Originally Posted by Penforhire
Hmm, the obelisk(s) in some of Clarke's most famous novels conflict with your definition of hard SF and nearly push those novels into fantasy.
It can seem like that on the surface. However, his thinking on elements like these reflects his famous theorem, "Any technology, sufficiently advanced, is indistinguishable from magic." This does not mean it is magic... just that the observer cannot, in his experience, tell the difference.
The fact that these are alien technologies, which do things we cannot understand, is as understandable as the idea that a car dropped into the Jurassic age would confound a Gallimimus. (Feel free to debate the Hard-SF likelihood of advanced aliens, however.) The concept of technology too advanced for us to understand is not the same as fantasy, or utilizing "magic," and can still be Hard-SF.