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Are we alone in this universe, yes or no?
No. First, there is the panspermia theory, which is a hypothesis that states that single-cell life can be transported from one place to another in the universe, either through asteroids, comets, or other means. If the theory is true then it automatically says that we are not alone and in fact, it could be that life came to Earth from somewhere in the universe. If we don’t subscribe to this theory then there is the mathematical probability that says there have to be billions (even trillions) of life-supporting planets in the universe and the mathematical probability also says that some of these planets must be supporting or have supported life in the past….
The observable universe is 93 billion light-years in diameter and 13.7 billion light-years in depth, but the whole universe is much bigger than that, so big that it is close to what our mind can comprehend to be infinity.
The universe contains trillions of planets, so the chance that life exists, has existed or will exist in the future on numerous other planets is a mathematical certainty but there are caveats… life can only evolve within a planet system where several unlikely conditions occur, i.e. the law of averages says “yes”, these unlikely conditions or events can repeat multiple times, but the other caveat is less likely to result in a “yes”…. evolution is very slow, really slow, civilizations are not eternal, planets themselves are not guaranteed to remain life-supporting, and stars themselves do die taking its planets with it, so the existence of other intelligent life simultaneous to our own is a much slimmer probability. The likelihood of intelligent alien life being so much more advanced than humans is much less a certainty. So, although we can say that other life exists, the probability that extra-terrestrial intelligent life is advanced enough to have developed a propulsion system that allows interstellar travel now (while humans exist) is doubtful. This explains why humans have not seen alien life.
The laws of physics and entropy apply to the whole universe so it is safe to say that if Earthlings can’t travel at light speed nor can they reach immortality, then neither can aliens. With this in mind, our current technology would take a manned spacecraft 70,000 years to reach our nearest star (Proxima Centauri), to make that journey within a human lifetime (or even a clam’s or shark’s lifetime, if aliens evolved from these long-living creatures) will need a quantum leap in technology that might not even be possible. All said, we can only think within the data fields we know.
An afterthought… quantum mechanics has entangled particles that mimic each other’s movements instantly regardless of the distance that separates them, if an advanced species had cracked the process of entangling particles, it might be (theoretically) possible for living beings to be in two places at once and explore interstellar bodies this way… it is far fetched but so is quantum mechanics that humans are far from mastering or even understanding at this point.
Added Feb 2024… I need to emphasize that I do not subscribe to the notion that evolution is the factor that progressively makes a species more intelligent as I do not believe that technological progress is tied to increasing intelligence. In the 4,000 or so years that the written word existed, there is nothing to indicate that humans were less intelligent 4,000 years ago nor that monkeys or elephants, etc have become more intelligent in that time. This makes me subscribe to the theory that there needs to be a freak, unique event in the history of a species that projects a species from dumb to intelligent instantly.
Progress has more to do with the ability of intelligent beings having the capacity to learn and build on the discoveries & inventions of their forefathers rather than the intelligence itself naturally increasing with time without external influences. If time travel existed and we teleported a 3-year-old from the pre-bronze age to now and gave him the same education modern kids get, I’m certain he would grow up to be indistinguishable from the average modern human.