In 1957 a Princeton graduate student named Hugh Everett III, working under the physicist John Wheeler, proposed the boldest answer to the measurement problem. What if there is no second rule at all? What if the smooth equation is the whole story, applied to everything, including detectors, scientists and the universe itself?

Everett's idea

Everett called his approach the "relative state" formulation. He proposed that the wavefunction never collapses: the smooth Schrödinger equation is the complete law, with no exceptions for measurement. Follow that rule through a measurement and something striking happens. The detector becomes entangled with the system, recording "up" in one part of the wavefunction and "down" in another. The scientist who reads the detector becomes entangled too. On Everett's view, each possible outcome ends up paired with a version of the observer who sees that outcome, so every outcome occurs, each in its own branch. Each version of the scientist sees one definite result and has no access to the others. From the inside, it looks exactly as if collapse happened.

Everett's work was largely ignored at first. Bohr's circle was cool toward it, and Everett left physics to work as a defense analyst; he died in 1982. The name "many worlds" was popularized in the 1970s by the physicist Bryce DeWitt, who championed the idea and brought it to a wide audience. Today its leading defenders include David Deutsch, David Wallace, Simon Saunders and Sean Carroll.

The case for many worlds

Its main appeal is simplicity of law: it takes the standard mathematics literally and adds nothing to it. There is no collapse rule, no special role for observers, no hidden particles and no new constants of nature. Physicists already accept the Schrödinger equation; many worlds simply refuses to switch it off when measurements happen. It also keeps the theory local in an important sense and applies cleanly to the whole universe, where there is no outside observer.

The obvious objection is that it multiplies worlds beyond reason. Defenders answer that the theory is economical in its laws, even if it is lavish in what exists. Science has made this move before: when astronomers learned that the stars are suns, the universe grew vastly bigger, but the laws stayed simple. The branches are not added by hand; they are already present in the mathematics every physicist uses.

The problem of probability

The hardest objection concerns probability. Quantum mechanics says that some outcomes are more likely than others, and experiments confirm these odds precisely. But if every outcome happens, it is unclear what it means to say that one outcome had a 90 percent chance. Simply counting branches does not work, since a 90 percent outcome and a 10 percent outcome would each get one branch.

Deutsch in 1999, and later Wallace, offered an answer using the theory of rational decision making. They argued that a rational agent who believes many worlds should weight each branch by the standard quantum rule when deciding what to expect or how to bet. Defenders think this solves the problem. Critics reply that the argument quietly assumes what it set out to prove, or that caring about weights is not the same as the weights being probabilities. The debate is lively and unresolved.

Other criticisms

A second problem asks what defines a branch. The wavefunction can be split up mathematically in endlessly many ways; why do branches correspond to cats alive and dead rather than to strange blends? Modern defenders answer with decoherence, the process covered in Level 7, which makes certain divisions stable. Critics note that this makes the number of worlds vague rather than a definite count. Finally, some argue that a theory whose other branches can never be observed has wandered beyond science, though defenders reply that the branches are predictions of a well tested equation, not extra assumptions.

Worked example: does anything matter?

Imagine a doctor deciding whether to give a risky treatment. Someone who has just learned about many worlds might reason: "In some branch the patient lives and in some branch the patient dies, whatever I choose. So my choice makes no difference." Is that right?

Defenders of many worlds say no, and the reason is instructive. The theory does not say that every outcome gets equal weight. Choices still change how much of the total weight goes to branches where things go well. A careful doctor raises the share of branches in which the patient recovers, just as, on a single world view, a careful doctor raises the odds. Wallace and others argue that rational action under many worlds looks exactly like rational action under ordinary uncertainty.

So the theory gives no license for carelessness, and the people in every branch are as real and as owed care as you are. What the example shows is that interpretations, even when they agree on all predictions, can change how people picture their own choices, which is one reason it matters to get them right.