Indeed, theoretically, your invention must have been "reduced to practice" and not just a good idea. However, adequately describing your invention in some embodiment in the patent application is "reduction to practice". It is up to the examiner (much overworked) to make sure that your description of a "preferred embodiment" is actually possible, and that it actually embodies the invention.
It IS also possible to invent and patent something that cannot be physically realized with today's technology, although it might be in the future. For instance, nuclear powered airplanes have been patented (by Feynmann, et.al.), as were nuclear powered ships and subs before their actual physical construction.
Or, you could come up with some invention that requires a 1 Teraflop computer in a cubic meter to do the necessary calculations. While you couldn't buy or build such a computer today, it is certainly a realistic expectation that in the not too distant future you might be able to build one.
The take home summary is that you CAN patent a good idea, for that is what an invention really is.
Created on Tuesday, February 20, 2001 1:14 PM EDT by James Lux