The Difference between ‘True Science’ and ‘Cargo-Cult Science’
Your definition is good, as long as “build something with it” includes experiments to verify some prediction. It is not necessary to build practically useful devices to be a “hard science”. QM is behind so many useful devices it is “hard” in spades. But QM is extremely messy and inconsistent, and most physicists are convinced that there is a more elegant logic behind QM.
String “theory” is attractive because it was elegant and unifying. It is not yet a real theory because it has yet to make any verifiable predictions different than QM. It is not even “shortcut” QM like Copernicus’ theory greatly simplified epicycle calculations. String theory calculations are actually much harder to do than QM.
However, the time and money spent on String theory is not by any means wasted. The mathematics of String theory is hard, and many mathematical breakthroughs have been made in its pursuit. And mathematical breakthroughs always tend to turn up in practical applications *somewhere*. So the mathematics of String is actually “hard science”. It is only the question of whether the Physics it models has any basis in reality that is “soft”. That is more than I can say for the time and money spent on AGW.
