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Knowledge Event Horizon
Introduction
It takes time to learn new things, but it takes far more time to break new ground. Therein lies the root of the problem. Back around 1900 Planck, Einstein Dirac et al were young when they made their breakthroughs. These days one needs to cover a lot of their work to reach new grounds. Thus, there will be a leveling out unless someone makes learning a whole lot faster.
In Academia
A throrough and mathematically based analysis is provided in Age and Great Invention by Benjamin F. Jones (April 2008), based on a working paper from 2005.
On the Net
Earliest discussions are from 2001.