# General Relativity & Newton's Gravity - the fit?

1. Apr 29, 2010

### C_Dawg

Hey Guys,

I'm a finance guy, who's developed a fascination with physics in recent years and read quite a few of the books available to non-scientist readers who want to understand this stuff.

Here's my situation:
- I understand Newton's gravity, in broad terms.
- I understand GR in broad terms.

To me, they seem contradictory and yet, from what I have read, they are seen as complementing one another. I've not been able to get my head around that. If anyone has a simple, broad explanation, or can refer me to books or other resources, I would appreciate that.

Thanks
C

2. Apr 29, 2010

### Nabeshin

Newtonian gravity compliments general relativity in the sense that GR encompasses the Newtonian point of view. That is, Newtonian gravity is a subset of the more general theory of gravity. Specifically, Newtonian gravity is valid for small masses moving slowly. Indeed, from the full theory of GR when you make the appropriate approximations I just mentioned you recover the classical picture of gravity.

3. Apr 29, 2010

### resaypi

Newtonian gravity accounts for a flat euclidean space. Provided that a particle is far away from a strong gravitational attraction and moving slowly GR provides the same results with Newtonian gravity. They are after all approximations of what really happens. GR is a better approximation than the Newtonian g. The ideas in them are rather contradictory, where Newton accepts a universal time and GR doesn't. But still if there is no strong gravitation, it is almost impossible to detect the time dilation.

4. Apr 29, 2010

### haushofer

That depends how you loop upon it ofcourse; Newtonian gravity can be casted into a geometric theory in which it is spacetime curvature.

5. May 2, 2010

### C_Dawg

Thanks for taking the time to respond to my question, all - I have a better understanding now.

I guess at the most fundamental level, the thing which still confuses me (a finance guy with no training in physics) is - Newton said that gravity is a force of attraction between all objects. To my knowledge, he was not clear on how that attraction took place. Einstein found that energy/mass bend spacetime, thereby causing changes to the trajectories that objects take.

Here is a very simple example to illustrate my question. If I drop my cup, does it fall to the ground as a result of a force of attraction from the earth, or because the mass of earth has bent the spacetime that my cup (and I) are in, thereby causing the motion when I let go of it?

Thanks

6. May 2, 2010

### Ich

The first part of the sentence is a description of Newtonian gravitation. The second part is a misrepresentation of GR.
From a GR viewpoint, the cup is not accelerated, it's still moving inertially, with no force changing its trajectory.
It's you and the ground that are accelerating, towards the cup.
That's the easy explanation for the equivalence principle: the "falling" bodies don't know whether the ground is accelerating or not, so they can't possibly react to that fact. Thus, everything "falls" at the same rate.
And it's consistent with observation: an accelerometer will read 0 for the cup, and 1g for you and the ground. GR simply says that that's how it really is, that it's not a coincidental cancellation of forces.