depth first search

The first part of my response to this.

When did “skeptic” become a dirty word in science? When did a skeptic require quotation marks around it?

I’m going to interpret this rhetorical question as an invitation to skepticism. Let’s first consider the argument against SETI.

I’m going to cut through directly to the punchline.

The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion. Faith is defined as the firm belief in something for which there is no proof.

In fairness, I need to summarize the argument preceding these statements. Fortunately, XKCD has already done so:

Now let’s decompose the punchline into its component parts. The first sentence:

The Drake equation cannot be tested and therefore SETI is not science.

For those of you who don’t know, SETI stands for the Search for Extra-Terrestrial Intelligence. The project involves listening to radio signals on radio telescopes, though the project has broadened somewhat to describe a constellation of different approaches, each of which is attempting to find evidence of extra terrestrial life. The “listening” usually involves pattern analysis over large swathes of radio frequency, usually with computers. Though this hasn’t led to the discovery of intelligence, it has led to some novel advancements in distributed computing technology.

Michael Crichton’s argument against the status of SETI as science makes sense if the hypothesis that motivates the SETI program, that noisy intelligent life exists in radio range of Earth, had anything more than a cursory relationship to the Drake equation. Fortunately for science, SETI, and us, but unfortunately for Michael Crichton’s argument, the relationship between the Drake equation and the SETI hypothesis is far more benign.

Actually, I should be more precise on this point, because it is an important one. Let’s examine two possible outcomes from SETI work. In the first, SETI discovers patterns in radio frequencies that exhibit, with little doubt, the existence of intelligence elsewhere in the universe. What are the implications for the Drake equation? Well, N > 2 instead of N > 1. And … everything else remains unchanged.

The discovery of intelligent life would be an astounding scientific discovery. It would also tell us next to nothing we don’t already know about the Drake equation. This should indicate just how useless the Drake equation is, even if SETI turns out to be spectacularly successful.

In the second case, SETI finds nothing. This is a more difficult case, because it could happen that intelligent life does exist in the universe, even if it is not noisy life within range and frequency of SETI telescopes and pattern recognition software. Then the narrow hypothesis under which SETI operates turns out to be false, but it is false in a very unsatisfying way. It is disproved by induction.

Disproved by induction.

I want to dwell on this point a bit, because I think it lies at the heart of a number of popular criticisms of science. The problem is that induction is not logically valid. Plato would not approve. Induction does not come to us from the world of perfect forms. In fact, induction is the cause of our separation from this world, living as we do beyond the Humian divide.

Let me make this clear: If you don’t allow for induction in science than you have to throw away everything except math.

So let’s accept that exhaustive search is sufficient to disprove a sufficiently narrow claim. What does a false SETI hypothesis say about the Drake equation? Nothing.

So, with this in mind, let’s return to the point of contention:

The Drake equation cannot be tested and therefore SETI is not science.

This statement has the logical form A (the Drake equation cannot be tested) then B (SETI is not science). As I’ve argued, A (the Drake equation cannot be tested) happens to be true, but B (SETI is not science) is not something we can conclude from the truth of A.

I’m close to running over some kind of self-imposed word limit. So I’ll stop here for now.

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