The Great Filter

Why haven't we heard from other intelligent civilizations across the cosmos? Is it just too big? Separated by distances too great? Or is something greater that bears some thought? The Great Filter.This would be a barrier that perhaps all advanced civilizations would ultimately evolve towards. It would present a filtering system that would end their advancement and could be the reason we have never heard from them. They just aren't there anymore!Is this the direction we here on Earth are heading? Of course, nobody knows, but is an subtle warning that may be worth considering.I hope that this sparks your own curiosity and for teachers, I hope this is material that you can use in your classroom to ignite great discussion and further inquiry.