Probabilities of Catching Alien Radio

As we continue to find extra solar planets around increasing numbers of stars and continue to find liquids (water, ammonia, methane…) in increasing places in our own solar system, one has to wonder when will we find life. Answering this question is a complex dance that requires us to first ask, “What is life?” and follow up with the question, “How can we detect it?” A lot of minds around the globe are focusing on these questions both theoretically and observationally, as scientists work to both find the most extreme life here on Earth, and to guess at what might be waiting to be found on other worlds.

Our search for life can be broadly divided into three different areas:

  1. Direct discovery of bacteria/microbes/other life with robot probes in our own solar system. This is the reach out and touch someone with a NASA/ESA rover way of finding life.
  2. Discovery of atmospheric components associated with life on another world. For instance, the unique signature of oxygen and smog indicative of the plants and polluters we have in our own atmosphere.
  3. Dialing up the radio emissions of alien civilizations using radio dishes (or alternatively optical or microwave beamed signals). This is the ET phones Earth scenario of movies like Contact.

Few science programs have lead to as much public involvement as the Search for Extra-terrestrial Intelligence (SETI) program. With their flag ship “SETI at Home” screen saver, they gave people a tangible way to help find life out there. Despite receiving no public funding, SETI has maintained an active research program, and their scientists have managed to consistently get telescope time and money to search the stars for radio signals.

At a certain level, I think people would endlessly search the heavens with a radio dish in search of a signal no matter how few planets were ever found, and how little liquid was ever discovered. We would keep searching even if we thought the search was futile. We want to know – Are we alone? That single question is one we have asked, at least once, every one of us. As children frightened of the dark we asked, “Are we alone?” as we watched distant planes and more distant satellites criss-cross their way through the pitch black sky. As adults, watching wars on TV, watching celebrity gossip, and watching 1000 small indignities and injustices being played out big on the little screen, we have asked “Are we alone?” We want someone else to be out there among the stars. And we are afraid there is someone else out there among the stars.

And so we search.

And, as scientists, some of us ask, what is the chance we will find that someone out there? If you were the last person on Earth, how would you know? If we were the only civilization in the galaxy, how would we know? Just as the lone survivor of some post apocalypse novel might say to himself, “I wonder if anyone survived in Australia?” and then run calculations based on guessed at wind and radiation and food ration levels and guess at the likelihood anyone else exists, a civilization like our own can look at the stars and calculate, based on guessed values, how likely it is we’ll detect life.

In a new paper by Marko Horvat of the University of Zagreb, calculated the probability of detecting radio signals from an alien civilization is carefully laid out way. In this cleanly written, and numerically thorough work, he starts from the Drake Equation and goes on to say that the number of civilizations out there waiting to be found is the total number of civilizations given by the Drake Equation minus 1. So, once we know what we’re looking for, which was defined long ago, we now need to ask, who might we hear? This is going to be a function of how long each civilization lasts and what fraction of that time they are leaking radio signals. For instance, in the situation that a civilization lived 1,000,000 years and developed and continued using (before moving on to other more advanced technologies) radio communications for 4000 years, we’d have a 0.4% probability of being around when that 1 civilization was giving off radio signals.

One nice thing about the Drake Equation is it considers the total number of civilizations to be constant while star formation is at a constant rate. This means that for every aged, advanced, dieing civilization there is another upstart group of evolving individuals getting ready to build a new society. If we want to find life, it is actually better if each civilization is short lived and uses low tech radio for as long as possible. If civilizations could only survive 50,000 years and used radio communications at some level for 4000 years, we’d have an 8% chance of being around when radio signals were being sent.

The next question is, if there is such a civilization out there, is it near enough and loud enough that our meager radio detectors can hear it? This is a complicated probability – A civilization that is loud, with powerful more-than-mega-watt transmitters could be detected at a much greater distance than a planet covered in boy scouts with ham radios. As our abilities to build more sensitive detectors get better, we can also change the probability. So – here we have to make estimates – based on our average radio leakage and our current technology, how near does that planet need to be, and what is the probability that will happen?

And if they are close enough to hear, and they are currently transmitting, what is the chance we are listening at the same wavelength they are sending? For practical reasons, a telescope can only listen to so many frequencies at once. What if we aren’t tuning in where we should be? So here, again, we have to look at the entire wavelength spectrum and consider how much of it we can tune it at one, and come up with a probability.

And even if we are listening at the right time at the right frequency and they are near enough, what if we just happen to be looking off in the wrong direction?

