Old Twentieth — Joe Haldeman

The twentieth century lies hundreds of years in humanity's past. But the near-immortal citizens of the future yearn for the good old days - when people's bodies were unable to spontaneously heal, and disease and age were actual causes of death. Immersing themselves in virtual reality time machines, they are addicted to exploring the life-to-death arc that defined a lifetime so long ago.

Jacob Brewer is a virtual reality engineer, overseeing the time machine's operation aboard the starship Aspera. On a thousand-year voyage to Beta Hydrii, the eight-hundred-member crew escapes the tedium of the trip within the artificial environment of twentieth-century Earth. But they get more reality than they expected when people entering the machine start to die.

For the time machine has become sentient, evolving far beyond what its creators imagined. It has become obsessed with humanity -- and wants Jacob Brewer to enter its confines and discuss this fragile state of being called life....