Honestly, I don't see what seperates the two genres. What's the difference between a wormhole and a portal? Potions and performance-enhancing chemicals? Magic wands and firearms? If I take a science fiction story and replace every instance of the word "technology" with "magic", does it become fantasy?
I feel like you're being purposefully obtuse here. While there isn't a clear-cut, unambiguous way of categorizing the two genres, there isn't really any clear-cut way to categorize the majority of genres. While there is some overlap in some universes that employ both genres, and some stories where the line between science and fantasy are somewhat blurred, you'd be really grasping at straws to argue that you couldn't categorize the vast, vast majority of science fiction and fantasy into their respective genres. In general, science fiction focuses more on concepts that are steeped in our current understanding of the universe. Of course, a lot of these are embellished and used to handwave, but the explanations for fiction technology is based upon science that seems plausible enough to the audience that it could potentially work in the real world. Fantasy, on the other hand, uses more supernatural explanations that aren't really expected to exist in the real world. Now, I know it could be argued that some people do believe in magic, and the proving of magic would bring it from the supernatural to the scientific, but anyone who is that desperate to be contrary that such an issue even needs to be addressed is probably just going out of their way to be confused. I've also heard it suggested that science fiction is more speculative and forward looking, often addressing the effects certain technology would have on society, while fantasy tends to be more backward looking, based more on history, society and issue that we've already faced. Genres are an imperfect way of communicating to the audience the type of story it will be, they're not really something that needs to be perfectly representative of the story because nothing could do that short of reading it yourself.
technology and terms which take from real science, of course there are stories that incorporate both and use magitech what your saying is that a wormhole is just fantasy, why not call real life astrophycists wizards then but thats true in a way isnt it? whats science to us is magic to a primitive society like those groups in ww2 who worshipped planes as gods.
Perhaps I should have phrased that question differently. "Despite the massive overlap, why are science fiction and fantasy viewed as completely separate genres rather than opposite ends of the speculative spectrum?" There's a science fiction hardness scale, but fantasy does not have its equivalent. Shouldn't hard fantasy extend into soft sci fi?
I'm not really sure what hard fantasy would be, but I presume you mean fantasy that is lacking supernatural elements, making it basically a primitive society that does not exist on Earth. If so, then that's the point where it starts to get a bit questionable as to what category it would be, but I would presume it would still be categorized as fantasy until it begins to reach more modern technological levels.
I don't know about these days; maybe there isn't any difference now. But 30 years ago, science fiction stories were centred around ideas; fantasy stories were about the milieu. You're not the only one having trouble distinguishing the difference, either. I haven't been in a bookstore in a long time that didn't have both mashed together into one section.
I think Void nailed it. While some science fiction, like Frank Herbert's "Dune" are almost fantastic in its use of telepathic mental powers, spice necessary for navigators to traverse interstellar space, etc. these were introduced as factual things founded in the natural world. He used those as a construct to depict the uprising of an obscure religiously-inspired desert nomadic people led by the Mahdi against a highly developed technological dominant power. This is eerily prescient against today's events in the Middle East; the Mahdi of Herbert's book is the Enlightened One, the Muslim end-time person who prepares the world for the second coming of Jesus and global Islamization. Herbert's work would, I think be far science fiction, with only tenuous connections to today's technology, though as noted, the scenario seems ripped from today's headlines. Gives me goosebumps, as these things were unheard of when he wrote it fifty or more years ago. Robert Heinlein, on the other hand, is the premier of the near science fiction, closely connected with today and exploring what if scenarios for future history... for example, in "The Roads Must Roll," the impact truly cheap, dirt cheap, solar power would have on the global infrastructure, and how in the near future, how mankind might have to adapt to life in space, or to life on nearby planets, all using technology that is not a reach beyond what we have today. Diane Gabaldan, in her Outlander series, used a fantasy construct, mysterious stones in various parts of the world that allowed a connection between the 20th and 18th century, but she used these sparingly, and mainly to put the eyes of the female protagonist from the 20th century on a very realistically depicted and harsh world Scotland subdued after Culloden, and revolutionary America. Fantasy, on the other hand, involves supernatural powers, some connections to non-physical worlds that we generally assign to the realm of gods or god-like beings. I am firmly grounded in science and history, so I am not a fan of fantasy in general. But I don't want to disparage it, because many are fans of the genre.
This seems like a novel way of looking at it. I (and I suspect most people) don't think about fantasy and sci-fi lying on different points of the same spectrum, i.e sci-fi is supposed to be "more believable" somehow than fantasy. Rather, sci-fi and fantasy can each be believable (or not) in their own ways. The real issue, I think, is being internally coherent, and getting the audience to suspend their disbelief. If you're looking at it in terms of "Well, science exists, and magic doesn't." then I can see how you could reach the conclusion you have. But people don't think like that when they're reading fiction! Instead it's more a matter of, "Well, I've been told that magic exists in this universe, so I'll go along with it, as long as it works in a consistent way and the author doesn't break his/her own rules for reasons of convenience or deus ex machina." So, to answer the OP's questions--what's the difference between a wormhole and a portal? Or potions and advanced chemistry? In a sense, you're right that there isn't any--they're both fictional constructs, of course. But it seems to me that the logical end of that argument is that the entire idea of "genre" is inherently meaningless, which seems like one of those post-modern arguments that is true but still meaningless. Because it's not like we're all going to suddenly stop categorizing things, etc.
I agree with @Robert Musil - I think it's pretty unusual to view scifi and fantasy as completely distinct - hence the SF/F designation, the lumping together of both under the Speculative Fiction umbrella, etc. They have a lot of similarities. They have a few differences. That's all.
Inherently it is all just an imaginative telling of made up events/people/places, so to a degree they are similar. Though there are delinquents to the spectrum. Jules Verne was considered Fantasy, and Star Wars is considered Sci-Fi. Exception being that Jules Verne wrote about things that were unknown at the time, that later turned out to be realities roughly 100 years later. Star Wars is a blurring of the two, using advanced tech and magic. And as arguably neat a light saber is, it is actually bad for a realization. Think about it a beam sword would be a horrible idea, because it would be basically a lethal flashlight of fairly long range making it impossible/impractical to use as a 'sword'. Ultimately the genre labels are arbitrary, considering Fantasy gets lumped into the Science Fiction section any way. Thus rendering the whole point moot by default, and only relevant for 'marketing' purposes. Since this isn't much of a debate, unless someone has something they need to prove. This is irrelevant.