There is a method which distinguishes SF from fantasy, though a lot of the science fiction written does not go by that method, and some fantasy does.
Science fiction is the very literature of change.
I'm pretty catholic about what constitutes science fiction.
You don't think progress goes in a straight line, do you? Do you recognize that it is an ascending, accelerating, maybe even exponential curve? It takes hell's own time to get started, but when it goes it goes like a bomb.
The way you write science fiction is: you sit down at your writing machine and you open your mind to the first thought that comes through.
A lot of the cosmologists and astrophysicists clearly had been reading science fiction.
Stephen Hawking said he spent most of his first couple of years at Cambridge reading science fiction (and I believe that, because his grades weren't all that great).
People ask me how I do research for my science fiction. The answer is, I never do any research.
A lot of science fiction is science-based, and it comes about because people notice something interesting about science and work it into a story.