The question of whether we are alone in the Universe is one that on its surface seems academic. Until we meet another species other than humans what difference could it possibly make? Yet the answer to this question could have a more profound effect on human civilization than any question that has been asked in humanity.
One of the first truly serious attempts to answer this question in a scientific manner took place in 1960 in Green Bank, West Virginia. A group of scientists that would later become known as the order of the dolphin were meeting to establish SETI as a scientific discipline. The United States government was still considering if it was even worth funding SETI and the answer came in the form of the Drake Equation which is still the best answer we to the question of whether we will find signals from other lifeforms in the universe.
The equation states N = R* x Fp x Ne x Fe x Fi x Fc x L where N is the number of species in the universe that communication might be possible with.
The first of the variables is R* or the rate at which new stars are formed in our galaxy. This number is one we can roughly guess at though we do not have exact numbers. In addition the true needed number is not the amount of stars being formed now but the amount being formed a few billion years ago when ours was. The current estimation is about 7 per year.
Next is F subset p. This number represents the fraction of those stars that have planets. We have more information than Drake did. He guessed 50% and our surveys of stars so far appear to show that it is no less than 30% and since we can still only detect very large planets in orbit of other stars it is likely far more.
After that is N subset e which is the number of planets that can potentially support life per star that has planets. So far in looking at space we have discovered that many planets have eccentric orbits that would make life similar to ours unlikely, or are gas giants with no surface or are simply to close or far from their sun. Our best guess from current evidence is that Ne > .005 though other factors may plan in making this number far smaller.
F subset L is the number of planets that could potentially support life that actually develop life. This is an assumption of microbes. Drake at the time estimated this number to be 1, meaning that every planet that could potentially support life would. Less optimistic scientists in Australia have come up with a number of > .13
F subset I is the number of those that would develop intelligent life. We have no real information at this point besides earth and possibly mars or Europe but scientists best guess is 0.01
F subset c is the number of those planets that would be willing and able to communicate with us. This number becomes even more of a guess but scientists are willing to assume 0.01 for the sake of the equation.
The final number is L and the one that has the most relevance to us, though perhaps marginally less than it did to the US government in the 1960's. This number is the expected lifetime of a species that can communicate across interstellar distances. That is to say the amount of time a species can keep from blowing itself up or being killed by a natural disaster or collapsing society. We have only ourselves to estimate this number. Drake guessed 10,000 years. Others have guessed lower or higher depending on their level of optimism. Using all available information which is us we can state that the L is at least 70, as we sent our first signals into space in 1938.
If we assume those numbers are correct the equation results in 2.31 civilizations in our galaxy at this time that could communicate with us. Not a high number, but also not 0.
And by changing any of these numbers even a fraction that number can rise or plummet.
If you wish to play with the numbers of the drake equation yourself to discover how many intelligent civilizations willing to communicate with us exist based on your opinions of these numbers you can check this drake equation calculator and decide for yourself if we are alone in the universe.