Old Twentieth by Joe Haldeman
The background for the story is the development of a biological self-repair system that makes it possible for humans to be free of old age and disease. It doesn’t prevent people from dying of traumatic injuries, but otherwise they tend to live indefinitely. The treatment was very expensive - limited to the wealthy. Resentment of this lead to a war, during which a man-made plague killed over 90% of the population. Most of the survivors were the "immortals". New society(ies) grew among those remaining.
In this context, an interstellar expedition is sent to another star that appears to have an Earth-like planet. (The planet appears to have characteristics that might make it habitable, but its true habitability can’t be determined in advance.) A group of starships fly together in formation at relativistic speeds. There are shuttle ships that can take people from one starship to another in the group. It’s an interesting starship configuration. The trip will take hundreds of years.
A major pastime for the travelers is what is called "the time machine," but it is actually a very life-like virtual reality system that interacts with a person’s brain. It's called "the time machine" because the virtual reality scenarios are mostly 20th century settings that have been carefully crafted for accurate portrayal of the time and place. The 20th century is a time that many are fascinated with because it is before the availability of “immortality” and before the war / plague that decimated the human population.
As the expedition flies off to the stars, they continue to receive transmissions detailing how to portray additional times and places during the 20th century, and additional information to improve the realism of scenarios they already have. When new or updated scenarios are prepared for use, crew members who work with "the time machine" do test runs and look for aspects that need to be fine tuned. While preparing 1929 New York City, they find some people "going there" return with headaches and test runs also suggest that the scenario lacks background odors of the city.
The crew know this is an indication more work needs to be done on the scenario, but it’s not clear that it should be unavailable to users. However, when a user dies while in the “time machine”, a serious issue arises. It’s not clear why the person died and what role the machine did or did not play, but death is such a rare thing that it draws a lot of attention. The travelers become divided. There are those for whom “the time machine” is an important part of their lives. They oppose having the machine shut down and don’t want to give up their scheduled appointments to use the machine. Other people, who don’t tend to use the machine, view the machine’s users as addicts. They advocate shutting down the machine at least temporarily – and some think it should be dismantled.
Jacob is one of the central crew members maintaining the “time machine” and he enjoys using it both for his job and leisure. He is especially motivated to find the explanation and find a way to keep the machine operating. As things develop, another person dies, while not using the machine – raising more questions. On top of that, while Jacob is hooked up to the system to check things out, a person in the virtual reality speaks to Jacob claiming to be the intelligence of the “time machine.” Not only is that not normal for the system, it’s not supposed to happen. All leading Jacob to a strange solution to the puzzle.
The book has some interesting food for thought, but the book’s short length limits its potential. For those of us who are nitpickers for consistency, you may find that not all the threads match up at the end. The book is worth reading, but you may wish Haldeman developed some parts further.