<i>&gt; How long would an economist expect there to be exponential growth in electricity consumption? Steel production? Indium usage?</i><p>Irrelevant, because exponential growth of wealth does not depend on exponential growth in any of these things, or in anything material. Wealth is what you can do, not how much stuff you have; as technology advances, we figure out ways to do the same things as we did before, using fewer material resources (not to mention doing new things that no one could do before--like posting here).