Discussion Overview
The discussion revolves around the conditions under which gravitational waves are emitted by orbiting objects, specifically exploring whether certain symmetric orbital configurations can avoid gravitational wave emissions. The scope includes theoretical considerations of gravitational wave generation and examples of systems that may not emit such waves.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
Main Points Raised
- One participant states that any object orbiting another emits gravitational waves unless the system has a time-invariant and symmetric quadrupole moment, providing the example of four planets symmetrically arranged around a star.
- Another participant reiterates that gravitational wave emission is driven by the third time derivative of the quadrupole moment, suggesting that systems where this derivative is zero will not emit gravitational waves.
- A different participant acknowledges the role of higher order multipoles in gravitational wave emission, suggesting that while the third time derivative is significant, it may not completely eliminate emissions.
- One participant emphasizes the need for examples of systems where the third time derivative of the quadrupole moment is zero, rather than leaving it to the reader to deduce.
- A proposed example includes a spherically-symmetric ball of fluid that oscillates radially, which, according to Birkhoff's Theorem, would not emit gravitational radiation.
Areas of Agreement / Disagreement
Participants express varying views on the conditions for gravitational wave emission, with some agreeing on the significance of the quadrupole moment while others introduce the complexity of higher order multipoles. The discussion remains unresolved regarding the completeness of examples provided and the implications of different configurations.
Contextual Notes
There is an acknowledgment of the need for further examples and clarification on the conditions under which gravitational waves may or may not be emitted, indicating potential limitations in the current understanding of the topic.