These stories may use time travel to change the past.Some set a story in a universe with a different history from our own.
Also, Hard SF can show worlds that more advanced technology may make possible.
Each subgenre is a group of stories that uses similar ideas or styles of writing.
Publishing companies and critics put works of SF into different subgenres to help describe the work to help readers choose which books to read or movies to watch. Some stories can be in two or more genres at the same time. In Alternate (or alternative) history stories, writers imagine how the past might have been different.
Landis, while mathematician authors include Rudy Rucker and Vernor Vinge. Clarke, Hal Clement, Isaac Asimov, Greg Bear, Stanislav Lem, Larry Niven, Robert J.
Sawyer, Stephen Baxter, Alastair Reynolds, Charles Sheffield, and Greg Egan.