when rome receded, europe saw a long period of people living in the ruins of a more advanced civilization. bridges fell and no one knew how to fix them. stone homes were repaired with thatch. medicine and technology were lost. crop yields fell. sanitation became once more vile. social and cultural cohesion disintegrated. the dark ages decended. "that could never happen again" seems a popular view, but is it true? to be sure, in modern times, outside knowledge like "how to make a cell phone" will be preserved by global communication and markets, but if we're going to be seriously...