How do we know that physics in distant galaxies is the same as we observe here on Earth? Do we just assume it's so, or can we do some experiments, make some observations to test the idea? In the last decade or three, quite a few tests of the idea have been done; an example discussed recently in PF is the work done to show that [tex]\alpha[/tex] (the fine structure constant) has indeed remained constant over cosmological times. What was it like 50 years ago? a hundred? Did physicists and astronomers in the 1920s and 1930s just assume that the physics in distant galaxies* was just the same on Earth? or did they try to find ways to 'prove' it? *interestingly, it wasn't until the 1920s that the question of whether galaxies were distant (beyond the Milky Way) was settled.