Hard science-fiction is the branch of the sci-fi genre which concerns itself chiefly with strict scientific plausibility. This should not be mistaken for strict scientific realism, as even the hardest sci-fi will still include technology far outside the current grasp of human ingenuity. Rather, hard sci-fi offers up a set of technologies and then strives to demonstrate how and why those technologies would be achievable, given an hypothetical starting condition that enables the tech we currently cannot create.
In Ringworld by Larry Niven, the concept of a megastructure encompassing the entire circumference of an Earthlike orbit is beyond absurd... with current materials and construction methods. Imagine, however, there is a substance with effectively infinite tensile strength. Yes, that is also absurd, but imagine it: suddenly this structure becomes plausible, albeit unlikely. Niven's novel presses hard against the limits that would allow it to be defined as hard sci-fi, but it does give a sincere attempt at conforming to the subgenre despite this borderline quality.
Artemis by Andy Weir sits at the mild, hyper-conforming end of the genre: it's set near enough into our future that every technology described already exists save one, and that one is the MacGuffin driving the plot.
Hard sci-fi often closely intersects rationalist fiction, due to the adherence to plausibility (if not realism).
Sam Hughes, Isaac Asimov, Arthur C. Clarke, Hannu Rajaniemi, Ted Chiang, Carl Sagan, Philip K. Dick, Poul Anderson, Michael Crichton, and Peter Watts all qualify as authors of hard sci-fi, although many of them range outside the subgenre (or at least verge toward the edges of it as Niven does).
Films in the hard sci-fi genre include The Andromeda Strain, Jurassic Park, Blade Runner, 2001: A Space Odyssey, and Contact. An observant reader will note that all of these films are based on novels written by authors named above; this is no coincidence. A majority of hard sci-fi to reach the silver screen is adapted from beloved written works.
It should be noted that the "hard" label implies only an effort at scientific plausibility. It implies nothing about whether the narrative voice uses first or third person perspective; it also doesn't impose a time or a location on the narrative. Person of Interest is a hard sci-fi television show set in the present day on Earth; the Culture series of novels by Iain M. Banks is set in a deep-future, post-singularity, post-scarcity version of the universe, in which interstellar travel is perfectly ordinary. Star Trek: The Next Generation is set midway between these extremes: a countable number of centuries into the future, with interstellar travel, but post-scarcity society has only been achieved within the Federation, and machine intelligence has only been elevated to sapience in a mere handful of cases. All of these works contextualise their technology in a way that aims at plausibility and immersion in the narrative, avoiding claims so outlandish that they'd pull the reader's attention away from the story.
Science fiction which does not strive for plausibility, either by failing to explain and contextualise the technology demonstrated, or by explicitly hand-waving it as "magic" or "too advanced for human minds to grasp," is called soft sci-fi, and it includes most works by Douglas Adams, Ray Bradbury, Yoon Ha Lee (an interesting blend of quasi-rationalism with soft sci-fi), Ursula K. LeGuin, and Frank Herbert.
These terms were coined by Australian scholar Peter Nicholls in the late 1970s. He assigned them one additional point of differentiation, on top of scientific plausibility: social and psychological emphasis. Nicholls asserted that sci-fi was "hard" if it focused on the "hard sciences" of physics, chemistry, astronomy, mathematics, pathology, forensics, and evolutionary biology... and it was "soft" if it focused on the "soft sciences" of linguistics, memetics, sociology, anthropology, philosophy, and psychology. In particular, it is "hard" or "soft" if it defers more strongly to one of these categories than the other, or focuses on one to the exclusion or "hand-waving" of the other. Many soft-science focused works of sci-fi are still hard sci-fi, and vice-versa, and many works, such as the films Her and Ex Machina, qualify as both, due to any combination of scientific (im)plausibility and foci of disciplines. A work of sci-fi can be "hard" in one respect and "soft" in another.
Iron Noder 2017, 16/30