De: I am grateful to you for keeping this thread so active and interesting with all the questions and answers!
http://www.explainingwhy.com/natural-sciences/physics/explain-why-time-slows-down-in-case-of-high-gravity/
Actually being IN a gravity field is enough -- you don't need to be standing on the ground or anything like that.
The amazing and wonderful thing is that the contributions of both speed and gravity have been tested extensively. As mind-blowing as GR is, it bears up to the tests every time.
GR is actually no longer just a "THEORY of Relativity" but is considered a Law. Very few theories in science ever get enough substantiation to be recognized as a Law. What's the difference? Once something is recognized as a Law, it is expected that it will NEVER be disproven/falsified -- although it may well become a subcase of some broader, more encompassing theory.
So, for example, Newton's Law of Gravity is still a law, it is just a subcase of GR, where things are non-relativistic (speed and gravity may be safely ignored WRT scale).
If you are orbiting a planet with a significant gravity field, I expect both the strength of the gravity field (vs. black hole event horizon) and the speed of travel (vs. speed of light) would contribute toward slowing down time. And interesting question is how much would each factor contribute?
Now you have me curious. I am going to see if I can find a way to quantify each factor.
Zim: I also want to thank you for your interest in the topics. Inertia really is just the cumulative effect of all the gravity in the universe. This has been tested extensively, also.
http://physics.stackexchange.com/questions/390540/what-is-the-relationship-between-gravity-and-inertia
"Inertia and gravity are phenomena identical in nature.ā€¯ - Albert Einstein
http://www.mathpages.com/rr/s7-01/7-01.htm. "Is the Universe Closed?"
You questioned my logic that if the universe was infinite -- thus filled with infinite mass -- the inertia would also be infinite...and you would not be able to change speed/direction. The only aspect of my own logic, and Einstein's quote, which gives me pause is that the propagation of gravity is, itself, limited by the light-cone. So, I would think that mass in galaxies far enough away has not yet reached the earth ...and in a universe expanding fast enough it maybe never will?
I haven't yet found sufficient substantiation for my logic, so I will leave it open for now. Just so I know what to search for, please clarify what you mean by infinite: are you saying it is "already" infinite or only that it MAY be open (go on expanding infinitely)?
But the assertion that inertia IS gravity stands. That is what Einstein's theories of relativity were all about. And again, these are now "Law"; they may be amended but will not be overturned.
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