The first of Arthur C. Clarke's "laws" (like Newton, he decided to have three of them) was: "When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."
Actually, when a distinguished scientist states that something is impossible, he is almost certainly right. There have been a few exceptions -- pronouncements on heavier-than-air flight, space travel, and so on -- but they've been pretty rare.
I was reminded of Clarke's law at a recent public lecture on nanotechnology by the distinguished scientist Professor Mark Welland, FRS. In the midst of talking about the potential applications of nanotechnology, he displayed the cover of an issue of American Scientist magazine depicting a space elevator, a geostationary satellite connected to earth by a cable which could be used to send payloads up to space. He complained that any undergraduate physics student could tell you why it was nonsense, and that this sort of irresponsible hype in the press had done great damage to nanoscience.
It was particularly amusing because the space elevator is a concept long championed by Arthur C. Clarke, who wrote about it in a 1979 novel, "The Fountains of Paradise". (Soon afterwards, Charles Sheffield published "The web between the worlds" based on the same device, and Clarke supplied an afterword absolving Sheffield of plagiarism and calling the space elevator an idea whose time had come.)
So does Clarke's law apply to our distinguished scientist? I couldn't think of any physics reason why the space elevator was impossible. It would require an enormously strong and light cable, very delicate positioning and carefully-monitored corrective movements of the geostationary satellite, and a counterweight at the other end. I have no doubt that the engineering challenges would be formidable. But those can be surmounted (greater problems have been overcome through the history of space science). I know of no laws of physics forbidding such a thing.
The stumbling block has always been finding a material strong enough and light enough. Carbon nanotubes could, theoretically, fit the bill, but in practice nothing close has been achieved in the laboratory. There are many other difficulties. The Wikipedia article is quite detailed and has a good set of further links (including this article on space.com).
Professor Welland is hardly the only skeptic, but his statement that "an undergraduate would tell you it's nonsense" is unusually strong. Is there a fundamental reason, that has eluded me (and many engineers and investors), that it won't work? I'd love to know. Or will Prof. Welland prove to be an instance of Clarke's law? Time will tell.