Does SETI Make Sense? Part IV: Communicating

Harry SETI header

We are likely to be the only civilization in our galactic cluster if the chances of forming such a civilization are but one in ten trillion. These are not encouraging numbers. For the sake of argument put the odds up to having 100 civilizations in our galaxy. That’s lots more than I would expect and means that the estimates are way off, by a factor of 10,000. Such an improvement in estimates certainly is friendly to SETI. But, can we contact one of these other civilizations?

Although I think that SETI is a colossal waste of resources, I cannot fault those who pursue this dream.

The galaxy is a very large place, about 100,000 light-years across. Any civilizations will be in an annulus around the center because being close to a galactic center is inimical to life. We won’t be able to communicate to the other side of our galaxy due to the extreme noise originating at our galactic center. Our potential range for communication is probably about 20,000 light-years, but this range again is limited by the number of noisy objects between us and our target. 

The area of an annulus with an outside radius of 50,000 light-years and an inside radius of half that is about six billion square light-years. If our communication range is 20,000 light-years, then our area of access is about 1.2 billion square light-years, making it about 20% of the total area in which a civilization may be found. There had better be more than five such civilizations in our galaxy to give us a chance. However, there’s the matter of that signal.

Because of the inverse-square law for transmitting radio signals, any signals from more than a few hundred light-years would be extremely weak and buried in noise. Ignoring this problem, there remain several more.

The other possible civilization will be on a rotating planet. Any signal from that planet will be moving. While there may be a great many such signals originating from the civilized planet, we cannot detect a coherent signal if it keeps changing constantly as the planet rotates. SETI must distinguish signal from noise. It must use an algorithm to determine that the data it receives have some sort of intelligent purpose and are not merely random.

A few decades ago, we might well end this discussion here, but we have learned much about transmitting information in the intervening time. Once, there was only AM (amplitude modulation) radio. Then, FM (frequency modulation) came along. Today, we are seeing a transformation to digital transmission, which is indistinguishable from noise unless you have the decoding algorithm. Who knows what will come next? We have to hope that we will encounter signals from a civilization in its early phases of radio transmission. How likely is that? Not very. We are moving out of ours in just a century. Also, today much of our information travels by cables or highly focused microwaves.

Ultimately, SETI people must assume that a distant civilization is attempting to reach out to us using technologies that are old and also focusing a very strong signal in our specific direction. Would we do such a thing? There are many who are concerned that extraterrestrials already have picked up our signals and are on their way to despoil our planet and enslave or kill us all. Therefore, other civilizations may choose to hide themselves instead of advertising their existence.

We must also assume that somehow this other civilization found us and is sending the signal specifically to us. How did they find us? If by our own radio signals, then they must be within 50 light-years of where we are because that’s how long it takes for our signals to reach them and for theirs to return and be noticed by SETI. If our search radius is 50 light-years, then we have a search area of about 8,000 square light-years. Compare that to the six billion that may contain maybe as many as 100 (very extreme upper limit) civilizations. That’s one civilization in every 60 million square light-years. Not very promising compared to 8,000. We have one chance of 8,000 that a civilization is there — at best! It’s probably more like one in 80 million. Add to that the likelihood that the other civilization has noticed us and decided to contact us, and you can see that there’s just no way.

Of course, the other civilization may have seen our solar system and noticed that it has three rocky planets in the appropriate range for life. It probably has noticed quite a few others as well. The chance of intelligent life on those planets may have been downgraded due to the circumstances. For Venus and Mars, there’s the closeness to our sun and the small size that count against them. In our case, the very large moon means that our planet was wiped clean at some time in the past. If they are very good with their observations, they may have noticed oxygen here.

Let’s say that we found a planet out there with oxygen. Would we assume intelligent civilizations? We’ve had oxygen for over two billion years and radio transmissions for one hundred. That’s a very tiny fraction of the total time (one in 20 million) and the two billion would have been much longer were it not for a rare asteroid impact. I suspect that we’d be searching for some verification of intelligence before blindly spending time and money sending signals to that other body. So would our would-be communicators. They would check for radio signals, for example.

My conclusion is that SETI does not make sense, except to those of great faith. Faith is not a part of science, unless you count faith in one’s own ability. In a phrase, SETI is scientific nonsense. However, I can understand the determination to move forward against overwhelming odds. It’s what has consistently defined our species. We determined to go to the poles. We determined to go to the Moon. Now, we are determined, some of us, to go to Mars.

The chances of other intelligent civilizations in the universe are very good, almost guaranteed. The chances of such a civilization within contact distance of us are remote, almost zero. It’s good that the human spirit can persevere and keep trying. Fly nonstop across the Atlantic? Ridiculous! Not to Lindbergh. Break the sound barrier? Too dangerous! Not to Chuck Yeager.

Although I think that SETI is a colossal waste of resources, I cannot fault those who pursue this dream.

Does SETI Make Sense? Part I: Numbers
Does SETI Make Sense? Part II: Life
Does SETI Make Sense? Part III: Evolution

One Response

  1. You point to the critical failing but don’t expound on the significance – that the sort of communication we can intercept and understand is essentially limited to span of a century or so (if using our civilization as a typical yardstick for how civilizations develop – itself a leap of faith.) So the chance of two civilizations being the right distance apart and overlapping by 100 years in their millennia long evolution path needs to be considered as well.

    The flip-side may be that other civilizations could be specifically targeting us to let us know of their presence. If going by our own development, this is perhaps not such a good thing – any time we encountered other civilizations less technologically developed it didn’t turn out too well for the other civilization.

    Finally, consider that if another advanced civilization had noticed us, or even noticed a potential here – that is, observed that there is a chance that life could develop here, and while they may not yet have observed or detected that life, operate under the hope that life could develop and instigate a system to let us know they are there. Even given the challenge of generating a signal that could be observed by us it would be child’s play for a sufficiently advanced technology. If such technology was interested in letting us know they are there, they could do so. Therefore, we must conclude that either there is no such technology, or they are not concerned with contacting us. From the analysis presented in the article, there is technology out there, just from a probability analysis. So they are not interested in contacting us, and the chances of us detecting signals that essentially are leaks of what they are up to are infinitesimal. And they may actively be obscuring their presence in order to avoid detection. The chances of us being able to notice a technology that is intent on being undetectable are even less than infinitesimal.

    Are we actively interested in targeting signals at nearby systems with the right configuration of planets with the intention of letting potential life there know we are here? I haven’t heard of any such initiatives, and for sure it would be centuries before such expenditure would have results, if ever. The Voyager probe is one of the early attempts to do this, in a random and potshot way.

    Possibly a more useful occupation of the SETI resources could be targeted at looking at what is considered noise and examining if there is signal there. Perhaps the hydrogen noise we filter out of everything we observe is the equivalent of our cellular baseband signal, if only we could decode it.

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