How much government debt is too much?

Robert J. Samuelson: Up Against a Wall of Debt - How much can governments borrow?

Excerpts:

The idea that the government of a major advanced country would default on its debt—that is, tell lenders that it won't repay them all they're owed—was, until recently, a preposterous proposition. Argentina or Russia might stiff their creditors, but surely not the likes of the United States, Japan, or Great Britain. Well, it's still a very, very long shot, but it's no longer entirely unimaginable. Governments of rich countries are borrowing so much that it's conceivable that one day the twin assumptions underlying their burgeoning debt (that lenders will continue to lend and that governments will continue to pay) might collapse. What happens then?

...

Deprived of domestic or international credit, defaulting countries in the past have suffered deep economic downturns, hyperinflation, or both. The odds may be against a wealthy society tempting that fate, but even the remote possibility underlines the precariousness and novelty of our present situation.

Comment: Sounds like the answer is "we don't know but we don't want to find out".