A machine that administers sedatives recently began treating patients at a Seattle hospital. At a Silicon Valley hotel, a bellhop robot delivers items to people’s rooms.Last spring, a software algorithm wrote a breaking news article about an earthquake that The LosAngeles Times published.

Although fears that technology will displace jobs are at least as old as the Luddites, there are signs that this time may really be different. The technological breakthroughs of recent years – allowing machines to mimic the human mind – are enabling machines to do knowledge jobs and service jobs, in addition to factory and clerical work.

And over the same 15-year period that digital technology has inserted itself into nearly every aspect of life, the job market has fallen into a long malaise. Even with the economy’s recent improvement, the share of working-age adults who are working is substantially lower than adecade ago – and lower than any point in the 1990s.

Economists long argued that, just as buggy-makers gave way to car factories, technology would create as many jobs as it destroyed. Now many are not so sure.

Lawrence H. Summers, the former Treasury secretary, recently said that he no longer believed that automation would always create new jobs. “This isn’t some hypothetical future possibility,” he said. “This is something that’s emerging before us right now.”

Erik Brynjolfsson, an economist at M.I.T., said, “This is the biggest challenge of our society for then ext decade.”

Mr. Brynjolfsson and other experts believe that society has a chance to meet the challenge in ways that will allow technology to be mostly a positive force. In addition to making some jobs obsolete,new technologies have also long complemented people’s skills and enabled them to be more productive – as the Internet and word processing have for office workers or robotic surgery has for surgeons.

More productive workers, in turn, earn more money and produce goods and services that improve lives. “It is literally the story of the economic development of the world over the last 200 years,”said Marc Andreessen, a venture capitalist and an inventor of the web browser. “Just as most of us today have jobs that weren’t even invented 100 years ago, the same will be true 100 years from now.”

Yet there is deep uncertainty about how the pattern will play out now, as two trends are interacting. Artificial intelligence has become vastly more sophisticated in a short time, with machines now able to learn, not just follow programmed instructions, and to respond to human language and movement.