Jules Verne, the story goes, disliked being compared to his English rival, H.G. Wells. In an interview he once said:
I do not see the possibility of comparison between his [H. G. Wells] work and mine. We do not proceed in the same manner. It occurs to me that his stories do not repose on a very scientific basis. ... I make use of physics. He invents. I go to the moon in a cannon-ball, discharged from a cannon. Here there is no invention. He goes to Mars in an airship, which he constructs of a metal which does not obey the law of gravitation. Ça c'est très joli ... but show me this metal. Let him produce it.
In this comment we can see a tension that has existed almost from the beginning of Modern Science Fiction: the conflict between Hard and Soft SF.
Verne had a deep interest in science and technology; and his publisher was devoted to producing educational works for family audiences; so it's not surprising that his works would have a strong didactic vibe to them. He called his novels "Voyages Extraordinaires" ("Extarordinary Journeys"), and each book was a geography lesson and an adventure combined. He would often consult his brother, an engineer, to get his figures accurate when, say, calculating how much pressure the hull of the Nautilus would have to withstand, or how much force would be required to propel an artillery shell at escape velocity.
Wells, on the other hand, was more interested in people and society than in places and devices. He'd use scientific ideas and jumping-off points for stories, but for the most part was unconcerned with the nuts 'n' bolts of how the science worked.
Now, granted, this over-simplification does injustice to both men. Verne was no purist when it came to science and was certainly not above fudging things when it came to the plot. He would certainly been aware that the force of his space cannon in From the Earth to the Moon would have killed its passengers; and the entire plot of A Journey to the Center of the Earth depends on throwing out nearly everything 19th Century science knew about geology -- and the narrator comes out and says so, frequently! And Wells, although he does not get out the slide rule and rattle off figures with Verne's persnicketiness, has enough familiarity with the scientific ideas with which he plays to give us some grounding and the sense that he knows what he's talking about. The chapter explaining Time as the Fourth Dimension from The Time Machine is essential reading for any fan of Time Travel stories; and Griffin's description of his invisibility from The Invisible Man has just enough scientific technobabble sprinkled in to make it plausible... at least for the span of the novel.
Hugo Gernsback, publisher of Amazing Stories, coined the term "Science Fiction" (although he preferred to call it "scientifiction"). Gernsback was a radio experimenter and and inventor as well as a magazine publisher, and like Verne before him, he saw science fiction as a way to educate the public about science, and insisted on scientific accuracy in the stories he published. It bugged him when readers seemed to prefer more fantastic stories with dubious educational value.
John W. Campbell, editor of Astounding Science-Fiction, reigned over what has been called the Golden Age of Science Fiction and was possibly the most influential person on the genre of the early 20th Century. He insisted that his writers have a strong grasp of both the gadgetry and technology of SF, but also on the human part of the equation and how the science will affect society. Campbell's protegés included Robert Heinlein, Isaac Asimov and other greats of the Golden Age.
These men established the ground rules for the the genre of Science Fiction and forged the assumption that there must be a solid foundation of Science underneath the fantastic Fiction.
But as time went on, things began to shift. I think part of it was that science had advanced to the point where it could not be as easily grasped by an adolescent with a high school knowledge of physics; part of it, no doubt, was that Einstein's Theory of Relativity had forever banished Interstellar Travel to the realms of Fantasy and Handwavium. But I bigger part, I think, was that a new generation of writers had come up who were looking beyond the territory staked out by the Golden Age writers for new worlds to conquer. They turned from the "hard" sciences like chemistry and phyisics and began looking more at "soft" sciences like sociology. Even Campbell himself, always seeking to challenge both his writers and his readers, moved more and more to areas like psionics and paraspychology and even mysticism in the 1950s and '60s.
About the same time, science fiction began to filter more into popular culture; first in comics, then in radio, movies and television. In these media there wasn't time to explain the theory behind a rocket. As Gene Roddenberry put it, whe Joe Friday pulls out his gun, he doesn't pause to explain the chemical reaction that propells the bullet. And so in these media, science took a back seat to storytelling, and Hugo Gernsback rolled over in his orbit.
If one is so inclined, it is possible to sort these out into a scale of "hardness", much like the Mohs scale in geology rates the comparative hardness of different types of rocks. In fact the website TV Tropes has done just that: (WARNING: do not click on this link unless you have several hours to kill!!!): the Mohs Scale of Science Fiction Hardness At the hardest end of the scale we have Reality, with no speculative science thrown in at all. Next we have things extrapolated from and plausible according to current technology. The further along we go, the more hypothetical we get, and the more impossibilities we are willing to accept. At the softest end, we have the type of sci-fi described in Mystery Science Theater 3000:
"If you're wondering how they eat and breathe
And other science facts;
Just repeat to yourself, it's just a show;
I should really just relax."
When I was in college, many of my friends in the campus Science Fiction Club were Hard SF fans; fans of Heinlein and Asimov and Niven; the disciples of John W. Campbell who insisted on having a rigorous foundation of science undergriding their fiction. And maybe some of them were even a bit snobbish about it. The Oldest Member of the group when I was there liked to sneer at the term "Sci-Fi", which he felt should only be applied to the kind of garbage that usually came under that name in popular culture. He liked to quote the limerick:
"You ask me the reason why,
I call it 'S-F' not "Sci-Fi"
I think there's a fine line
Between Robert Heinlein
And 'Son of the Two-Headed Fly'
It was a kind of transitional period back then, between the pre-Sputnik era when science fiction was considered "That crazy Buck Rogers stuff" and the present day when it's pretty much mainstream. I think some of us were sensitive about others labeling science fiction as inferior to "real" literature. I know I was.
And so I became a SF zealot. Science fiction was NOT escapist drivel because it was based on SCIENCE! It was focused not on the past but on the Future!
Except... when I stopped to think about it, that seemed awfully pretentious. And I realized that a lot of my favorite science fiction was not based on hard science at all, or was based on old, outdated science that was no longer considered accurate, like my beloved old Jules Vernes. I realized that what I treasured most about science fiction was not that it is the Fiction of Science, but rather that it is the Fiction of the Imagination.
Indeed, that is one of the reasons why my friends considered 'SF' a better abbreviation for the genre than 'Sci-Fi'; because it could equally stand for 'Speculative Fiction', or for 'Science Fantasy'.
That was Hugo Gernsback's guilty little secret. There were basilisk eggs in the cuckoo's nest. The Garden of Science which he had so optimistically planned had tares sown among the wheat. Mixed in with his Engineers' Tales of formulae and equations, were also stories of Magic, of Whimsey, of Demons and Sorcerors, and of Dragons.
Some of the Hard SF Masters of the Golden Age wrote fantasy as well; but their approach was informed by their appreciation for science. L. Sprague de Camp wrote a novella in which an engineer finds himself trapped in fairyland. At first he considers it a "Land of Unreason" because it does not follow the laws of physics that he understands. Eventually, he comes to realize that magic does follow laws; he just need to suss out what they are. His Harold Shea stories, written with Fletcher Pratt, uses a similar conceit, that other magical worlds can be reached through the use of higher mathematics. Robert Heinlein wrote a novella entitled "Magic, Inc." set in a world where magic and technology co-exist, and Poul Anderson developed the same theme in his novel Operation Chaos. And if we go back to a previous century, probably the most famous writer of Victorian Era fantasy, Lewis Carrol, was in his day job a professor of mathematics.
And of course, we can't forget Arthur C. Clarke who formulated the ultimate loophole for writers wishing to slip fantasy into SF: "Any sufficiently advanced technology is indistinguishable from magic."