At some point as TLs progress the science fiction has to become more handwavium due to the need to invent fictional physics. The important thing for fictional physics is that it achieves the desired new technology in a consistently applicable manner, the new physics needs to have parameters established.
Jump drive - opens a rift into extra-dimensional hyperspace. Is a hyperspace with different physics and different dimensions a scientific possibility? As things stand we can write the maths for it but we haven't a clue as to the technology that could make it a reality, nor the ontological explanation for it.
Damper and meson tech - manipulation of the strong and weak interactions... scientifically possible, but he mechanisms and technologies not so much...
Gravitics and acceleration compensation are the outliers - there is no handwavium for how gravitics even works or what the limits on the technology limits are, it is the real handwavium of Traveller. This is likely due to that here in the real world gravity is still being argued about. Is is quantisable? Can the gravitaional interaction be unified with the other interactions? You can read textbook after textbook where physicists describe gravity as a force, while others say it isn't, and then all the handwavium as they discus interactions...
I was reading about "mercury engines" as the motive power behind the UAPs and ARVs so beloved of UFO conspiracy theorists. Mercury has the unusual property that its electrons are moving close to the speed of light, so could adding angular momentum to mercury tip the relativistic effects of mercury electrons into the realm of new physics?
No, just no, but fun to speculate