We Are Not Saved

There are lots of very smart people who tell us we don't have to worry about the national debt, despite the fact that it recently passed $20 trillion. Is this true, are we over-reacting? Or is this the classic case of deciding that because something is not a problem now that it will never be a problem? The US can borrow at historically low rates, there are countries worse than us, but what is the endgame? How long can we keep this up?