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Sunday, December 1, 2019

SETI's "We've Only Just Begun" Excuse Versus Historical Reality

SETI (which stands for Search for Extraterrestrial Intelligence) is the scientific attempt to search for extraterrestrial intelligence (civilizations on other planets) by astronomical efforts.  Such efforts have mainly consisted of attempts to pick up radio signals coming from other planets, although sometimes SETI scientists use other techniques such as searching for optical signals.   Thus far SETI has failed to detect any signals from extraterrestrials.

Radio telescopes like this are used for SETI

SETI proponents have long resorted to a rhetorical technique designed to make us think that we should not let such results dampen enthusiasm for making radio searches for signals from other planets.  The technique is to say something along the lines of "we've only just begun to search for life in outer space."  If someone thinks that "we've only just begun" to listen for radio signals from civilizations on other planets, he won't be too discouraged by the lack of success so far. The "we've only just begun" meme works pretty well as an excuse for a lack of results.  But if a person tries to bolster such a meme by greatly misstating the number of stars that have been searched for extraterrestrial radio signals, dramatically understating such a number, then the "we've only just begun" meme may start to sound less than honest.

On July 1, 2019 there appeared an article on the site of the SETI Institute, a leading organization in the search for extraterrestrial life.  The article was written by Seth Shostak, a senior scientist of the SETI Institute. The article reported on the failure of the Breakthrough Listen project to find radio signals from extraterrestrial civilizations.  The article was entitled, "Search for space aliens comes up empty, but extraterrestrial life could still be out there."

In the article Shostak attempts to persuade us that only about 2000 stars have been searched for extraterrestrial radio signals.  He states, "The new results from Breakthrough Listen — an examination of roughly 1,300 nearby stars — has approximately doubled the tally of reconnoitered real estate." If a search of 1300 stars had "doubled the tally of reconnoitered real estate," then it would have to be that only about 650 stars had been previously checked for extraterrestrial radio signals, and that a total of only about 2000 stars had been searched for such radio signals.   Later in the article, Shostak makes a statement that clearly teaches that the number of stars searched for extraterrestial radio signals is less than 5000.  He states, "Their long-term goal is to target a million star systems — exceeding by hundreds of times the total number of targets scrutinized by SETI since the birth of the field 60 years ago."  A million divided by 200 is 5000, so if a search for a million star systems were to exceed "by hundreds of times the total number of targets scrutinized by SETI since the birth of the field 60 years ago," it would have to have been that the number of star systems searched for extraterrestrial radio signals was 5000 or less. 

The summary numbers Shostak gave us in this article are far from true.  The actual number of star systems that have been searched for extraterrestrial signals using SETI techniques prior to 2019 is vastly in excess of 5,000.  In 2014 a paper was published with the title "SETI Observations of Exoplanets with the Allen Telescope Array."  The paper described 19,000 hours of observations of 9293 stars, searching in "multiple bands."  No extraterrestrial radio signals were found.  The paper was co-authored by Shostak himself. 

In 2016 numerous press stories reported that Shostak's SETI Institute was going to start conducting a search of 20,000 red dwarf stars to look for signs of extraterrestrial signals. This was a different set of stars than the 9293 stars searched by the previously mentioned survey.  In 2019 Shostak was quoted as saying this: 

"What would be the best strategy to find ET? We’ve been looking at a list of about 20,000 so-called red dwarf stars."

So why did Shostak tell us in his 2019 article that only about 2000 stars had been searched? The page here can be used to examine previous radio SETI searches. The page here can be used to examine previous optical SETI searches. It is clear from such pages that very many stars have been checked using SETI, many times more than Shostak indicated.  For example, the paper here discusses how optical SETI searches were made on more than 6000 stars.

Many of the SETI searches were full-sky or portion-of-the-sky surveys in which an instrument simply scans a specific fraction of the sky (or the whole sky) rather than targeting specific stars. Such searches end up being equivalent to a survey of more than 100,000 stars, often vastly more. For example, in the SETI paper here we read the following:

"These initial observations covered 1% of the sky....Using a model of ∼ 107 Sun-like stars in range (and 108 total stars in range), 1% sky coverage implies that ∼ 105 Sun-like stars—a factor of ∼ 200 more than that in the targeted search—were surveyed."

