| I think you're right, but with a tiny handful of notable exceptions in astrophysics, where gigametres can be a characteristic or upper length scale. LISA is practically always described as a gigametre-scale observatory. One example: "LISA, a gigameter-scale space-based gravitational wave observatory, will explore the gravitational wave universe in the band from below 0.1 mHz to above 0.1 Hz." <https://about.cern/news/announcement/physics/cern-colloquium...>, and a trawl through arxiv will show a common association of Gm and LISA. Assuming LISA is successfully deployed, gigametre may be seen more commonly. The only other place I've seen Gm scales in common use is in galactic physics, particularly with respect to turbulence and star and planetary nebula formation (stars have ~ Gm diameters; very large stars like VY Canis Majoris have ~ Tm diameters; star systems like ours have gravitationally bound rocky and icy objects at ~ Tm diameters). 1 petameter ~ 0.1 lightyear; 30 petameter ~ 1 parsec, so those are obvious cutoffs for the SI unit of length in astronomy and astrophysics. Gigaparsecs and (less frequently) gigalightyears are commonly used in physical cosmology (e.g. <https://duckduckgo.com/?q=gigaparsec+site%3Aarxiv.org&ia=web>). Penultimately, truly long lengths are typically measured in cosmological redshift z, which is unitless (being a ratio \frac{\delta\lambda}{\lambda}, or 1+z = \frac{a_{now}}{a_{then}} where a is the scale factor), leading to such things as a comoving volume (1 + z)^3. For z > 0.2 one would be using Gpc lengths, or in SI units Ym; or when working in these sorts of comological volumes in 2023's Britain, trevigintillions of acre-feet. Finally, cf. the excellent printable table at <https://arxiv.org/abs/1303.5961>, the leftmost column (redshift) and the r_comov column (megaparsecs) being the most directly relevant. |