Discussion Overview
The discussion revolves around the concepts of length contraction and time dilation as they relate to galaxies moving away from us in an expanding universe. Participants explore whether we observe contracted lengths of galaxies due to their motion and how this relates to adjustments made for time dilation in supernova observations.
Discussion Character
- Debate/contested
- Conceptual clarification
- Mathematical reasoning
Main Points Raised
- Some participants propose that galaxies moving away from us should appear contracted in length, similar to how time dilation is adjusted for in supernova light curves.
- Others argue that Lorentz contraction does not apply to galaxies due to their separation and the curvature of spacetime, which prevents them from being in the same inertial frame.
- It is suggested that noticeable contraction would only occur if a galaxy is moving away at a significant fraction of the speed of light.
- Participants question the meaning of "going away from us" in the context of curved spacetime, noting the lack of a single inertial frame covering both us and distant galaxies.
- Some clarify that adjustments made for supernova observations relate to redshift rather than time dilation, emphasizing that time dilation cannot be properly defined in an expanding universe.
- A participant introduces a hypothetical scenario involving a galaxy at redshift z and Hubble velocity v, questioning the relationship between cosmological time dilation and the Lorentz factor.
- Another participant challenges the definition of "Hubble velocity," suggesting it may not have a well-defined meaning in curved spacetime.
- There is a discussion about the factor B in relation to redshift and its comparison to the Lorentz factor, with some asserting that they are not directly related.
- A later reply points out that maintaining a constant distance to a home galaxy is only feasible if one is within a certain range, otherwise it would require superluminal speeds.
Areas of Agreement / Disagreement
Participants express multiple competing views regarding the application of length contraction and time dilation to galaxies. The discussion remains unresolved, with no consensus reached on the implications of these concepts in the context of an expanding universe.
Contextual Notes
Limitations include the dependence on definitions of velocity in curved spacetime, the ambiguity surrounding the application of Lorentz contraction, and the unresolved nature of time dilation in an expanding universe.