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Contemplation of Benoit Mandelbrot’s famous fractal oft brings to my mind the opening stanza of William Blake’s *Auguries of Innocence*:

** To see a World in a Grain of Sand
And a Heaven in a Wild Flower,
Hold Infinity in the palm of your hand
And Eternity in an hour.**

No matter how much one zooms in within the Mandelbrot set, intricate patterns can be discerned that bear a striking similarity to the whole set. (The self-similarity in the Mandelbrot set can be seen in this amazing video.) This inability to reach some simple structure or quiescence reminds me of the search for some final fundamental particle or superstring in physics. The application of ever greater amounts of energy seems to create just more particles out of the void.

Of course, one cannot really zoom forever in actuality. Both computer power and time constraints form natural limits. Such practical considerations, however, do not limit the mathematical contemplation of infinity. It wasn’t always so. For the ancient Greeks, infinity was anathema and they went to elaborate lengths to avoid it. When it did come up, as in Zeno’s paradox, it was often used to demonstrate some perceived fundamental philosophical flaw with the concept of infinity. In fact, the problem of infinity led Aristotle to differentiate between potential infinity (such as being able to count indefinitely) versus actual infinity (which he denied existed). Aristotle’s distinction lasted right down to the 20th century, with the exception of those who allowed for an infinite divinity.

Galileo was one of the first to notice that the set of counting numbers could be put in a one-to-one correspondence with the apparently much smaller set of their squares. He similarly showed that the set of counting numbers and their doubles (i.e., the set of even numbers) could be paired up. Galileo concluded that “we cannot speak of infinite quantities as being the one greater or less than or equal to another.”

Such apparent contradictions did not long deter mathematicians from developing the study of instantaneous change and irregularly-shaped spaces, or the calculus (with infinitesimal increments). Of course, this created many new paradoxes involving infinity. For example, Gabriel’s Horn (also known as Torricelli’s Trumpet) is formed by rotating the curve *y* = 1/*x* for *x* > 1 about the *x*-axis. Remarkably, the volume of the resulting three-dimensional figure is infinite, while the area of the two-dimension surface of the horn is finite. In other words, Gabriel’s Horn could be completely filled with a finite quantity of paint, but no amount of paint would suffice to paint the horn’s surface. A modern example, known as the Banach–Tarski paradox, comes from set theory and requires the axiom of choice. In 1924 it was shown that a solid sphere can be decomposed into a finite number of pieces (originally 6, subsequently 5) that, when suitably moved about by rigid motions, can be reassembled into two new solid spheres, each of which is the same size as the original sphere. In essence, to put it another way, a billiard ball could be cut apart and put back together to fill the same space as the sun (or any other object, for that matter).

Contemplation of infinity always reminds me of the joke about the relative who thinks that he is a chicken. When asked why his family doesn’t get him some treatment, the response is that they need the eggs.

William L. Hosch – January 12th, 2007 (C) open.britannica.com

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