 Why can't Loy Chuk use time travel to send Ned back to Earth before his death?
A scream of brakes, the splash into icy waters, a long descent into alkaline depths ... 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.... Movement was slower now, as he began to sink, trapped inside his wrecked car. 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. Time, the Brother of Death, and the Father of Change, seemed to wait.... The icy breeze tore tiny puffs of dust from grotesque, angling drifts of soil, nearly waterless for eons. A launching catapult for the last space ships built by the gods in exodus, perhaps it was—half a million years ago. Glacial ages, war, decadence, disease, and a final scattering of those ultimate superhumans to newer worlds in other solar systems, had done that. Loy Chuk had flown his geological expedition out from the far lowlands to the east, out from the city of Kar-Rah. He could think and dream and invent, and the civilization of his kind was already far beyond that of the ancient Twentieth Century. The dry mud that had encased it like an airtight coffin, had by now been chipped away by the tiny investigators; but soiled clothing still clung to it, after perhaps a million years.
 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.