Discussion Overview
The discussion revolves around the nature of simultaneity in the context of General Relativity (GR) and Special Relativity (SR), particularly in relation to the expanding Friedmann-Lemaître-Robertson-Walker (FLRW) universe. Participants explore the implications of observer-dependence in simultaneity and the role of different families of observers in defining global simultaneity slices.
Discussion Character
- Debate/contested
- Technical explanation
- Conceptual clarification
Main Points Raised
- Some participants suggest that the OP may be referring to GR rather than SR, emphasizing the importance of the "everywhere at once" notion in the context of the expanding universe.
- Others argue that simultaneity is inherently observer-dependent, regardless of whether the discussion pertains to GR or SR.
- A participant claims that applying the relativity of simultaneity to extended objects like the universe negates the possibility of a Big Bang model.
- Some participants highlight that the expansion of the universe, as described by comoving observers in FLRW spacetime, is independent of any choice of simultaneity convention.
- There is a contention regarding the existence of global synchronization procedures in GR, with some expressing discomfort in generalizing this concept across all spacetimes.
- Participants discuss the implications of the last scattering surface and the Cosmic Microwave Background Radiation (CMBR), with differing views on their independence from coordinates and simultaneity conventions.
- One participant notes that many spacetimes can admit global coordinates, while others assert that this is not generally true in GR.
- There is a discussion about the definition of "absolute" versus "relative" simultaneity, with some seeking clarification on the simultaneity of the CMB.
Areas of Agreement / Disagreement
Participants express multiple competing views regarding the nature of simultaneity in GR and SR, and the discussion remains unresolved with no consensus reached on key points.
Contextual Notes
Limitations include the dependence on definitions of simultaneity and the unresolved nature of the mathematical implications of different coordinate systems in GR.
