I think it would be useful to explain a bit more about gravity, since there are some obvious misunderstandings here. I leave it to the rest of you to
decide whether a force that we see acting everywhere in the universe is created by aliens or not.
Originally posted by chickeneater
But no one can explain what gravity truly is beyond giving this phenomenon a name. I can name anything too, I mean, Newton didn't discover gravity,
he just gave it a name, d'oh.
Gravity is a empirical phenomena completely determined by what we see. We like to describe it using some mathematical theory, because this allows us
to give predictions, but the theories always remain falsifiable and are never absolute truth. Gravity in essence is the effect we measure and there is
no further purpose or meaning to it, according to science.
In modern physics, gravity is seen as a effective force between massive objects caused by the theory of general theory. This is a theory that thinks
of empty spacetime like a pseudo-Riemannian four-dimensional manifold, which is a fancy way of saying curved space, where the curvature is determined
by the matter present. Now, you may think this is a view of the world which is completely preposterous, but the fact remains that it explains a lot of
physical phenomena on medium (several centimeters) to extremely large scales (intergalactic distances) very accurately. Misner, Thorpe & Wheeler's
book, Gravitation, which you should be able to get at any university library, has a large section (starting at page 1045, note especially box 40.4 on
page 1129) about experimental evidence.
Especially, by looking comparing the motion of stars in other galaxies with those of stars in our own galaxy and comparing this motion of planets in
our solar system, we see that general relativity explains these equally well. This shows that we can determine with high precision that the motion of
distant stars goes by the same mathematical rules that apply in our solar system. The aliens should be able to control gravity in the entire universe
for this to be true.
Originally posted by Chiiru
IMO gravity's been around for quite a long time, not only present on earth, but like everyone says, in the universe. I think its our orbit around the
sun that determines our gravity, which is why on earth we may weigh a certain number, while on a different planet we'd weigh significantly less, or
more, depending on the planet's orbit around the sun. Atmosphere also determines gravity, I think. Another reason we weigh less on the moon. Our
weight is determined on gravity pull into the 'center'.
Furthermore, one can show a good approximation to general relativity at low densities and low speeds is Newtonian gravity. In most everyday situations
(except GPS, where very high precision is needed) Newtonian gravity will give you very accurate predictions. This shows that the gravity we experience
on earth is determined largely by the mass and radius of the earth, with some very small contributions by the rotation of the earth (variation of
local gravitational constant g by latitude), the moon (tides) and the sun (spring tides).
Our orbit is determined by the mass of the sun, our distance to the sun and our angular momentum with respect to the sun. However, this orbit doesn't
influence the gravity on earth in any meaningful way (one could imagine some extreme cases with stars rotating closely around black holes where it
does matter, but here these effects are very small). Earth rotating at twice the distance around the sun would have almost identical gravity for its
The atmosphere of the earth has a very small mass in comparison to the rock and iron the earth is composed of. Furthermore, it is almost spherically
symmetric and above us. Newton's theorem for the gravitational field of a spherical shell of mass, says that inside a spherically symmetric shell of
mass, the contribution of this shell to the gravitational field is zero. Both its small mass and its symmetry show that the atmosphere of the earth is
negligible for the gravity on the surface of the earth. There is a atmospheric pressure around the atmosphere around us, but it exerts forces to all
sides of us (unless you have a vacuum between your shoes and the ground, similar to what happens with those plastic suckers you can put on the wall),
thus not contributing a net force in any direction.