We’re probably alone

I didn’t want my first real post here to be about file formats, so I’ll go with the next best thing: space aliens.

I don’t know whether extraterrestrial intelligent life (ETI) exists in the observable universe. A lot of very smart people think it does. But based on humanity’s current observational and theoretical knowledge, it is quite possible that it doesn’t.

I’ll place my bet on “we’re alone”. I do this this in part just to be contrary, and balance out what I think is an overly optimistic prevailing opinion that ETI is all over the place.

Our scientific assessment of the likelihood of ETI changes over time, as we learn more about exoplanets, abiogenesis, etc. But I feel like that assessment has pretty much been moving in only one direction: downward. As we study other planets, do computer simulations, etc., the properties of Earth and the Solar System look more and more special.

In recent years, we’ve confirmed that a lot of extrasolar planets exist, so doesn’t that move our assessment in the positive direction? Yes, but not by much. Given that the Sun has a bunch of planets, it was always a good guess that most stars have at least a few.

Here are some examples of dubious reasoning that I’ve seen used to defend the “ETI is common” belief:

  • Wishful Thinking. I too have a romantic notion of a Star Trek universe teeming with interesting alien life forms. But that is not logical. The universe does not care about my romantic notions.
  • Appeal to Personal Incredulity. A.k.a. “I just can’t believe that such a big universe could have only one planet with life!” Sorry, but the universe does not care about your inability to believe something.
  • Innumeracy. A.k.a. “It’s a statistical certainty that ETI is out there.” Well, show me your math. Your instincts may tell you that the number of planets hospitable to life, multiplied by the probability of abiogenesis on such a planet, is a number substantially larger than 0. But human instincts are notoriously bad when it comes to extremely large and extremely small numbers. Why couldn’t it be that ETI is so rare that it only occurs once per thousand universes? If so, there’s a 99.9% chance we’re alone.

I want to emphasize that I’m talking about the observable universe, which has, and will only ever have, a finite number of stars in it. If by “universe” you mean the hypothetical infinite universe beyond the observable universe, that’s different. If the universe is infinite, and even remotely homogeneous, then yes, it must contain ETI. Along with, for example, an infinite number of atom-for-atom identical copies of planet Earth. Infinity is very difficult to reason about.

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