They might have been covering information that would have led to the discovery until they could no longer stop it. At that point in the rise of any culture the discovery of such a factor would be almost automatic....
Taking any science as a parallel, civilization makes its discoveries as it is ready for them. The discovery of radio would have been impossible before the knowledge of electricity. Nuclear physics would have been impossible without a working knowledge of simple chemistry.
Each science stood upon the shoulders of the other. Electronics aided astronomy, mechanics aided electronics and chemistry aided mechanics. Physics gave men more information about chemistry and chemistry was a foundation stone for electronics.
How long that had been here Carroll did not care. The pertinent thing at present was the simple fact that now they were gone!
Gone because they dared not stay!
Carroll cursed. It was his fault. Whatever was being done to eliminate Terra as a threat to the aliens' ideas of aggrandizement was being done because James Forrest Carroll had been instrumental in uncovering their schemes. Had he remained in ignorance there would have been no reason for their latest plan—conquest for aggrandizement does not include extermination.
To exterminate an enemy spells economic failure. There is little glory in being the Lord of All when All consists of burned planets, dead cultures and the hollow grinning skulls of a billion billion intelligences.
Homage comes not from a skull.
There, in the moonlight of the clearing where once stood a large alien edifice, Carroll took from the back seat of his stolen car the knocked-down teleport and set it up alongside the road. He stepped into it and emerged in his asylum laboratory.