Republican presidential contender Tim Pawlenty has put forth a plan in which he hopes the US economy will grow at 5% a year for decades.  
Such growth, Pawlenty suggests, will solve all of our problems. (And in the meantime, in Pawlenty's plan, we'll get to enjoy lower taxes to boot!)  
Over the short-term, economies certainly can grow 5% a year, or even faster. And Pawlenty's projected growth rate seems achievable in part because, in 2006 and 2007, the global economy grew at 4.5% a year. Thanks to that happy growth spurt, in fact, economic growth of 5% a year seems not just achievable but normal.  
But lest we all become too enamored of the idea that we can grow at 5% a year forever, it's worth pointing out that we can't. Over the long term, this growth rate is not just unlikely--it's impossible.  
Nothing can grow at 5% a year for long without eventually overwhelming everything else.