## July 30, 2009

### Microgravity (debate, geekery)

by at 7:55 AM
I really hate the term "microgravity." It is supposed to convey that people in orbit around the Earth are only subject to a small, immeasurable amount of gravity, but that simply isn't the case. They're actually subject to a rather large amount, it's just that their "ground" is also subject to it and is falling just as fast as they are. There are subtly immeasurable variations in the local gravity field due to the spaceship itself, but there is nothing "micro" about the total gravitational force, aside from how it would be measured without a known external reference (like the big honking planet right outside the window, which I think qualifies).

Personally, I prefer the term "freefall," since it explains exactly what's going on and follows every implication perfectly (relativistic effects included). What do other people think?

#12274 07/30/2009 09:22 am Relativity
If I recall my relativity physics correctly, there is no conceptual difference between situations where you are falling and thus don't experience forces, situations where you are balanced between masses and thus don't experience forces, and situations where you are between two masses, and thus don't experience forces.
#12275 07/30/2009 10:45 am
Right, exactly why I prefer "freefall."
#12277 07/30/2009 06:29 pm
The way I see it is, microgravity is just a term describing how many Gs an object is experiencing, and isn't inherently related to space travel. For example, those flights where they simulate weightlessness by having the plane fly in a parabolic arc. The plane and its passengers are all under the effects of the same gravitational forces as always, of course, but the passengers are effectively weightless at 0.1 Gs or whatever. For the duration of the parabola, the passengers are experiencing microgravity.
#12279 07/30/2009 08:36 pm
Right, but the vessel is falling at the same rate as them - they aren't experiencing "micro"gravity but no gravity, at least per the equivalence principle (which is one of the underpinnings of general relativity). Any "gravity" you experience on the vomit comet (within your reference frame, anyway) is due solely to subtle fluctuations in the airplane's path due to turbulence and the like.

From the external reference frame, however, in the vomit comet you still experience gravity of roughly 9.8m/s^2.
#12289 08/02/2009 03:24 pm
What about negative connotations e.g. "the economy is in freefall"? 'Microgravity' is redundant except in not implying imminent disaster.

On the topic of using more accurate but less intuitive terms for things, i always liked Gene Wolfe's use of "Urth-rise" instead of 'sunset' in the New Sun books (in the books the Sun has dimmed so much that you can see the stars during daytime, so it's obvious that it's the Earth that's moving).
#12290 08/02/2009 04:49 pm
Yeah, I guess "freefall" does have a "splat" connotation involved, doesn't it... there needs to be a better term, I suppose. Neutral acceleration?

In Unity they say "zero-V" (at least in some of my scripty plotting) because they think of things in terms of tangent velocity, but there's a lot of basic physics they've forgotten (or rather, never had a reason to rediscover).
#12292 08/03/2009 01:19 am
Given that we already use "Gs" to describe force with respect to the immediate surroundings, I guess "micro-G" would be reasonable, and shorter, as a bonus.
#12293 08/03/2009 08:32 am
I think the reason for "microgravity" wasn't because of negative connotations about free fall but as a response to "zero gravity", which implied a complete absence of forces.
#12294 08/03/2009 09:11 am
Right, "zero-G" is clearly incorrect. I was proposing "freefall" as a better alternative to "micro-gravity" which is still incorrect for many of the same reasons as "zero-G," IMO. But then xbat pointed out that "freefall" has a connotation which makes it maybe not better after all.