Does gravity bend gravitational waves?

[Bosko]

TL;DR Summary

1. Does the gravity of massive space bodies bend gravitational waves on their way to Earth?
2. Does gravity change the direction of gravitational attraction?

Reading the article,
GRAVITY BENDING GRAVITY: ARE ANY OF THE O3A LIGOVIRGO DETECTIONS GRAVITATIONALLY LENSED?
I think about the following two questions:

1. Gravity bends light and all other electromagnetic waves. Does it also bend gravitational waves?

2. Does gravity change the direction of gravitational attraction?
Consider the following thought experiment.
Imagine two exercise weights A and B at the same distance from a massive object.
By rotating the weights, small gravitational waves can be produced.
Does the gravitational attraction of the weight act in the direction from which the gravitational waves come?

Let me draw...

 

[Ibix]

1 - yes, as it says in the flyer you linked.

2 - I don't think so, no, at least not in general. For bodies in orbit the gravitational attraction direction is towards where they are, not where they appear to be. Where they appear to be is not where they are because of light speed delay, but general relativistic gravity includes velocity dependant terms that, for a body in orbit, turn out to point its gravitational attraction towards its present location.

Note that this isn't a faster than light effect, just a coincidence for bodies in orbit. If the planet whose gravitational attraction we feel is a light minute away and suddenly fires rockets and takes off into deep space the gravitational attraction would continue to point at where we expect it to be for a minute after ignition. Only after light and gravitational radiation reaches us would we see the gravitational pull start to shift.

 

[Bosko]

For bodies in orbit the gravitational attraction direction is towards where they are, not where they appear to be.

The direction should be orthogonal (at 90 degrees) to the gravity wave front.
Both electromagnetic and gravitational waves move at the same speed through the same curved space-time.

 

[Ibix]

The direction should be orthogonal (at 90 degrees) to the gravity wave front.

This is wrong. I told you that in the post you quoted. I also told you why, and preemptively addressed your objection about the light speed limit.

Perhaps you should re-read my post. If you didn't understand it, ask questions about the parts you didn't understand. Don't just make wrong statements, or we'll be going backwards and forwards with "you're wrong", "no, you are", "no you are" all day.

 

[Bosko]

This is wrong.

Why?

 

[Ibix]

Why?

See post #2. What part of it did you not understand?

 

[Bosko]

See post #2. What part of it did you not understand?

How the gravitational pull of object B "knows" where object A is and vice versa.
The path of light is "straight" in curved space-time.
I understand your post but I don't understand how that direction can be determined

 

[Ibix]

How the gravitational pull of object B "knows" where object A is and vice versa.

Gravity is analogius to electromagnetism in some ways. Newtonian gravity is analogous to electrostatics, and that acceleration does point towards where the other object was. However, there's also an analogue of the magnetic component of the field, which modifies the free-fall direction. If the bodies are in free fall orbit, it modifies it so that the attraction is towards where the other body is now.

As I explained in the last paragraph of #2, the "points to where it is now" is a special case since it is assuming that the body did not undergo acceleration due to some other force. But the "magnetic" effect will be there to some extent or other in general, and therefore the force does not point in the same direction as the gravitational radiation in general.

Steve Carlip's paper goes into more detail. https://arxiv.org/abs/gr-qc/9909087

 

[Bosko]

...
Steve Carlip's paper goes into more detail. https://arxiv.org/abs/gr-qc/9909087

I didn't find anything relevant to question 2

 

[Ibix]

I didn't find anything relevant to question 2

It explains why gravity isn't aberrated - it points to where the source is now (given stated caveats). That's addressing your question 2.

 

[Bosko]

It explains why gravity isn't aberrated - it points to where the source is now (given stated caveats). That's addressing your question 2.

Where? Question 1 or 2 ? How ?

 

[Drakkith]

With respect, your 2nd question is a little vague, as it's actually asking two different things:

1. Does gravity change the direction of gravitational attraction?
2. Does the gravitational attraction of the weight act in the direction from which the gravitational waves come?

The answer to the first question is that no, the gravity of one body doesn't change the direction of gravitational attraction from another body towards a third. The answer to the second questions is also no. In general, the direction of 'attraction' does not need to be perpendicular to the gravitational wavefront carrying the change in gravity from one body to another. At least as far as I know.

 

[PeterDonis]

Does gravity change the direction of gravitational attraction?

This is the relativity forum, so there is no such thing as "gravitational attraction". Gravity is not a force in GR. So you need to rethink what you are actually asking about.

 

[PeterDonis]

The answer to the first question is that no, the gravity of one body doesn't change the direction of gravitational attraction from another body towards a third.

Actually, the answer is "mu", since, as I said in post #13 just now, gravity is not a force in GR.

However, the question can be reframed to ask about something that is relevant in GR, for example in which direction geodesics will tend to "fall"--but in that case the answer is yes, the presence of additional bodies does change it--it must, since additional bodies change the spacetime geometry.

In general, the direction of 'attraction' does not need to be perpendicular to the gravitational wavefront carrying the change in gravity from one body to another. At least as far as I know.

With "attraction" reframed to something that is relevant in GR, yes, this is true--gravitational wavefront propagation does not, in general, take place along the same directions as, for example, timelike geodesics tend to fall.

 

[PeterDonis]

For bodies in orbit the gravitational attraction direction is towards where they are, not where they appear to be.

Note that "gravitational attraction" has to be properly reinterpreted, as I have pointed out in my two previous posts.

The Carlip paper basically reinterprets it as "the relevant connection coefficients in an appropriately chosen coordinate chart".