I'm not familiar with the numbers either, but your estimate sounds plausible to me. Tom Murphy has a nice presentation from which a more exact figure can proba

I'm not familiar with the numbers either, but your estimate sounds plausible to me. Tom Murphy has a nice presentation from which a more exact figure can probably be extracted, but I don't see it there directly: http://physics.ucsd.edu/do-the-math/2013/09/the-real-population-problem/ http://physics.ucsd.edu/do-the-math/2013/09/the-real-populat...