But intelligent civilizations don't exist on evolutionary timescales, they exist on the timescales of intelligent beings, like us. We struggle to maintain proje

But intelligent civilizations don't exist on evolutionary timescales, they exist on the timescales of intelligent beings, like us. We struggle to maintain projects for more than a few years, a tiny fraction of our lifetimes. Our oldest institutions are only a few thousand years old, dozens of generations. Many of our civilizations rose and fell within centuries. Our species is much less than a million years old, and while it is unclear how long we will last, it seems pretty optimistic to think we are in the first few percent of our species history, nonetheless the first few thousandths of a percent of our civilization's history. If we had built ships travelling at 1% of lightspeed to colonize the galaxy, and launched them when the common ancestor of humans and chimps lived, some of them would still be travelling to their destinations. Maybe there are rare species that live on timescales tens, hundreds, even thousands of times longer than ours, but even for them these would be long time periods. And it's not enough to simply reach these distant star systems, you need to survive there indefinitely. A billion years is a long time, much longer than complex life has existed on Earth. Most of Earth's crust is less than 250 million years old. Objects in space would be pulverized to dust by a billion years of collisions, assuming their orbits were even stable for anywhere near that long. On timescales like that, even structures made of solid granite would erode away on a planet's surface. Maybe by looking for weird minerals which couldn't have formed naturally or microfragments of broken down structures we could identify a civilization as having existed in the deep geologic past, but there is no way we could do that sort of analysis from light years away, even if we knew what to look for. Realistically, there are only a few technosignatures we could hope to find. Of course there are active radio transmissions, which requires the civilization to be active in an incredibly narrow timeframe. Exoplanet atmospheres containing short lived gases with no known natural production mechanism would be good, and may outlive a civilization, but anything stable enough to last for geologic timescales would be impossible to rule out as natural. Probably the longest lived evidence that we could detect would be the remnants of alien megastructures in space. No one really knows how long say a dyson sphere would last after it stops being maintained, maybe millions of years. But unless we make some extremely tenuous assumptions about what and how alien civilizations would construct megastructures, we can only make extremely rough guesses as to what a functional megastructure would look like, nonetheless one that has suffered millions of years of wear and tear. Of course eventually they'll all start to look like clouds of dust and debris, but how do we distinguish them from the clouds of dust and debris that we find all over the place with a presumed natural origin?