# Einstein and Newton and gravity

1. Apr 24, 2013

### Ahmed Samra

According to GR gravity is not a force so what is gravity according to GR? And according to Newton gravity is a force. Who's right and why?

2. Apr 24, 2013

### Crazymechanic

hi there , this question has been up here many times , i'm sure doing a search in this forums history or google will reveal alot of what you asked but to put it out for you

According to GR gravity is spacetime curvature , as to who is right and who is wrong well they both are right and they both are wrong , it depends on what scales and situations you look.
If you are concerned with everyday objects here on earth Newton gravity is perfectly fine , if you want to understand and calculate the gravitational pulls exerted in space by massive objects like stars and big planets you take GR also GR is helpful in many other aspects , GR just goes deeper than Newton could in his time with his understanding , it takes on phenomenon like redshift and that mass can bend space just as much as energy can and so on.

Have you took a look on
http://en.wikipedia.org/wiki/General_relativity ??

3. Apr 24, 2013

### Staff: Mentor

They're both right.

In the mathematical formalism of Newton's theory, gravity is represented as a force and its effects are described by the equation F=ma. In the mathematical formalism of GR, gravity is just objects moving in a straight line at a constant velocity as you'd expect them to do when there's no external force acting on them.

The predictions of the two theories are experimentally indistinguishable under a very wide range of conditions. They make different predictions across cosmological distances and under extreme gravitational conditions, and in those cases experiments generally support the GR prediction. Neither works when the masses involved are large and the distance and time scales are very small.

That makes them both right in their respective domains of applicability and only there, like any theory.

Last edited: Apr 24, 2013
4. Apr 24, 2013

### Staff: Mentor

This is not the difference between GR and Newtonian gravity. Because the force is proportional to mass Newtonian gravity can also be geometrized, this is called Newton Cartan gravity.