# Problems w/ Model of Gravity

1. Aug 18, 2004

### Tom McCurdy

Alright,
I as all students have heard the model of space and the idea of gravity as the sheet being pressed down by the bowling ball, which represents a fluxuation in space time which causes other objects to be attracted to it. The problem I have with the model is it really sells the theory short on what we are trying to get away from. The whole point of the model is to move away from our inherent belief that things just should go "down" and yet in the model itself what is it that "forces" the other objects surrounding the bowling ball down... its the same idea that things just should go down. Also a lesser problem is a the dimensional reduction of the model reducing it from 3 d to 2 d. I was wondering if there were any alternate models to describe gravity. It just seems to be missing the point with the current model.

2. Aug 18, 2004

### mijoon

I agree ! That is a horrible, misleading model.
I sometimes try to illustrate the phenominum by drawing a euclidian grid in one colour to illustrate spacetime as seen by a small free-falling body and then super-imposing a curved grid in another colour to illustrate the same spacetime as seen by an observer on the surface of a near by large body. I then draw a motion vector and try to explain how the same vector is seen differently.
But I only have varying degrees of success.There must be better ways.

3. Aug 19, 2004

### chroot

Staff Emeritus
The rubber sheet analogy is not a model, it's an analogy of a model. It has limitations, and you are complaining about one of them.

A more accurate description of how gravity works is to think of two ants crawling on an apple. The two ants both start at the equator of the apple, and both march due north. The ants follow great circles on the apple's surface -- the straightest possible lines on the apple. Both left the apple's equator at right angles, travelling due north, but -- remarkably -- their paths continue to get closer and closer together, until they bump into each other at the north pole of the apple.

The ants could just as well have said that some force pulled them together, even though both did nothing other than following the straightest possible lines on the apple. They might call the force that pulled them together 'gravity.'

As you can now understand, general relativity does not really consider gravity to be a force at all, and nothing pulls anything else together. Space is (positively) curved by mass, and mass moves in the straightest possible way through that curved space. The result is an apparent force that pulls things together.

- Warren