The Preferred Basis Problem

The Preferred Basis Problem

The Everett approach was not able to fully account for the measurement problem until the theory of decoherence was discovered by German physicist Heinz-Dieter Zeh and Polish physicist Wojciech Zurek in the 1970s and 80s. This is because there are an infinite amount of ways in which we could divide the universe into branches. Although Everett is right in saying that different branches will not interact, some will involve indeterminate macroscopic objects.

American physicist Henry Stapp stated that "if the universe has been evolving since the big bang in accordance with the Schrodinger equation, then it must by now be an amorphous structure in which every device is a smeared-out cloud of a continuum of different possibilities. Indeed, the planet earth would not have a well defined location, nor would the rivers and oceans, nor the cities built on their banks" (Stapp, pp.1052). We never experience macroscopic objects in this state and so there appears to be a preferred way to divide the universe so that these branches do not occur. The preferred basis problem asks why this is.

Schrodinger first considered this problem when he discussed the realist implications of his theory in 1952. He stated that "nearly every result [a quantum theorist] pronounces is about the probability of this or that...happening - with usually a great many alternatives. The idea that they be not alternatives but all really happen simultaneously seems lunatic to him, just impossible. He thinks that if the laws of nature took this form for, let me say, a quarter of an hour, we should find our surroundings rapidly turning into a quagmire, or sort of a featureless jelly or plasma, all contours becoming blurred, we ourselves probably becoming jelly fish." Schrodinger thought that this idea must be flawed and referred to "the compulsion to replace the simultaneous happenings, as indicated directly by the theory, by alternatives" as "a strange decision" (Schrodinger, pp.19-20).

Decoherence solves the preferred basis problem by showing that the same mechanism that is responsible for the suppression of interference effects in the quantum realm is also responsible for this suppression in macroscopic objects. It does not solve the measurement problem when combined with the collapse approach because it does not provide any collapse dynamics, or explain how quantum and classical objects could be two distinct substances. It does not solve the problems faced by the Bohm approach because it does not explain why all but one world is suppressed (Deutsch, pp.225).

Zeh and Zurek both argue, however, that the measurement problem can be resolved when the theory of decoherence is combined with the Everett approach (Zeh, pp.221-233 and Zurek, pp.1793-1820). Decoherence shows that a natural basis will form which prevents us from experiencing branches that involve indeterminate macroscopic objects. Mathematically, these branches are said to decay exponentially, but they do not disappear completely and so decoherence can only give us an approximate appearance of definiteness. One response to this argues that the approximate nature of decoherence does not matter because our minds have evolved to only comprehend definite objects, another argues that the approximate nature of decoherence does not matter because decoherence is precise enough to explain our observations when combined with a functional approach to the mind.

References

Deutsch, D., 1996, 'Comment on Lockwood', British Journal for the Philosophy of Science, Vol.47, pp.222-228

Schrodinger, E., 1995, 'July Colloquium, 1952', The Interpretation of Quantum Mechanics: Dublin Seminars (1949-1955) and other unpublished essays, Bitbol, M. (ed.), Ox Bow Press, Woodbridge, pp.19-37

Stapp, pp.1052 H. P., 2002, 'The Basis Problem in Many-Worlds Theories', Canadian Journal of Physics, Vol.80, pp.1043-1052

Zeh, H. D., 2000, 'The Problem of Conscious Observation in Quantum Mechanical Description', Foundation of Physics Letters, Vol.13

Zurek, W. H., 1998, 'Decoherence, Einselection, and the Existential Interpretation (The Rough Guide)', Philosophical Transactions of the Royal Society of London