Gravity...Please explain.

[4_teddybears]

When reading about gravity, I notice that people like to use the analogy of a stretched rubber sheet whith marbles or ball bearings on top; The balls are drawn together, as they cause depressions in the sheet...

I have also read that objects in space cause a curvature of spacetime, and that the more massive the object, the greater the curvature.

I can understand the use of the rubber sheet analogy up to this point.

But now I read that objects follow the curvature of spacetime (the rubber sheet), and that this is what the force of gravity is.

My problem is, that if an object is going to "follow" the curvature of spacetime, then it has to be moving. This works fine in the analogy because there is always an external force pulling the balls into the sheet(the balls move due to the force, and therefore follow the curve.) It does not work in Reality, as there is no "down" in space.

If the object is not moving, then surely gravity (the curvature)will have no effect.
Basicy, due to the fact that one object does begin to move towards another due to gravity, according to the analogy, there must be another force(or the analogy is wrong in this respect). What is it? Don't say gravity, because thats what we're trying to explain - and how can gravity cause gravity?

The problem I have is that the analogy actualy uses an example involving gravity- to explain gravity. Sure, on a rubber sheet on earth the balls will cause depressions, But that is in one direction (down) and is caused by an external force outside of the sheet and balls(the gravity of the earth). In space there is no up and down and certainly no force pulling planets into the invisible "rubber sheet" of spacetime in order to cause the depressions(curvature).

Maybe I am trying to get more out of a simple analogy than it was intended to explain.

Please let me know what you think

[ian]

Gravity wells in 3d space are difficult to visualise in 3d so a 2d model is used. There is no 'up' or 'down' in the rubber membrane model - just 'sideways' x and y co-ordinates (ie no z-axis). Curvature of the model's surface by a mass is seen as a 'well' but the model still remains a 2d surface. This 2d model now occupies an extra dimension to show the effects of spatial curvature and gravitational attraction - viewed from above the surface still looks 'flat'. The mass makes lines curved in relation to 'straight' reference grid lines so objects travel in curved paths instead of 'straight' lines. Does this help?

quote: "It does not work in Reality, as there is no "down" in space. "

But there is gravity due to the mass creating the curvature.

quote: "If the object is not moving, then surely gravity (the curvature)will have no effect."

Why will the object not move in a gravity field?

[4_teddybears]

You ask "why will the object not move in a gravity field"

I think you misunderstood my point. I was saying that if there were
two objects in space, and that the space was curved because of them,
then why is that curvature going to cause them to move. You seem to be saying that the the curvature is the gravity. I am now thinking that the gravity causes the curvatue.

I now understand that the analogy is only trying
to explain how masses curve space, and not how they actually cause the force of gravity.

Thanks, your post has cleared that up for me.

quote: "Curvature of the model's surface by a mass is seen as a 'well' but the model still remains a 2d surface. This 2d model now occupies an extra dimension to show the effects of spatial curvature and gravitational attraction - viewed from above the surface still looks 'flat'."

Are you saying that gravity waves permeate another/other spacial dimensions, or just that if we wanted to visualise it in 3d, we would have to imagine a fourth?

[ian]

quote: "... why is that curvature going to cause them to move" - Because of the gravitational attraction between those objects.

quote: "I am now thinking that the gravity causes the curvatue" - I think you're right. As far as I understand it, the spatial curvature is the effect gravity has on space in the gravity field. Gravity causes light (photons) to travel in curved lines compared to the 'straight' path it would take if there was no gravity. This can be interpreted as gravity curving space because light has no mass.

quote: "Are you saying that gravity waves permeate another/other spacial dimensions, or just that if we wanted to visualise it in 3d, we would have to imagine a fourth?" - If you wanted to visualise gravity wells in 3d space you would have to think in 4d (although gravity wells don't occupy a 4th spatial dimension in 3d space).

Try this link for a better explanation... http://en.wikipedia.org/wiki/Gravity_well

Howzat?

[Bahnyen]

My problem is, that if an object is going to "follow" the curvature of spacetime, then it has to be moving. This works fine in the analogy because there is always an external force pulling the balls into the sheet(the balls move due to the force, and therefore follow the curve.) It does not work in Reality, as there is no "down" in space.

Yes, it is moving. If you saw its not move, it might be the condition of viewing like angle or direction of moving or speed ......

[relativelyspeaking]

It does not work in Reality, as there is no "down" in space.

could the "down" (or up) be in hyperspace which causes the balls to move?

I am now thinking that the gravity causes the curvatue.

from what ive read the curvature *is* gravity

quote: "I am now thinking that the gravity causes the curvatue" - I think you're right. As far as I understand it, the spatial curvature is the effect gravity has on space in the gravity field.

dont thinks thats right - my understanding is that gravity *is* the curvature caused by presence of mass

This can be interpreted as gravity curving space because light has no mass.

mass curves spacetime - the curved spacetime being gravity