Talk by James P. Sethna at ASC (Jan 16)

[Dardashti, Radin]

Speaker: James P. Sethna (Cornell University)
Thursday 16th Jan 2014
Location: Room A348/349, Theresienstr. 37/III
Time: 12:15 - 13:45

Title: Sloppy Models and How Science Works

Abstract:
"With four parameters I can fit an elephant; with five I can make it 
wag its tail.” Systems biology models of the cell have an enormous 
number of reactions between proteins, RNA, and DNA whose rates 
(parameters) are hard to measure. Models of climate change, ecosystems, 
and macroeconomics also have parameters that are hard or impossible to 
measure directly. If we fit these unknown parameters, fiddling with them 
until they agree with past experiments, how much can we trust their 
predictions? Multiparameter fits are sloppy; the parameters can vary 
over enormous ranges and still agree with past experiments. Nonetheless, 
they can often make useful predictions about future experiments, even 
allowing for these huge parameter uncertainties: a few stiff 
combinations of parameters govern the behavior. Third, these sloppy 
models all appear strikingly similar to one another – for example, the 
stiffnesses in every case we’ve studied are spread roughly uniformly 
over a range of over a million. We will use ideas and methods from 
differential geometry to explain what sloppiness is and why it happens 
so often. Finally, we shall show that models in physics are also sloppy 
– that sloppiness makes science possible.