david jon furbish

@davidjonfurbish.bsky.social

emeritus professor studying statistical physics of sediment transport & science philosophy essays and ebook preview at: https://my.vanderbilt.edu/davidjonfurbish/ author of Fluid Physics in Geology: https://academic.oup.com/book/40895?login=false

Excited to share two preprints for companion papers that examine the statistics of bedload motion! We used high-resolution imagery to capture grain tracks from flume experiments across a range of fluid shear stresses. We also subject the sand bed to "stress history" to see how grain motion changes.

Three-panel figure from a scientific paper. The top panel (a) shows example sediment trajectories from a particle-tracking analysis overlaid onto an image of the underlying sediment bed; note the appearance of fluorescing sand grains illuminated by the ultraviolet light. These trajectories represent transport conditions over a subset of the sediment bed that is 1.2 times the threshold for grain motion. The middle panel (b) shows the same sediment trajectories as depicted in panel (a) with markers indicating periods of motion (blue) and rest (red); one of the trajectories is highlighted in yellow. This highlighted track is further examined in panel (c), which shows a timeseries of streamwise position (black line) and velocity (red/blue markers) for the example trajectory illustrating segments of motion and rest.

I usually compose footnotes in a manner befitting their normally intended purpose: to support or elaborate material in the main text. But I also view certain footnotes as opportunities to drop serious snark. Here is one of my favorite recently composed footnotes, with context.

Consider rain splash transport as an example, and recall the one-dimensional entrainment form of the Exner equation...

Focusing on the advective term in (1)...

where D = k E / c_b.  Thus the advective term looks like diffusion as defined mathematically.  Whereas it is correct to say that (2) is a diffusion equation, it is incorrect to say that this equation represents the effects of particle diffusion.  The effects of particle diffusion are described by the last term in (1), a point that we fully examine in Chapter 11.  A similar interpretation applies to soil creep, where (2) arises from the assumption that the vertically integrated soil flux is proportional to the local land-surface slope.

The derivative on the right side of (2) often is referred to as a diffusive term, likely in analogy with a conventional advection--diffusion equation.  This is correct in a mathematical sense, and perhaps intended to refer to the idea that ``diffusive smoothing'' of the function < zeta > occurs according to the mathematical algorithm defined by a diffusion equation.  But in a physical sense related to transport, the derivative on the right side of (2) is actually an advective term.  It looks mathematically diffusive only because the average particle displacement is proportional to the land-surface slope.\footnote{\textcolor{blue}{This point of confusion seems to reflect a broader misunderstanding of the meaning of advection.  The popular, heuristically confused ``stream power model'' of land-surface evolution includes a ``diffusive'' term as in (2) and a term involving the product of the local land-surface slope and the upstream catchment area.  This latter term is often referred to as an advective term, but it has nothing to do with advection.  It is in fact a local sink term.  Moreover, for physical reasons the two terms are not additive as normally assumed.}}

What is the value of human beings studying the cosmos? For @michaelgreshko.bsky.social at Science, I dug into how practitioners of humanity's oldest exact science are metabolizing the latest developments in AI...and how it seems to be forcing them to wrestle with philosophical questions.

Amid a flood of AI advances, astrophysicists are questioning the soul of their field

Researchers see enormous power in new tools—but also the potential end of astrophysics as a human endeavor

science.org

A younger West-Coast academic colleague asked about any professional changes in my life. My reply: “So that you know, they immediately give you tenure when you retire, and there are no promotions. Well… maybe a promotion from “ok dog walker” to “pretty good dog walker,” but that’s it.” lol

As an emeritus faculty I minimally maintain a Windows system for reasons related to legacy software and engagement with campus IT. But I have otherwise nearly completed my migration to Linux and open-source software. (I have avoided most MS software for years.) 1/3

Yes, the central limit theorem might just be as close as one can get to real mathemagic! Indeed, here is the opening sentence of material on the CLT that I recently posted (the link appears in 2/2 below): “The central limit theorem is among the crown jewels of mathematics and science.” 1/2

Quanta Magazine@quantamagazine.org · 5mo ago

The central limit theorem is a mathematical truth so powerful that it often strikes newcomers as impossible, like a magic trick of nature. Through it, the most random chaos can lead to striking predictability.