There are clear differences, I don't disagree, I just think the difference isn't "rooted in real physics" vs "rooted in imaginary physics". The difference is more a matter of tone and general setting. Space? It's scifi. Medieval culture? Fantasy.
An author once wrote an intro to some short story. The story is part of a much larger futuristic scifi universe in which people have developed telepathy and other things through genetic means. And the specific short story was the first one he wanted to publish, and it was about a specific planet in that world, in which the whole story is basically a telepath coming into town and interacting with the population.
And the publisher returned the note that this wasn't scifi, it was fantasy. Because of course he did - stripped of the broader futuristic setting that the story takes place in, it's just a story of a wizard coming into town. Never mind that there are solid science fiction explanations for the "magic" - you don't get that in the short story.
An author once wrote an intro to some short story. The story is part of a much larger futuristic scifi universe in which people have developed telepathy and other things through genetic means. And the specific short story was the first one he wanted to publish, and it was about a specific planet in that world, in which the whole story is basically a telepath coming into town and interacting with the population.
And the publisher returned the note that this wasn't scifi, it was fantasy. Because of course he did - stripped of the broader futuristic setting that the story takes place in, it's just a story of a wizard coming into town. Never mind that there are solid science fiction explanations for the "magic" - you don't get that in the short story.