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Another Look at the Drake Equation
or
E.T. Phone Home, But Was Anyone There? (Part I)

by Billy Cooke

"There is a creature, called a Quakipoo, that lives on Mars. It is green, has three legs, four eyes, and hops around a lot."
This was a surprising remark from my committee chairman, Dr. Heinrich Eichhorn.

"You’ve got to be kidding!" I responded.

"Why not? Can you prove to me such a creature doesn't exist? …"

First of all, let me state, for the record, that Dr. Eichhorn does not seriously advocate the existence of such a creature. In fact he is a steadfast proponent of the "We are alone in the universe" principle. The above statement was made during a conversation we had some time ago concerning SETI (the Search for Extraterrestrial Intelligence). Dr. Eichhorn was posing about the error in what seems to be the governing principle of SETI these days, which can be simply expressed as

Absence of evidence is not evidence of absence.

While I am by no means convinced that we are alone in the universe (unlike my boss), I do take exception to the above, for one big reason:

It's not science!

Scientists are supposed to follow the scientific method, which can be summarized as follows:

1. Formulate an hypothesis to be tested.

2. Devise and perform a set of experiments to test the validity of the hypothesis. If the experiments validate the hypothesis, then we can be comfortable that it is right (at least under the conditions of the experiments), and now can call the hypothesis a theory. If the experiments do not validate the hypothesis, then we have two choices:

a. Discard the hypothesis (i.e., it is wrong).

b. Modify the hypothesis and start over with the experiments.

Let us apply the scientific method to the case of the Quakipoo. We postulate the existence of such a creature and must now devise a set of experiments to see if such a creature exists. Our experiment is simple - send space probes to Mars to search for Quakipoos. This has been done. In 1976, two Viking landers set down on Mars and searched for signs of life. They found none, not even one organisms. Confronted with the Viking results, most scientists would say that Quakipoos do not exist on Mars, that the hypothesis is wrong. However, a SETI proponent would cite the above "Absence of evidence is not evidence of absence" principle and suggest that we send more spacecraft to Mars to search the entire planetary surface for Quakipoos. He would even propose that we search beneath the martian surface, just in case the Quakipoo is a subterranean creature. The probable outcome: we would waste billions of dollars and not find a single Quakipoo! Sure, based on the Viking results, one cannot state with absolute certainty that Quakipoos do not exist on Mars -- but we can say that it is highly improbable that such creatures are there. We can’t rule out the absence of Leprechauns with absolute certainty but you don't see any scientists searching for them! In science, a null result is often just as valuable as a positive one. These null results help set limits and establish probabilities, from which scientists can decide whether further experimentation is necessary.

Let us now look at SETI in the context of the above discussion We have an hypothesis,which can be stated as:

There are other intelligent organisms somewhere in the galaxy, and we can detect these organisms.

The latter part is most important as the whole of SETI is based upon it. If intelligent species do exist out there, and cannot be detected by our instruments then SETI is a waste of time and money. Following the scientific method, SETI scientists have devised a set of experiments to test the validity of the above hypothesis. These experiments are quite simple in nature, employing radio telescopes to scan the sky, listening for some sort of "signal" that can be identified as originating from what we would consider an intelligent species. To date, forty-two of these surveys have been performed, and nothing has been detected. I all fairness, these surveys have only scanned a limited range of radio frequencies (those that we consider to be the "most likely" channels for interstellar communication) and have concentrated on nearby stars, so we cannot regard the negative results as proof positive that the SETI hypothesis is in error. But these results do lead to one of the following conclusions

1. There are no intelligent detectable civilizations in the near vicinity of the Solar System,

OR

2. We are listening at the wrong frequencies.

The first of these presents no real problems if we are to invest the time and money to build an array of radio telescopes (known to SETI scientists as CYCLOPS) that can detect signals from civilizations that may be located farther out in the galaxy. The second conclusion causes a few more problems. Let's say we assume conclusion 1 is correct and build CYCLOPS (at a cost of several hundred million dollars). The radio telescopes making the CYCLOPS array can only listen for signals transmitted within a certain range of radio frequencies. If the alien civilizations do not transmit in this frequency range, then they could be practically next door and we still would not detect them, even with CYCLOPS! The electromagnetic spectrum consists of an infinite number of frequencies, any one of which could be used as a possible communications channel. We cannot possibly scan all frequencies looking for intelligent signals, and hence run the risk of not detecting at least some communications. We have uncovered a hidden assumption behind the SETI hypothesis, namely

In order to be detectable, an intelligent species must transmit signals in the frequency ranges applicable to our search instruments (radio telescopes).

What most people fail to realize is that mankind has unwittingly performed another SETI experiment by engaging in global communications. Our radio and television signals have reached out to a distance of about 20 parsecs (a parsec is equal to 3.26 light years). Any civilization with sensitive radio telescopes would have surely received these signals, as they span a broad range of frequencies. If we allow time for a reply, this means that we should have heard from any communication within 10 parsecs of the Sun. We haven't! In my opinion, this tends to support conclusion 1 -- we are the only intelligent civilization (according to our standards) within a 10 parsec radius of the Sun. If we assume that there is nothing special about our region of the galaxy, then this places an upper limit of 1 million intelligent civilizations in the Milky Way. Certainly not billions, as some people would have us believe.

A million is still a very large number! Then why do many astronomers believe that we are alone in the galaxy? I’ll get to that in Part 2 of this presentation. But for now lets be content with the fact that the radio surveys seem to place an upper limit of one million intelligent detectable species in the galaxy. Next month, we shall look at the equation that expresses the probability of such civilizations and watch this number of one million shrink enormously.

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