-- by Horatio Algeranon
Scientists abide by physics
Aware of its constraint.
Economists abide by metricks
Which physics doesn't taint.
It's not just that individual economists have a poor understanding of physics (which many of them do, as exemplified by Superfreakonomics author Steve Levitt), but, even more critically, that mainstream economics itself is based on the idea of "eternal growth", which is a physical impossibility.
There are economists who are notable exceptions -- Herman Daly (see Beyond Growth: The Economics of Sustainable Development), Manfred Max-Neef (author of "Outside Looking In: Experiences in Barefoot Economics") , E.F. Schumacher (author of "Small is Beautiful") and a few others -- but they are definitely in the minority.
For another ditty on the same subject, read Wreckonomics.