"Aliens...If they exist, where are they? Why aren't they contacting us? What are they waiting for?"
When discussing the possible existence of extraterrestrial green men (the kind that possibly romp around the universe, visiting star systems and enslaving the unsuspecting lifeforms that are unfortunate enough to get detected), the above questions will almost certainly arise at some point. Aliens.. You've seen them in movies. A multitude of different books have been written about them. Our ancestors built huge monuments in their honor...but we've yet to make "official" contact with any of them.
How Can This Be?
The truth is, Earth is one tiny planet in an ordinary solar system in an average size galaxy. The universe is so mind bogglingly vast, it's a mathematical likelihood that we are not the only advanced civilizations out there (though that says nothing about the likelihood of one of them being located close enough for us to physically travel to them in, not only our lifetimes, but in the lifetimes of our children's children). Didn't Hawking remark that's it's more likely any kind of life exists (any kind that you can imagine) instead of none at all, even in the most basic forms? Let's look at some the explanations behind why such civilizations would not be apparent to us...
What would you say if I told you that advanced alien races could be monitoring our planet without being directly detected? It sounds silly, right? Assuming such a technologically advanced race exists, why would they have to crawl through a worm hole or gas up the starship to visit our solar system when they could build a self replicating device (such as a Von Neumann probe) to do all of the work for them? Besides, who knows exactly how advanced their technology is. Molecular nanotechnology would make it possible for such devices to leave no discernible trace to our human eyes.
We all know dolphins are pretty smart, what happens if we elaborate on that. What if an advanced civilization was forged in a water world where it was difficult to create fire or mine for other precious metals that are necessary for technology to communicate with other civilizations?
It could be that intelligent species see other forms of life as competition and they systematically destroy them as they are discovered to avoid confrontation or competition. After all, humans still destroy other cultures on a consistent basis. Once we overcome that hurdle (with one race reigning supreme), would we seek to destroy other lifeforms if we feel threatened by them? This might be the case for other intelligent beings. (it would also be bad news for us in the event that we are sussed out)
Extraterrestrials might use radio frequencies that are not used by human-kind. Said frequencies are unlikely to be discernible after traveling vast distances. Or certain signals might not penetrate our atmosphere. For instance, with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light years. Clearly detecting an Earth-type civilization at great distances is difficult. A signal is much easier to detect if the signal energy is limited to either a narrow range of frequencies, and/or directed at a specific part of the sky. Such signals can be detected at ranges of hundreds to tens of thousands of light-years distant. However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent.
Could it simply be they choose not to communicate with us? For fear of what we may do upon discovery? Or the galaxy could be full of civilizations eager for contact, but everyone is listening and no-one is transmitting.