Given the limited state of our knowledge of the process of technological change, we have no way to estimate what the upper bound on the feasible rate of growth for an economy might be. If economists had tried to make a judgment at the end of the 19th century, they would have been correct to argue that there was no historical precedent that could justify the possibility of an increase in the trend rate of growth of income per capita to 1.8% per year. Yet this increase is what we achieved in the 20th century. (p. 226)
Romer, Paul M. “Should the Government Subsidize Supply or Demand in the Market for Scientists and Engineers?” In Innovation Policy and the Economy, vol. 1, Cambridge, MA: MIT Press, 2001, pp. 221-252.