The Universe: Can we really be the only ones in it?

We know the core is iron based on observations, principally of seismic waves helped by knowing Earth's overall mass. We have no reliable, repeatable observations that give evidence of aliens, still less for their nature. And we can't "know" anything based just on mathematics.

Life, or intelligence, could be extremely rare, to the point that it arising on one planet within a 13.7 billion light-year radius is extremely unlikely. There's nothing stopping life on Earth being an extreme fluke. There's also nothing stopping the Universe being much larger than we observe, or even infinite, meaning that intelligent life is highly likely to arise somewhere and ALSO that said intelligent life is highly unlikely to see any other intelligent life in its observable radius.

And while it could take a long time for messages to be passed back and forth, that doesn't preclude communication. Consider that in pre-modern times it could take years for information to travel around the world. Instantaneous communication is the exception, not the norm.
 
I thought this would be interesting to point out:

Most abundant elements in the observed universe, in order:

Hydrogen
Helium
Oxygen
Carbon
Nitrogen

Most abundant elements in the human body, in order:

Hydrogen
Oxygen
Carbon
Nitrogen

Basically, life is made of the most common stuff in the universe, except for helium, which is not chemically active and thus not very useful.

Note that these lists are in order of abundance by mole-ratio (number of atoms), not by weight. For instance, the mole-ratio of oxygen to hydrogen in water is 33% (1 in 3) while the weight ratio of oxygen to hydrogen is ~89%. I think mole-ratio is more important in terms of what chemistry is likely to happen.
 
On the other hand, life doesn't draw from the Universe as a whole. It draws from what's on Earth. Abundances in Earth's crust, the oceans, or the atmosphere are rather different. Carbon is rare in the crust, though we believe it was abundant in the atmosphere before plants pulled most of it out. Elements like silicon and aluminium are much more common in the crust than in life, and chlorine is common in seawater but again sees little biological usage (while the near-equally present sodium is far more important for life).
 
If anyone has read the Ender's Game series, I think Xenocide and Speaker for the Dead are great examples of what could happen if the stars aligned and we met another sentient alien race. The Hierarchy of Exclusion that Orson Scott Card proposes is really pretty interesting to me. The basic idea can be found here.

The obvious problem with it is that it is subject to the opinion of the race deciding whether the other race is ramen or varelse, but we have voting bodies for that :)
 
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