At this past summer’s Lindau meeting of Nobel Laureates, Christian Rene de Duve, who is over 90 years old, gave a talk on population growth and its potential dire effects on the world. Part of his talk was broadcast on the Science Show. His talk prompted me to think more about growth. The problem is not that the population is growing per se. Even if the population were stable, we would still eventually run out of fossil fuels if we consume energy at the same rate. The crucial thing is that we must progressively get more efficient. For example, consider a steady population where we consume some finite resource at the rate of . Then so long as , we can make that resource last forever since is finite. Now, if the population is growing exponentially then we would have to become exponentially more efficient with time to make the resource last. However, making the world more efficient will take good ideas and skilled people to execute them and that will scale with the population. So there might be some optimal growth rate where we ensure the idea generation rate is sufficient to increase efficiency so that we can sustain forever.