Arthur C. Clarke? Arthur Kantrowitz? Apocryphal?

Question for Quote Investigator: Science fiction luminary Arthur C. Clarke described the audacious idea of building an elevator from the surface of the Earth straight up into space and beyond geostationary orbit in his 1979 novel “The Fountains of Paradise”. The megaproject would require extremely strong lightweight material, and some engineers and scientists have questioned its feasibility. Clarke puckishly said that the space elevator would be built a few decades after people stopped laughing. Would you please help me to find a citation?
Reply from Quote Investigator: In 1979 Arthur C. Clarke addressed the 30th International Astronautical Congress and surveyed the concept of a “space elevator” which has also been called an “orbital tower” or “heavenly ladder”. Clarke’s speech was printed in the journal “Advances in Earth Oriented Applications of Space Technology” in 1981:1
What I want to talk about today is a space transportation system so outrageous that many of you may consider it not even science-fiction, but pure fantasy. Perhaps it is; only the future will tell. Yet even if it is regarded as no more than a ‘thought-experiment’, it is one of the most fascinating and stimulating ideas in the history of astronautics.
Clarke speculated about the time needed to develop the space elevator. Clarke stated that his vivid quotation was modeled after an earlier remark made by Arthur Kantrowitz who was an influential proponent of an innovative idea for space transportation called laser propulsion:2
And when will we have that? I wouldn’t like to hazard a guess, so I’ll adapt the reply that Arthur Kantrowitz gave, when someone asked a similar question about his laser propulsion system:
The Space Elevator will be built about 50 years after everyone stops laughing.
A separate article focused on the remark attributed to Kantrowitz is available here.
Below are additional selected citations in chronological order.
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