I recently read an article in Spectrum magazine about the ongoing search for extraterrestrial life. The idea behind most SETI (Search for ExtraTerrestrial Intelligence) projects is we assume that if there is another intelligent civilization with technology at or beyond our level they might be trying to contact other life (us!) by sending out messages on electromagnetic waves. In the 60's until now these people assumed other civilizations would be using radio waves, and so have been searching radio frequencies with no real success. The article I read says the the search is now moving to optical light, another bandwidth of the electromagnetic spectrum, which is a more likely communications tool. I feel like I don't even need to mention the hubris involved in believing that other civilizations will be using the same technologies as us, but apparently I do since the SETI people are putting a lot of time and money into it.
So lets put that arrogance aside and assume that alien cultures are using lasers to contact us. These alien cultures don't know that there is life on earth, or they would be trying to contact us much more directly. So, these laser messages are not going to be sent out to the earth constantly. Again assuming the aliens have the same types of technology as us, these beams will be sent out in the direction of different star systems that these aliens think might have life on them, each star system getting a message at a different time. Now, let's assume that they're sending the messages towards earth at all, which is not a trivial assumption, otherwise the whole issue goes away. So, every once in awhile the laser message is pointed towards our star system. What are the odds that at that time we have a clear line of sight to the alien star system? That some other planet or piece of space junk or the moon isn't in the way? Or the sun, even? Then, assuming that the laser actually reaches the earth, what are the odds that it hits a point on the earth were we have a receiver listening? We're talking about light year distances here, so a change of a few fractions of a degree at the sender translates to huge differences in hit location on the earth. Unless we put receivers all over the earth its almost impossible to believe that we will intercept such a message, if you can even believe that a message will hit the earth at all.
But, lets be arrogant again and assume that we do receive a message. Then it will take us years, if not decades, to even recognize that it is a message. But lets assume that some day we receive a message, realize that it is a message, and even pinpoint where in the universe it came from. What now? What was all this about? I guess it proves that there is other intelligent life in the universe -- probably. But to really prove that, or do anything meaningful with it, we have to contact this life and communicate with it.
So, we need to start beaming messages back to them. This is much easier than their task of beaming them to the entire universe because we know exactly where they are. So we can built a laser-message space craft and put it somewhere in space always transmitting right to their planet. Now, this planet will be at least ten light years away. That means that when we received the message it was already ten years old. When we send our message to them it will take 10 years for them to receive it, assuming that they receive it right away. Then, assume that they recognize our message and send one back. That takes another 10 years. So now its been 20 years since we first detected the message, and we get something back.
Now what? What good is this? We have spent billions of dollars and who knows how many hours and we have proved that extra-terrestrial life exists. What does that matter? What can we do with that? We can communicate with this culture, you might say. Not likely any time soon. Assuming they even have the same concept of language as us, its very unlikely their language will be anything we can understand or decipher without a great deal of correspondence. And considering that it takes 20 years for a single round-trip message, its going to be a long while until we can actually have any sort of "meaningful" conversation with such a culture.
Of course, all this assumes that any other intelligent life is benevolent. You can easily imagine some uber-fascist alien race trying to detect intelligent life in the universe in order to subjugate it. So we spend billions of dollars to build a laser-messaging satellite that is really just a homing beacon for some oppressive overlords to find our little planet. Nice move guys. But, a lot of assumptions have been made to this point, so why not continue and assume these guys are nice?
All of this to say that SETI is one of the biggest wastes of time and money I can imagine. Thankfully, the government stopped funding SETI in 1993. It is a project that makes many, many arrogant assumptions to even make itself viable. Then, assuming all of the assumptions are correct all we get out of it, best case, is the knowledge that there is some other intelligent life out there. Worst case, we become enslaved by some much more powerful race. Nice. I say we wait until we have the technology to visit or communicate more quickly with and hopefully stand toe-to-toe militarily with any alien race before we spend the time and money to try to contact them.