Think of the United States as a latter-day Titanic. Super-modern. Super-rich. And heading for an iceberg of colossal proportions. Unthinkable? So, too, was it unthinkable that the Titanic — at 46,329 tons, the largest, most luxurious, and arguably, most modern passenger ship afloat in 1912 — would ever go down. But go down she did, in 13,200 feet of frigid waters, 600 miles east of Newfoundland, with a loss of 1,500 lives. One looming danger that, by itself, could sink the American Titanic is called Social Security. Everybody knows it is in serious trouble, but virtually no one in public life has the political fortitude to do anything about it.