Back in the 1960s we had a tv show called “Star Trek.” It starred William Shatner as James Tiberius Kirk, the captain of the starship Enterprise. And for one hour every week Kirk and his crew of four hundred able-bodied men (and women) warped their way through the galaxy seeking out new life and new civilizations and boldly went where no men had gone before.
And wherever they went Lieutenant Uhura kept them (mostly) in contact with the Federation using a form instantaneous contact called sub-space communication. I say mostly because something called sub-space interference would be invoked whenever it would be useful to the plot for the Enterprise to be incommunicado. This would allow Kirk to show initiative and break with inconvenient policy like the Federation’s Prime Directive against interfering in the technological development of more primitive civilizations. Of course, Kirk broke this rule about a million times in the course of the show. But you know, there was dramatic tension while they sorted out their feelings about this rule.
But getting back to sub-space communication, it was instantaneous. Which was needed to make the stories work. After all, radio waves are electromagnetic waves and propagate at the speed of light. The Enterprise travels many times the speed of light (maximum about 1000 times the speed of light). So normal radio communication is essentially useless for them. They could beat their radio message back to Earth by a factor of as much as a thousand-fold.
But here’s the thing. Suppose you want to write a story where faster than light space travel is impossible but instantaneous communication with other stellar systems is possible? What would be the dramatic possibilities of such an arrangement. Well, basically, it allows the writer to imagine radically different cultures and organisms sharing ideas and information across the entirety of the cosmos.
You could have a supremely old race teaching a young one how to avoid all of the dangers that it experienced. And they could teach them about their future. Suppose a race on a planet is circling a star that is about to explode. What would they want to pass on to the universe? Maybe some religious insight or a supremely beautiful musical composition or maybe just their name; so, they would not be forgotten after they die.
Now of course instantaneous communication is as completely untethered to scientific reality as warp drive is. But I can still see how using it in a story without FTL space flight would present story lines that aren’t as well-explored as ones where FTL space flight exists but “sub-space” communication does not.
Now all this being said, I will say that Lieutenant Tawny Madison, played by Sigourney Weaver in the movie Galaxy Quest really improved on Lieutenant Uhura’s performance as communications officer. Her costume was significantly skimpier and showed much more cleavage than Lieutenant Uhura ever did. And Madison didn’t need to wear an ear piece to hear the computer tell her what the ship was doing. All she had to do was repeat what everyone else could hear over the ship’s speaker system. Now that was progress.