According to Wikipedia, a human body can resist g-force of about $5 g$. It can be a greater value in some circumstances, but even as low as $2 g$ would be unpleasant after several seconds. This means our bodies set rather low limit to manned space travel. Even if we built a spaceship that could accelerate steady with $20 g$, it would be fatal to its crew.
We know that it's not the acceleration itself that is harmful; internal forces inside our bodies are. When a pilot experiences $5 g$, the force comes from their seat, through compressed skin and flesh; then the bones apply forces to every other pieces of flesh, they "feel" the flesh inertia; the heart builds pressure to accelerate blood; the skull pushes the brain, the brain pushes the skull; and so on. These forces create the impression of acceleration and may be harmful. Yet if accelerating forces were applied directly to every particle, the pilot would feel nothing. Gravity works this way. In free falling we experience zero-g even though we accelerate towards the Earth (or another body) according to a distant observer.
This observation led me to following concept:
.-------. /== | engine <=== everything accelerates '-------' \== to the left | | / \ / \ / large \ free falling | mass | . passenger craft, \ M / small mass m \ / \ / | | .-------. /== | engine <=== '-------' \==
The manned craft of mass $m$ falls freely towards unmanned large mass $M$. Engines accelerate $M$ just enough for the distance between the two masses to remain constant. There is of course some force from $m$ that pulls $M$ to the right. The engines act against this, so in fact they accelerate $M+m$. It's no surprise, no magic. Gravitational pull between $M$ and $m$ might be replaced by a rope or another structure that would connect the engines to the passenger craft.
The difference is: with the rope we have a force applied to a joint, then fuselage, its structure, seats, flesh, skeleton – all those harmful forces that can make screws break and lungs collapse. With gravity we have free fall, no tensions, no harm done.
I'm not asking about engines, energy source, tidal forces, economy etc. Let's assume we can build and power this set of crafts, even if $M$ should be of planetary scale.
Primary questions: Will passengers experience zero-g regardless of the whole set acceleration as I expect? or am I missing something? Do Newton's and Einstein's answers differ here?
Secondary questions: Has the concept already been discussed by scientists? What is its name (if any)?