Human space stations aren’t zero gravity (it’s actually 0.89g). The apparent weightlessness is due to continually falling and never hitting the ground. You don’t need to be in zero gravity to see the water form a sphere shape either, use an eye dropper and a high speed camera and you’d see similar results.
Getting a little off topic here, but general relativity is based on there being no observable difference between inertial motion and free-fall within a gravitational field. The weightlessness from free-fall isn’t just apparent. It’s identical in every way to the weightlessness experienced if you were sitting in one of the universe’s vast voids.
It is of limited meaning to say the gravity in LEO is 0.89G. That’s an Earth-centric statement, and ignores the gravitational contribution of the sun, the galaxy, the local group, and so on.... At best, you can say “a body initially at rest relative to the Earth at altitude 300km experiences an acceleration of 0.89g towards Earth”. Earth itself is in free fall around the Solar System’s barycenter, which in turn is in free-fall around the galaxy’s center of mass, which in turn is in free-fall around etc... etc...
The only gravitational experiment that could be done on the ISS to determine they were orbiting Earth (vs sitting in empty space far from any other bodies) would be to measure gravitational tidal forces.
When you create a drop of water and let it free-fall, it forms a sphere precisely because, from its frame of reference, there is zero gravity.
Getting even more off topic, your comment is completely right except for this:
“a body initially at rest relative to the Earth at altitude 300km experiences an acceleration of 0.89g towards Earth”
which is false in the general-relativistic way of looking at things. An accelerometer initially at rest at 300km would show no acceleration as it starts to fall towards earth. If the atmosphere and the earth could be removed without affecting the shape of spacetime the accelerometer would continue speeding up till it reached where the center of the earth was, then slow down, reaching a speed of zero at a point opposite from where it started, at which point it would repeat the motion in reverse. At no point would it register an acceleration. Nor would an accelerometer in orbit register an acceleration. In the general-relativistic framework, free fall means no acceleration, and the motion of a body in free fall is determined solely by the shape of spacetime. So, at best you can say,
“The shape of spacetime at 300km affects a body's motion the same way an acceleration of 0.89g towards Earth would if spacetime were flat.”
By the way, an accelerometer on the surface of the earth, e.g., the one in your smartphone, registers a constant acceleration of 1.0g in the up direction caused by the normal force of whatever the smartphone is resting on. If someone knocks the phone off a table, the acceleration becomes zero till it hits the floor. (Air resistance can also apply an (upward) acceleration to the smartphone, but is negligible at the speeds reached during a fall of only a few feet by an object as heavy and dense as a smart phone.)
> The weightlessness from free-fall isn’t just apparent. It’s identical in every way to the weightlessness experienced if you were sitting in one of the universe’s vast voids.
Also agree with this as well (I again just wasn’t as detailed in my response). I used the word apparent because that’s the established convention from all scientific papers on the topic, but you are 100% correct that it’s identical. The VomitComet flights are a great example, but a lesser known example is going downhill on a roller coaster.
> When you create a drop of water and let it free-fall, it forms a sphere precisely because, from its frame of reference, there is zero gravity.
Correct, which is what I said (just not as explicit). I now realize that the “you” could be ambiguous though, the intended “you” is the person performing the experiment doesn’t need to be themselves in zero gravity.
