You'll have to explain how seeing spectra from distant planets conclusively proves intelligent life. I don't see how it does. Spectra will tell you that a pla

You'll have to explain how seeing spectra from distant planets conclusively proves intelligent life. I don't see how it does. Spectra will tell you that a planet has molecular oxygen, for instance, carbon dioxide, etc., which would indicate an atmosphere like ours, which would indicate life similar to ours (doing photosynthesis), but you'd get that on a world occupied solely by plants, or with only non-intelligent life. Anyone looking at Earth before about 2M years ago (or anyone looking at it now, but more than 2M ly away) will see exactly this: a life-bearing planet with no life intelligent enough to build a civilization. Finally, even if we could conclusively detect a civilization on a distant planet, that still doesn't explain why they have any obligation to contact us. They can easily just ignore us, and I for one can see why they would. The only time they'd really need to strongly consider establishing contact is when we have sufficient capability to visit them, and that's never going to happen the way we're going. It'd probably by prudent on their part to spy on us though, since we're a potential threat.