Many-worlds interpretation
Explanation
The many-worlds interpretation was proposed by Hugh Everett III in 1957 as a radical alternative to the Copenhagen interpretation. Everett was uncomfortable with collapse: it seemed arbitrary to him to posit an extra process, not described by the Schrödinger equation, that intruded at certain special moments called measurements. His proposal was to remove it altogether: the Schrödinger equation describes the entire evolution of the universe, and there is no collapse.
If there is no collapse, then when an observer measures a system in superposition, the observer himself ends up in a superposition of having seen each of the possible outcomes. From outside, the total universe (system + apparatus + observer + environment) remains a single unitary wave function. From inside, each branch experiences a definite outcome and is unaware of the others. Bryce DeWitt and others popularised this idea with the image of a universe that continually branches into many parallel worlds.
For consciousness, this interpretation has surprising consequences. If every measurement produces branches, each with its own observer, then your conscious flow, lived as unique, would be one branch among many. Other versions of yourself live, with full experience, all the outcomes you did not see. This raises deep questions about personal identity, moral responsibility and the sense of self: am I just this branch, or am I, in some way, the whole family of branches?
The interpretation has considerable technical virtues. It is mathematically parsimonious (only the Schrödinger equation), deterministic (without fundamental randomness), local (no non-local collapses) and avoids any mention of special observers. For many theoretical physicists, especially in quantum cosmology and foundations, those virtues make it preferable to alternatives with collapse or extra ontologies. Authors such as David Deutsch and Sean Carroll publicly defend it.
The criticisms are numerous. Ontologically, it seems extravagant: a universe multiplying itself every microsecond into countless branches. Epistemically, it is hard to give meaning to probability: if all results occur in some branch, in what sense is one more probable than another? Several approaches (Deutsch-Wallace, self-locating approaches) try to answer this, but the debate remains open. Phenomenologically, it is hard to see why we experience only one branch if all are equally real.
In relation to consciousness, the many-worlds interpretation invites us to think of it in an unusual way. The conscious self would not be a solid entity in a single world, but a kind of thread in a giant tree of possible quantum histories. Although it is not a theory of mind proper, it forces any theory of consciousness to say something about personal identity in a branching cosmos. Even if rejected as a literal description, it works as a powerful conceptual laboratory on subjectivity and metaphysics.
Strengths
- Eliminates the collapse problem.
- Compatible with the linearity of quantum mechanics.
- Elegant mathematical articulation.
- Influential in quantum cosmology.
Main critiques
- Massive ontological multiplication.
- The Born-weights problem not entirely resolved.
- Hard to square with the experience of personal identity.
- Lack of clarity about when consciousness 'branches'.