# Physics in the distant universe - same as on Earth? (history question)

1. Mar 8, 2004

### Nereid

Staff Emeritus
How do we know that physics in the distant universe 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 $$\alpha$$ (the fine structure constant) has indeed remained constant over cosmological times.

What was it like 50 years ago? a hundred? Did physicists in the 1920s and 1930s just assume that the physics in distant places* 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.

2. Mar 8, 2004

### Janitor

Good question

I suppose spectroscopic observations by astronomers have been important.

Also, if nuclear forces (for instance) were different long ago and oh so far away (Karen Carpenter?), that would probably affect things like how/when/if a star of a certain mass can become a white dwarf or neutron star or can go (super)nova. So by looking at other galaxies and compiling statistics, astronomers would probably find some significant anomalies in the population of this or that type of star, and the frequency of this or that type of event (such as a supernova), which would give them a clue that something was different out there & back then compared to here & now.

3. Mar 9, 2004

Staff Emeritus
Before the 1920s astronomers thought there was only the milky way "galaxy" (they didn't have the word.) They couldn't really resolve exterior galaxies and classified them as nebulae, along with the real nebulae. Beyond the milky way star swarm was thought to be empty space. Infinite of course, because they didn't have any concept of "curved space" either. They had no concept of cosmology or of a history of the universe. It was just thought to extend back in time, steady state, beyond the mental horizon. This was the Newtonian universe.

4. Mar 16, 2004

### Nereid

Staff Emeritus
Certainly, historically, study of 'the heavens' lead to much new science on 'the earth' - from Newton's insight about apples and the Moon, to the discovery of helium and much more.

If we re-define 'deep space' to be 'at least to the edge of the solar system, and maybe as far as stars whose parallex we can't measure', to what extent did physicists of the ~1850s to 1920s assume the physics must be the same, vs actively seek to test their assumptions?

A deep puzzle must have been 'what powers the stars?' How did physicists - including Einstein - feel about this paradox (until the late 1920s)? The physics of stars is different than the physics here on the Earth?