How does gravity comply with the law of conservation of energy?

[koti]

I wasn't sure where to post this but I presume that the answer to my question is either very simple or there is no answer so I decided to go for speculations. I apologise in advance for the crudeness of my argumentation:

If I kick a ball, my body is depleted of a certain amount of energy which is being transfered onto the ball. If an atom of U235 is being struck by a neutron it falls apart emmiting energy but at the same time its loosing some of it's mass. The mass is depleted and transformed into energy. If a star emmits radiation it looses some of it's mass which exactly correlates to the amount of energy that it emmited. Again, the stars mass is depleted. As far as I understand we can even make models of black holes which comply with the law of conservation of energy. How does gravity come into the picture here? For simplicity, lets consider a stationary meteor stuck in "empty" space far away from any other sources of gravity. The meteor has mass. If I find myself in proximity of this meteor I will feel the gravitational force pulling me towards it. What source is depleted and of what is it depleted in favour of the force that I am feeling?

Or is the answer that I am simply experiencing the curvature of space-time and there is no force that needs to to be considered by the law of the conservation of energy?

In that case, why is gravity considered as one of the 4 forces?

Edited October 24, 2016 by koti

[Endy0816]

How did you and the other mass come to be separated in the first place?

Edited October 24, 2016 by Endy0816

[geordief]

I wasn't sure where to post this but I presume that the answer to my question is either very simple or there is no answer so I decided to go for speculations. I apologise in advance for the crudeness of my argumentation:

 

If I kick a ball, my body is depleted of a certain amount of energy which is being transfered onto the ball. If an atom of U235 is being struck by a neutron it falls apart emmiting energy but at the same time its loosing some of it's mass. The mass is depleted and transformed into energy. If a star emmits radiation it looses some of it's mass which exactly correlates to the amount of energy that it emmited. Again, the stars mass is depleted. As far as I understand we can even make models of black holes which comply with the law of conservation of energy. How does gravity come into the picture here? For simplicity, lets consider a stationary meteor stuck in "empty" space far away from any other sources of gravity. The meteor has mass. If I find myself in proximity of this meteor I will feel the gravitational force pulling me towards it. What source is depleted and of what is it depleted in favour of the force that I am feeling?

 

Or is the answer that I am simply experiencing the curvature of space-time and there is no force that needs to to be considered by the law of the conservation of energy?

In that case, why is gravity considered as one of the 4 forces?

I thought your final point about curvature and the non treatment of gravity as a force was in line with GR.

As a thought experiment, if the two bodies in question are far enough (ie infinitely) removed from all (ie absolutely all) other sources of gravitational attraction does the space time curvature describing the two bodies remain fundamentally unchanged?

Edited October 24, 2016 by geordief

[koti]

How did you and the other mass come to be separated in the first place?

What does that have to do with anything?

I came in my spaceship, stepped out of it and I'm now in proximity of the meteor.

 

I thought your final point about curvature and the non treatment of gravity as a force was in line with GR.

 

As a thought experiment, if the two bodies in question are far enough (ie infinitely) removed from all (ie absolutely all) other sources of gravitational attraction does the space time curvature describing the two bodies remain fundamentally unchanged?

 

Let's pretend we have a universe just like ours with all the physical laws intact with one exception - it consist of only me and the meteor.

If I am in proximity of the meteor the spacetime curvature pulls me onto the meteor. If I am on one side of the universe and meteor is on the orher, the spacetime curvature is not pulling me onto the meteor but the curvature remains. Is that what your trying to convey ? Or in other words...if your thought experiment (in my above universe) implies that spacetime curves only if Im in close proximity to the meteor then I dont buy it.

 

(I was not aware that GR doesn't treat gravity as a force)

Edited October 24, 2016 by koti

[geordief]

 

What does that have to do with anything?

I came in my spaceship, stepped out of it and I'm now in proximity of the meteor.

 

Let's pretend we have a universe just like ours with all the physical laws intact with one exception - it consist of only me and the meteor.

If I am in proximity of the meteor the spacetime curvature pulls me onto the meteor. If I am on one side of the universe and meteor is on the orher, the spacetime curvature is not pulling me onto the meteor but the curvature remains. Is that what your trying to convey ?

 

Yes that is what I am wondering.

 

(I was not aware that GR doesn't treat gravity as a force)

Well yes ,I think that is considered to be the salient feature of GR (that it does not treat gravity as a "force" ). As usual I hesitate to pronounce on GR when I have only learned it from forums like this but I would be extremely surprised to learn otherwise.

Edited October 24, 2016 by geordief

[koti]

Well yes ,I think that is considered to be the salient feature of GR (that it does not treat gravity as a "force" ). As usual I hesitate to pronounce on GR when I have only learned it from forums like this but I would be extremely surprised to learn otherwise.

ok, that I think answers my question. I have difficulties with parsing GR (this is a clear case of it) I wonder why we are tought that there are 4 forces of nature when clearly we should be tought that there are 3. That would have saved me confusion.

As for your thought experiment, I find it ridiculous that the meteor could curve spacetime only if I'm in proximity of it (assuming my universe for simplicity)

[swansont]

For simplicity, lets consider a stationary meteor stuck in "empty" space far away from any other sources of gravity. The meteor has mass. If I find myself in proximity of this meteor I will feel the gravitational force pulling me towards it. What source is depleted and of what is it depleted in favour of the force that I am feeling?

 

 

You will start moving, and have kinetic energy. The amount of PE removed will be equal to the KE gained for the system. Nothing is "depleted".

 

It might make for a clearer discussion to not superimpose relativity and Newtonian physics.

[koti]

 

 

You will start moving, and have kinetic energy. The amount of PE removed will be equal to the KE gained for the system. Nothing is "depleted".

Yeah, I realised that just now.

 

It might make for a clearer discussion to not superimpose relativity and Newtonian physics.

I agree. Sory, it wasn't intentional.

Edited October 24, 2016 by koti