Okay, first of all, no, that's not the fundamental principle of science. That's the fundamental principle of science according to Karl Popper. Karl Popper is

Okay, first of all, no, that's not the fundamental principle of science. That's the fundamental principle of science according to Karl Popper. Karl Popper is probably the single most influential philosopher of science, but his perspective is not the whole story. I know, I know, your high school science teacher told you otherwise, but I promise you there's more to philosophy of science than that. There's a reason they teach Kuhn and Feyerabend and so on. But even if you ignore that, or don't believe me, or whatever... You're conflating two different meanings of "speculative" here, which I think is probably whence your confusion stems. On the one hand, you're (reasonably) saying that theoretical physics is speculative, in the sense that, once you finish your modelling work, your output could be said to be a speculation about what would happen in an experiment. On the other hand, you're (again, reasonably!) describing certain proposed engineering projects as speculative, in the sense that if someone says "we'll have economically viable fusion within N years", that's a speculation. My point is that those are just very different types of speculation, they really don't have much to do with one another. It's perfectly reasonable to be someone who wants to use theoretical (a.k.a. model-based, a.k.a. speculative) techniques to try to advance human knowledge about the nature of reality, while also being someone who is skeptical about the practicality of various (speculative) proposed exploitations of that knowledge. "I'd like to advance physics in areas P and Q; I don't think engineering is likely to advance in areas X, Y, and Z". For example, Einstein was a theoretical physicist (very much not an experimentalist). He wasn't (initially) particularly bullish on the plausibility of doing engineering based on his work (and when it became clear that atomic bombs were gonna happen, he was horrified). For a later famous example of theoretical physics, Peter Higgs (and others who did the same work) were not concerned with whether or not it would be possible to design an experiment to validation the existence of the Higgs Boson, while still being vaguely hopeful that someone might one day figure out that experiment (using equipment invented decades after the proposal of the Higgs mechanism). ALSO, btw, just because someone is a theoretical physicist doesn't mean they don't think that they're uninterested in experimental validation. It means they're not working on experimental validation. Maybe someone else will figure out how to do experiments, maybe not, it's just not the theorist's problem.