Double slit

Most Copenhagen readings treat the wave function as a tool for computing probabilities rather than a thing in the world — though how far Bohr himself went that way is still argued over, and there was no single Copenhagen doctrine until Heisenberg named one in 1955. Between source and screen there is only the calculus of amplitudes; reality enters at measurement, and the theory is deliberately silent about anything more. Bohr insisted the experiment must be described half classically — the slits and the screen belong to our world, the electron to the quantum one — with the boundary set by what the instrument has to do rather than by the observer's choice. The freely movable cut belongs more to Heisenberg and von Neumann.

Take the equation literally and never add anything: superpositions never resolve, they spread. Decoherence explains why branches stop interfering and why each one looks like a single classical world from inside. The one dot you see is not selected — it is indexed. All the others are seen too, by other versions of you. The hard part is probability: if everything happens, what does “a 5% chance” even mean?

De Broglie proposed it in 1927; Bohm completed it in 1952. Particles have positions; the wave function is a physical field that guides them. Measurement is nothing special — just particles moving. The catch is that the guidance is instantaneously sensitive to distant events: the nonlocality Bell later proved unavoidable is written on the theory's face rather than hidden.

GRW-type models change the equation itself: each particle suffers rare, random localization events. Entanglement makes them contagious — one hit localizes a whole apparatus. Size, not consciousness, draws the quantum–classical line. Because the dynamics differ from standard quantum mechanics, these models are testable in principle: precision interference experiments with ever-larger molecules keep tightening the allowed collapse rates.

QBism treats quantum states the way a Bayesian treats probabilities: as personal degrees of belief, exquisitely disciplined by the quantum formalism. Two agents may honestly assign different states to the same electron. The measurement problem dissolves — nothing physical collapses when beliefs update. What remains is a live question: what is the world like, that it forces agents to bet this way and no other?

Rovelli's move is to treat every system as an observer of the systems it interacts with. Variables take values only in interactions, and a value for one observer need not yet exist for another — like simultaneity in relativity, definiteness is relative. Collapse never happens absolutely; it happens for someone. The bookkeeping stays consistent because whenever two observers compare notes, that comparison is itself an interaction.

Griffiths, Omnès, Gell-Mann and Hartle recast quantum mechanics as rules for assigning probabilities to families of histories. A family counts as consistent when its histories don't interfere, so ordinary probability rules apply. Measurement devices appear as physical systems inside histories, not as special acts from outside. The unsettled question is famous: many incompatible families are equally consistent, and the formalism itself never says which one describes what actually happened.