Ha! I was so gonna say something akin to this. So nice Except I think this is a small error in logic. In a sense all stories "involve" science as science is a part of all worlds fictional or not. So I think science fiction is more when a story is about something we currently deem un-reachible by science. Sorry if I am nit picking.
While the science may be perceived as unreachable, there are also many circumstances within sci-fi where the science involved is very much in the here and now but approached in a very different way. Hence the 'what if' scenarios. The point, however, is that, in science fiction, that science must be a pivotal part of what moves the story. Yeh. If you reduce everything to its most basic elements, everything has a certain element of science and every story asks 'what if'. But, in science-fiction, it is that very science that makes the story. It is at the core of whatever the story is about. It is the "Why?" to go with the "What if?"
To the original poster, I recommend you read Eric Raymond's (author of The Cathedral and the Bazaar) definition of science fiction, and why he defines it to be what he does: http://esr.ibiblio.org/?p=6005 In the main, I agree with Mr. Raymond.
When I call a story "science fiction" I generally mean it tries to help the reader to imagine that science and technology have advanced from the real world level to a new level, thereby making certain things possible, and the story depicts those things. The more the story focuses on things made possible by fictional advances in science and technology, the likelier I am to call it sci fi. (If a story has one scene where a character casually talks to a voice recognizing AI system on his phone that happens to be self-aware, then I am unlikely to call the entire story sci fi. By that measure, CSI would be sci fi. On the other hand, if characters do that throughout the story and the story is about the limitations of human-computer interaction, then it is definitely sci fi.) The "hardness" of sci fi is a measure of how hard the story tries to help the reader to imagine such progress. Even something as simple as a character who uses a time machine instead of casting a time traveling spell makes the story sci fi instead of fantasy, although just barely. That is because a depiction of machinery causes the reader to focus less on the plot and more on the applied phlebotinum that makes the plot possible (and to notice the distinction between explanation and hand-waving); therefore, it is more likely that the reader imagines, however vaguely, some kind of progress from the real-world level of science and technology to the point where time travel is achievable. (I consider it to be very soft sci fi if that is all it does.) If the story explains how the time machine works, with accurate references to real-world theories of theoretical physics (with well-thought-out modifications to the theories where appropriate), then that makes it hard sci fi, because it is trying much harder to walk the reader through the process of imagining scientific and technological progress.
The sub-genre 'hard sci-fi' involves only achievable science- so for example interstellar travel would be at sub-light speeds, take decades and involve realistic time dilation, and probably technology along the lines of sun-powered lasers accelerating/ decelerating sail craft; rather than the Star Trek/ Star Wars approach of ignoring physical rules or what is practically possible if it gets in the way of the plot. Most science fiction involves many plot devices which are more akin to magic/ fantasy- such as Faster than Light space travel. When FTL is used it is often dressed up in theoretical science which is practically unachievable due to insurmountable obstacles regardless of technology, such as worm-holes or warp drives. Taking a warp drive as one example which 'warps' space time around a craft to enable it to travel in a bubble at faster than the speed of light; there are many reasons why this is not possible, the requirement of unproven (and vanishingly unlikely) negative energy is one, the huge quantities of energy required (more than our sun produces in its lifetime) is another; but most damningly the inability to transmit a requirement to alter the warp to the wave front at faster than the speed of light means you would never be able to stop the craft. No future technology will overcome these obstacles ever.
I wouldn't think so. There's no plausible mechanism at work there. Of course, that brings up other classic examples, like The Time Machine, which also features no plausible mechanism for the time traveller to move through time.
I offered Connecticut Yankee as kind of exception, to test the question. But The Time Machine that's been considered Science Fiction for a long time, even thought you're right: it has no plausible mechanism. The Speculative Fiction pile, for those?
Yeah, I think they definitely fall under Speculative Fiction. I suppose whether something is considered science fiction, or at least hard science fiction, can change over time as the state of the science advances.
Science-fiction is: The possible and impossible. It is the collective continuum of reality through which there can be seen the possibility of the existence of possible and impossible possibilities. ...I know right? Such a mind boggler, right? Hehehe. In fact, to me, Science-fiction and fantasy are under the same category: undiscovered possibilities and impossibilities----fiction; possibilities that are not currently known, but theorized to be possible; impossibilities to be not known---possibilities to be understood as 'impossible'---that are in fact unknown and misunderstood possibilities. In much simpler words, science-fiction and fantasy are subgenres of 'fiction', but 'fiction' is really 'undiscovered/unknown non-fiction'. That's my take on fiction at least.
Rod Serling's definition is "fantasy is the impossible made probable. Science fiction is the improbable made possible."
In science fiction the concept constrains the story. Connecticut Yankee isn't constrained by the method of time travel, the time machine in The Time Machine is extremely constraining.