These last two factors: frequency and sky coverage are things we as observers can control. In theory, given enough money and enough telescopes it would be possible to look at every star in every wavelength in almost every moment in time. (In theory.) We can also improve on the sensitivity of our telescopes until the one lost explorer with a pocket walkie-talkie on a world on the other side of the galaxy can be heard. (In theory.) These human controllable elements are the variables in our equations. We can change their values, given time, money, and technology. What we can’t change are the number of civilizations out there and how long they last and how long they transmit radio signals. Those are external constants that are just waiting to be determined.

So, faced with a beautiful equation laid out in Horvat’s paper, we could guess at values and plug our way through, but he’s already done that for us. On the bleak or safe (depending on perspective) side of the story, if there are between 5 and 30 civilizations who live from 500,000 to 4 million years but only use radio for 500 years… well those are civilizations probability says we will never find (probability < 0.6%). On the other hand, if there are 300 civilizations out there, and they live 250,000 years and use radio for 2500 years, then there is over 90% probability of finding them if all else is optimal.

So, we are in the interesting position, thanks to Horvat’s careful thinking, of knowing how to predict mathematically what our chances are of knowing “Are we alone?” What we don’t know are the numbers we need to put in to find our probability. How long does a civilization last? How long does it send radio waves into space? And how many worlds do these civilizations fill? We can guess. For now the truth may be out there, but we are still waiting to learn it down here.


  1. Jorge Schrauwen July 10, 2007 at 2:58 am #

    Very interesting read…

    What came to my mind after reading this is…

    Why does a civilization have to use radio waves?
    Is there a significant reason why we use radio?

    And say if we move on to something new in say 1000 years… will be leave a radio beaken behind for others to find us via radio waves?

    If another civilization looking for ETL has been searching radio it seems somewhat logical that they would.

  2. John Hoxter July 11, 2007 at 4:22 am #

    The biggest factor, I think, is the silence! Surely, if there are other civilizations out there, how could all of them be less advanced than we are?
    If there are more advanced worlds out there, why no contact whatsoever?
    Isaac Asimov postulated that the main reason we do not hear anything (or have not been contacted by them) is that the speed of light is truly insurmountable.
    Whatever the case, the “thunderous silence” is a very convincing factor indicating no other life out there.

  3. Vijayaraghavan. E July 11, 2007 at 4:42 am #

    If we consider the sources of radio transmissions to be located in concentric “shells” surrounding us, by the time the transmissions from one shell stop reaching us, those in the next can start reaching us and so on the duration for which a particular civilisation transmits wil not have much bearing on the probability. As star formation and development of alien life can go on and on in each shell for a very long time (say during the active life of the galaxies) the prospect of civilisation replacing civilisatin ineach shell need to be built in.

  4. Rick E. July 11, 2007 at 9:11 am #

    As always doc you make an inordinate amount of sense. If they call I hope you get a chance to talk to them.

  5. astrojr1 July 11, 2007 at 3:36 pm #

    Absence of evidence is not evidence of absence. Just because we can’t hear anything doesn’t mean it’s out there. To conclude the thunderous silence is evidence is to particularly ignore the history of stellar parallax, and the lack of it’s detection until the 19th century viewed prior to then as evidence of a heliocentric universe. There are many reasons why we may not be able to hear anyone yet, clearly presented in the article. We are, technologically, three days out of the tree. It’s easy to believe we have powerful instruments. The intererometric instruments of the future will make Aricebo look like cans and string.

    IMHO we’re just still way behind down on the interstellar ladder, an anthill in the middle of a very big field.

  6. Tim Eby July 11, 2007 at 4:18 pm #

    Just a thought;
    Suppose the ‘aliens’ chose to communicate amongst themselves by modulating the polarization of an EM signal instead of modulating frequency or amplitude. Would we know that a given, perhaps ‘white noise like’ signal was a communication?

  7. Ryan March 7, 2008 at 10:09 pm #

    Maybe the us infered frequencies tat are modulated.


  1. A Ler…-- Rastos de Luz - July 11, 2007

    […] “Probabilities of Catching Alien Radio“, no Star Stryder; […]

  2. Star Stryder » Blog Archive » Panspermia is in the Air- by Pamela L. Gay - July 12, 2007

    […] For some strange reason, the ideas of moving life around the solar system on rocks / comets / other random objects and alien life have been coming up a lot with me this week. First blogged on the probabilities of finding radio signals from aliens (hat tip to Fraser on that one), then Fraser and I talked with Swoopy about the new National Research Academies Report on things we need to think about in trying to find life on off of this world. And now… The Universe is jumping in and bringing up alien life as well. […]

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