Is Frame Dragging an Example of a Gravity Wave?

[tony873004]

Is Frame Dragging an Example of a Gravity Wave?

 

[ohwilleke]

No. Frame dragging is example of gravitomagnetism (which is one of several phenomena predicted by GR).

 

[Chronos]

A contrasting view: Yes. It is not one of several GR predictions... it is a fundamental prediction of GR. If it is wrong, then so is GR.

 

[Chronos]

I'm willing to take a stand on that. Got to put your foot down sooner or later. If I'm wrong, I will freely admit it.

 

[LURCH]

A contrasting view: Yes. It is not one of several GR predictions... it is a fundamental prediction of GR. If it is wrong, then so is GR.

But I think the OP was asking if Frame Dragging is a subset of "Gravity Waves". Both Frame Dragging and Gravity Waves are predictions of GR, but the two are not the same thing, right? After all, Frame Dragging can't be a form of gravity wave, because it isn't a wave at all, is it?

 

[tony873004]

I ask because I have a question on my latest Astronomy homework. If the moon collapsed into a black hole, how big would it be? Would the event cause gravity waves.

The size is less than 1 mm for the event horizon!

I think the teacher is expecting an answer of no to gravity waves.

But I'm saying yes for 2 reasons. First, something that is not perfectly spherical like the moon emits a less-than-perfect gravity field. But after collapsing into a singularity, the imperfections in the gravity field would vanish, propagating outward at the speed of light in the form of a gravity wave. Very small though.

And the other part of my yes answer is frame dragging. Since the Moon has spin already, it will speed up when it collapses to a singularity. The rapidly spinning singularity will cause frame dragging.

But would the amount of frame dragging at the distance of the old moon's radius still be the same as it was before the collapse? I think it would, so I might get rid of this part of my answer.

Any thoughts?

 

[ohwilleke]

But I think the OP was asking if Frame Dragging is a subset of "Gravity Waves". Both Frame Dragging and Gravity Waves are predictions of GR, but the two are not the same thing, right? After all, Frame Dragging can't be a form of gravity wave, because it isn't a wave at all, is it?

This is what I was thinking.

GR predicts lots of things: Black hole singularities, lensing, frame dragging, and gravity waves. All are phenomena predicted by GR. If anyone of them is wrong, GR is wrong. But, it doesn't follow that Frame Dragging and Black hole signularities are the same phenomena, and likewise doesn't follow that gravity waves and frame dragging are the same phenomena.

 

[pervect]

Is Frame Dragging an Example of a Gravity Wave?

We can draw an analogy to electromagnetism here, and ask the anologous quesiton:

"Is the detection of a magnetic field from a rotating sphere of charge an example of an electromagnetic wave?

I would say the literal answer is no, it is not - but if one had never detected a magnetic field of any sort before, detecting the existence for the first time of a magnetic field would be a confidence-booster in one's predictions of the existence electromagnetic waves.

 

[Nereid]

We can draw an analogy to electromagnetism here, and ask the anologous quesiton:

"Is the detection of a magnetic field from a rotating sphere of charge an example of an electromagnetic wave?

I would say the literal answer is no, it is not - but if one had never detected a magnetic field of any sort before, detecting the existence for the first time of a magnetic field would be a confidence-booster in one's predictions of the existence electromagnetic waves.

I like this answer!

In general, we can use analogies to EM when discussing (some aspects of) GR ... it can often make things seem 'more obvious'.

The danger, of course, is that the analogy cannot be taken too far (and sometimes there is no valid analogy).