Boston has a long tradition of innovation, and some of America’s great inventions have been born there. “There’s a continuous thread, stretching across centuries, of powerful new ideas developed and commercialized in Boston. But that thread is always vibrating, at least faintly, with worry: Will there ever come another idea as big as the last? What new industry will create jobs sufficient to replace those lost as older industries fade? And how does the environment for company-creation here compare with other parts of the country, particularly Silicon Valley?”