Outline
Although several versions of MWI have been proposed since Hugh Everett's original work,[1] they contain one key idea: the equations of physics that model the time evolution of systems without embedded observers are sufficient for modelling systems which do contain observers; in particular there is no observation-triggered wavefunction collapse which the Copenhagen interpretation proposes. The exact form of the quantum dynamics modelled, be it the non-relativistic Schrödinger equation, relativistic quantum field theory or some form of quantum gravity or string theory, does not alter the content of MWI since MWI is a metatheory applicable to all quantum theories and hence to all credible fundamental theories of physics. MWI's main conclusion is that the universe (or multiverse in this context) is composed of a quantum superposition of very many, possibly infinitely many, increasingly divergent, non-communicating parallel universes or quantum worlds.
The idea of MWI originated in Hugh Everett's Princeton Ph.D. thesis "The Theory of the Universal Wavefunction",[5] developed under his thesis advisor John Archibald Wheeler, a shorter summary of which was published in 1957 entitled "Relative State Formulation of Quantum Mechanics" (Wheeler contributed the title "relative state";[9] Everett originally called his approach the "Correlation Interpretation"). The phrase "many worlds" is due to Bryce DeWitt,[5] who was responsible for the wider popularisation of Everett's theory, which had been largely ignored for the first decade after publication. DeWitt's phrase "many-worlds" has become so much more popular than Everett's "Universal Wavefunction" or Everett-Wheeler's "Relative State Formulation" that many forget that this is only a difference of terminology; the content of all three papers is the same.
The many-worlds interpretation shares many similarities with later, other "post-Everett" interpretations of quantum mechanics which also use decoherence to explain the process of measurement or wavefunction collapse. MWI treats the other histories or worlds as real since it regards the universal wavefunction as the "basic physical entity"[10] or "the fundamental entity, obeying at all times a determinstic wave equation".[11] The other decoherent interpretations, such as many histories, consistent histories, the Existential Interpretation etc, either regard the extra quantum worlds as metaphorical in some sense, or are agnostic about their reality; it is sometimes hard to distinguish between the different varieties. MWI is distinguished by two qualities: it assumes realism, which it assigns to the wavefunction, and it has the minimal formal structure possible, rejecting any hidden variables, quantum potential, any form of a collapse postulate (i.e. Copenhagenism) or mental postulates (such as the many-minds interpretation makes).
Many worlds is often referred to as a theory, rather than just an interpretation, by those who propose that many worlds can make testable predictions or that all the other, non-MWI interpretations, are inconsistent, illogical or unscientific in their handling of measurements; Hugh Everett argued that his formulation was a metatheory, since it made statements about other interpretations of quantum theory; that it was the "only completely coherent approach to explaining both the contents of quantum mechanics and the appearance of the world"[1].
[edit] Wavefunction collapse and the problem of interpretation
As with the other interpretations of quantum mechanics, the many-worlds interpretation is motivated by behavior that can be illustrated by the double-slit experiment. When particles of light (or anything else) are passed through the double slit, a calculation assuming wave-like behavior of light is needed to identify where the particles are likely to be observed. Yet when the particles are observed in this experiment, they appear as particles (i.e. at definite places) and not as non-localized waves.
The Copenhagen interpretation of quantum mechanics proposed a process of "collapse" in which an indeterminate quantum system would probabilistically collapse down onto, or select, just one determinate outcome to "explain" this phenomenon of observation. Wavefunction collapse was widely regarded as artificial and ad-hoc, so an alternative interpretation in which the behavior of measurement could be understood from more fundamental physical principles was considered desirable.
Everett's Ph.D. work provided such an alternative interpretation. Everett noted that for a composite system (for example that formed by a particle interacting with a measuring apparatus, or more generally by a subject (the "observer") observing an object (the "observed" system)) the statement that a subsystem (i.e. the observer or the observed) has a well-defined state is meaningless -- in modern parlance the subsystem states have become entangled -- we can only specify the state of one subsystem relative to the state of the other subsystem, i.e. the state of the observer and the observed are correlated. This led Everett to derive from the unitary, deterministic dynamics alone (i.e. without assuming wavefunction collapse) the notion of a relativity of states of one subsystem relative to another.
Everett noticed that the unitary, deterministic dynamics alone decreed that after an observation is made each element of the quantum superposition of the combined subject-object wavefunction contains two relative states: a "collapsed" object state and an associated observer who has observed the same collapsed outcome; what the observer sees and the state of the object are correlated. The subsequent evolution of each pair of relative subject-object states proceeds with complete indifference as to the presence or absence of the other elements, as if wavefunction collapse has occurred, which has the consequence that later observations are always consistent with the earlier observations. Thus the appearance of the object's wavefunction's collapse has emerged from the unitary, deterministic theory itself. (This answered Einstein's early criticism of quantum theory, that the theory should define what is observed, not for the observables to define the theory[12] .)
Since Everett stopped doing research in theoretical physics shortly after obtaining his Ph.D., much of the elaboration of his ideas was carried out by other researchers and forms the basis of much of the decoherent approach to quantum measurement.
. However, argument 3 is incorrect. There are in fact two other alternatives. The first is a universe that is both random and causal (that is to say, some events are random, and others have causes), but the random events and the deterministic ones are do not interact. The other possibility is a universe that is both random and causal, where the random events can sometimes cause other events to happen. Conversely, deterministic events can affect the chances at which a random event will occur.
But seriously: determinism doesn't say, of any individual time, that only one state of the universe is possible at that time. It only makes statements of possibility/impossibility about combinations of different times.
