According to Wikipedia, "fantasy is a genre of speculative fiction involving magical elements, typically set in a fictional universe and sometimes inspired by mythology and folklore."

To answer your title question, no, I don't believe science fiction is always fantasy. Yes, some fantasy has space aliens and robots, but not all fantasy, just as not all science fiction has "magical elements." That's why I think something set in space like Star Wars is fantasy as well as science fiction while something like Alien is only science fiction. There's nothing magical about Alien. It's set in space and in the future with Xenomorphs and androids running around, but no one's using anything comparable to the Force or any other magic.

Now much like science fiction, those rules don't seem to always apply. Ghostbusters is considered fantasy. Okay, that's fair I guess. Ghosts can probably be considered magical, especially if you don't believe in them. But why then aren't Amityville Horror or Poltergeist considered fantasy? And what about Indiana Jones? Raiders of the Lost Ark and Last Crusade both feature fantasy elements by the end and Temple of Doom has those glowing stones, but they're only labeled action and adventure. Same with Kingdom of the Crystal Skull, which you'd think would be considered science fiction since it's about aliens and was inspired by science fiction of the 1950s.

In my mind, fantasy should be easy to define. Are there magical elements? If so, it's fantasy. Sword & sorcery and some space operas mainly come to mind, but obviously those two subgenres don't cover everything. And even if it was that simple, the internet would still disagree on what should and shouldn't be considered fantasy.

Maybe Amazon has the right idea.

The Indiana Jones films are considered "low fantasy" which is a subgenre defined by magical elements intruding into our every day world, as opposed to the Middle-Earth of The Lord of the Rings or Westeros of Game of Thrones, etc. which are obviously "high fantasy". Although apparently Harry Potter is considered "low fantasy" too, I'd argue it was high fantasy because the magical and fantastical elements are dominant throughout those movies.

Interesting point about Ghostbusters and Poltergeist. Supernatural events can fall under fantasy, so it should really apply to both. The tone of the movies might have something to do with it.

I guess the "low fantasy" label for Harry Potter comes from the fact our everyday world still exists in that world. I agree with you though. It's far more about the wizarding world than the everyday world.

What if the force in Star Wars isn't magical at all, but a science that is not yet understood? What if it works via some sci-fi/wi-fi nanobot system that sends signals that cause a reaction in other, or some other scientific jargon? Couldn't most of Star Wars' fantasy elements be explained away by some unseen gadgetries?

I suppose Ghostbusters gets the fantasy tag because of its ramifications with the afterlife. Dieties and whatnot, which are generally considered fabled to our distant generation, so I can understand that. The Amityville thing is a fair point though. I guess Ghostbuster's isn't horrific enough to get away with a horror label, so the ghosts (being more light-hearted) get it lumped into the fantasy label whereas Amityville can be written off as creepy coincidences or psychological horror. But why wouldn't Ghostbusters get the Sci-Fi tag? Proton packs and ectoplasm seem very sci-fi to me, but the mystic elements somehow negate that aspect.

You aren't far off about nanobots, except Star Wars calls them midi-chlorians.

Midi-chlorians were microscopic, intelligent life forms that originated from the foundation of life in the center of the galaxy, and ultimately resided within the cells of all living organisms, thereby forming a symbiotic relationship with their hosts. The Force spoke through the midi-chlorians, allowing certain beings to use the Force if they were sensitive enough to its powers.

As you can see though, you're wrong about how the Force works. The Force is its own thing not created by any gadgetry and nothing in Star Wars has ever indicated otherwise.

The Force was an energy field created by all life that connected everything in the universe. The Force was created by life, and therefore, resided in all life forms.

You could still call it a science that isn't yet understood. For most people though, it's a magical element that will never exist. Therefore, it's fantasy.

And yes, Ghostbusters should be considered science fiction.

Sometimes I don't mind if they try to merge sci-fi and fantasy in one story, like in Krull. Other times it really bugs me. For example, I really enjoyed the book Mrs. Frisby and the Rats of NIMH as a child. Supposedly there is a Disney movie based on it, called The Secret of NIMH, and it looks like it has a bunch of fantasy bullshit worked in. I've never seen it, the movie, I mean, because I wory that it would wreck my appreciation of the original book. I know, you aren't really asking about childrens' books or movies, but it's a pretty similar issue.... I guess sometimes I feel like they are cheating me, when they introduce magic into a story that I expected to be mostly rational and possible (albeit highly unlikely, in the case of the rats becoming as intelligent as the rats of NIMH). I feel cheated because I expected to be able to understand the overall situation and the actions and plans of the characters in a completely rational way, and bringing magic into it just spoils it all for me. If I recall correctly, some of the Madeleine L'Engle books do similar things... they bring up a lot of scientific-sounding ideas, but then they story ends up being much more about magic.

This is the same type of thing that bugs me about Terminator 2. I basically do like that movie, of course... who doesn't. But, as I have mentioned here before, it bugs me that in T2, suddenly there is a whole bunch of biblical shit going on that wasn't in the first movie at all. Like, talking about Judgement Day, and the whole theme of redemption for the original model of the Terminator. It still works, and I really do like the movie, but the WAY that it works is dramatically different from the first movie.... the first movie was just a straight sci-fi story about time travel and runaway technology, with a shitload of action and general Arnold awesomeness. T2 felt like it had been written by some kind of Council of Church Elders, as a way of taming the anarchy of the first movie, and making it safer for public consumption. T2 also basically set the tone for the whole future of the franchise, in a lot of ways, with lots of biblical references to Genesis, Salvation, Arnold's character being a good guy, etc.

I don't know, this is isn't exactly what you asked, but it's what comes to mind for me when I think about your question. There's just something that bugs me about being led to expect one kind of story, and then finding out that I was misled..... but, I do like Krull, with its combination of sorcery and alien robots with blasters... I'm not sure exactly why sometimes I enjoy the combination, and other times I dislike it.

Speaking of this general type of thing, if you are a GoT fan, I happened to find an interesting early Geroge R.R. Martin story in which he seemed to be playing with the concepts that eventually became GoT. The story is called Bitterblooms, and it is set in a harsh world in which the laws of physics apply, and... I don't think there is any magic. But, having said that, it is very similar to GoT, and probably laid a lot of the groundwork for it. It takes place on a planet with unbelievably long seasons, and has large clan families who have complex relationships. It also has a few characters that seem at first to be magical, but... I won't totally spoil it, you get the general idea.

So, anyway... I think I know what you're getting at. In some ways, if it is done well, a story can be both sci-fi and fantasy. But it is also possible for the fantasy stuff to just feel like a way that the author is cheating, or using it as a crutch.