Sanitation workers, firefighters, teachers and other state and local government employees have performed their duties in the public sector for decades with the understanding that their often lackluster salaries were propped up by excellent benefits, including an ironclad pension. But Moody’s Investors Service recently estimated that public pensions are underfunded by .4 trillion. That amount, which is equivalent to the economy of Germany , accounts for one-fifth of national debt. It’s a significant concern for public employees who were banking on a fully funded retirement to get them through their golden years. But the issue has wider implications for all taxpayers, who likely will be tapped to make up the shortfall. The Knowledge at Wharton radio show, which airs on  Wharton Business Radio on SiriusXM , asked two experts to explain how governments dug themselves into such a deep hole — and whether they can ever get out.…