# Is gravitational energy continuous?

1. Jan 21, 2013

### MikeGomez

It seems to me that gravitational bodies radiate gravitational energy continuously, without losing mass/energy. It that true?

Here are my reasons for thinking as such.

First of all, when I say “radiate” I just mean that in a general way. I don’t mean radiation as in electromagnetic radiation, and I don’t mean gravitons or anything else. I just mean gravitation, whatever that might be.

1:
The gravitation of a body weakens as the inverse of the square of the distance from that body, all the way to infinity. If a second body it attracted to the first body due to gravitation, then energy is transferred (potential to kinetic) and work is done.

Now if the first gravitating body is moved to another location (ignore the energy required to do this), then I assume it’s gravitational effects with respect to it’s new location are felt only after a delay, since gravitation is restricted by the speed of light. This indicates to me that new “gravitation” is in some sense being “emitted” or “radiated” continually. In fact the gravitating body doesn’t need to be moved at all. It seems as though it is emitting gravitation continuously, and can (does) expend (gravitational) energy when acting on other bodies in it’s vicinity.

Does it have to do with zero point field or something?

2:
Also, due to the equivalence principle, wouldn’t charged particles emit Larmor radiation in a gravitational field the same as due to an equivalent acceleration? If that is true, is the gravitational body less massive after emitting Larmor radiation, or does the continuous gravitation provide the energy?

BTW, I’m not looking for perpetual motion or tapping into infinite energy source or anything stupid like that. Just trying to understand. Thanks.

2. Jan 21, 2013

### Staff: Mentor

No, there is no such thing as "gravitational energy", as you describe.

Gravity is a force or field, surrounding a body. It isn't emitted, it is just always there (theoretical gravitons notwithstanding).

3. Jan 21, 2013

### Staff: Mentor

No, they do not radiate in the way you think.

There are systems which emit gravitational radiation (usually called gravitational waves), but this is completely negligible for most systems. It can be notable in double neutron stars, for example.

Mass (or, more general, energy) has a gravitational field around it, this can exist forever without requiring any "new" energy to be emitted or whatever.

It depends on the way you move the object - this motion might lead to radiation.
No

That was discussed here recently, and it seems to be an unsolved problem for a constant acceleration. Gravitational acceleration is not constant (in space), however, and you get electromagnetic (together with negligible gravitational) radiation. This causes a loss of the total energy of "mass+orbiting charge" - if you consider them as a single object, this object loses mass.

4. Jan 21, 2013

### MikeGomez

I don't really get how the gravitational field is just always there, as you say. That doesn't seem physical to me.

I've seen people say that if the sun were to suddenly disappear, the earth would not just fly away immediately. There would be a delay of about 8.5 minutes (or the same amount of time it takes light to reach us from the sun). That example in reverse means that if our sun somehow increased it's gravitation say ten-fold, we wouldn't know about it here on earth for about 8.5 minutes.

5. Jan 21, 2013

### MikeGomez

O.k., yes, the question of whether a charged particle under constant acceleration emits radiation is still a matter of ongoing research. I actually found a pretty good article regarding that here.
http://www.mathpages.com/home/kmath528/kmath528.htm

6. Jan 21, 2013

### Staff: Mentor

There is no problem with that. a field can be there just like a particle.

While this is not completely wrong, the sun cannot just disappear within the laws of General Relativity, so it is meaningless to ask what this theory would predict there.
If you modify the mass distribution of the sun in some way, those changes would need about 8 minutes to get noted on earth.

7. Jan 23, 2013

### MikeGomez

And from From Wikipedia:
"In the context of classical theories of gravitation, the speed of gravity is the speed at which changes in a gravitational field propagate."

So by your comments, and Russ’s comments, and from the Wikipedia source, it appears that the consensus in the physics community is that the gravitational field is in general static, but when the gravitational field changes due to a change in the motion of a gravitating body, then that change in the field propagates at the speed of light.

Is that correct?

8. Jan 23, 2013

### Buckleymanor

The Earth is not what.If there is an energy difference and there is, work is done!
Rotational KE of the Earth would not stop the needle of the compass turning if the field flipped so I don't understand what you mean.

9. Jan 23, 2013

### Staff: Mentor

The moving compass needle slows or accelerates the rotation of the earth (or makes it wobble? not certain of the direction of the force/torque). A teeny tiny little bit. That's what Dale meant by "vastly overwhelmed by the rotational KE of the earth".

10. Jan 23, 2013

### Staff: Mentor

Yes, this is correct.

11. Jan 24, 2013

### MikeGomez

Ah, but I think there may be a slight problem with that view.

As the ocean currents move there is motion of mass, and with it the gravitational field is in a state of flux. As the tidal forces change the shape of the earth, the gravitational field is in a constant state of flux.

As the wind blows, as the rain falls, as the tectonic plates move, as a car passes by, as a feather falls, as a bowling ball rolls, as a baseball flies, the gravitational field due to these objects is in a state of flux.

And it’s even more than that. In addition to all the constant motion of mass on a macro scale, consider the cumulative motion at the atomic level. All the atoms that the entire earth is composed of are in constant motion. The gravitation contributed by every atom on earth is in a constant state of flux, and that continuous change takes place at the speed of light.

12. Jan 24, 2013

### Staff: Mentor

Why is any of that a problem? I agree that it is "slight" but don't understand why you think it is a "problem".

13. Jan 24, 2013

### MikeGomez

Well ok, not a “problem” if you don’t want to use that term.

Anyway, I guess this thread has run its course. You can close it out if you wish.

I appreciate everyone’s help.

14. Jan 24, 2013

### Staff: Mentor

OK, but perhaps if you can explain what it is that bothers you then I might be able to help.

15. Jan 24, 2013

### MikeGomez

Ok then. Thanks Dale.

In my original post I said that it seems to me that gravitation is emitted continuously.

I was told I was incorrect, and that gravitation is (with a few exceptions) a static field, and not emitted continuously.

Then, in post #11 I put forth my arguments to support my original statement regarding the gravitational field as continuous, not just sometimes, but in all circumstances.

So when I used the term “problem” there I didn’t mean it in a bad way. I just meant there seems to be a flaw in the logic somewhere that needs to be resolved, either with my arguments, or with the counter arguments.

I had rather hoped that someone would either validate my position, or give me a solid example showing where a true static gravitational field could exist.

16. Jan 24, 2013

### Staff: Mentor

It is static for static objects, and nearly static for nearly static objects. Its "updates", if you like that concept, happen all the time.

17. Jan 24, 2013

### Staff: Mentor

Well, the first point in your OP you appeared to think that the 1/r² term in gravity represented some form of gravitational radiation which would carry energy away from the system. That is not the case, it is static meaning that it does not carry gravitational energy away from the system. The gravitational fields that determine the observable motions in the solar system are all of this non radiative character.

The fluctuations that you are talking about in 11 are so infinitesimally small that they cannot even be measured, despite decades of attempting to do so. However, in theory, they do occur. They have yet to be confirmed, and do not represent a significant amount of energy lost (or even a measurable amount of energy lost).

The original answers to your question were not addressing this miniscule feature, although mfb did mention it in passing. They were more focused on the bulk 1/r² part of gravity which does not radiate but which you seemed to think represented some sort of continual energy flux.

18. Jan 24, 2013

### Buckleymanor

It won't make it wobble if there is an even distribution of compasses or magnets.Which there probably is.
If energy is conserved when the Earth's magnetic field flips again in the other direction won't it speed up if it ever slowed down in the first instance.

19. Jan 25, 2013

### MikeGomez

Aha, that is what I needed to know!

Thanks all…