Superposition
What is a system doing before you look?
Spread quantum state: superposition amount 100 percent, amplitudes for both outcomes. Probability 50 percent for zero, 50 percent for one.
Explained by
Copenhagen
The orthodox viewFor Copenhagen, the two humps are not two things in the world — they are weights in a forecast. The system does not have a value yet; “which one?” becomes a meaningful question only when a measurement ends the sentence.
Many Worlds
Everything happensThe amplitudes are physically there, both of them. Turning the superposition up is not mixing in ignorance — it is widening the part of reality that will branch when a measurement couples you to it.
Pilot Wave
Particles, guidedUnderneath the amplitudes, the particle already sits at one definite value; the wave carries both humps and steers it. The sliders shape the wave. The particle's answer is fixed before you ask.
Objective Collapse
Collapse is physicsA small, isolated system can genuinely hold both amplitudes — spontaneous collapse events are far too rare to matter. Scale the system up and the new dynamics bites: large superpositions destroy themselves. That, on this view, is why you never see a spread-out chair.
QBism
Probability is personalThe sliders are about you: they set your odds for what you will experience on looking. A wider superposition is not a wider object — it is a more even bet.
Relational
Facts are relativeRelative to you, the system is undecided; relative to something it has already interacted with, there may be a fact. Superposition is not absolute — it is a statement about which relations exist so far.
Consistent Histories
Histories, not measurementsAmplitude questions and value questions belong to different frameworks. In one family the humps interfere; in another the value is definite from the start. Consistency, not observation, decides which questions may be combined.
Key assumptions
- The wave function is a predictive tool, not an object
- Measurement is a special, unanalysed act
- A classical world of labs and screens is assumed, not derived
What it gets right
- Matches every experiment with the least machinery
- No extra worlds, particles, or equations
- The working physicist's default for a century
What it gets questioned on
- Never says what counts as a measurement
- Puts a movable, unexplained cut through reality
- Not one doctrine — Bohr and Heisenberg never fully agreed
Key assumptions
- The wave function is physically real — and is all there is
- The equation holds always, with no exceptions for observers
- Branching structure emerges through decoherence
What it gets right
- No collapse, no special role for measurement
- Fully deterministic at the level of the whole
- Removes a rule rather than adding equations, supporters argue
What it gets questioned on
- An enormous inventory of worlds no one can see
- The Born rule must be argued for, not assumed
- What exactly counts as one branch?
Key assumptions
- Particles with definite positions at all times
- A real guiding wave obeying the usual equation
- An initial statistical distribution (quantum equilibrium)
What it gets right
- A single definite world — no collapse, no branching
- Observers appear nowhere in the laws
- Recovers the non-relativistic predictions exactly
What it gets questioned on
- Openly nonlocal — awkward with special relativity
- Only position is definite; spin and the rest are contextual
- Extending it to quantum field theory is hard work
Key assumptions
- The wave function is a physical object
- The standard equation is slightly, stochastically wrong
- Collapse rate grows with the number of constituent particles
What it gets right
- Observers play no role at all
- Draws the quantum–classical boundary quantitatively
- Makes predictions that experiments can chase
What it gets questioned on
- The wave function never quite vanishes — the problem of tails
- Standard versions heat matter slowly — later variants patch this
- Each null experiment shrinks its living room
Key assumptions
- Probabilities are personal degrees of belief
- A measurement is an agent's action on the world
- The formalism is a normative rule for betting, not a map
What it gets right
- The measurement problem simply dissolves
- No nonlocal mechanism — updating isn't physics
- Takes the probabilistic core of the theory seriously
What it gets questioned on
- Seems to abandon the third-person picture science expects
- The world between agents is left undescribed
- Critics hear solipsism; QBists insist it is not
Key assumptions
- States describe relations, not intrinsic properties
- Every physical system can serve as an observer
- Facts are complete only relative to an interaction
What it gets right
- No collapse, no extra worlds, no new dynamics
- Dissolves paradoxes by forbidding the view from nowhere
- Kinship with the spirit of relativity
What it gets questioned on
- A world of many partial facts takes getting used to
- Comparing observers' accounts needs delicate care
- Critics ask: relations between what, exactly?
Key assumptions
- Probabilities attach to histories, not measurement outcomes
- Only mutually consistent families of histories may be combined
- The formalism applies to closed systems, observers included
What it gets right
- Measurement loses its special status entirely
- Applies to the whole universe — no outside observer needed
- Stays close to the standard formalism, adding no new dynamics
What it gets questioned on
- No rule selects which framework to use
- Different frameworks license incompatible descriptions
- Critics ask what, if anything, actually happens