 Why can't Loy Chuk use time travel to send Ned back to Earth before his death?
THE ETERNAL WALL By RAYMOND Z. GALLUN A scream of brakes, the splash into icy waters, a long descent into alkaline depths ... "See you in half an hour, Betty," said Ned Vince over the party telephone. "We'll be out at the Silver Basket before ten-thirty...." His old car rattled and roared as he swung it recklessly around Pit Bend. Another car leaped suddenly into view, its lights glaring blindingly past a high, up-jutting mass of Jurassic rock at the turn of the road. Dazzled, and befuddled by his own rash speed, Ned Vince had only swift young reflexes to rely on to avoid a fearful, telescoping collision. An incredible science, millions of years old, lay in the minds of these creatures. His car hit the white-painted wooden rail sideways, crashed through, tumbled down a steep slope, struck a huge boulder, bounced up a little, and arced outward, falling as gracefully as a swan-diver toward the inky waters of the Pit, fifty feet beneath.... Ned Vince was still dimly conscious when that black, quiet pool geysered around him in a mighty splash. Movement was slower now, as he began to sink, trapped inside his wrecked car. Nothing that he could imagine could mean doom more certainly than this. The Pit was a tremendously deep pocket in the ground, spring-fed. The edges of that almost bottomless pool were caked with a rim of white—for the water, on which dead birds so often floated, was surcharged with alkali. As that heavy, natronous liquid rushed up through the openings and cracks beneath his feet, Ned Vince knew that his friends and his family would never see his body again, lost beyond recovery in this abyss. The welt, left by the blow he had received on his forehead, put a thickening mist over his brain, so that he could not think clearly. Presently, when he could no longer hold his breath, bitter liquid was sucked into his lungs.
 No one has figured out time travel.
 The government of Kar-Rah turned down Loy Chuk's request to use time travel.  There is no such thing as time travel.  No one has figured out time travel.  Humans took the secrets of time travel with them when they left Earth.