Acceleration is a vector. A vector is magnitude and direction. "Down" is a direction. If there's no acceleration, there's no "down". That there is no acceleration in "zero g" is a critical difference between "falling" and "zero g" in this context. Therefore, they aren't the same.
True, but which kind of acceleration are we talking about?
A cat in the "zero g" in the experiment described in the video has no coordinate acceleration relative to the Earth. Whereas a cat falling off a ledge to the floor does have coordinate acceleration relative to the Earth.
But both cats have zero proper acceleration--they are both weightless. (Air resistance will become significant at some point during a fall from a height to the floor, but cats are heavy enough that I don't think that would be significant in most falls where cats are observed to land on their feet.) And "zero g" means zero proper acceleration, not zero coordinate acceleration. So the GP is correct and my original comment was in error: cats in both situations are in "zero g" so that can't be what is causing the different behavior in the two situations.
I've never thought critically about this - but in free-fall on earth, you are falling through the air which could be used to measure the direction of the fall.
I think most people would consider “falling” to be going “down” a gravity well. Stable orbits around a gravity well, or at sufficient distance to not be influenced by it, are not what most would consider to be “falling.”
In the sense that matters for this discussion, zero g is the same as falling--both are weightless conditions. So the GP is correct and my original comment was in error; "zero g" can't be what is making the difference.
I think more precisely, the traditional definition of an orbit (stable or not) is that it's influenced by gravity. You could be in a situation where the influence of gravity was negligible (say, far beyond any galaxy) but it wouldn't be considered an orbit at that point.
I guess certain multi-body situations like Lagrange points might make it debatable about which "direction" you're falling though.
I agree with mrexroad. Falling implies downward velocity. Free-fall is a different term that implies downward acceleration. For example, a ballistic projectile fired up that then falls back down first climbs, then falls, but it is in free-fall the whole time.
> Falling implies downward velocity. Free-fall is a different term that implies downward acceleration.
The GGP didn't say zero-g "is" falling. They said it's "the same as" falling. Which, for purposes of this discussion, it is, for the reason I gave--the key common property is being weightless, i.e., free falling.
Note, btw, that "free-fall" does not necessarily imply "downward acceleration". It just means "weightless". You could be weightless, in free fall, far out in deep space well away from all gravitating bodies, so that there is no well-defined notion of "downward acceleration" in your vicinity.
Regarding your second point, I think everywhere in the universe has some direction of the gravity field, however weak it is, and that would be the downward direction. But yea in some ideal place with exactly zero gravity, there's that special case.
