Disclaimer: I have not read the book either.

One thing I don't understand about the r and g thing is how it makes sense to compare these two values at all. Isn't capital a measure of accumulated wealth, while GDP is a measure of wealth produced in a certain unit of time? For example, what if we just maintained a perfectly steady GDP that exceeded our consumption needs; wouldn't that yield a positive r and explain r > g? Does someone who read the book have a better understanding of exactly what these two numbers mean?

P.S. I find it implausible that Piketty would make such an elementary mistake as the arithmetic mean vs. geometric mean issue discussed in the article. But again someone who has actually read the book should weigh in.