The search for extraterrestrial intelligence (SETI) is entering a golden era. As astronomers find more and more Earth-like planets orbiting other stars, the excitement is growing over the possibility that one of them may harbor something of interest.
An important question is what form these signals might take, and one possibility is that any advanced civilization will develop lasers and point them, deliberately or inadvertently, in our direction. So an obvious strategy for SETI is to scour the heavens for the telltale signals that lasers produce.
Today, Nathaniel Tellis and Geoff Marcy at the University of California, Berkeley, reveal the results of the most comprehensive survey of this kind to date. These guys say that if even a tiny fraction of the stars they studied were orbited by planets beaming lasers toward us, they would have seen them. And yet, they find no compelling evidence of laser signals from any of these stars.
The method behind this work is straightforward. Astronomers have been recording the spectroscopic signals from numerous stars for decades. Tellis and Marcy have got hold of a historical database of starlight measurements made at the Keck telescope in Hawaii between 2004 and 2016.
This database records the starlight from 5,600 stars in the near vicinity of the sun, most of them less than 100 parsecs away (about 300 light-years). Indeed, these stars have been viewed many times over the years, so the entire database consists of over 67,000 spectra.
Tellis and Marcy then developed an algorithm to examine each spectrum for the signs of a laser signal. They assume this signal would be superimposed on the background signal from the star. So the signal would consist of a higher number of photons than the background level, and these photons would appear in several adjacent pixels in the spectrographic image.
They set their algorithm loose on the data set. But to make things more interesting, Tellis and Marcy also added their own laser signals to a portion of the data to test the efficacy of the algorithm and to gauge the detection threshold of their technique.
The results make for interesting reading. The detection thresholds of their search correspond to alien lasers with continuous power in the range of a few kilowatts to 13 megawatts, depending on the type of host star. These kinds of lasers are well within our technological capability.
Of the 67,000 spectra studied, the algorithm identified around 5,000 that warranted further investigation. Tellis and Marcy then analyzed these by hand, ruling out all but 12 signals, which warranted more detailed study.
However, it turned out that none provided compelling evidence of extraterrestrial laser activity. Indeed, all the candidates fell into three categories of false positives. More than half were caused by well-known emissions from molecules in Earth’s atmosphere. Another group of false positives were the result of emissions from the parent stars themselves.
The final group of false positives, about 1,000 of them, were caused by extraneous signals within the telescope itself—either internal reflections or the impact of high-energy particles such as cosmic rays.
Tellis and Marcy are able to rule out many of these by eye. For example, high-energy particles tend to leave worm-like tracks across the pixel array and so are straightforward to spot. Nevertheless, care must be taken with this process, since the spectra highlighted in these ways could also be signals of other kinds.
At the end of this process, Tellis and Marcy were able to rule out signs of laser signals in all the spectra they studied. “We found no compelling evidence for extraterrestrial laser emission among any of our 5,600 stars at power levels of 3 kW to 13 MW,” they say.
That’s a result that can help place limits on how prevalent intelligent civilizations might be. Given the rate at which astronomers have found Earth-like planets orbiting other stars, Tellis and Marcy can estimate the number of Earth-like planets their survey must cover. “As [these star systems] contain roughly 2000 lukewarm, Earth-size planets, we rule out models of the Milky Way in which over 0.1% of warm, Earth-size planets harbor technological civilizations that, intentionally or not, are beaming optical lasers toward us,” they conclude.
To put that in perspective, if technological civilizations emerge on just 1 percent of such Earth-size planets, then Tellis and Marcy’s survey must have sampled roughly 20 such civilizations. If ETs are out there, they are not pointing their lasers our way.
And that raises some interesting questions about the nature of extraterrestrial intelligence. One line of thought is that we see no evidence of ETs because we are alone in the universe. In other words, there is no evidence to find.
But another possibility is that if there are intelligent civilizations out there, they are many millions of years more advanced than we are, because any civilizations less advanced than ours wouldn’t yet have developed the technology to produce laser signals. In that case, this and other negative SETI results may be evidence of a decision or agreement by advanced beings not to contact civilizations as primitive as our own.
Of course, there is little in the way of evidence to back either idea.
This work now sets the stage for more ambitious projects. One of these is the Breakthrough Listen project, which over the next few years will record spectra from a wider range of nearby stars and galaxies. In particular, this survey will study cooler stars and brown dwarfs, which were omitted from the current work.
We’ll look forward to seeing what this golden age reveals.
Ref: arxiv.org/abs/1704.02535 : A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars
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