The question keeps on getting asked, but the reality is a long way short of Star Trek, and distances between stars so vast that any encounter would have to be pure chance.
In another supposed explanation of the lack of contact with any kind of alien intelligence, it has been mooted that maybe we humans are simply being ignored, for mysterious reasons that all science-fiction writers and scientists can spend years speculating about.
There was a book by Arthur C Clarke – The Pale Blue Dot – which brilliantly explained just how rare it is likely to be, within known space, for life to have evolved to a level of intelligence equal to or better than that of humanity.
Even if you take the conclusions of the iconic physicist and sci-fi giant Isaac Asimov, that the laws of probability demand the existence of such life forms – in the milky way galaxy – our own – 100billion stars exist. If only 1% of those had planetary systems ( It is actually much more) then that would be a billion.
If only 1% of these had a planet capable of supporting life, that would be 1billion such, and if only 1% of those supported intelligent life (at some level), that would be 100million. Even if only 1% of those supported civilizations of some kind, that would be 1million, and if only 1% of those had developed space travel, there would be ten thousand other advanced races within our galaxy alone.
Considering that ours is one of countless billions of galaxies within the universe, then everywhere has to be absolutely teeming with life, so why are still untroubled, even after a century of radio waves blasting out into the ether? Light-speed is about 186,000 miles per second, and accepted by physicists as the ultimate velocity at which one could travel.
Quantum physics and ever more technological breakthroughs might possibly, in the next century, allow us to travel to the outer edges of our solar system, but interstellar travel is at present, quite frankly an impossible dream.
Consider just how long might be needed by alien societies to accumulate resources and technology to let them to leave their home worlds and travel to other stars. Even 1% percent of light-speed would leave them, or even us, taking centuries to reach the nearest star, which may well not have planets.
Then again, we humans tend to think that intelligent life will be something like ourselves – carbon based, bipedal etc., but there is no reason why it should be, and the strangeness of some earth life-forms which exist in hostile environments can bear witness to that fact.
Consider also that the curiosity which is such an inherent part of the human psyche is not going to be a universal trait. Perhaps there are much older races throughout the galaxy who simply see no need to explore too far beyond their own worlds.
Any way in which we humans care to look at this issue, we simply cannot assume that other races out there are anything like us in appearance or temperament, or even ambition where exploration is concerned. Whilst humanity wants to go inter-galactic just because it is there to be done, others may find it a waste of valuable time and resources.
I personally have not the slightest doubt that other races out there in space are aware of humanity, but not interested in making contact. It may be that we are just too young as a species to be of concern to them, or that in truth, civilizations as intelligent as our own are actually pretty rare throughout the universe, and when discovered are shunned for obvious reasons.
We are, truth be told, a very aggressive species, and that could well count against us where ET is concerned. Perhaps the human race needs to learn humility and act peacefully around the globe before any alien race would initiate first contact.
Space is called space because distances between stars are so vast, and will remain an insurmountable obstacle to human ambition until our science is much more advanced. ET is unlikely to be calling anytime soon, so if you want to buy a great collection of science fiction shorts, go to either of these for a genuine bargain evening of electric entertainment.