Why? They are not a form of EMR, are they. Or are the rapidly changing ones the only ones we can detect?

  • $\begingroup$ @Qmechanic Please be careful to read what you declare as duplicate. That question is for a special case and the only answer, if not wrong, ignores the need for asymmetry in order to have a quadrupole ( in contrast to the dipole of EM) $\endgroup$ – anna v May 26 at 10:56
  • $\begingroup$ @anna v. Thanks for the feedback. I hereby encourage the Phys.SE community to vote to reopen/close as they see fit. $\endgroup$ – Qmechanic May 26 at 11:35
  • $\begingroup$ @Qmechanic - I voted to reopen this post but my previous vote (in REVIEW QUEUES) is still there (I voted to be reopened after my previous vote). Why I cannot retract it? $\endgroup$ – SG8 May 27 at 6:25
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    $\begingroup$ Hi @SG8: I've experienced the same. Try see if there's some mother meta post about it. $\endgroup$ – Qmechanic May 27 at 7:47

Are gravitational waves produced only when a mass accelerates?

Acceleration of a mass is a necessary but not sufficient condition for the energy to transform to gravitational waves. A symmetric accelerating mass does not radiate,

If an object changes shape asymmetrically, the spacetime ‘dents’ travel outwards like ripples in spacetime called ‘gravitational waves’. Gravitational effects that are spherically symmetric will not produce gravitational radiation. A perfectly symmetrical collapse of a supernova will produce no waves, but a non-spherical one will emit gravitational radiation. A binary system will always radiate.

or a symmetric system . See here.

An object's gravitational dipole is a measure of how much that mass is distributed away from some center in some direction. It's a vector, since it had to convey not only how much the mass is off-center but also which way. Considering some object in the abstract, the natural 'center' to pick is the center of mass, which is the point around which the dipole is zero.

The quadrupole represents how stretched-out along some axis the mass is. A sphere has zero quadrupole. A rod has a quadrupole. A flat disk also has a quadrupole, with the opposite sign of the quadrupole of a rod pointing out from its flat sides. The rod is a sphere stretched along that axis and the disk is a sphere squashed along that axis. In general, objects can have quadrupole moments along three different axes at right angles to each other. (The quadrupole moment is something called a tensor.)

The quadrupole moment can definitely change. Think of two balls attached by a spring. If they are stretched apart and then allowed to oscillate, the quadrupole moment will get smaller and bigger in the oscillations.

In contrast to electromagnetic waves which have a dipole radiating, the gravitational waves need quadrupoles ,

You ask:

Why? They are not a form of EMR, are they. Or are the rapidly changing ones the only ones we can detect?

If you read the answers to this question suggested as duplicate in your previous question, and are able to follow the mathematics, you will understand that general relativity has similar mathematical structure with electromagnetism, but the gravitational waves have in common to the electromagnetic waves the velocity c ,(as all massless particles,) and are solutions of a wave equation. Everything else differs.

Gravitational waves have been detected by LIGO, where the masses accelerating are order of sun masses. The gravitational constant is too small to be able to detect small masses radiating gravitational waves. My answer to a relevant question here may help.

  • $\begingroup$ I think it may be good if you put this link (one of your previous posts) in your answer: physics.stackexchange.com/questions/229213/… $\endgroup$ – SG8 May 27 at 6:25
  • $\begingroup$ Please explain what you mean by symmetric accelerating mass. $\endgroup$ – ProfRob May 27 at 7:20
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    $\begingroup$ @ProfRob a sphere, for example. To get gravitational waves one needs a quadrupole moment, either in the single mass, or in the system accelerating. $\endgroup$ – anna v May 27 at 7:24
  • $\begingroup$ Your answer is currently ambiguous. A dipole is "symmetric". $\endgroup$ – ProfRob May 27 at 7:31
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    $\begingroup$ @SG8 thanks, I have included the link $\endgroup$ – anna v May 27 at 7:31

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