October 8, 2012

Hugh Everett's "Many Worlds" Theory (VIDEO)

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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