The interpretations so far share one assumption: the wavefunction describes something real out in the world, whether a wave, a guide or a branching universe. A different family of views starts by questioning that. Perhaps the wavefunction is not a thing at all, but a record of information or a tool for reasoning. These views have roots in Copenhagen, but they have been developed in new and precise ways since the 1990s.

QBism

QBism, pronounced "cubism," began as "quantum Bayesianism." It was developed in the early 2000s by Christopher Fuchs, Carlton Caves and Rüdiger Schack. It takes its name from the Bayesian view of probability, named after the eighteenth century minister Thomas Bayes, in which a probability expresses how strongly someone expects something rather than a property of the world.

QBism applies that view to quantum theory. Quantum probabilities are an agent's personal degrees of belief, and the wavefunction is a tool an agent uses to organize expectations about their own future experiences. Two physicists can assign different wavefunctions to the same system if they have different information, and neither need be wrong.

On this view, the measurement problem dissolves. Collapse is simply the agent updating their beliefs after a new experience, just as a weather forecaster revises a forecast after seeing the sky. Nothing physical happens to the system when the wavefunction changes, so nothing travels faster than light when an entangled partner is measured far away. The physicist David Mermin, once known for urging colleagues to stop worrying and calculate, became one of QBism's best known supporters.

The case for QBism and its critics

Defenders say QBism removes paradoxes rather than adding new machinery. It needs no hidden particles, no branching worlds and no new constants. It also puts the person who uses science back into the picture, reminding us that probabilities guide real decisions by real people.

Critics reply that QBism seems to turn physics into a theory about agents rather than about the world. If the wavefunction only records beliefs, why do those beliefs work so well? Some charge it with solipsism, the view that only one's own mind is certain to exist. QBists deny this firmly. They hold that the world is real and shows itself by pushing back on us with surprises we cannot control. Critics also ask what counts as an agent. A physicist, surely; but a cat, a bacterium or a computer? QBism's answer here is less settled.

Relational quantum mechanics

In 1996 the Italian physicist Carlo Rovelli proposed relational quantum mechanics. His starting point was an analogy with relativity. In Einstein's theory, a speed is not a property of an object alone; it is a speed relative to something else. Rovelli suggested that quantum facts work the same way. The properties of a system are not absolute but are relative to another system it interacts with. An electron does not simply have a spin; it has a spin relative to the detector that measured it.

A crucial difference from QBism is that the relational view does not need agents or minds: any physical system, even a rock, can play the role of the observer. Facts are created in interactions between systems, whether or not anyone is aware of them.

Defenders like the view's economy: it uses the standard theory, adds no new laws and avoids both collapse and many worlds. Critics raise one main question: if facts are relative, it is unclear how different observers' facts come to agree. Rovelli answers that they agree once they interact and compare notes, which is itself a physical process. Some philosophers found this vague, and in 2022 Rovelli and the philosopher Emily Adlam added a new principle intended to guarantee that observers who compare notes always find their records match. Whether this fix fits the spirit of the original idea is still debated.

Worked example: Wigner's friend revisited

Return to Wigner's friend from Level 1. A friend inside a sealed lab measures an electron's spin and sees "up." Wigner, outside, describes the lab as a superposition.

A QBist says there is no contradiction because there were never two descriptions of one objective state. The friend's wavefunction encodes the friend's expectations; Wigner's encodes Wigner's. Each is a personal guide to future experience. When Wigner opens the door and asks, he has a new experience and updates.

A relational theorist says the spin is "up" relative to the friend and has no definite value yet relative to Wigner. Both statements are true, just as a train can be moving relative to the platform and still relative to a passenger. When Wigner interacts with the lab, a fact relative to him is created, and it will match the friend's report.

In both cases the puzzle is solved by denying that there is a single, observer independent answer to the question "what is the spin?" Whether that is a deep insight or too high a price is exactly where defenders and critics part ways.