> Every moving thing on the planet does the same thing. However the net effect
> is
> zero..
Reciprocity is obviously broken for effectively-reactionless
accelerations however. Let me try restate the conundrum more clearly:
• gravity's a mutual attraction between masses / inertias as observed
from the zero momentum frame
• from within either inertial frame it's a uniform acceleration
(Galileo's principle)
• a hovering UFO exhibiting no reaction matter is nonetheless a
massive body in a gravity field, thus being accelerated downwards at 1
G like anything else
• ..it's just also applying a cancelling 1 g upwards acceleration..
• ..yet because this acceleration is effectively reactionless, the
craft is now towing the planet
So although it appears, from ground observation, that the craft is
hovering motionless, in reality it is specifically holding height
relative to the ground / planet, and although it's not accelerating
towards the earth, there's nothing to stop the mutual gravitation of
the planet back into the gravity well of the suspended craft..
The instant you have a unilateral force or momentum change active in
an otherwise-closed system, the net system momentum is no longer
constant..
So if a ship's hovering over earth, counteracting its own gravitation
does nothing to impede the mutual gravitation of the planet, relative
to which if it is holding distance, it must, therefore, be
accelerating away from at equal speed to its approach.
Hovering ('anti-gravity' in the naive conception), reactionless
propulsion or energy creation / destruction via the exploitation of
unilateral forces, alters the planets resting momentum state.
You could arguably undo a change afterwards by applying an equal
opposing change some time later, but any non-zero period between
alters our trajectory or axis or spin rate or whatever over what it
would've been if we'd stuck with aerodynamics and rocketry..etry
