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Vaniver comments on Open thread, Oct. 12 - Oct. 18, 2015 - Less Wrong
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Given that the universe started a finite amount of time ago, and supposing there is easy space travel, then there is an interval during which the first colonists have intrastellar space travel but have not visibly done interstellar space travel, and we can estimate how long that interval is. They're in that interval, or there isn't easy space travel.
We cannot argue "because there is one, there must have been a previous one," you can't do that sort of induction on the natural numbers, eventually you hit one. We can argue it's unlikely, sure, and we weigh that unlikelihood against the unlikelihood that interstellar travel is hard in order to determine what our posterior ends up being.
But that's a lot of information. It's a very short interval. Since it's so unlikely to be in that interval, this is large evidence against easy space travel.
It's a probabilistic argument. But what isn't? There's no argument that allows infinite certainty. At least, I'm pretty sure there isn't.
I agree that it's a lot of information. But it's also the case that we have a lot of information about physics, such that interstellar space travel being difficult is also unlikely. Which unlikelihood is larger? That's the question we need to ask and answer, not "the left side of the balance is very heavy."
And that's why my conclusion is "that wasn't made by aliens."