Propagation of changes in a gravitational field

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Discussion Overview

The discussion revolves around the propagation of changes in a gravitational field, particularly in the context of black holes and gravitational waves. Participants explore theoretical scenarios involving black holes, neutron stars, and the implications of gravitational wave emission in relation to event horizons.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that gravitational waves are responsible for propagating changes in the gravitational field, as indicated by the linked Insight.
  • Others argue about the feasibility of an intruder black hole grazing the event horizon of a stationary black hole while escaping, questioning the consistency of this scenario.
  • A participant suggests that the gravitational influence of a black hole can be traced back to events before its formation, complicating the understanding of gravitational wave propagation.
  • There is a discussion about whether the gravitational field changes felt by an orbiting body are influenced by the black hole's movement due to external perturbations.
  • Some participants express confusion regarding the implications of gravitational waves not escaping the event horizon and how this affects external objects.
  • A later reply clarifies that gravitational radiation will be emitted during interactions, regardless of whether the objects involved are black holes or neutron stars.
  • One participant emphasizes that the propagation of gravitational influence does not require anything to cross an event horizon or exceed the speed of light.
  • Another participant attempts to clarify their understanding by framing the question about how gravitational changes propagate when a black hole is involved.

Areas of Agreement / Disagreement

Participants generally agree that gravitational waves propagate at the speed of light and that they play a role in changes to the gravitational field. However, there are multiple competing views regarding the specifics of how these changes affect external objects, particularly in scenarios involving black holes and event horizons. The discussion remains unresolved on several points, particularly concerning the implications of gravitational waves in relation to event horizons.

Contextual Notes

Participants express uncertainty about the definitions and implications of gravitational waves and event horizons, leading to a complex interplay of ideas that are not fully resolved. The discussion highlights the challenges in conceptualizing gravitational interactions in systems involving black holes.

  • #31
Lord Crc said:
Ok, now we're getting somewhere. So presumably these changes propagates at c as well?

It depends on what you mean by "propagate". The gravitational field can change with time. That doesn't imply anything "propagating". Propagation is really applicable to perturbations. If you jiggle a mass, then that will cause a jiggle in the gravitational field, and that will propagate out at the speed of light.

I thought it was the mass inside the black hole that emitted the waves, and that these waves escaped the event horizon.

No, gravitational waves are disturbances in the gravitational field. The "source" of a gravitational wave isn't a mass, it's a little jiggle in a gravitational field. So the waves don't propagate from the center of the black hole; they propagate from the field surrounding the black hole.
 
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  • #32
stevendaryl said:
It depends on what you mean by "propagate". The gravitational field can change with time. That doesn't imply anything "propagating". Propagation is really applicable to perturbations. If you jiggle a mass, then that will cause a jiggle in the gravitational field, and that will propagate out at the speed of light.
Right, and my misunderstanding was equating that jiggle with gravitational waves.
stevendaryl said:
No, gravitational waves are disturbances in the gravitational field. The "source" of a gravitational wave isn't a mass, it's a little jiggle in a gravitational field. So the waves don't propagate from the center of the black hole; they propagate from the field surrounding the black hole.
Gotcha. I'll have to spend some time reprogramming my brain but I think I got the point now.

Thank you all for your input and patience. Clearly I need to work on my question-asking skills.
 

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