Posted 10 August 2006 - 04:32 AM
While I have never been scuba diving, and hence can't speak from personal experience, I still must question how close it comes to zero g. While you are bouyant enough to not be falling relative to your surroundings, you nevertheless are being accelerated at almost 10 m/s^2. Your own weight, pressing into your wetsuit, can be felt. I can stand, or do a handstand, or lie on my back, my sides, or my stomach: all six primary directions along the three main axes of my body. Each is subtly different, but in a way pretty much the same. Once in free fall, e.g. on a roller coaster ride or while jumping from a height sufficient to provide a near-second of free-fall (e.g., from a 3m springboard), however, the sensation in my body is extremely different from the sensation I feel along any of those six directions of full gravity. If that sensation is so different from the sensation of standing on one's feet, one would expect the sensation of lying on one's side, or of doing a headstand, to be just as fantastically different. But it isn't.
That pressure, compressing your body through any gradient (top to bottom, left to right, front to back, etc.), has a very distinct sensation that you notice when it's gone, or even just slightly reduced (like when an elevator starts to descend and your body goes from 1.0 g's to 0.9 g's). And it's not just sensed in one's ears: a slight drop in the road and I can feel it in my gut and my, er, nether regions.
While I understand that scuba diving has many features which can be disorienting and/or unique, I still don't see how it can really prepare someone for true free-fall weightlessness. I can see how it can help one appreciate the "bouyancy" of one's limbs, etc., for better motor control. But that's not the same as preparing one for the sensation of free fall.