system
Given the following post, respond as if you're commenting on Reddit:
human
The earth is about 4,5 billion years old, and the universe about 14,5 billion, if life isn't special, then shouldn't we have already been contacted?

At what point can we say that the silence is an indication of the rarity of intelligent life?
gpt
This is a large part of the Fermi Paradox. The galaxy is only about 100,000 light years across, so even at 1% of the speed of light, it takes 10 million years to cross the galaxy. We evolved from small mammals to tool-using humans with space rockets over less than 100 million years. The invention of writing to the Apollo Program is maybe 10,000 years or less. All of these time-scales are much shorter than the age of the Earth, let alone the universe. This means that if life intelligent evolved anywhere else within the galaxy, it's unlikely that it appeared at the same time as us - it's almost certain that any intelligent life would be millions of years more advanced or millions of years less advanced.

This tells us that galaxy-colonising advanced life must be rare, as if there is intelligent life that has the capability and intent to colonise the galaxy, anywhere within the galaxy, anywhere in the past X million or billion years, they should have reached Earth a very long time ago.

Of course, there are multiple reasons why galaxy-colonising advanced life might be rare.

- they lack the intent, i.e. they could colonise the galaxy, but they choose not to leave their home planet, or they do explore the galaxy but leave us alone (basically the Zoo hypothesis)

- they lack the ability, i.e. even with millions of years of advancement it's not practical to leave a solar system in mass migrations, or a more advanced society generally becomes more at risk of destroying itself before it reaches that stage ("the great filter")

- *intelligent* life is rare. Life has thrived on Earth for billions of years before one species developed spaceflight. Evolution doesn't inevitably lead towards developing life that can invent advanced technology. There may be many planets out there full of animals and plants, or even just bacteria, but it's possible that humanity is a bit of a freak accident.

- life is rare in general. We don't really know how common life is. We know the ingredients seem to be fairly abundant, but how often do these combine to make something we would reasonably call "life"?

- the conditions for life are rare. However, as we discover more and more exoplanets, it looks like there are quite a few planets that seem like they would be hospitable to life, so this is less of a factor than we used to think.

So this isn't really a "paradox" in the common sense, because there are many ways to resolve it. But each of the resolutions involves stuff we just don't know - we don't know how frequently life evolves in the right conditions, we don't know how frequently life evolves to form intelligent space-faring species, and we don't know how often a space-faring space faring species would have the intent and capability to explore the galaxy. Any of these are plausible, and it could easily be a combination of everything.