# SR/GR Space-Time

1. Mar 5, 2008

### Michamus

Hello everyone,
I am a self taught (dare I say Physicist?). I have been struggling with a specific concept of GR/SR. This concept is "Gravitation" and it's explanation. As I understand it, classic Newtonian gravity in which mass attracts to other mass is not technically correct. There are "geodesics" in space-time that objects of mass follow. I have two questions in reference to this concept.
1) Why does space-time even bend for massive objects? In Illustration 1a I have provided my conceptualization of the "Rubber sheet" of space time.
[View 1a]

This is what I would initially view the rubber sheet as. Now the trouble I am having is why would the sheet bend? What compels the massive objects to bend this sheet?

2)In the event that space-time does bend (perhaps as volume displacement?) why would a lesser object (B) be compelled to move toward the more massive object (A) given a scenario in which Object B has no momentum (velocity)?

[View 2a]
As I understand it, in a zero-G environment neither object would move regardless of the curve in the rubber sheet. What makes space-time different?

I look forward to your replies as these are questions I have never had properly satisfied.

2. Mar 5, 2008

### A.T.

Curved spacetime is just a model to describe how massive objects affect their surroundings. The why-question cannot be answered without creating the next why-question.

Very well observed . The very popular marbles-rolling-on-a-rubber-sheet-analogy is misleading, because the rubber sheet represents curved space not curved spacetime. To understand the difference look here:
http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html

And then check out the links I gave in this post:
https://www.physicsforums.com/showpost.php?p=1557122

And then ask further questions, if any.