According to the relativity theory, it’s physically not possible to determine if you’re on Earth, or in a spaceship accelerating at 1 g, just by observing physical processes.
I think you're confusing special and general relativity here. The equivalence principle in accelerated systems only holds true for infinitesimal regions of spacetime.
In the sense that an infinitesimal region of spacetime is a "perfect" reference frame, sure. But for the scales we're talking about, the equivalence principle gives a very strong approximation of the situation. Remember, we are extremely small!
(Some people don't even believe the Earth is round. They think that their local tangent space is all there is too it! That's how small we are.)
The equivalence principle is the opening from special to general relativity, I don't think ajconway was confusing the two at all!
But then I got to wondering if our bodies aren't critically entangled with our Earth ecologies in various time sensitive ways, and realized that I don't know the answer to this question.
So my second question is treating the human body as being meaningfuly physically entangled with it's environment and as being in some sense unified with it. Accelerating a body away from Earth at 1g might be akin to cutting off a limb and expecting it to survive on its own. I don't know if this is the case, but it seems possible.
