Gravity: How do objects communicate?

[fez]

I understand that small objects are attracted to large... but how do two objects know which is which?

How does the moon know that the Earth is bigger, thus it should be attracted to earth?

 

[phyzguy]

All objects attract each other. The force of the moon on the Earth is the same as the force of the Earth on the moon.

 

[fez]

Thanks for the answer, but I'm still a bit confused. If the force is the same, why doesn't the Earth revolve around the moon?

 

[fez]

And why doesn't the Earth go chasing after passing comets?

 

[fez]

It's because the Earth is larger than comets... but how do they know that? How does my body know it should be attracted to the ground? Better yet, how does my body even know the ground is there?

 

[phyzguy]

The Earth and the moon both revolve around their common center of mass. Every object feels the gravitational force due to all other objects in the universe. It then responds to this force according to Newton's second law: F=ma . So the comet has a much large acceleration than the Earth, even though the forces are the same, because the comet's mass is much smaller than the Earth's

 

[jtbell]

If the force is the same, why doesn't the Earth revolve around the moon?

Both the Earth and the moon revolve around their mutual center of mass. The Earth has a lot more mass than the moon, so their center of mass is much closer to the center of the Earth than to the center of the moon. In fact, the center of mass is about 4700 km from the center of the earth, whereas the Earth's radius is about 6400 km.

So the center of mass of the earth-moon system is inside the earth, therefore the moon orbits around the earth, whereas the Earth just wobbles in synchrony.

 

[fez]

Aaaah, so objects are attracted to a mutual center of mass! I always thought Little was simply attracted to Big.

I now understand your first answer, phyzguy. Thanks jtbell for clarifying that for me.

Can I beg your patience to let me re-coach my original question (How do objects communicate?):
I can accept Newton's Second Law and I can accept that objects must obey, but my question is how do objects announce their presence to each other?

Lets say two balls are placed on the floor in a low gravity environment. One ball is large enough that the smaller ball is attracted to a center of gravity that is inside the big ball (slightly off center). Given low enough outside resistance, these two will warble toward each other.

But what caused Newton's Second to happen? Did the big ball pick up an atom sized cell phone and ring the little ball? "Um, yea, listen dude... I just happened to look out my window and see you over. Not sure if you know it, but we're kinda close to each other. My density is X and volume is Y... I'm pretty sure I'm a lot bigger than you. So if you don't mind, I think you should go ahead and start rolling toward me before the physics cops come by and write us up."

 

[Drakkith]

Without getting into a complicated explanation of spacetime and curvature, I'll just say that it happens because that's how it does. They are both attracted to each other. How does an electron communicate to a proton that it wants to be attracted to it? It just is.

 

[Lsos]

Yeah, Einstein said that basically a massive objects warps space-time in such a way that it alters the motions of other massive objects around it. And this model works for almost every possible scenario we can test against it.

Almost.

So ultimately, we don't know. That's just the way it is.

 

[syhprum]

Drakkith

The coulamb attraction between the electron and the proton is mediated by virtual photons, it is suggested that the gravitational force between masses is medated by gravitons but there are difficulties to this theory as the attraction appears to operate at a vastly greater speed than c.

 

[Drakkith]

Drakkith

The coulamb attraction between the electron and the proton is mediated by virtual photons, it is suggested that the gravitational force between masses is medated by gravitons but there are difficulties to this theory as the attraction appears to operate at a vastly greater speed than c.

Then I could ask the question, how do virtual photons mediate or communicate with particles for the attractive force? They just do.