# Einstein's warping of space-time as a theory of gravity

1. Sep 24, 2004

### Overdose

Heres what i dont understand, a massive body in space causes the space around it to warp in accordance with its shape, fine i understand that.
What i dont understand is why the massive body would warp the space beyond the close surface of it's body. Which, according to einstein it does and causes other planets near it to follow the path of this warped space.

I tried likening it to a ball placed in a bath, the water (like space) wraps itself around the ball to accomodate its shape. But the water isnt displaced beyond the ball's shape...unless the ball is rotating like a planet, in which case it would create a swirling current around it.
Maybe a bad analogy but...is this what einstein meant? that planets by rotating create ripples and swirls in space-time, and it is these that the other near by planets 'follow' and are carried along by?

Hope someone can help, as this model of gravity has never really made much sense to me...

2. Sep 24, 2004

### PRodQuanta

Place a marble that weighs as much at 1 ton in the middle of a trampoline. This warps the trampoline. Now role another normal weighing marble around the edge. It will start going in circles, or rotating around the larger mass. Maybe this analogy helps a bit more.

3. Sep 24, 2004

### Tide

The curving of space-time due to the presence of mass is greatest near the object and weakens as one moves further away. In that sense it is similar to Newton's theory.

In neither Newton's view nor Einstein's view is the rotation or movement responsible for either the force or the curvature of space although the two theories predict different effects when rotation of a body is taken into account.

4. Sep 24, 2004

### Overdose

This is what i suspected, i just couldnt wrap (no pun intended) my head around why this should be the case atall, and why space shouldnt simply close its self around the surface area of the object.
I suppose my question could be re-phrased why does mass disrupt space beyond the object's position? (in relation to heavy bodies like planets)

Last edited: Sep 24, 2004
5. Sep 24, 2004

### Tide

Now that is the million dollar question! Why is anything the way it is? In Newton's picture of gravitation the fundamental question of why mass exerts a force on another mass is not answered. Einstein merely replaces the question with a new one - why does matter curve space-time? In essence, both address the question of how the world works but the question of why it works that way may or may not even be answerable!

6. Sep 25, 2004

### juju

Hi,

It isn't necessary to believe that space/time is really warped.It is only necessary to understand that the space/time coordinate system used to define space/time in the GR model is warped with both axes being curved relative to flat space/time.

juju

7. Sep 25, 2004

### Overdose

Thats a pretty shocking answer but one i half expected,
For me its incredibly important to understand if space-time is warped or not, otherwise im just dealing with a conceptual niceity thats a waste of my time.

8. Sep 26, 2004

### juju

Hi Overdose,

The concept of the actual warping of space/time is one conceptual mapping of the theory on to reality.

Other possible mappings say that the presence of mass affects the properties of the space around the mass in a different manner, but produces the same results.

Some of the properties that have been theorized to be affected are refractive index; energy density; coherence level and intensity and phase of vacuum fluctuations; and even permittivity and permeability.

These changes might change the structure of space/time in such a manner as to produce the same results as a warping of space/time.

juju

9. Sep 26, 2004

### Garth

If the Gravity Probe B satellite experiment, being performed at this time in a polar orbit around the Earth, confirms the predictions of GR of the geodetic precession and the Lense-Thirring frame dragging precession of its four gyroscopes then it may make the curvature of space-time more than just a conceptual mapping.
Those who believe space-time curvature does not really 'exist' but is just a conceptual model that obtains the right answers are called Instrumentalists, those who actually believe in the reality of the geometry are called Idealists.
One way of distinguishing between the two is the Idealist position does not require gravitons as there is no such thing as gravitational force, just geometrodynamics.

10. Sep 27, 2004

### pervect

Staff Emeritus
Curved space-time seems to confuse a lot of people. Hopefully, at least some of the people confused by the idea of curved space-time would not be so confused by the idea of tidal forces. The idea is pretty simple, if gravity is attracting another body, and the force is going down as the distance is going up, there must be at any given point some "gradient" of the force, the rate at which the force changes per unit distance.

But here's the kicker - in empty space, space-time curvature and tidal forces are just two different ways of describing the same concept! This is probably the hard part to grasp, there are numerous illustrations of rubber sheets, and ants marching around apples, that try to illustrate it graphically, with varying degrees of success.