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I've asked Under what circumstances might gravitational waves impart linear momentum to an object? (e.g. Quasar 3C 186) which cites the recoil as one example, but continues:

My question is not only about this event, but about the effect of gravitational waves incident on objects in general as well. If I have two test objects, a black hole and a star, and a gravitational wave produced from a separate event passes through them, would either of these test objects receive an impulse from the passing wave and begin to move away from their initial position due to the wave?

Background material in my question covers the case of BH merger recoil, so I've asked about a situation where a wave produced from a separate even is incident on a star and a black hole. I've asked earlier about a passing gravitational wave transferring energy to an object and received an excellent answer.

I'm having difficulty understanding the answer to my current question. It starts:

The momentum bestowed by a passing gravitational wave (GW) on an object is always going to be negligible (there may may be situations when the energy deposition is non-negligible... but rarely and frankly unlikely). The key is in the momentum carried away by anisotropic GW emission from the object itself.

Is that an unqualified yes? As far as I know, "negligible" does not necessarily mean finite or nonzero.

I put a bounty on the question, and I'd like to award it. I've asked the poster to simply add a sentence with a clear yes. I did get a yes in a comment, but comments should be considered temporary and not integral parts of answers.


enter image description here


When the dust settles I was hoping for an answer to Under what circumstances might gravitational waves impart linear momentum to an object? that would be easy enough for future readers to get an answer from without having to go in and start reading the citations of the 7 peer-reviewed references.

The poster is responsive in comments, but resistant to alter or add in any way to the originally posted wording of the answer.

Question: Is there something wrong with me asking in this case for the answer to be clearer? Is there any other ways I can ask?

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    $\begingroup$ There is nothing wrong with you asking for an answer to be clearer, but the answerer is under no obligation to conform. What exactly are you looking for as an answer to this meta post? $\endgroup$ – ACuriousMind Mar 31 '17 at 11:57
  • $\begingroup$ @ACuriousMind in meta I've often received insightful answers that I couldn't have anticipated in advance. That said, I'd like to have an answer there that is more helpful to me, and I'm asking for advice here how to work with the person who's posted one answer there so far. It's possible someone here may have helpful suggestions or recognize some aspect of the situation that I've missed. $\endgroup$ – uhoh Mar 31 '17 at 12:50
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Frankly, I think that your formulation for the core of the question,

Under what circumstances might gravitational waves impart linear momentum to an object?

is still ambiguous: it could equally well apply to gravitational waves emitted by the object itself as opposed to waves emitted by some external source. If you want to restrict your question to the latter case then that needs to be made much more clear in the body of the question.

In addition to that, if your question really is about the momentum deposited on a system by a passing GW, then the mentions of quasar 3C 186 are entirely off topic and only serve to derail the discussion, since nobody is suggesting that this source got its momentum from GWs derived from a separate source. By making the majority of your post about this source, including a gratuitous image and whatnot, you implicitly make the title question apply to GWs emitted by the object under consideration.

That said, I would interpret the first line of the answer,

The momentum bestowed by a passing gravitational wave (GW) on an object is always going to be negligible

is a pretty unambiguous response to the explicit question you posed,

would either of these test objects receive an impulse from the passing wave and begin to move away from their initial position due to the wave?

and I don't think you can really ask for more with the current formulation of the question. It's true that "negligible" does not equate to "zero", but it does mean "so small that it will be swamped by a bunch of things that you're not even considering at the moment", or in other words "so small that you can and should ignore it". This means that, at present, the answer does a pretty good job at addressing the question you have posed.

If what you really want is an answer to the question

Under what circumstances might gravitational waves from an external source impart linear momentum to an object?

then you really need to alter your question to reflect that - and, frankly, if that is what you really want to know, there's no call at all to have any discussion about the recent quasar discovery, because there's no external source in play there.

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  • $\begingroup$ If the first sentence of the answer answers the incident wave from external source, then it seems the derailment was at least incomplete. I appreciate your suggestions how to improve the question. As soon as I saw the answer there I realized it was an excellent answer to a question that I didn't ask but wished I had, but with a bounty and an answer already present I felt major edits to the question could also be problematic. I think you're recommending how the question could have been better written, not that I should go in and do it now. $\endgroup$ – uhoh Mar 31 '17 at 15:03
  • $\begingroup$ So it seems to me the best way to move forward from here in terms of the current question would be to happily accept the answer that is there - I do find it a great answer in general, and I found the description of the recoil is very helpful. I can then do some reading on momentum transfer from incident gravitational wave, and for that, find a way to "ask better". $\endgroup$ – uhoh Mar 31 '17 at 15:06
  • $\begingroup$ I'll also try to find a way to make it clearer that I'm not asking primarily about the size of the effect, or how easy it would be to observe. I think adding the negligibility is a stackexchange reflex anticipating how someone else might run with a concept. The friction felt by Feynman's beads would fall into the "negligible" category except that it was the zero or non-zero-ness that was the central issue, but there used to be at least one other answer there that went for the "new form of energy" angle. $\endgroup$ – uhoh Mar 31 '17 at 15:25

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