Return to Tomorrow was published in 1954, towards the tail end of Hubbard's "Golden Age pulp writer" period but before his "criminal cult leader" period began in earnest. It actually debuted four years earlier as To the Stars, a two-part story in the monthly Astounding Science Fiction, which is why the only Wikipedia page for "Return to Tomorrow" refers to a Star Trek episode. And I think this means that the book cover is lying to me when it claims this story is "An ACE BOOK Original Novel, Never Before Published." Not even an asterisk explaining "as a complete story" to cover their asses.
As you can see from that cover, this here's a vintage sci-fi story. We've got classic rocket ships landed on their tail ends, we've got sleek weapons that can only be ray guns, and we've got useless fins on the helmets of the "space suits," which I refer to with sarcastic quotation marks because the things don't even have gloves. At least put on some mittens, dearie, space is cold.
Our story hook on the back cover is "Shanghaied Into Eternity" - our protagonist Alan Corday is doomed to be captured by a Captain Jocelyn to crew a spaceship, since "not even wealth and women were sufficient to fill his crew vacancies." That's because of the terrible price you pay for sailing between worlds, which will be elaborated on later. So we're due for a mutiny, swashbuckling pirates, and "the acrid fumes of a war of annihilation" as Mr. Corday tries to get home.
Also, I'm tickled that "women" is listed as a lure to entice sailors.
"You there, strapping young lad! Would you like to leave your old life behind and sign on for months of labor inside a cold steel shell wrapped in the colder void of space, possibly to never see your home world again?"
"You might get laid."
But what is this fearsome price that all star sailors must pay? Perhaps the prologue will elaborate.
Space is deep, Man is small, and Time is his relentless enemy.
Do we have to be so disparaging? Can't we say that Man is big, but Space is just bigger?
In an ancient and forgotten age,
The early 20th century.
he first discovered the barricade. Before space travel first began he knew the barricade was there. It was an equation. Without the equation, the basic equation of mass and time, Man could not have progressed beyond barbaric fire.
Er... I think we had at least made it to the Medieval Age, maybe even the Industrial Era, before busting out this particular physics equation.
But he did progress
Oh, well, objection withdrawn then. I mean, if you are going to immediately contradict yourself...
and he used fission and his mechanicians
Did you mean "machinations?"
became mighty and his hopes large. But the terms of his salvation were the terms of his imprisonment as well.
AS MASS APPROACHES INFINITY, TIME APPROACHES ZERO.
Ah, I see. As Man sits on the couch and eats microwave dinners, he gains weight exponentially, but his time runs out and he dies of heart failure.
The narrator credits this equation to Lortentz and "Fitzgerald" and especially a "theoretical philosopher" named Einstein. With the power of this equation that Man fellow was able to colonize the Solar System, yet at the same time it "almost denied to him the stars." And by "almost" I mean "didn't."
And yet, despite the difficulty derived by these great men and confirmed first by nuclear physicists and then by actual use, there were still those who accepted and yet defied the law, a small cohort of ships and men who, throughout the ages have kept the routes alive. The outcasts and pariahs of extra-atmosphere travel, the cursed and shunned by Man, they course their lonely ways, far-bound but prisoners too, shackled fast by Time.
Knowing well their waiting fate, who would volunteer to become part of that thin group?
Er, the outcasts and pariahs, those cursed and shunned by-
But amongst the societies of man there are always those who are outcasts from character or force of circumstance, and there are adventurers who will not heed equations. And so the stars were reached and partially explored despite the fact of those who made the runs.
This narrator is going to keep answering his own questions by repeating information he gave before asking them.
Also, my hunch is that's supposed to be "despite the fate of those who made the runs."
But let's get down to the brass tacks, whatever those are. These outcasts take what's called "the long passage" to places like Alpha Centauri, which a spaceship can reach in six weeks. Sometimes they're earnest adventurers, sometimes they're criminals and fugitives trying to evade capture. At any rate, if they ever get back to their port of origin, they'll find it irrevocably changed, because while it's a six-week voyage for them, many years will have passed planetside. This can be a bit inconvenient.
He who is gone for a century cannot well return. He knows too little. His people are dead. He has no place and he does not fit. And what may have begun as an adventure for a crew invariably ended in the same way - another passage out while behind them further age accumulated while the crew stayed young.
The only fraternity was within the ship.
The only hope was that some day someone might discover another equation, a solution to the barricade:
AS MASS APPROACHES THE SPEED OF LIGHT, TIME APPROACHES ZERO.
The outcasts of the long passage, those that stayed alive, never ceased to hope.
See, this is a story about time dilation. And true to form, the author is muddling it up.
As I've said before, my career in hard science was cut short by some bad experiences in high school, so I'm not the most qualified person to discuss this topic. But as far as I've been able to piece together, the idea goes like this - the faster you're moving compared to another frame of reference, the less time seems to pass for you by their standards. On your spaceship zipping around at Warp Whatever, you wouldn't notice anything weird, but if you went back to Earth, you'd find that your ship's clocks marked less time on your voyage than what your departure and return times on Earth would indicate. Thus, flying around at high speeds is a form of one-way, only-forward time travel.
As for where mass enters his equation, err... I think Hubbard's latched on to the notion of "relativistic mass," but I don't think this actually means you get bigger or anything as you approach lightspeed. And the "time approaches zero" bit is also perplexing, since it would imply that what, as you move fast you get bigger and then stop moving at all? It's probably better to ignore this cruel equation altogether and just keep in mind that weeks aboard a spaceship can equal years for your family back home.
Except it sounds like this story is going to be about trying to find a breakthrough new equation. This could be problematic, a story based on solving a misunderstanding of advanced physics.