Astronomy

given the age of the Universe and the relatively short time it would take for an advanced civilization to spread across the Milky Way Galaxy (650,000 years, by Hart’s estimate), Earth should have been visited by an extraterrestrial civilization (ETC) by now.

It took humans 30,000 years to cross the Atlantic. Using modem propolsion systems, it takes us two years to get to Mars and 40 to reach the edge of the solar system. This seems like an extremely generous estimate considering the Milky Way has a 50,000 light year radius.

I'm as bullish about extraterrestrial life as anyone, and I think a fuller survey of even just the current Solar System has potential. But I have no idea how you get a full galactic survey in so short a time, given what we know about the soft limits on speed of travel and communication.

By Tipler’s refined estimate, an ETC would be able to explore the entire galaxy in “less than 300 million years.”

That definitely feels like it's more in the ballpark. But, again, it presumes a certain amount of steady cartography by the hypothetical fleet of Von Neuman probes.

There's a Sci-fi series called The Bobverse that explores the idea of a sentiment fleet of Von Neumans exploring the galaxy, and the various trial and tribulations involved. One point it discusses is that even with a saturation of probes, you don't get real time communication. So even in a hypothetical universe where alien life did exist and survey earth, what are they odds they'd be watching us at the moment of our development. What would an alien AI be looking for and what would it do when it was discovered?

We could still be too primitive to bare noticing. Or we could be living in between blinks of an alien camera that only reports back every 1000 years.

As we look out at the cosmos, we could be looking at things we don't understand. After all, what does a star surrounded by a Dyson Sphere look like to a telescope that is searching for glimmers of light, heat, and gravity? SETI is operating purely on conjecture. That's assuming alien civilizations are even capable of creating these hypothetical superstructures. Or that the structures would function as we intuit.

At some level, I have to question if we know what we're looking for. Because so much of this feels like we're searching for humans deep in space. Perhaps the reason we can't find aliens is that they are simply... too alien.