How Things Break

Solids fail through the propagation of cracks, whose speed is controlled by instabilities at the

smallest scales

Michael Marder and Jay Fineberg

Galileo was seventy-two years old, his life nearly shattered by a trial for heresy before the Inquisition,

when he retired in 1635 to Florence to construct the Dialogues Concerning Two New Sciences. His first

science is the study of the forces that hold objects together, and the conditions that cause them to fall

apart, the dialogue taking place in a shipyard, triggered by observations of craftsmen building the

Venetian fleet. The second science concerns ``local motions:'' laws governing the movement of

projectiles. The two subjects Galileo founded have fared differently. One is a respectable branch of

mechanical engineering, while the other is a core subject that physicists learn at the beginning of their

education. Although now, as in Galileo's time, ship-builders need good answers to questions about the

strength of materials, the subject has never yielded easily to basic analysis. Galileo identified the main

difficulty: ``one cannot reason from the small to the large, because many mechanical devices succeed

on a small scale that cannot exist in great size.[1]'' Nearly three hundred years elapsed after Galileo

wrote these lines before science reached the atomic scale and began to answer the