Fermi and Von Neumann suggested the galaxy should be completely populated by aliens if alien advanced civilizations exist, because they could build machines (Von Neumann machines) that would travel in interstellar space, find usable materials, duplicate themselves, and continue to expand in number and in space until the galaxy as full of them. since the process is exponential Von Neumann suggested it should take less than 100 million years, which is a short time compared to the age of the galaxy. Since we don’t see such machines, they must not exist, so advanced alien civilizations must not exist.
But there is a problem. Chemical rockets can only escape planets a little bit bigger than earth, because the greatest chemical rocket specific impulse (lithium and fluorine!) is not high enough to even lift the rocket fuel away from the pull of gravity. look at the payload of the Saturn rocket compared to the rocket size–it is 98% fuel. if a planet were 20-30% more massive than earth no chemical rocket could escape, so the alien advanced civilization would be planet-bound.
of course there is also a minimum size planet that can support sentient life, and sentient life that can build rockets need an insulating atmosphere (see The Anthropic Cosmological Principle, Barrow et al.). This minimum size is not much smaller than earth. so civilizations that can do interplanetary and galactic travel are a very small fraction of such civilizations.
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