Hard SF : About The Genre : What Is SF?
If you're looking for a clear, unambiguous and definitive answer, you might as well stop reading now. I don't have one.
Science Fiction is a category within culture. One rarely, if ever, finds an area in culture that is pure, distinct and isolated from all other categories. Therefore, at best one has some "gray areas" around the edges where it's hard to place a work in just one category.
By at least some accounts, one factor which will cause a work to be "science fiction" is the inclusion of a gizmo which has not yet been constructed by humans. I find this definition unsatisfying. There are just too many books/movies in which no more than 10% of the story involves talking about a time machine and describing people using it to travel to the past, while the other 90% or so describes what happens in the past. Granted, the time travelers probably don't perceive the conditions in the past in the same way the residents of that time and place do, but this different point of view could be conveyed by characters who are travelers from a different country at that time, by a resident of that time and place who has become alienated from the status quo, or even in the way a narrator phrases things. As far as I'm concerned, these works should have the time travel part edited out and be put on the "historical fiction" shelf. While I don't tend to read historical fiction, there are plenty of people who will be happy to see another book in the historical fiction section.
Of course, there's no way to stop writers from writing "time travel" historic fiction, or a day-time soap opera that's set on a planet called Flarf that has 2 moons and a green sun, or a Sherlock Holmes-style detective story where the characters are brain-augmented dogs, or such. Once in a while, such might make a worthwhile novelty for someone like me. But if you film a movie twice, once when the actors faces are left natural and the second time they wear masks to look like aliens; does that make the first film "not SF" and the second film "SF"?
From my personal tastes, a story about a spaceship that lands on a planet full of dragons, wizards and elves would be better off without the spaceship and be written strictly as a fantasy.
On the other hand, suppose a book is strictly about a spaceship studying a red giant star. The writer may employ hopelessly impossible "technology" for faster than light travel and laser beam guns that can work non-stop for hours powered by 2 AA batteries, etc. It's certainly not hard SF, but there's no mixing of genres.
One is inclined to think that to be SF, the book should have at least one of the following:
In addition to the above, we have to ask where we include alternative histories. The setting might be (sort of) the past, but not the past we knew. This genre is associated with SF readers and writers, but is it SF? Perhaps this is a grey area where each person needs to decide what appeals to them.
In clothing, should a pair of shorts be considered "pants"? They are the same as the upper part of pants. A person who would wear "pants" in the winter might in summer wear "shorts" to serve the same purpose. Yet, you would not want to confuse them and wear shorts in the winter. At a given temperature or event, some people will think either pants or shorts would be OK, other people will believe only one is appropriate. Sometimes categories don't work when applied too rigidly on a broad scale.
As best as I can figure it, it's not enough for there to be something from beyond our planet or our time. To be SF, that element has to play a significant role. A story about spies in 2012 destroying secret documents using a "disintegrator" rather than a paper shredder has to have a meaningful difference from a story using a shredder -- or there's not much point calling it SF. A story about extraterrestrials needs to use them in a way significantly different from Earth human characters. (Perhaps when they are used in a parable, where in the past animal characters would have been used, in order to make a comment about humans more palatable that is a "necessary" use of aliens.)
To some degree, SF may be able to discuss ideas that it isn't very meaningful to handle in a book that lacks SF elements. However, for the most part, SF stories deal with people and those things related to people's needs and desires. The ideas in a SF story about humans getting insights into humans by seeing themselves thru interactions with aliens might be conveyed by a story about Americans getting insights into themselves thru interactions with an isolated stone-age tribe.
A book in which humans find the ruins of an interstellar civilization and deduce the issues that led to it's collapse won't interest people much if the causes of the collapse have no parallels or significance to human civilizations and choices. If it does have parallels to humanity, then the story could be told in terms of a fallen ancient civilization on Earth (real or imaginary).
And then there are SF books that are essentially about interpersonal relationships or an individual's internal issues.
Because so much of what we choose to write in an SF format could be written in non-SF formats, it may be hard to discern when the SF elements define the book's genre and when they are merely a language in which the author has written because he is more fluent or feels it is more aesthetically pleasing.
So, to be SF it does not have to be impossible to convey the underlying ideas in non-SF terms. However, the SF elements need to be of importance, not merely incidental to the story.