A Do Moving Masses Slow Down Due to Gravitational Waves?

[Cato]

TL;DR Summary

Gravitational waves are produced by accelerating masses. Since all space is curved -- more curved near large masses stars, less curved in intergalactic space -- all moving masses are being accelerated to some degree. Do all moving masses therefore produce gravitational waves? If they do, will all moving masses lose energy and slow down?

Gravitational waves are produced by accelerating masses. Since all space is curved -- more curved near large masses stars, less curved in intergalactic space -- all moving masses are being accelerated to some degree. Do all moving masses therefore produce gravitational waves? If they do, will all moving masses lose energy and slow down?

 

[phinds]

Technically, yes, gravity waves are produced but they are so weak in the kind of situation that you describe that their effect is probably something like a rounding error in the 15th decimal place.

 

[Ibix]

Objects in free fall aren't accelerating in any meaningful sense. The source of gravitational radiation is stress-energy with a changing quadropole moment.

That does mean that any pair of objects ought to emit gravitational radiation unless they are at rest with respect to one another (I think - there might be exceptions). And that does mean that they'll slow down with respect to one another, but that may or may not mean slow down with respect to whatever coordinate system you are using.

Back-of-the-envelope, the kinetic energy of Earth in its orbit is 10³¹J. The power output from gravitational radiation is around 100W, if memory serves. So I think that this effect is rather weaker even than @phinds says - even for a system as massive as our planet.

Finally, extending this argument to "every object" is risky. We strongly suspect that GR is not an accurate description of gravity when quantum effects are important for its source. So gravitational radiation may or may not be emitted by very small objects.

 

[timmdeeg]

That does mean that any pair of objects ought to emit gravitational radiation unless they are at rest with respect to one another (I think - there might be exceptions). And that does mean that they'll slow down with respect to one another, but that may or may not mean slow down with respect to whatever coordinate system you are using.

Looking at the the merger of two compact objects I think they speed up independent of the chosen coordinate system. Or do you think of non-inertial frames here?

 

[Nugatory]

speed up independent of the chosen coordinate system.

Can “speed up” ever have a coordinate-independent meaning? “Experiences proper acceleration” has a coordinate-independent meaning and in an inertial frame does imply what most people would mean by “speed up”... but we’re talking gravitational effects here, so are considering regions of spacetime that aren’t properly described by any inertial frame.

 

[Cato]

I don't know this kind of physics enough to discuss the details. "Finally, extending this argument to "every object" is risky. We strongly suspect that GR is not an accurate description of gravity when quantum effects are important for its source. So gravitational radiation may or may not be emitted by very small objects." So perhaps objects where quantum effects are important don't produce gravitational waves. Thanks for all your answers.