Robert Solow found that piling up capital barely explains long-run growth

Solow's 1956 model showed that simply adding more machines and workers hits diminishing returns and can't sustain growth forever. When he ran the numbers on the US economy, most of the long-run rise in output per worker was left unexplained by capital or labor at all. He credited the leftover, the 'Solow residual,' to technological progress.

— Robert Solow, A Contribution to the Theory of Economic Growth — Quarterly Journal of Economics, 1956