Do we know or is there any way to know...

[finster]

if a moving object, such as a planet, has more gravity than an object not moving?

 

Or, since isnt everything supposed to be moving??, does an object like a planet with more velocity?? have more gravitational attraction than an object the same size with less velocity?

 

I'm trying to develop an explanation on what is causing gravity and the answer to this question may help me depending on what it is.

 

 

Thank you. You guys are great!

[ajb]

if a moving object, such as a planet, has more gravity than an object not moving?

 

The question is flawed. The planet is moving with respect to what? And, what do you mean by more gravity?

[finster]

The question is flawed. The planet is moving with respect to what? And, what do you mean by more gravity?

 

I guess in respect to the object the planet is attracting.

 

And by more gravity I mean the strength of gravitational pull.

[ajb]

Classical Newtonian gravity is a conservative force, so the force cannot depend on velocity only position.

[finster]

Classical Newtonian gravity is a conservative force, so the force cannot depend on velocity only position.

 

By position does that include mass? Because I thought mass was the major factor in determining the force of gravity.

 

Also, perhaps I'm using the wrong term. If I said speed instead of velocity would that change your answer?

Edited February 20, 2012 by finster

[DrRocket]

if a moving object, such as a planet, has more gravity than an object not moving?

 

Or, since isnt everything supposed to be moving??, does an object like a planet with more velocity?? have more gravitational attraction than an object the same size with less velocity?

 

I'm trying to develop an explanation on what is causing gravity and the answer to this question may help me depending on what it is.

 

 

Thank you. You guys are great!

 

I suspect that the origin of this question involves the notion of relativistic mass which in general is greater than the rest mass.

 

That is a relativistic effect. The relativistic theory of gravity is general relativity.

 

In general relativity, what we call gravity is the effect of curvature of spacetime, and that curvature is determined by the stress-energy tensor. The stress-energy tensor does indeed include mass/energy and hence relativistic mass. Therefore one might naively expect that relative motion might result in a stronger gravitational effect, but that naive expectation is incorrect.

 

The stress-energy tensor and the Einstein curvature tensor are, obviously, tensors. In particular that means that, while the expression in specific coordinates is dependent on the observer, the actual tensor quantity itself is not. Tensors are invariants. Thus curvature and the attendant gravitation are not dependent on the observer or on any relative motion. The individual components depend on the observer, but, as with the matrix expression of a linear transformation in terms of different basis vectors, the underlying function is independent of that expression.

 

You cannot, for instance, cause a body to form a black hole simply by choosing a reference frame in which it has sufficiently high velocity. Being a black hole is a physical event, and events are invariant.

[ajb]

By position does that include mass? Because I thought mass was the major factor in determining the force of gravity.

 

Newton's law tells us that the force is proportional to the product of the two masses divided by their separation distance squared.

 

Also, perhaps I'm using the wrong term. If I said speed instead of velocity would that change your answer?

 

No difference.

 

In general relativity, what we call gravity is the effect of curvature of spacetime, and that curvature is determined by the stress-energy tensor. The stress-energy tensor does indeed include mass/energy and hence relativistic mass.

 

 

Issues of gravity always become more subtle in general relativity.

Edited February 20, 2012 by ajb

[finster]

I suspect that the origin of this question involves the notion of relativistic mass which in general is greater than the rest mass.

 

That is a relativistic effect. The relativistic theory of gravity is general relativity.

 

In general relativity, what we call gravity is the effect of curvature of spacetime, and that curvature is determined by the stress-energy tensor. The stress-energy tensor does indeed include mass/energy and hence relativistic mass. Therefore one might naively expect that relative motion might result in a stronger gravitational effect, but that naive expectation is incorrect.

 

The stress-energy tensor and the Einstein curvature tensor are, obviously, tensors. In particular that means that, while the expression in specific coordinates is dependent on the observer, the actual tensor quantity itself is not. Tensors are invariants. Thus curvature and the attendant gravitation are not dependent on the observer or on any relative motion. The individual components depend on the observer, but, as with the matrix expression of a linear transformation in terms of different basis vectors, the underlying function is independent of that expression.

 

You cannot, for instance, cause a body to form a black hole simply by choosing a reference frame in which it has sufficiently high velocity. Being a black hole is a physical event, and events are invariant.

 

Well, actually what I was leaning toward was the idea of a "super vacuum" caused by mass vacating a position in space as a explanation of what causes gravity. Thus objects are not drawn to the actual object but forever trying to occupy the immediate empty space trailing behind the larger object. Wild speculation based on not enough training on my part but I figured I'd run it by the experts on here just in case they spot any value in it.

[Delta1212]

Well, actually what I was leaning toward was the idea of a "super vacuum" caused by mass vacating a position in space as a explanation of what causes gravity. Thus objects are not drawn to the actual object but forever trying to occupy the immediate empty space trailing behind the larger object. Wild speculation based on not enough training on my part but I figured I'd run it by the experts on here just in case they spot any value in it.

If that were the case, you'd expect gravity to be directional with the strength of the force corresponding to the direction of travel.

 

Additionally, there'd have to be some medium that the object is traveling through and displacing in order to create a sort of vacuum behind it. Vacuum doesn't so much suck matter as matter gets "pushed" into vacuum by greater pressure elsewhere.

 

It's an interesting thought, but there isn't really any way to make the observed effects of a vacuum mesh with those of gravity. It just doesn't work.

 

I'd very much recommend looking into general relativity which provides both a very accurate model for the observed effects of gravity and a corresponding conceptual model for what gravity "actually" is.

[finster]

If that were the case, you'd expect gravity to be directional with the strength of the force corresponding to the direction of travel.

 

Additionally, there'd have to be some medium that the object is traveling through and displacing in order to create a sort of vacuum behind it. Vacuum doesn't so much suck matter as matter gets "pushed" into vacuum by greater pressure elsewhere.

 

It's an interesting thought, but there isn't really any way to make the observed effects of a vacuum mesh with those of gravity. It just doesn't work.

 

I'd very much recommend looking into general relativity which provides both a very accurate model for the observed effects of gravity and a corresponding conceptual model for what gravity "actually" is.

 

 

Ah, this makes more sense. Yes, I guess it would be directional in that case.

 

I read somewhere we know what gravity is but we have yet to ascertain why anything is attracted to anything else. But the book might have just been discussing attraction at the quantum level.