Here is an excerpt from my letter (the link to the graph is added here):
From 1950 to 2006, the period covered by Prof Gordon’s analysis, the rate of growth in US per capita GDP changed very little if at all, according to the Bureau of Economic Analysis. Debating the prospects for future economic growth in the context of innovation and productivity is important; however, such arguments should be grounded in accurate data. Prof Gordon describes his thesis as a “provocative fantasy” – a characterisation that I find to be completely accurate.The full letter is here. Background on this issue can be found here and here and here.