Life’s Devices

by Kieran Healy on June 26, 2005

“Teresa Nielsen Hayden”: links to a “terrific paper”: by E.M. Purcell called “Life at Low Reynolds Number.” The Reynolds Number is, roughly, the ratio of intertia to viscosity in fluids, and if you want to learn more about it I strongly urge you to read the rest of the talk for yourself. I learned about the Reynolds Number in graduate school. It’s not something they teach sociologists, as a rule, but I discovered during my first year that Princeton University Press often had sales at the University Store. Because I am in inveterate dilettante — er, I mean, polymath — I picked up a great book by “Steven Vogel”: called “Life’s Devices”:

It’s a course in biomechanics, the subject at the intersection of biology, physics and engineering. But it’s also a great read. It’s terrifically well-written. Don’t worry if you (like me) haven’t had anything in the way of physics since the early years of high school. Vogel is sympathetic, noting that biologists themselves are often essentially in this position,

bq. except that most of us were subjected to a few college courses in physics and mathematics, so we can’t easily admit either innocence or fear. As James Thurber said about the founding editor of _The New Yorker_, “Ross approached things mechanical, to reach for a simile, like Henry James approaching Brigitte Bardot. There was awe in it, and embarrassment, and helplesslness.”

Here’s an excerpt, courtesy of Amazon, from a section about insects and other creatures that have the ability to walk on water. Vogel says

It’s no trick at all to concoct a dimensionless index to the practicality of standing or walking (not floating or swimming) on the surface of water, held up by the force of surface tension on from hydrophobic (nonwettable) foot. All that is needed is the ratio of the force holdin gthe creature up to that pulling it down. The upward force is simply the surface tension γ … times the length of the air-water-foot contact line (usually the combined perimeter of the feet). The downward force is gravitational, the usual _F = mg_ or approximately the density (of the organism) times length cubed times gravity. We get, then, as an index (call it Je, the Jesus number):

… With that mild bit of sacrilege (apologies to the offended), we have probably plumbed enough of these indices to persuade any skeptic that discrete dimensions are no prerequisite for applicability to the physical world.

Great stuff. I think you should do yourself a favor and “buy it”:



mondo dentro 06.26.05 at 11:06 am

Vogel’s book is a delight. For me personally, though, it all started with D’Arcy Thompson’s On Growth and Form. The first time I saw that (as an undergrad) I was enraptured.

Another beautiful book is Winfree’s The Geometry of Biological Time. It is less concerned with mechanics and scaling, and more purely dynamical systems oriented, but the fluid writing and the entrancing illustrations of “time crystals” for flys and plants pack a mighty esthetic wallop.


P O'Neill 06.26.05 at 11:28 am

I thought the Reynolds number was the number of “Hehs” that appear on Instapundit on any given day. So a low Reynolds number equals a bad day for Bush.


ogged 06.26.05 at 12:11 pm

That’s the Heh’d Count.


Joshua W. Burton 06.26.05 at 12:22 pm

Credit where it’s due: M. Gordon posted a link to the Purcell paper here on CT (in the slushie thread) a few days ago, and that rippled it across the net, presumably to Hayden’s blog which picked it up a few hours later. It’s a very well-known paper in the physics community, and should be more widely read on any excuse.


Joshua W. Burton 06.26.05 at 12:25 pm

Oh, and while we’re recommending books on biomechanics and the structural engineering of everyday life: J. E. Gordon’s _Structures, Or Why Things Don’t Fall Down_, followed by his meatier _The New Science Of Strong Materials_.


Paul Orwin 06.26.05 at 12:27 pm

I remember learning about Reynold’s number in a biophysics class, and it was a revelation. It is something I try to pass on to my biology students (understanding how different it is for bacteria to move through water as opposed to how we do it), but it is frankly pretty tough to imagine. I usually describe it as swimming through molasses (which I am pretty sure is not my own invention!), but that is just not that informative. Purcell’s paper is a classic (was it synopsized or recap’d in Berg’s random walks in biology? It seems like it’s connected somehow…but I can’t remember). I have to start collecting all of these great books!


Keith M Ellis 06.26.05 at 12:32 pm

I immediately followed and read Gordon’s link from his comment. I then forwarded it to a couple of friends with a note that my intuition tells me that Purcell was right in his “outrunning diffusion” hypothesis. Was he?


Paul Orwin 06.26.05 at 12:35 pm

From Purcell’s paper (which I am reading right now! Not sure if I read it many moons ago or not…)

Well you put him in a swimming pool that is full of molasses, and the you forbid him to move any pare of his body faster than 1 cm/min.


Consider this a belated attribution! Cheers all.


J. Ellenberg 06.26.05 at 1:41 pm

Purcell’s “Electricity and Magnetism” textbook is an absolute delight.


andre 06.26.05 at 4:46 pm

A recent book that has an excellent discussion of Reynolds Number in general and specifically as it relates to biology is Philip Nelson’s Biological Physics. The book also covers many related topics, including random walks in the spirit of Berg’s Random Walks in Biology. About a year of college level calculus is required to really appreciate the discussion, but it flows nicely and would probably be a pleasure for the interested layman.


M. Gordon 06.28.05 at 1:29 pm

jwb is always watching my back. Mad props. To bring things full circle, LaLRN was first brought to my attention by Peter Meyers, when I was taking E&M at Princeton. He taught it out of Purcell’s book, because he had himself taken it from Purcell. I graduated from P’ton in ’98, and recall vivdly, on numerous occasions as an undergrad, paging through the editorials in the Daily Prince and asking myself, “Who is this wanker grad student Kieran Healy who has all this time to be writing editorials? Shouldn’t he be grading papers or something?”


M. Gordon 06.28.05 at 1:49 pm

I should also point out, while we’re on the topic, that the “Death at Low Reynolds Number” joke is not originally mine. It was made by Howard Berg (see #10 above) at a talk he gave here at UIUC about bacterial motility. He originally scribbled it on a photocopy of a newspaper article about the Boston molasses explosion of 1919, cited by jwb in the referenced thread, and mailed it to Purcell.

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