If one was to look at the things the Romans built a few hundred years after the fall of Rome. Then look round at the local inhabitants one might think there's no way their ancestors built those things. World history is full of civilizations that raise to power, built fantastic cities, buildings, roads, acuducts and whatever., then fall for one reason or another. After a civilization falls the people left in the area somehow loose all the technical knowledge. One example concrete. The Romans had concrete, it took hundreds of years to rediscover freaking concrete after the fall of the Roman Empire. In the course of human inventiveness, concrete, one would think is quite simple. When our era ends, and it will, how long will it take to rediscover the technology we now have? Like this means we're communicating with right now for instance. I'd imagine when that happens, someone will look to the stars and say the IPhone came from outer space. Oh yeah, they already are.