Oscillating masses and gravitational waves

[PeterDonis]

what I have in mind is that no matter if the field is changing or not, the "time delayed basis" is there, doing its work. updating sequentially the field values.

As I said, since this is just a matter of words, not physics, I can't say you're wrong. But since the only way to test the "time delayed basis" is to change something and watch the change propagate, there's no way to get evidence about how lack of change "updates" anything.

 

[DaTario]

As I said, since this is just a matter of words, not physics, I can't say you're wrong. But since the only way to test the "time delayed basis" is to change something and watch the change propagate, there's no way to get evidence about how lack of change "updates" anything.

Ok, we are done. Sorry for this drifting away from the OP. I would like to thank you, for your words helped me to shake some ideas which were rather static in my head.

Best wishes,

DaTario

 

[mfb]

Basically, yes. But one waveform can't really describe a GW. GWs have two possible polarizations, as shown here:

https://en.wikipedia.org/wiki/Gravitational_wave#Effects_of_passing

So to fully describe a GW, you need two waveforms, one for each of the polarizations; there is no necessary connection between them. (I don't know if LIGO was able to measure both waveforms; they may only have been able to measure one, since one waveform from each detector is all that I've seen in published info.) Also, you have to bear in mind that even if you have both waveforms, that is not a complete description of spacetime curvature, i.e., it's not a complete description of the effects of gravity; see my previous post.

LIGO is sensitive to relative changes in the lengths of the arms only, so each detector is limited to one polarization direction. A gravitational wave that changes both arms with the same amplitude is completely invisible to this detector. Two two LIGO observatories have different orientations for the arms, combining both they can get more data about the polarization. The Livingston site has roughly SSE/WSW orientation, while the Hanford site is SW/NW. The curvature of Earth has to be taken into account as well as the detectors are not at the same place.

 

[PAllen]

One thing to point out that is that for an oscillating mass, a test body's attractive response is to the quadratically extrapolated retarded position. This is different from EM, where the coulomb response is to the linearly extrapolated retarded position. In both cases, this velocity/acceleration dependent effect mimics responding to the instantaneous position of the source, but the mimicry is much more 'precise' for gravity. This is precisely why solar system motions, up to very small effects, seem consistent with instant action at a distance NOT delayed propagation. This is also related to why gravitational radiation has no dipole component; quadrupole is the lowed order.

So, in simple physical terms, the near field effect of an oscillating mass would be change in the center of attraction that seems to have almost no propagation delay because of the quadratic position extrapolation. The effect of the GW is not even approximately a change in center of attraction - it is compression in one direction, expansion in another. This distinction is in addition to the points about completely different distance dependence of strength of the effect.