophole in the 1970s and 1980s and found it implausible. Nobel laureate Gerard 't Hooft has championed it since the 2000s, and Sabine Hossenfelder and Tim Palmer argued the case in 2020.
The idea
Chapter 5 listed three assumptions Bell's theorem says can't all hold. Superdeterminism drops the third: that each lab chooses its setting independently of how the source made the particles.
If the settings and the particles share a common past that correlates them, a local, fully deterministic theory could produce the quantum results. Nothing is random and nothing travels faster than light.
Measuring our 70/30 qubit, step by step
- The qubit carries hidden detail, and their shared history links that detail to the detector's setting.
- The result follows from the detail and the setting, with no chance involved.
- "Up" happens; across many runs the share of histories leading to "up" comes out at 70%.
Where the 70% comes from
From the spread of the underlying hidden states. There's no complete superdeterministic theory yet, so no derivation of the 70% exists.
What supporters like
- It keeps locality and determinism, which many physicists value highly.
- Hossenfelder has proposed tests: measure the same set-up again and again, fast, with small and very cold devices. Results might then be less random than quantum theory predicts.
Main objections
- Critics call it a conspiracy: the universe would have to arrange the experimenters' "free" choices, or the random number generators, to match the particles. Many argue that undermines how science tests anything.
- It's a loophole, not yet a theory with equations to check.
Where it stands today
A small minority, much discussed online. People also ask whether CSD is superdeterministic; CSD's own answer is on the CSD website. Is CSD superdeterministic?
Which assumption does each view give up?
- Results are set by local instructions or detailoff
- Nothing travels faster than lighton
- Settings are chosen independentlyon
Copenhagen: Results aren't written in advance, so there are no instructions to carry.
Every interpretation pays somewhere. They differ on where.
Which assumption each view gives up, in words
- Copenhagen: switch 1 off (results are set by local instructions or detail). Results aren't written in advance, so there are no instructions to carry.
- Many-Worlds: Gives up something before Bell's theorem applies. It drops "one outcome": every result happens, each in its own branch.
- Information-based: switch 1 off (results are set by local instructions or detail). The wavefunction is information about outcomes, not instructions the particles carry.
- Pilot wave: switch 2 off (nothing travels faster than light). Its guiding wave links distant particles instantly, though it can't send messages.
- Objective collapse: switch 2 off (nothing travels faster than light). A collapse changes the whole joint state at once, however far apart its parts.
- CSD: Quantum Without Dice: switch 1 off (results are set by local instructions or detail). Its outcomes depend on the whole measurement context, not on instructions fixed in advance.
- QBism: Gives up something before Bell's theorem applies. The wavefunction is your expectations, not the world, so nothing physical has to give.
- Relational: Gives up something before Bell's theorem applies. There's no single outcome for everyone: facts are relative to each system.
- Consistent histories: Gives up something before Bell's theorem applies. Only histories within one consistent framing get odds; there's no framing-free story to test.
- Ensemble: Gives up something before Bell's theorem applies. It says nothing about single runs, so it never meets the question.
- Superdeterminism: switch 3 off (settings are chosen independently). Settings and particles share hidden links from the past.
Every interpretation pays somewhere. They differ on where.
How do the five compare?
| Interpretation | One world? | What picks the result? | The wavefunction is… | Main objection |
|---|---|---|---|---|
| QBism | Yes | Nothing physical; your experience | Your beliefs | What is the world itself like? |
| Relational | Yes, but facts are relative | Chance | Relations between systems | Relative to what, ultimately? |
| Consistent histories | Yes, within a chosen framework | Chance | A tool for histories | Which framework is "true"? |
| Ensemble | Silent | Silent | Describes many runs, not one | Says nothing about single runs |
| Superdeterminism | Yes | Hidden detail, linked to the settings | A summary of something deeper | Undermines free choice of settings |
Which one are you?
1. After the measurement, is there one world, or one for each outcome?
2. Was "up" decided by real chance, or by detail we can't see?
3. The wavefunction is…
How each view answers the three questions
- Copenhagen: One world; Real chance; Our knowledge or information.
- Many-Worlds: One for each outcome; Detail we can't see; A real thing in the world.
- Information-based: One world; Real chance; Our knowledge or information.
- Pilot wave: One world; Detail we can't see; A real thing in the world.
- Objective collapse: One world; Real chance; A real thing in the world.
- CSD: Quantum Without Dice: One world; Detail we can't see; A summary of something deeper.
- QBism: One world; Real chance; Our knowledge or information.
- Relational: One world; Real chance; Our knowledge or information.
- Consistent histories: One world; Real chance; Our knowledge or information.
- Ensemble: One world; Real chance; A summary of something deeper.
- Superdeterminism: One world; Detail we can't see; A summary of something deeper.
Check
Two quick ones
Collect
Your aha card
Every interpretation pays somewhere.
Bell's theorem and the measurement problem force each view to give something up. They differ on what.