"What do people do when you give them time travel? They go right to the unhappiest moment of their life and they go back again and again and again trying to fix it," she said. "As an older person, there are more of those events. I'm 54. Do I really want to go back to the core terrible experiences and reengage with them, or do I feel like I've moved past them or around them?"

James Gunn, 90, is the founding director of the Gunn Center for the Study of Science Fiction. Gunn says for the first time in his life he can now acknowledge the fact that he won't read everything he wants to read. He'd no longer take a trip to outer space if given the chance. But he would like to revisit his childhood and see his parents when they were young, maybe offer some romantic advice to his younger self.

"If I had an opportunity to use a time machine, I probably would," Gunn told me. "I'd maybe tell my younger self to be a little more sociable, less bookish."

Later, having given the question more thought, he emailed with more ideas about his itinerary through time. "My older son died of problems associated with colon cancer. I wish I could go back and make him have a colonoscopy as soon as problems began to appear. Or put a light in the hall so my wife didn't fall over our cat in the middle of the night and break her leg. Or [tell] my younger self not to go out golfing with my brother in shorts and get a bad sunburn on my legs... Nothing that would affect the course of life but would avoid pain."

But the Pew study offers another interesting layer by which to assess differences in generational attitudes because researchers also asked how people feel about real-life technology.

The older respondents who were less likely to say they wanted time machines were also much less interested in future inventions of any kind. Some 15 percent of 50- to 64-year-olds and 15 percent of those who are 65 and up said they weren't interested in any futuristic inventions. Even larger percentages of those groups (25 percent and 41 percent, respectively) said they simply didn't know what futuristic device they might want.

These older adults were as optimistic as younger people about long-term changes associated with technology, they just weren't as engaged with the specifics. In other words, Americans older than 50 are less enthusiastic about both emerging technologies and imagined ones.

"Technology changes people," Gunn said. "I find myself at this stage in my life saying I don't really need a cell phone. I don't need a tablet. I'm far more interested in simplifying existence rather than complicating it."

The interplay between science fiction and reality can be revealing. How we think about technologies that don't exist is directly connected to how we think about the devices that already do.

Maybe it's just the realism that comes with life experience. The Pew study asked respondents for all kinds of predictions about the next 50 years of science technology, and the oldest cohort tended to have more modest ideas about what might be achievable.