Our species has been around for only a flash of Earth's 4.5 billion year history: and capable of electromagnetic communication for yet a fraction of that time. What are the chances of another species evolving and achieving communication on the same time scale as humanity? Perhaps that is even the wrong question; in order for us to hear anything from distant planets and solar systems, it would most likely have traveled hundreds of thousands of light years. Whatever--or whoever--sending the signal would be that much older when we receive it. Can we expect a civilization to survive that long? Can we expect their planet, or solar system, to remain stable for the time it would take us to respond? Knowing how much time our own development has taken, many astronomers think it's unlikely.
"Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world."
-Albert Einstein
It has been said that for all practical purposes, we are alone. But that does not stop the dreamers...those among us who are inspired by the vastness of the cosmos; those who imagine the intense solitude of humanity being finally broken...
In a sense, aliens are already with us. They have lived in our consciousness since the moon and nearby planets were populated by imaginative writers and even scientists who saw "evidence" of extraterrestrial life, such as the canals on Mars.