I think if you consider that the mass of our Galaxy is approximately 100 Billion times that of the Sun, and an estimated 90% of that is in the form of stars, most of which are smaller than our parent star, researchers have conservatively estimated that the Galaxy contains anything from 300 to 500 Billion stars.
Now if you rule out all the systems that are very unlikely to have resulted in planetary systems at all, such as multiple stars, contact binaries, Galactic Hypergiants and likely the majority of type 1a and 1b Supergiants, then you still left with around 250 to 430 Billion stars that may have planetary systems.
Now lets rule out all those stars that are unlikely to have planets orbiting at a distance from them which would allow for life to evolve and flourish (for simplicity we will deal with life which are water based), or they have planets in this "habitable zone" but the life expectancy fo the star rules out sustaining life for extenbded periods. This will remove all the flare stars, most of those falling into spectral types A,F,B,K & M plus all the Brown Dwarves, pulsars, Neutron stars, Magnetars and White Dwarves, regardless of spectral type. This is a large figure, and depending on whose figures are used, this will likely leave between 15 and 35 Billion stars around which planets may exist that are within the so called "habitable zone"
Now we get into really speculative territory, simply because we only have one example to go on, ours, so many researchers tend to estimate that 10% of this number will have planets capable of supporting life, and 10% of this figure will have life, then 10% will have life which has evolved to a higher order, and 10% of this will have an advanced and dominant species on it, but then only 10% will become technically advanced and form the type of society we are currently in.
So if we take the middle figure of the range, say 25 Billion, and do the math from there, then our Galaxy could have about 250,000 planets upon which resides a technically advanced species.
Now the chances of those species being at the same level of development as us is tiny..someone did attempt to calculate it a few years back..don't know if the came up with anything though..realistically many will be behind us and an equal number could be ahead of us technically.
Now from a Technology perspective we have only just climbed onto the tree and are out of the roots, but firmly in the lower branches. Those behind us are unlikely to be able to communicate, let alone travel here, so they can be ruled out, so we are left with those that are similar or more advanced than us technically. The problem is that we don't use the same types of radio that we did 50 years ago, and so long as the frequency is the same and the way in which the signal is generated is the same, then there will be no problem, but in reality much of our communications is now done at frequencies that 50 years ago they would never have been able to recieve..even many of our civilian broadcasts would be unrecievable to those with 50 year old radio sets as many are now Frequency Modulated rather that amplitude modulated.
Now lets take it further...would a species say 1000 or even 10,000 years more advanced than us even use radio as we know it? Unlikely is the answer, our technology now is changing radio comms at an alarming pace, so that is another 50 years everything we think is High tech, will be stoneage to us...
I support SETI, we have to start somewhere, but I do not think aliens are visiting Earth, have visited Earth, and nor are they planning too.
I firmly beleive they exist, somewhere out there, but they are incommunicado at this time for us..and it is likely to remain that way for the forseeable future.
