# Two Faces of Gravity

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#### BuzzLY

##### Guest
I hear gravity defined in two ways; 1) an attractive force between objects or masses (ala newton) and 2) the wharping of spacetime around an object or mass (ala einstein).

In the first case it is two objects pulling at each other. It's the force that holds planets in orbit; the string if you will, that counteracts the centrifugal force of orbithing bodies. In the second case, the planets aren't being directly acted upon at all, they are just moving straight through "curved" spacetime. We know this view has merit because we have tested for time slowing near large mass objects, so with time as a fourth dimension, spacetime is wharped. You don't need "attraction per se" to explain the motion of planets.

My question is: are these two descriptions consistent with each other, or is there a conflict? If gravity is wharped spacetime and nothing more, it seems that it is not actually a "force," so can it ever become part of a unified theory of everything and "merged" with the other forces?

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#### MeteorWayne

##### Guest
They are consistent. They are just two different ways of describing the same effect. One uses older Newtonian physics, and the other using Einstein's more refined view. Either works within their own domains.

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#### ramparts

##### Guest
The curved spacetime view is the most "right" one that we know of. If you look at weak curvatures of spacetime, it ends up looking exactly like a force, so in that domain the Newtonian force view holds.

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#### yevaud

##### Guest
BuzzLY":1554xeg3 said:
...so can it ever become part of a unified theory of everything and "merged" with the other forces?
Yes. For most simple calculations involving gravity, Newton works just fine. It's only certain more complicated situations that Einstein must be applied. They are fully consistant with each other.

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#### Saiph

##### Guest
It's this very consistency that lends weight to the stranger ideas of Einstein's Relativity. When boiled down to simple terms, under normal conditions (mild gravity, typical distances etc) you can actually DERIVE newtonian gravity from it. It's called a special case. Plug in a few specific conditions and viola! the equations your used for physics since newton dreamed them up!

If relativity couldn't do that in simple cases...it would be a lot harder to believe.

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#### Beam_me_up

##### Guest
I have a problem with gravity. It seems to be a product of mass and density. For example escape velocity is the speed needed to break free from an objects gravitational field, that of the earth being around 11kp/s. Yet if the earth was compressed smaller than its schwartschilds radius or event horizon it would still have the same mass but an escape velocity greater than the speed of light which is obviously greater than that of the sun which has a far greater mass. Do any of the gravitational calculations account for this in understandable terms?

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#### MeteorWayne

##### Guest
Yes indeed. The gravity is related to the mass and the distance from the center of mass. If the earth were much smaller, the surface gravity would be different, but the escape velocity at a distance would be the same if the mass was the same.

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