A 2011 United Nations report said the practice can amount to torture and called for a ban on terms longer than 15 days. In this country, there are an estimated 25,000 prisoners in long-term solitary in supermax prisons; in California, the average stay is nearly seven years.

The inmates are isolated because prison officials have determined that they pose a threat to the safety of the guards and other prisoners, despite a growing body of evidence that such use of solitary does not reduce prison violence or promote safety.

At Pelican Bay, the overwhelming majority of the men in solitary don’t even have a record of violence; they are placed in solitary for their “gang associations,” despite the fact that such associations have hardly any predictive value for a prisoner’s likelihood to be violent.

The little hope these inmates have of leaving solitary lies mostly in what prison officials call “debriefing,” or snitching on other gang members. (California officials say that about 200 inmates statewide have been classified for return to the general prison population under a pilot program that considers behavior and other factors besides debriefing.)

Opponents of solitary do not deny that certain inmates are too dangerous or disruptive to live among the general prison population. The issue is whether depriving thousands of people of virtually all human contact for years on end, without real opportunities to get out, goes beyond any reasonable standard of proportionality in punishment. “They want to make these people suffer — it’s exactly what the goal is,” said Bryan Stevenson, executive director of the Equal Justice Initiative in Alabama. “Whose interests are being undermined if you let someone for the first time in a year talk to their mother?”