I am a strong supporter of SETI – the search for extraterrestrial intelligence. To me this is a fascinating scientific endeavor with a potentially huge payoff. And yet, I find that even within skeptical circles I hear grumblings that SETI is not real science.

The primary complaint stems from the misapplication of an important scientific principle – that a necessary criterion for any scientific hypothesis is that it needs to be falsifiable. If you cannot make an observation or conduct an experiment to prove an idea wrong, then that idea is not useful to science.

SETI critics argue that the notion of extraterrestrial intelligence is inherently unfalsifiable. No matter how much you look with negative results, one could always argue that it is not enough, we have to look deeper into space, for fainter signals, in more EM frequencies.

But this, however, does not render SETI unfalsifiable in the that is meant by philosophers of science, because it runs up against another scientific principle – the notion that you cannot prove a negative. It is logically impossible to prove completely that a phenomenon does not exist (unless it can be demonstrated that it violates know laws – but even then the proof is only as good as the degree to which the violated laws are established). Evidence for the lack of a phenomenon is always only as good as the thoroughness and efficacy of the search methods.

You can apply this to anything is science, even drug trials. If a large well-designed clinical trial shows that drug A does not improve disease B what it really showed is at the dose and duration given it did not have an effect big enough to be detected with statistical significant by the study. You cannot rule out a small effect, a subpopulation effect, or that a higher dose would not have shown an effect. What the study does, in effect, is set statistical limits on the size and scope of an effect.

The search for ET (or any new phenomenon) is the same. We can never prove that there are no ET’s in the universe – but that’s not the point. The more searching we do with negative results the more statistical limits we can set on the number of ET civilizations that are engaging in specific behaviors – like broadcasting at certain frequencies. Right now, for example, we can say there aren’t numerous nearby ET civilzations or artifacts braodcasting in our direction in the radio frequencies we have listened to.

In a recent Nature commentary, Philip Ball also feels the need to defend SETI. He discusses another method (other than listening for radio signals) for looking for ET, writing:

Dyson suggested that a sufficiently advanced civilization would baulk at the prospect of its star’s energy radiating uselessly into space. They could capture it, he said, by breaking up other planets in their solar system into rubble that would form a spherical shell — known as a Dyson sphere — around the star, creating a surface on which the solar energy could be harvested.

He is talking about the work of Richard Carrigan, who asked a specific question: are there any detectable Dyson spheres within about 1000 light years. That is not the same thing as asking if there are any ET’s, rather its a proxy question. It’s a method of looking where the light is good. What behaviors might ET be engaged in that we can detect. Well, they may be broadcasting at us in the radio frequency. Or they may have buit something like a Dyson sphere around their star – perhaps we can detect that.

Of course, these methods require speculation about what an ET civilization might do. This is problematic because really we have no idea. All we really can do is assume that humanity is somewhat representative of technological intelligence and then speculate about what a more advanced version of our own civilization might do. It’s a pretty thin line of reasoning, although it is legitimate as far as it goes. We just know we are speculating from too little information.

Another recent SETI “innovation” was to assume that ET might be interested in looking for life elsewhere in the galaxy. They might start by looking for planets around other systems, and planets like earth would be easiest to detect from a direction in the plane of our system. Therefore, to those observers the earth would eclipse the sun and that is one way they could detect it. It might be more likely, therefore, that such civilizations are broadcasting at us, because they know we are here, or at least life might be here. Perhaps, then, SETI should focus its efforts by looking at stars in the plane of our system.

That is another long chain of speculation – but very interesting, and legitimate as far as it goes. This is an effective investigational logic to use – imagine what might be the case, then take a look.

It still strikes me as odd when people use this feature of SETI to argue that it is pseudoscience. No -it would only be pseudoscience if SETI proponents concluded that ET exists based on such reasoning, or to explain away negative results. But this is a perfectly reasonable hypothesis upon which to design a program for searching for data.

SETI programs also use a very reasonable protocol for dealing with signals when they encounter them. They consider such signals an anomaly, then they systematically rule out possible known causes. So far, every candidate signal has turned out to be something known – no enduring anomalies. This behavior is very different from pseudoscientists, who might call such signals “ET signals” and then proceed to only rule out a couple of obvious known causes, or then dismiss non-ET explanations for the signals.

Take ghost hunters, for example.They typically look for anomalous photographic effects, declare such anomalies to be “ghost globules” and then dismiss any non-ghost explanation.

I think it is important to properly demarcate legitimate science from the “cheap imitation.” In my book, SETI is legit.

And, of course, with SETI we just might hit the cosmic lottery and find genuine evidence for ET. That’s worth the tiny investment currently going to SETI projects. Even knowing that our galactic neighborhood is relatively empty is interesting information. Perhaps humanity is more rare than we imagine.

But of course nothing would be more fascinating than discovering that we are being bathed in a stream of information from an advanced alien culture. I like just knowing that SETI makes that possible.