Caption: An axiom of general relativity (presented by Albert Einstein (1879--1955) in 1915) is that there is NO way in principle to distinguish motion in a reference frame accelerated relative to an inertial frame with NO gravitational field and motion relative inertial frame in a uniform gravitational field.

Features:

1. In the image, one person is in rocket uniformly accelerated relative to inertial frame in outer space and the other is at rest in inertial frame with a uniform gravitational field. There is NO measurement they can do in their respective closed rooms that can tell which room they are in.

   So the person in the rocket (if they know where they are by looking out window for example) can convert their non-inertial frame into an inertial frame by invoking an artificial gravitational field.

2. In fact, in any non-inertial frame (i.e., any reference frame accelerated relative to an inertial frame) can be converted into an inertial frame by invoking a non-ordinary force that is mass-dependent like gravity and this done all the time when convenient (i.e., usually when the acceleration is smooth enough).

   Such non-ordinary forces are called inertial forces.

3. Note, ordinary forces are relationships between physical bodies or between a physical body and a force field.

4. Inertial forces in one sense are a trick for converting an non-inertial frame into an inertial frame. However, general relativity tells us there is a fundamental LIKENESS between inertial forces and gravity just as the image illustrates. This fundamental LIKENESS was, in fact, one of Albert Einstein's (1879--1955) starting points in developing general relativity (1915).

5. If it is NOT convenient to convert a non-inertial frame into an inertial frame, you can always find a different reference frame (often a larger reference frame) that is an inertial frame.

6. The upshot is that there is an inertial frame in which to analyze systems where you want one either by conversion to an inertial frame or by just finding an inertial frame.

   Sometimes getting an inertial frame for physical analysis can be tricky, but usually NOT.

7. Recall all physical laws in the observable universe are referenced to inertial frames, except general relativity which tells us what inertial frames are. And this statement include converted-to inertial frames using inertial forces.

8. You may wonder if all this formalism is rather hard to remember since it takes so many words to explicate. At first, yes, but physicists and astronomers just get used to.