How far could the sun possibly be?
Astrophysicists rely on a chain of indirect measurements and assumptions to estimate distances in space, starting with the Sun, and readers explore how techniques like parallax, planetary transits, and gravitational redshift fit into that “cosmic distance ladder.” Commenters also delve into deeper conceptual issues such as why the one‑way speed of light cannot be measured independently of clock synchronization, how anisotropy in light speed could in principle be hidden, and what this implies for modern puzzles like the cosmological “Hubble tension.” Overall, the exchange highlights both the ingenuity and the fundamental limits of observational cosmology.
Scientific Methodology & Cosmology Context
- Commenters praise the article as a clear example of real astrophysical practice: you don’t “know” distances; you iteratively rule out possibilities and narrow bounds.
- Some link this to the current “crisis in cosmology,” where different methods (e.g., involving JWST data) yield non‑overlapping parameter ranges, suggesting at least one major assumption or pillar of cosmology might be wrong.
One-Way Speed of Light Debate
- A major subthread explains that only round‑trip (two‑way) light speed is empirically measurable; one‑way speed is convention‑dependent due to clock‑synchronization issues.
- Multiple thought experiments are proposed: moving synchronized clocks apart slowly, using identical acceleration profiles, triangular paths, and later recombining clocks.
- Other commenters point to formal results (slow clock transport, generalized Lorentz transforms) showing that any attempt to measure one‑way speed sneaks in assumptions equivalent to fixing it.
- Some insist one‑way speed is in principle measurable (e.g., via circuits, or by referencing a “preferred” CMB rest frame); others strongly reject this and link to expository videos/papers arguing it is fundamentally unobservable. The disagreement remains unresolved in the thread.
CMB, Isotropy, and Anisotropic Light Speed
- One view: anisotropic light speed would spoil the observed uniformity of the cosmic microwave background (CMB).
- Counterpoint: with appropriate direction‑dependent time dilation, anisotropic light speed can still yield an isotropic CMB; a cited paper even considers extreme anisotropy (c/2 one way, instantaneous the other).
- Another commenter claims you could, in principle, define an effective one‑way speed relative to the CMB rest frame, but this is contested.
Measuring the Sun’s Distance & Size
- Several comments explore “from scratch” methods: parallax, eclipses, Venus transits, solar angular size, and shadow geometry.
- A recurring obstacle: many naive constructions require knowing either the Sun’s true size or Earth’s orbital radius, which are themselves unknown without a distance ladder.
- Historical techniques using Venus transits and Kepler’s laws are briefly mentioned as the first successful AU measurements.
Sun vs. Stars & Historical Intuition
- Discussion on whether ancient observers could infer that stars are distant suns.
- Arguments for: stars are dim point-like light sources, analogous to distant fires.
- Arguments against: lack of “intermediate” bright stars, color differences, apparent finite star sizes (pre‑telescopes), and multiple possible light‑generation mechanisms.