If radio signals such as our earliest radio broadcasts continue to flow outward into the universe, why are we not surrounded by such signals from other intelligent life forms if they exist. Surely all intelligent life did not suddenly become technological in the last 200 years. If there was intelligent life elsewhere even 2000 years ago and if it exists, I am sure that there was, surely their signals would be permeating the galaxy by now. I am mystified by the great silence out there, one begins to wonder are we something very rare indeed, or are we missing something here.
You are certainly not the first person to wonder where everyone is. The physicist Enrico Fermi asked the same question in the 1950's, and it was given the fancy-sounding name of "Fermi's paradox". There are two types of responses. One is that complex life is not as common as we may think. In the Drake equation, the calculation that estimates the number of radio-loud civilizations in the galaxy, there are several factors that we really have no idea on, all of them having to do with the rise of life. We can truly only guess at what fraction of life will progress from the basic to complex to intelligent. Right now the only planet that has life also has intelligent life. This is quite the bias. The Drake equation also includes a factor for the lifetime of a civilization. While the lifetime of an intelligent species might be long (millions of years), it may not be that they are radio-loud for the entirety of those million years.
This brings me to the second response. Even if there are civilizations out there right now emitting radio waves, their signals are hard to detect. For now let's assume that there has been enough time for extraterrestrial signals to reach us, and it's just a matter of us finding them. The difficulty in finding such signals is that they are likely to be weak, much weaker than many astrophysical sources. To give you a rough idea of what it takes, one of the brightest sources of radio emission on Earth is the Arecibo Observatory radar. An identical Arecibo telescope could detect the radar emission out to about 3,000 light years if it's pointed directly at us. As the diameter of the galaxy is about 100,000 light years, we're missing out on a huge fraction of the stars. Furthermore I emphasized that this works only if both transmitter and receiver are pointed right at each other, and what are the odds of that? In addition to considerations of signal strength and directionality, we don't know what kind of signal we should be looking for. The usual SETI philosophy is to look for signals with narrow frequency bandwidths. While there are good reasons for choosing this, ET might look totally different. So while it is disappointing that we haven't found any galactic neighbors, there are good reasons why we shouldn't have expected to see anything. We just have to wait for the next generation radio telescopes to come around and keep listening in the meantime.
This page was last updated on January 21, 2016.