So the paper is indicating that 100,000 Sun-like stars (the same as 10Sun-like stars) were searched for optical signals from extraterrestrial civilizations, when only 1% of the sky was surveyed.  We would get much larger numbers if we calculated the number of stars that have been searched for radio signals, in surveys that used full-sky or large portion-of-the-sky surveys. As the paper here notes, "Wide-field surveys serve as a way to sample 106 - 108 more stars than pointed surveys, albeit at lesser sensitivity.”

How many stars were searched from the searches below, involving much more than 1% of the sky?

  • The SERENDIP project, surveying a large portion of the sky, the portion depicted in Figure 4 of the paper here, a project which a Sky and Telescope article tells us surveyed "many billions of Milky Way stars."
  • The Southern SERENDIP project surveying a large portion of the sky, the portion depicted in Figure 2 of the paper here.
  • The SETI project discussed here, surveying a significant portion of the sky, the portion depicted in Figure 2 of the paper here
  • The all-sky SETI survey discussed here, which operated continuously for more than four years. 
  • The two-year southern sky SETI search discussed here, which observed for 9000 hours and "covered the sky almost two times."  

It would seem that when we consider both star-specific and full-sky or portion-of-the-sky searches, the number of stars that have been searched for extraterrestrial signals before 2019 is more than 100  times greater than the number suggested by Shostak in his July 1, 2019 article.  If we extrapolate from the SETI paper quotation quoted above, in which 1% sky coverage is equated with 100,000 sun-like stars, it would seem that SETI searches have actually checked millions of stars,  at least hundreds of times more than the number suggested by Shostak. So it is not at all true that SETI searches have "only just begun," or that only a small number of stars have been checked. 

Strangely, Shostak's claims in his July 1, 2019 article are very inconsistent with a 2017 written testimony he submitted to the United States Congress. In that testimony he stated the following: 

"To put that in context, if we conjecture that there are 100 thousand signaling societies in our galaxy, then we will have to scrutinize roughly one million star systems before detecting a transmission. This is approximately ten times the total sample of all SETI experiments undertaken since 1960."

Since a million divided by 10 is 100,000, Shostak here states that about 100,000 star systems have been searched by SETI, which is a number 50 times greater than the number he gives in his July 1, 2019 article.  Even that 100,000 figure is an understatement, for the portion-of-the-sky and full-sky surveys had a combined sample size of millions of stars, in the sense that they would have detected a strong consistent radio signal from any of millions of stars.

In his written testimony to Congress, Shostak failed to present a convincing case for thinking that SETI efforts will succeed, and he failed to clearly describe any great benefit that would come if extraterrestrial radio signals were to be found. But he does reveal a little about theological motivations of SETI.  He makes it clear in his first paragraph that SETI is about proving that "biology is not some sort of miracle."  

Given its very mysterious workings and its complexity far greater than the complexity of any machines humans can construct, I can understand why some people might fear that biology is some sort of miracle;  and I can understand why people might spend decades trying to remove their fear about this matter. But I don't understand why anyone would think that finding a second example of biology (on some distant planet) would prove that biology is not some sort of miracle. If a man points towards a mountain top, and the mountain top rises way up in the air, that would presumbably be a miracle. If the man does the same thing with a second mountain top, causing that mountain top to also rise way up in the air, this would seem to be two miracles, and not at all prove that the first case was not a miracle. So given its gigantic complexity and mysterious workings, why would we suddenly conclude that life is not a miracle merely because we observed it in a second very distant place? 

In his testimony to Congress, Shostak claimed that finding some extraterrestrial intelligence would prove that life is "commonplace" and "ubiquitous," but also states that "whatever intelligence is found will likely be hundreds of lightyears distant or more."  Why would we conclude that life is "commonplace" or "ubiquitous" because we had merely found one type of life hundreds of light-years distant? There are thousands of sun-like stars in a cubic volume of a few hundred light-years, so finding one radio signal hundreds of light-years distant would not actually prove that life existed on more than one solar system in a thousand. 

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