1970s author: this is hard sci-fi, so it stays only within the limits of what's scientifically proven as possible: slower-than-light travel, the impacts of weightlessness, and telepathic powers.
That always bugged me about Star Trek. It's established cannon, but mind melds and telepathy never seemed to sit well with me for fitting into the setting.
it was an easy shorthand for making characters that were clearly no longer human beings, even if the the people in the story on both sides hadn't realized it yet.
as soon as frank poole got his wildly enhanced telepathic powers, he was no longer human.
psi corps was ok with killing all the mundanes.
Poul Anderson, who was certainly guilty of the above, had my favourite definition, something like: "science fiction based on the most implausible premise: that current scientific theories are correct".
(I wonder if you could set up a version of Hempel's dilemma for definitions of hard sf...)
I've said this about STAR WARS a lot: a huge change in the context of how audiences respond to it is that audiences in the 1970s understood "The Force" to be something that plausibly existed in our own universe rather than a type of fictional magic.
The only works I can think of that meet all those qualifications are some Heinlein stories, which almost always introduce FTL in later stories of the same setting.
Omfg a real life relative of mine unironically said on the way back from Dune that it was interesting and about real science unlike Star Wars which was about how "lIgHtSaBeRs aRe cOoL!!!", hard sci-fi is when space cocaine gives you superpowers apparently
> Philip Kindred Dick has entered the chat.
{STRAWBERRY FIELDS FOREVER, MOTHERFUCKER!]
I mean, Phil ain't wrong. And if he's wrong, I don't want to be right when I write.
H. Beam Piper believed in reincarnation and wrote a very didactic short sci-fi story where he explained why reincarnation undermines the case for economic redistribution (because it allows anyone to be born to wealth and privilege if they so choose).
now that we are on our generation spaceship to another galaxy, let me bring out this film projector and show you a little movie about eugenics
the space stewardesses will bring you space coffee, gentlemen
have you ever read the RPG DAMNATION DECADE? It has the conceit that all 70s sci-fi/fantasy ideas are accurate, so you have things like a secret Atlantean cult with psychic powers mixed with Patty Hearst being kidnapped by the Stepford Wives but still becoming a violent bank robber/insurrectionist.
IIRC from James Nicoll reviews, telepathy is codified as a sci-fi thing because Campbell believed in it and would pay money if you put it in a story, and since he was half the regular market you bloody put it in a story. If he'd believed in astrology instead, Star Trek's wildly different!