Why aliens probably exist
There are around 2 hundred billion stars in our galaxy, and tens of billions of those haves planets that are kind of like Earth.
So even if the odds of complex live occurring on earth-like planet are one in a billion it’s likely we are not alone.
So, where is everybody?
Well, the galaxy is huge. It takes light 100,000 years just to make it from one edge to the other. And the fastest spaceships we can build would be lucky to reach even a tenth of a percent of the speed of light, so maybe the aliens just haven’t arrived yet. But that’s a little weird because the galaxy is really old, so old that even a slow going alien civilization would have had plenty of time to colonize the whole thing.
The first thing you need is a ship or at least a robot that can copy itself. Send two of those out to the nearest stars and when arrive they’ll set up shops and make two more copies each. Repeat this process for a while and even if it takes hundreds of years to travel from one solar system to another, it only takes about ten millions years to colonize every solar system in our galaxy. Compare to the 13 billion years that our galaxy’s been around, that’s nothing.
If technologically-advanced aliens exist there should be evidence of them everywhere, so why haven’t we found it? Well, maybe aliens don’t exist maybe life really is so rare that only one planet in the galaxy has it. Or maybe civilizations just can’t develop advanced technology without destroying themselves. Or maybe aliens really are everywhere and we just don’t know how to find or recognize them.
And that just underscores how difficult the idea of alien life is. We have to make assumptions about what aliens would be like, like maybe they’re carbon-based, need liquid water and use technology based on radios waves.
Otherwise, what do we look for?
But nothing says that aliens have to be anything like us. Or even like anything that we can conceive of before we meet them, which means that if we assume too much, then we’ll probably miss life that’s truly alien.