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Writing and publishing work on rivers


Working 'before the limit'
Benoît Mandelbrot Mathematician
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These Hausdorff-Besikovitch dimensions are really mathematical concepts, in the limits of very special objects, and one of the major advances was to see this playing a role in real objects that do not fit mathematical constraints. Very true. Which means that to make a major advance one still had to take a step back. In mathematics, Hausdorff defined his dimension in a very particular fashion, which includes a limit. Limits actually are not so bad in physics but also an inf, which is an impossible alteration - he wanted to obtain a measure, an outer character measure. It's a mathematical requirement of no significance outside of his field. Besides, Hausdorff being a mathematician, a very pure one, wanted to have the finish applicable to all shapes, at least all shapes in metric space. Therefore the search for generality, the search for a proper measure with which everything could be done with, was paramount in his mind. These two searches are totally irrelevant to physics and to my concern. In my discussions with Henry McKean about the Hausdorff Besikovitch dimension, I always kept telling him, "But why do you do something so complicated, so unnecessarily complicated?" And he would say, "We need it for mathematical purposes." And I always kept in mind that this may not be necessary, and the very important feature of my work from the very beginning was that different, how should I say, pre-asymptotic concepts, which are very much in the spirit of the Hausdorff dimension but very different and finite, are much more important than the limit itself. When I started talking about using the Hausdorff dimension in physics my friends, my mathematical friends, were making fun of me. They were saying, "You? That can't be used. It is, first of all, esoterica; secondly it is only a limit and it does not distinguish between shapes having otherwise very big differences. I said, " Yes, yes, yes. Except, that I'm not interested in arbitrary shapes, I'm interested in self-similar shapes. For self-similar shapes all these considerations appear. There's no local, there's no middle, there's no global. Everything is at the same scale." So in a way by walking back, by destroying some generality that the concept had from the very beginning, I was able to make it useable. And this is a lesson which got lost; very often since then, my work has consisted mathematically speaking in not going to a limit, of looking at what happens before the limit, and by doing so all kinds of very beautiful notions become meaningful which otherwise become meaningless. I would just mention in passing the idea of negative dimension as a measure of a degree of emptiness, which sounds almost like a ridiculous notion in itself, but a dimension can be negative, it measures it very effectively, and numerically, measurably, usefully.

Benoît Mandelbrot (1924-2010) discovered his ability to think about mathematics in images while working with the French Resistance during the Second World War, and is famous for his work on fractal geometry - the maths of the shapes found in nature.

Listeners: Bernard Sapoval Daniel Zajdenweber

Bernard Sapoval is Research Director at C.N.R.S. Since 1983 his work has focused on the physics of fractals and irregular systems and structures and properties in general. The main themes are the fractal structure of diffusion fronts, the concept of percolation in a gradient, random walks in a probability gradient as a method to calculate the threshold of percolation in two dimensions, the concept of intercalation and invasion noise, observed, for example, in the absorbance of a liquid in a porous substance, prediction of the fractal dimension of certain corrosion figures, the possibility of increasing sharpness in fuzzy images by a numerical analysis using the concept of percolation in a gradient, calculation of the way a fractal model will respond to external stimulus and the correspondence between the electrochemical response of an irregular electrode and the absorbance of a membrane of the same geometry.

Daniel Zajdenweber is a Professor at the College of Economics, University of Paris.

Duration: 3 minutes, 33 seconds

Date story recorded: May 1998

Date story went live: 24 January 2008