The many possibilities carried by quantum superpositions are spread out over space and time. 
However, Newtonian physics is an accurate description of ordinary experience. 
What is the relationship between the strange quantum world and the classical world of common sense? 
Clearly the difference occurs when we measure or observe a quantum system.
Whatever the process, it occurs at that time. The ``how and why'' of this process is unsolved and 
many believe modern physics will be incomplete until it is resolved.

By the 1950's, the ongoing parade of successes had made it abundantly clear that quantum 
theory was far more than a short-lived temporary fix. And so, in the mid 1950's, a Princeton 
graduate student named Hugh Everett III decided to revisit the collapse postulate in his Ph.D. 
thesis. Everett's idea is known as the relative-state, many-histories or many-universes
interpretation or metatheory of quantum theory. Dr Hugh Everett, III, its originator, called 
it the "relative-state metatheory" or the "theory of the universal wavefunction", 
but it is generally called "many-worlds".

Many-worlds is a re-formulation of quantum theory which treats the process of observation or 
measurement entirely within the wave-mechanics of quantum theory, rather than an input as additional 
assumption, as in the Copenhagen interpretation. Everett considered the wavefunction a real object. 
Many-worlds is a return to the classical, pre-quantum view of the universe in which all the 
mathematical entities of a physical theory are real. For example the electromagnetic fields 
of James Clark Maxwell or the atoms of Dalton were considered as real objects in classical physics.
Everett treats the wavefunction in a similar fashion. Everett also assumed that the wavefunction obeyed 
the same wave equation during observation or measurement as at all other times. This is the central 
assumption of many-worlds: that the wave equation is obeyed universally and at all times.

Quantum systems, like particles, that interact become entangled. If one of the systems is an observer 
and the interaction an observation then the effect of the observation is to split the observer into a 
number of copies, each copy observing just one of the possible results of a measurement and unaware of 
the other results and all its observer copies. Interactions between systems and their environments, 
including communication between different observers in the same world, transmits the correlations that
induce local splitting or decoherence into non-interfering branches of the universal wavefunction. 
Thus the entire world is split, quite rapidly, into a host of mutually unobservable but equally real 
worlds.

According to many-worlds all the possible outcomes of a quantum interaction are realised. 
The wavefunction, instead of collapsing at the moment of observation, carries on evolving in a 
deterministic fashion, embracing all possibilities embedded within it. All outcomes exist 
simultaneously but do not interfere further with each other, each single prior world 
having split into mutually unobservable but equally real worlds.