Super-rotation of Earth's inner core and the structure of scientific reasoning
Dept. of Philosophy, Northern Arizona University, Box 6011, Flagstaff, Arizona 86011, USA;
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Recent confirmation that Earth’s inner core rotates with respect to the mantle (cf. Zhang et al., 2005) presents a good case to highlight important aspects of the structure of observation and reasoning in geophysics, geology, and the physical sciences in general. In particular, it can help to clarify the relationship between theory and evidence. Determining the rotation of the inner core is a solution to a so-called “inverse problem” (Jacobs, 1987, p. 2). The defining characteristic of an inverse problem is the challenge of determining properties of an unobserved cause based on observed properties of the effect. This provides an opportunity to raise some worthwhile methodological questions.
The first question is about confirmation. How is solving an inverse problem different from other forms of confirmation in science? A second question is about the difference between explanation and description. Is an inverse problem distinct from common patterns of explanation—in particular those that explain observed effects in terms of their unseen cause? What, if anything, has been explained by the discovery of super-rotation? Or, is this a case of describing an aspect of nature without explanation?
And then there is a question of the empirical status of super-rotation. Is the image of the rotating inner-core a matter of observation (with information moving from outside in, from the physical world into our minds), or inference (with information moving from inside out, from ideas to implied situations in the world)? Following the flow of information in the case of super-rotation will shed some light on this difference.
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Manuscript received 5 Feb. 2010; accepted 26 April 2010