Difference: we know the laws of physics. People in the middle ages didn't. Hydrogen atoms aren't going to change their annoying properties because you ask nicely.
Correction: We know SOME of the laws of physics. We're still having difficulty with things like efficient fusion, to say nothing of hyperspace mechanics. We still haven't been able to reliably commute from Earth to the Moon, to say nothing to Mars and Jupiter.
The technologies in such efforts will make storage and transportation of necessary hydrogen more efficient. 100 years ago, one couldn't fly across any of the oceans, while we can do it today in hours (if one is willing to pay the expense).
It is hubris to think that all we know now is all that can be known. Such a declaration was made a little over 100 years ago, which has since seen the internal combustion engine, rocketry, nuclear fission, and the integrated circuit come in to development. The changes to the world of those technologies is absolutely staggering and has allowed the world's population to easily triple and more since then. If we don't lock ourselves in hubris or religious fanaticism, how far will we go in the next 100 years? The systems used in the Battletech setting were matured over 6 times that.
