Notebooks
http://bactra.org/notebooks
Cosma's NotebooksenWinding Number and Topological Explanations
http://bactra.org/notebooks/1995/03/22#winding-number
<P>Take a rubber band. Wrap it around a cylinder. Run your finger around the
band. Add up the number of times you circle the cylinder counter-clockwise,
and subtract the number of times you went round clockwise. That's the winding
number. You can twist and pluck and pull at the band all you wish without
changing it; to do that, you have to take it off the cylinder (possibly by
cutting and splicing). As they say in the trade, "winding number is invariant
under continuous deformations." There are fancier definitions of winding
number, with integrals and stuff, and related concepts for other sorts of
shapes, which are also constant under continuous deformations. All this falls
under topology.
<P>Toss a stone in a still lake. It makes waves. Even without friction, the
waves spread out and die away. In an infinite lake, they would eventually die
away to zero; the wave disperses. Under the right conditions (like in canals),
you can get other kinds of waves, called solitions, which don't disperse, but
travel, pulse-like, indefinitely. Things like solitons show up in lots of
physical theories, called various solitions, kinks, hedgehogs, monopoles and
lumps. (I like "lumps.") For many lumps, you can prove they won't disperse
by using things like the winding number. A flat lake, after a wave has
dispersed, has a winding number of zero. (It's actually one of the winding
number's cousins; let it go.) Lumps have non-zero winding number. Therefore
you can only get from a lump to a flat lake by a discontinuous transformation.
<P>Nature doesn't like discontinuities.
<P>Winding number also shows up in <a href="pattern-formation.html">pattern
formation.</a> If you put the right chemicals together --- a mix called the
Belusov-Zhabotinskii reaction is particularly famous --- you get a cyclic
reaction. If conditions are just right, different parts of the solution are at
different phases in the cycle, and the mixture spontaneously forms spiral
waves. (I may have to break down and get a picture for this. It's pretty.)
Your initial even mix of chemicals has winding number zero; spiral waves don't.
(And these are the real true original honest-to-Gauss winding numbers, too.)
So how can spiral waves form, if Nature dislikes discontinuities so much? The
trick is that spirals can point in two senses, clockwise and counter-clockwise,
and that these two senses give winding numbers of opposite sign. Winding
number will be conserved if spiral waves always emerge in pairs, rotating in
opposite directions. (Since one goes round clockwise, and the other
anti-clockwise, the total winding number is +1 -1 = 0.) And in fact this is
what we always see; you even see it in computer simulations of the reaction.
<P>Now at first glance this kind of topological argument may look like that
much-fabled and long-sought beast, a non-<a
href="reductionism.html">reductionist</a> explanation of an
<a href="emergent-properties.html">emergent property</a>. I don't
think it's anything of the kind. The conservation of winding number shows
up, after all, in computer models where we <em>know</em> only local,
micro-level rules are involved. What this looks like to me is that the
topological reasoning is a <em>hack</em>: a clever shortcut which lets us
see <em>consquences</em> of the micro-rules without elaborate calculations.
If you want to explain the emergence of any particular pair of
spiral waves microscopically, you can do so in detail by following
the application of the rules over time: and does it get more reductionist
than that?
<ul>Recommended:
<li>Sidney Coleman, "Some Classical Lumps and their Quantum
Descendants" in his <cite>Aspects of Symmetry</cite> [Excellent on lumps and
kinks and so forth in theoretical physics; unfortunately you have to know field
theory to follow him. I'm looking for an explanation for the laity.]
<li>David Griffeath, <a
href="http://math.wisc.edu/~griffeat/kitchen.html">Primordial Soup Kitchen</a>
[I wish he'd put his lecture notes on-line as well...]
<li>A. T. Winfree, <cite>Where Time Breaks Down</cite> [More on spiral
waves, self-organization and topology. Math-free.]
</ul>