Extraterrestrials might prove unintelligible, or even hostile toward humanity:  Were extraterrestrials to actually be contacted, an unlikely event in itself, the ability to share in any form of meaningful communication might well prove more difficult than might first seem. The extraterrestrials will have evolved on a different world, perhaps along lines so alien to that of Earth's life that it would be utterly impossible to understand what they said, even if they could be heard. Language is built largely on frames of reference, and when an extraterrestrial, evolving on an entirely alien world, perhaps possessing an entirely alien thought process, and certainly having an entirely alien linguistic frame of reference, no level of communication might be possible. With no conceptual similarities or cognitive common ground, the potential for sharing knowledge between species would likely prove impossible to decipher. They could simply be too alien. Furthermore, the hope that technologically advanced extraterrestrials would be friendly towards humanity is entirely unfounded. They might well prove hostile, and if they are more technologically advanced, they could prove an existential threat to the survival of mankind1. This is especially true if the extraterrestrials were actually capable of physically reaching Earth, which would require an understanding of physics and engineering centuries ahead of that of mankind. Welcoming such creatures to Earth could well spell ruin for humanity, as so advanced a civilization might well consider humanity to be of no higher an order of life than we might consider insects. It is far better than humanity not make such a habit of broadcasting its position. It could prove dangerous, especially if the Universe is not as friendly a place as scientists hope.  1 Brin, David. 2006. "Shouting at the Cosmos". Lifeboat Foundation.