Space flight, for example, is something that exists today. Some works of science fiction imagine new ways of crossing the universe that may seem impossible to us now, but still operate according to scientific principles extended from what we know now.
Ultimately the point of the best science fiction is not the technologies or inventions. Like all drama, science fiction explores the human condition, but it can look at it from unexpected angles. Science fiction imagines strange challenges and opportunities for us in order to delve deep into human nature. That's why some of the most provocative science fiction starts with men and women very similar to us -- what these people do when faced with unusual crises speaks directly to who we are in our everyday lives.
"Modern science fiction," wrote one of its masters, Isaac Asimov, in 1952, "is the only form of literature that consistently considers the nature of the changes that face us, the possible consequences, and the possible solutions. ... [It is] that branch of literature which is concerned with the impact of scientific advance upon human beings." That is the one aspect that always lies at the root of science fiction -- the impact on human beings is what is explored by true science fiction.