# Curvature of spacetime

1. May 31, 2005

### roger

we are doing forces and gravity , but what I dont understand is when the sun curves spacetime for the earth to orbit this curvature, what causes it ?

roger

2. May 31, 2005

### Dr.Brain

If you mean what causes the bend in spacetime fabric, it is the matter itself. In this case the Sun distorts the spacetime fabric and the earth moves under its influence.

As I have put earlier:

"Matter tells fabric how to curve, and distorted fabric tells matter how to move"

3. May 31, 2005

### roger

could someone give me a precise answer ?

4. May 31, 2005

### Ouabache

A simple analogy for what Doc has given you: lets use the surface of your bed for example, with the assumption you have a soft mattress.

Let's use a bowling ball (6.5kg) as the model for the sun and set it in the center of your bed. You will notice that it warps the blanket resulting in a depression around it.

Lets take a marble as model for a earth and place it near the depression of bowling ball. Depending how close you place it, it will roll and accelerate towards the bowling ball. If the marble were set in motion around the bowling bowl, it will circle around the depression (like an car on a banked curve).

All objects will cause a depression (warp) in the blanket to different degrees relative to its mass.
Though this is a 2-dimensional model, it is analogous of how the sun warps the fabric of space-time, causing any object orbiting near its depression, to experience a force towards it.

Last edited: May 31, 2005
5. May 31, 2005

### HallsofIvy

Staff Emeritus

How "precise" an answer do you want? In the very first response, Dr. Brain said "mass". That's about as "precise" an answer to the question "what causes gravity" as you are going to get!