Post-WW2. That's at least what I see referred to as "modern era" the most. I can't think of an invention (well, meaningful one, to be specific) that wasn't based on already previously posited or semi-proven theories. I'm not talking about inventing "the pet rock" or something like that. It's an interesting discussion, to say the least, as the process of R&D is extremely involved and very interesting.
I think it's easy to say that... given the enormous bulk of inventions in the modern era are generated from nations that prescribe to the classical university system which relies heavily on peer-reviewed academic papers and thorough research.
What about the transistor? What was it's theoretical precursor? ETA: The Lillenfield patent was before WWII -- and before production of semiconductor materials was feasible -- and the actual Bell Labs transistor was after WW2, but the latter invention/discovery didn't build on Lillenfield at all. They weren't aware of the Lillenfeld patent until they began looking into patenting the transistor on their own. http://en.wikipedia.org/wiki/Transistor
My stance is that of saying the transistor wouldn't have happened without the triode already being a proven (but inefficient) device. That, of course, goes without saying that the enormous amount of research done on electrical power and electronic signals. It's just like saying that the Wright Bros couldn't have done what they did had someone else not already invented the engine they used. They certainly didn't invent that nor did they revolutionize the process to manufacture the engine.
That's kind of why I see the most important and impressive inventions to be the ones made by some nameless ancestor tens of thousands of years ago. They had a lot less shoulders to stand on, and even the concept of "invention" was probably not around.
Wait til they get those Insight carriers up and running. We are all ****ed then. Well, not me (Hail Hydra).
