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[asterroc]

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Comments

[sildra.livejournal.com]

It sounds like you're mixing up Special and General Relativity. Special Relativity can be invoked when everything can be done assuming something is stationary in an inertial reference frame, i.e. there is no acceleration involved relative to any other inertial reference frame. It's usually used for objects moving at significant fractions of the speed of light relative to each other. Gravity, however, being force-like (and therefore appearing to cause acceleration) is necessarily non-inertial. In General Relativity, to deal with gravity as a not-force, rather than treating an object as stationary in some frame, we treat spacetime as warped and an object as moving along a geodesic of spacetime's new geometry (it's been a long time and I forget whether we pick the largest object or the center of mass of the entire system of objects to be the stationary reference frame, probably the latter). A geodesic through spacetime can only be stationary in space when spacetime is completely flat--that is, there is nothing with mass around (so in that case, it reduces back to Special Relativity).

[q10.livejournal.com]

you're right, of course, that i was mixing things up and/or being sloppy.

i was mainly remembering being taught to think of an elevator in free-fall as a canonical inertial frame, which is, i'm sure, wrong in the details, but which is really good enough a lot of the time.

[sildra.livejournal.com]

Well, it is true that if you decide to invoke curvature of spacetime rather than gravity, you can redefine inertial to mean anything moving along a geodesic in your curved spacetime, and it won't contradict the Special Relativity definition of inertial (just expand on it). In that case, an elevator in free-fall is an inertial frame, yes. It's an either-or thing, though. If you call it "gravity" instead, then it's an accelerating frame, which is not inertial. But the basic argument in favor of General Relativity (ignoring, of course, that now that the framework has been built up, GR was shown to predict new phenomena which have since been discovered) is that an object in freefall looks, in a lot of ways, more like an object in an inertial frame than like an object with a force applied to it.