How Do Orbiting Black Holes Generate Gravitational Waves?

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Discussion Overview

The discussion centers on the generation of gravitational waves by orbiting black holes, exploring the conditions under which such waves can be produced and the underlying physical principles involved. Participants are questioning the mechanics of gravitational wave generation in relation to the movement of the center of gravity of the system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how gravitational waves can be generated if the center of gravity of the black hole system is not oscillating.
  • Another participant draws a parallel to electromagnetic waves, questioning whether the same principles apply when charges move, even if their center does not.
  • A third participant notes that gravitational waves require a time-varying quadrupole moment, contrasting this with electromagnetic waves that need only a time-varying dipole moment.
  • Another participant introduces the idea that a single event can generate a range of frequencies, suggesting that a non-oscillating system can still produce gravitational waves through its reaction to these frequencies.
  • A participant reiterates the question about the generation of electromagnetic waves by moving charges, asking how to demonstrate that the waves do not cancel each other out.

Areas of Agreement / Disagreement

Participants express differing views on the mechanics of gravitational wave generation, with no consensus reached on the relationship between the movement of the center of gravity and the production of waves.

Contextual Notes

There are unresolved assumptions regarding the definitions of oscillation and movement in the context of gravitational and electromagnetic wave generation, as well as the implications of frequency generation from single events.

jnorman
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I am probably missing something, but it seems to me that the only way gravitational waves could be generated would be if the center of gravity of a massive body or system began oscillating somehow. The detection of such waves was recently reported and were supposedly generated by a pair of black holes orbiting each other. I do not understand how this system could generate gravitational waves when the center of gravity of the system is not moving/oscillating. Help? Thanks.
 
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jnorman said:
I do not understand how this system could generate gravitational waves when the center of gravity of the system is not moving/oscillating. Help?
Do you understand how two charges doing the same movement would produce EM-waves, even when the center of charge is not moving?
 
As I recall, for gravitational waves the source needs to have a time-varying quadrupole moment (of energy-momentum) whereas for electromagnetic waves the source needs to have only a time-varying dipole moment (of charge).
 
A single step function (a step function f(t)=0 when t<0; f(t)=1 when t>0) contains all frequencies. So a single event, even without oscillations, can generate all frequencies. Then the system will react to the frequencies differently. It will damp out some frequencies and will oscillate to other frequencies. Sort of like a single push on a suspended weight will make it swing in a cyclic fashion.
 
A.T. said:
Do you understand how two charges doing the same movement would produce EM-waves, even when the center of charge is not moving?
How do you show that the waves don't cancel each other out?
 

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