[annalex]

you'd have to have some way of "turning on" the mass behind the car. I don't know of any way to do this, so yes, the fact that you can turn a dynamic acceleration on and off rapidly would probably be a good indication that the acceleration was not gravitational in nature.

the distiction I was making: between acceleration due to gravity and acceleration due to some other force

Thank you, Physicist, for piling on Luther with me, it is always fun...

We are on the same page all three. Yes, in practical terms, uneven acceleration cannot come from gravity. I agree. At the same time, an observer in an unevenly accelerating car might conclude that a gravitational force was growing behind his trunk and have all valid laws of nature observed consistent with that hypothesis. He would not be able to explain what is the source of that gravity, because planets do not emerge behind cars.

The reality, as Einstein sees it, is that in the reference frame of the unevenly accelerated car the timespace is unevenly warped. Whether the warp is inertial or gravitational is a matter of differently labeling the same reality.

Imagine a man who grows up very fast: so fast that he reaches maturity while the car increases its acceleration. That man has no knowledge other than what he can observe in a windowless laboratory inside the unevenly accelerated car. He has superintelligence that allows him to master the laws of nature, but he has no knowledge of what planets or cars are. He will observe the uneven warp of the horizontal timespace, period. He will posit two hypotheses: the Inertial Hypothesis, -- the car is unevenly accelerating; and the Gravitational Hypotheses, -- there is a growing or nearing mass behind the car. Being a good scientist he will stop at that; he will not prefer one hypothesis over the other, because, having been brought up inside his lab, he lacks the knowledge that would tell him that accelerating cars are very common and planets chasing cars are not common at all.

[stripes1776]

The reality, as Einstein sees it, is that in the reference frame of the unevenly accelerated car the timespace is unevenly warped. Whether the warp is inertial or gravitational is a matter of differently labeling the same reality.

Yes, fictitious gravitational forces emerging mysteriously out of nowhere can be used to point to real effects. That doesn't mean the explanation is convincing.

Being a good scientist he will stop at that; he will not prefer one hypothesis over the other, because, having been brought up inside his lab, he lacks the knowledge that would tell him that accelerating cars are very common and planets chasing cars are not common at all.

This is why it is good for physicists to get out of the lab once in a while. Walk down to the local pub and have a beer.