Title isn't very helpful for what I wanted to ask, sorry. Also, the question has no basis in current technology and, possibly, no basis in reality ever.
The question I'm wondering about is in regards to the effects humans feel during acceleration, and possible scenarios where the felt acceleration would possibly be zero.
As we all know, accelerating the human body too rapidly has a variety of side effects, including loss of consciousness and death. This (typically, although extreme accelerations probably vary in exact cause) has to do with the blood exerting too much pressure on the brain, or too little blood making it into the skull.
With that being said, what would happen if the entire body (including fluids, like blood) could be accelerated at the same rate as the craft it resides in? I imagine there would be no perceived acceleration, but would there be any physiological responses? A technology capable of accelerating all matter inside its influence could, if I'm not missing some detail, be used to achieve insane linear accelerations in manned craft without killing the occupants.