Despite the apparent problems with quantum mechanics, there is one more option which solves both the measurement problem and the problem of instantaneous action at a distance
without relying on extra dynamics or hidden variables to explain our
observations.
In 1957, American physicist Hugh Everett III suggested
that we should simply take Schrodinger's wave equation
literally (Everett, 1957a). This means ending the search for collapse
dynamics and hidden variables and applying the theory to everything,
including the universe itself.
Everett argued that if there are no collapse
dynamics, and no hidden variables that suppress the effects of a
superposition, then everything will evolve in accordance with the
unitary evolution of Schrodinger's wavefunction.
This means that an
observer is not separated from the quantum system they are measuring.
When they measure a property of a quantum system, it will not collapse
into a single determinate state. Instead, every possibility given by the
Schrodinger equation is actualised and, because they too are in a superpositional state, they will observe them all.
When someone measures the spin
of an electron in the vertical plane, for example, it will have a 50%
chance of appearing 'up' and a 50% change of appearing 'down'. Both the Bohm and collapse approaches
predict that an observer will record only one result in accordance with
its probability, the Everett approach predicts that they will record
both.
Everett described how; "there is only one
physical system representing the observer, yet there is no single unique
state of the observer...Thus with each succeeding observation (or
interaction), the observer state 'branches' into a number of different
states," and "each branch represents a different outcome of the
measurement" (Everett, 1957b). Everett referred to his approach as the
"relative state formulation" (Everett, 1957b) because it shows that
everything we experience exists in relational terms. In the example
above, the experience of measuring the electron to be 'up' is only real
relative to the experience of measuring it to be 'down'. Neither branch
is more real than the other.
Hugh Everett's Parallel Universes and space time reality
Despite the elaborate nature of Everett's proposal, he made it clear that he was not suggesting an instrumental interpretation.
Everett argued that "it is completely unnecessary to suppose that after
an observation somehow one element of the final superposition is
selected to be awarded with a mysterious quality called 'reality' and
the others condemned to oblivion" (Everett, 1957c). Although Everett
only mentioned the word 'branches' in his 1957 paper, the realism
associated with them soon led to the term 'parallel worlds' being used
instead. American physicist, Bryce DeWitt was the first to do so when he
popularised Everett's approach in 1970 (DeWitt, pp.155-165) and by
1977, Everett was defending his theory in these terms (Deutsch pp.223).
DeWitt described how, from our subjective
perspective, it seems as if the universe is "constantly splitting into a
stupendous number of branches, all resulting from the measurement like
interactions between its myriads of components. Moreover, every quantum
transition taking place on every star, in every galaxy, in every remote
corner of the universe is splitting our local world on earth into
myriads of copies of itself" (DeWitt and Graham, pp.161).
Hugh Everett
The Everett approach does not contradict the
laws of energy convention, however, because the universe does not
literally split every time a quantum event takes place. Collapse
approaches to quantum mechanics cannot apply their theory to the
universe as a whole because an external observer would be required in
order to collapse its superpositional state. The Everett approach does
not face this problem and so it can describe the entire universe using
Schrodinger's wave equation. This superpositional universe is known as
the multiverse. Because there are no collapse dynamics within the
Everett approach, there is no distinct time when a measurement is said
to have been made. When we become aware of the result of a quantum
experiment we simply realise which world we are already in.
The Everett approach solves the measurement
problem because it does not suggest that there are two different
substances, the quantum and the classical, which obey two different
laws. This means that Everett avoids the problem of explaining how these
two substances interact.
Everett argued that we do not appear to
experience every possible result because we too behave like a quantum
object, and "all the separate elements of a superposition individually
obey the wave equation with complete indifference to the presence or
absence ('actuality' or not) of any other elements. This total lack of
effect of one branch on another also implies that no observer will ever
be aware of any 'splitting' process" (Everett, 1957b).
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