# Gravity

Discussion in 'Physics & Math' started by timojin, Nov 5, 2017.

1. ### Write4UValued Senior Member

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9,436
And with absolute precision.

3. ### Write4UValued Senior Member

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9,436
TY for the link. I understood most of the narrative, but I am a little stumped by the term Quadrupole.
In this context does quadrupole fundamentally mean a 360 degree radius, IOW. all points in a variable curved configuration between the four poles, rather than four specific individual points?

Is it possible to explain this in layman terms?

Last edited: Nov 10, 2017

5. ### Q-reeusValued Senior Member

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3,162
Have no idea what a '360 degree radius' means. Anyway, a finite mass quadrupole moment exists whenever there is a departure from spherical symmetry.
A spherical mass set in oscillatory motion between oblate and prolate spheroid shapes constitutes a mass quadrupole oscillator that will generate gravitational waves.

Much more typically, two co-orbiting bodies like 'BH's' or NS's generate huge time-varying quadrupole moments, and are the source of those recently deteced GW's.

Going back to the Earth & tides situation, you will generally have complex shifts in mass distributions going on and higher-order than quadrupole mass moments will be involved.
Nobody would bother to calculate the likely GW spectrum of such because apart from the enormous calculational complexities, any actual GW's given off will be far too feeble to detect. Ever. And whats more, since we are intimately immersed in the near-field source (Earth), it would be impossible to extricate 1/r GW components from the vastly greater near-field shifts in local g that just might be barely detectable.

7. ### Write4UValued Senior Member

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9,436
Sorry for my inadequate terminology, but you have put in an understandable narrative for me.....

Thank you.

8. ### Q-reeusValued Senior Member

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3,162
Thanks. Where I wrote in #43: "...a finite mass quadrupole moment exists whenever there is a departure from spherical symmetry.", that is not strictly accurate. It's possible to create e.g. an octupole moment without any quadrupole moment, and so on. But that is a highly unnatural situation and in almost all actual cases of interest the quadrupole moment is completely dominant. During LIGO detected merger events, there is additionally 'magnetic moment' analogues to consider owing to presence of spin. But it gets quite intensive to study.