Re: Adapting to an open universe

Nick Bostrom (
Mon, 19 Jan 1998 21:09:03 +0000

John K Clark <> wrote:

it causes another paradox. If we look at a galaxy 15 billion
> light years away and then look at another galaxy in the opposite direction
> that is also 15 billion years away, and if the universe is only 15 billion
> years old and has always expanded at the present rate, then the two galaxies
> could NEVER have had anything to due with each other. The trouble is that we
> can see that the two look very similar to each other, and the laws of physics
> seem the same in both places, that is, electrons produce photons in exactly
> the same way. How do electrons in such distant places know to act identically
> if they never ever had any causal relationship with each other?

I think the purported puzzel (this is the so-called horizon problem)
is not so much that the laws of physics are the same in causally
unconnected parts of the universe, but rather that these different
parts are in thermal equilibrium -- that the matter configuration and
the cosmic background radiation look roughly the same in all

Nick Bostrom