# Gravity

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please tell us the origin of Gravity

For Newtonian gravity, the origin is mass (add momentum/energy if you go to relativity). Is that what you meant to ask?

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I must have missed the revolution in Physics since I was under the impression that we had a concrete answer to "what is gravity ?"

Gravity is nothing more than the intrinsic curvature of space-time in response to a presence of mass/energy. The mass/energy then follows this intrinsic curvature in its motion along lines called geodesics, unless of course another force is acting on that mass/energy.

Anything else implies action at a distance, which I and a lot of others are unconfortable with, and is also the case with Newton's gravity, and though Newton's gravity is fairly accurate ( it put people on the moon, did it not ? ), he never could explain what it was. We had to wait for Einstein's GR for an answer.

I was under the impression that what you describe as a concrete answer to "what is gravity", is nothing more than a model that describes how objects behave in gravity's presence.

Is it possible to measure the curvature of space-time, or are we simply measuring/predicting/describing how an object behaves when it is a given distance from a mass?

If a better theory comes along that does not include space-time, I would assume that we would no longer talk in terms of space-time, but in the terms described in the new model.

As far as 'action at a distance', what else would you call the actions of space-time being curved at a distance 'x' from an object as that object moves through space?

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I was under the impression that what you describe as a concrete answer to "what is gravity", is nothing more than a model that describes how objects behave in gravity's presence.

No objections here.

Is it possible to measure the curvature of space-time, or are we simply measuring/predicting/describing how an object behaves when it is a given distance from a mass?

It's certainly possibly to measure the components of the Riemann curvature tensor.

If a better theory comes along that does not include space-time, I would assume that we would no longer talk in terms of space-time, but in the terms described in the new model.

Macroscopically, I don't think we'll get anything better than metric theories of gravity (though this is just my opinion of course). At large scales, we'll probably always work with a theory that relates curvature to stress-energy in some form.

Quantum gravity will almost certainly describe gravity in terms of spin-2 gauge bosons (gravitons).

As far as 'action at a distance', what else would you call the actions of space-time being curved at a distance 'x' from an object as that object moves through space?

You describe its motion as a result of the local geometry of spacetime. There are, though, well observed instances of non-locality in QM.

the answer to life, the universe and everything

Gravity is the aether wind. To measure the drag and direction of the aether wind, stand on a scale and look up.

given

Gravity is a state of force. Force is inertial pressure differential. Space consists of weakly interactive massive quantum particles. Weakly interactive means they are not easy to measure. Massive means they are subject to gravity.

before you unleash the hounds of dogma

Name one observation of gravity or van der vvaal's or casimere or weak or any other force that is not consistent with inertial pressure differential.

in the spirit of peace

ron

-Gravitons are massless

-Weakly interacting means that the particles only interact through the weak force + gravity. It doesn't mean they're hard to measure.

-Massive does not mean subject to gravity. Massless particles (e.g. photons) also interact gravitationally.

Edited by elfmotat
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No. The spinning actually results in the net force on you being smaller, depending on your latitude, but it is not the source of the gravitational force.

Interesting. What would the percentage increase be on an Earthbound object's weight if the Earth wasn't spinning?

Edited by StringJunky
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You describe its motion as a result of the local geometry of spacetime. There are, though, well observed instances of non-locality in QM.

What I meant to say was that as an object moves through space, it seems as if it is constantly changing the curvature of space-time around it. For example, the curvature of space-time midway between two stars is relatively small as compared to the curvature close to a large body. If a rogue planet was travelling between the stars, the closer it got to the midpoint, the more space-time would curve there. Meaning that any small bit of debris at the midpoint would be affected by the rogue planet while the rogue planet was still some distance away.

That is what I meant when I said that the curvature of space-time is still "action at a distance".

Thanks for the other feedback. I need to do a bit of reading to make sure I understand what you are saying.

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Interesting. What would the percentage increase be on an Earthbound object's weight if the Earth wasn't spinning?

The radial acceleration due to rotation is ω2R. At the equator, this is (7.2921150 * 10-5 rad/s)2(6,378137.0 m) = 0.033916 m/s2. Compared to the gravitational acceleration of 9.79 m/s2 at the equator, it is almost entirely negligible. The difference is about what you would get from moving from one of the poles (g=9.83 m/s2) to the equator (g=9.79 m/s2).

Edited by elfmotat
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The force due to rotation is ω2R. At the equator, this is (7.2921150 * 10-5 rad/s)2(6,378137.0 m) = 0.033916 m/s2. Compared to the gravitational acceleration of 9.79 m/s2 at the equator, it is almost entirely negligible.

Thanks...it's just of academic note then.

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If you (who claim that you are the most educated here) think as you were thought in school what is right and what is wrong, please give some freedom to all others. Looks like even most educated or experianced physicist can not be 100% right.

Everybody is free to post anything they want. There is no policy to shut up anyone here.

The stuff we all learned in school is not 100% right. In fact, the more you learn about physics, the more you will realize just how little we know.

However, you should accept that any new theory will be tested. And in science, we test things by attacking it... We will try to punch a hole in it, and if we fail to punch a hole in a new theory, then it might be something.

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Thanks...it's just of academic note then.

Back when people were first going to define the meter, an idea was floated to tie length and time together via a pendulum, such that a 1-meter pendulum had a period of exactly 2 seconds, by definition. The variability in g quashed that idea. But $T=2\pi\sqrt{\frac{L}{g}}$, so it turns out that the approximation $\sqrt{g}=\pi$ is good to better than 1%

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Back when people were first going to define the meter, an idea was floated to tie length and time together via a pendulum, such that a 1-meter pendulum had a period of exactly 2 seconds, by definition. The variability in g quashed that idea. But $T=2\pi\sqrt{\frac{L}{g}}$, so it turns out that the approximation $\sqrt{g}=\pi$ is good to better than 1%

I learned something today, thanks.

But

Where else in physics or geometry do we encounter Pi squared?

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<br />I learned something today, thanks.<br /><br /><br />But<br />Where else in physics or geometry do we encounter Pi squared?<br />
<br /><br /><br />There are so many places that I've lost track. Many many places in math that's for sure.
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Zapatos, if an object possesses an intrinsic property which affects space-time so as to curve or warp it, and this effect propagates outward at the speed of light, we know what is 'exchanged' between two objects and at what speed the curvature information is exchanged to account for gravity.

Using the Newtonian model for gravity, what exactly is exchanged between the sun and earth to compel the earth to follow its orbit ?

That is what is called action at a distance. Just to clarify the concept.

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