# Is Frame Dragging an Example of a Gravity Wave?

• tony873004
In summary: All analogies are imperfect, but they can be helpful in thinking about complex topics.In summary, Yes.
tony873004
Gold Member
Is Frame Dragging an Example of a Gravity Wave?

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

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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.

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.

Chronos said:
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?

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, propogating 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?

LURCH said:
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.

tony873004 said:
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.

pervect said:
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.

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).

## 1. What is frame dragging?

Frame dragging is a phenomenon in which a massive object, such as a spinning planet or a rotating black hole, can "drag" the fabric of spacetime around it, causing it to rotate along with the object.

## 2. How is frame dragging related to gravity waves?

Frame dragging is one of the possible sources of gravity waves. When a massive object rotates, it creates ripples in the fabric of spacetime, which are known as gravity waves.

## 3. Is frame dragging an example of a gravity wave?

Yes, frame dragging is considered as an example of a gravity wave, as it is one of the possible sources of these waves. However, not all gravity waves are caused by frame dragging.

## 4. How is frame dragging measured?

Frame dragging can be measured using a technique called Gravity Probe B, which uses gyroscopes to detect changes in the orientation of spacetime caused by the Earth's rotation.

## 5. What are the implications of frame dragging for our understanding of gravity?

Frame dragging provides evidence for the theory of general relativity, which states that massive objects can curve the fabric of spacetime and affect the motion of other objects. It also helps us understand the effects of rotation on spacetime and how it can generate gravity waves.

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