# Is gravity a force, or curvature of space-time?

## Main Question or Discussion Point

Simple question (I think). According to GR, is gravity no longer thought of as a force of attraction, but simply a curvature of space-time induced by mass? The earth, for example, creates some kind of "space-time well" that keeps us in our seats as we type on our keypads? (That's poorly phrased I'm sure, but I'm not schooled in GR, thus the question.)

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A.T.
Simple question (I think). According to GR, is gravity no longer thought of as a force of attraction, but simply a curvature of space-time induced by mass? The earth, for example, creates some kind of "space-time well" that keeps us in our seats as we type on our keypads? (That's poorly phrased I'm sure, but I'm not schooled in GR, thus the question.)
GR doesn't disallow you to see gravity as a force, it just redefines which frame is accelerated and which is inertial in a gravitational field. And so gravity becomes a inertial force present in accelerated frames only.

More on this topic in chapter 2.6 of this:
http://www.relativitet.se/Webtheses/tes.pdf

gravity is a force. before einstein scientists were confused about the nature of gravity. it seemed that gravity was an "action over a distance." so all that GR does is explain the nature of gravity, but it is still a force.

Fredrik
Staff Emeritus
Gold Member
According to GR, is gravity no longer thought of as a force of attraction...
(That's poorly phrased I'm sure...)
You have the right idea, and you're right that its poorly phrased. I would drop the words "no longer" because they suggest that GR has replaced Newton's theory as what we now believe is true. GR is just a better theory in the sense that it's better at predicting the results of experiments, but they're both good theories in the sense that they both make pretty accurate predictions, and they're both "wrong" (like all other theories) in the sense that they aren't exactly right. (GR describes matter as "classical" even though it's not).

I would say that gravity is a force in Newton's theory of gravity, and that it's not a force in GR. It doesn't make sense to think of "gravity isn't a force" as some sort of absolute truth. You need to specify a theory of gravity to even ask the question that makes sense: "Is gravity a force in this theory?"

Thanks, A.T. and Fredrik. A.T., I appreciate the various reference frames tying gravitational mass with inertial mass. I downloaded that dissertation and it looks quite relevant (but I'll wait to print it where I have a color printer available).

Fredrik, I appreciate the pragmatic "until we understand it all, we have to recognize that our current theories should be treated as provisional" attitude. The first sentence of your last paragraph succinctly expresses my impression. Most helpful (especially because you're recognized as an official science advisor here - not that I'm a worshipper of "authority" {;-)).

Ralilu, I suppose "curvature of empty space" plays the role in GR, of "explaining" that obvious "action at a distance" that aether theorists sought unsuccessfully to explain via mechanical means.

We know that the gravity on the moon is much less because the mass of the moon is much less than that of earth. All of the stars,moons, and even comets you can see rotate on an axis and rotate in same fashion around a star, black star, etc. What we are trying to define is a force of order coming out of chaos for millenia. As the particles draw closer pressure increases towards the center of the mass as does temperature. As this rotation increases gravity is the inevitable result. If we were to land a man on Jupiter the gravity would incapacitate his/her bodies musculature to overcome the gravity and even walk. This is an inertia explained by Newton in albeit fundamental terms.

Maybe this is the reason y today's physicists cant get a unified field theory that includes gravity?

We know that the gravity on the moon is much less because the mass of the moon is much less than that of earth. All of the stars,moons, and even comets you can see rotate on an axis and rotate in same fashion around a star, black star, etc. What we are trying to define is a force of order coming out of chaos for millenia. As the particles draw closer pressure increases towards the center of the mass as does temperature. As this rotation increases gravity is the inevitable result. If we were to land a man on Jupiter the gravity would incapacitate his/her bodies musculature to overcome the gravity and even walk. This is an inertia explained by Newton in albeit fundamental terms.
What has all this to do with the initial discussion? Also, I didn't understand that of " the inevitable result" ... was related to the rotation on axis or with traslational movement? Actually all assertions seemed so vague to me, liked much a precise statement (chaos for millenia?).

By the other very different side, it's very elegant and \$simple the response by A.T., thanks for that.

I think it would be better to think of gravity as neither a force nor a curvature of spacetime but rather as an effect.

Admittedly, whatever causes the effect requires a force to resist it and also makes what we consider a straight line to be not quite so straight. This, for me, makes the idea of the movement of dark energy as the cause of the effect rather appealing.