  This article is a follow-up of a short essay that appeared in Nature 455,
1181 (2008) [arXiv:0810.5306]. It has become increasingly clear that the
erratic dynamics of markets is mostly endogenous and not due to the rational
processing of exogenous news. I elaborate on the idea that spin-glass type of
problems, where the combination of competition and heterogeneities generically
leads to long epochs of statis interrupted by crises and hyper-sensitivity to
small changes of the environment, could be metaphors for the complexity of
economic systems. I argue that the most valuable contribution of physics to
economics might end up being of methodological nature, and that simple models
from physics and agent based numerical simulations, although highly stylized,
are more realistic than the traditional models of economics that assume
rational agents with infinite foresight and infinite computing abilities.
