https://www.technologyreview.com/s/61223...lien-life/
EXCERPT: . . . These guys have characterized the parameter space that astronomers need to search for signs of alien life. They say this space is so vast that SETI searches so far have done little more than scratch the surface. [...] “We develop the metaphor of the multidimensional ‘
Cosmic Haystack’ … into a quantitative, eight-dimensional model and perform an analytic integral to compute the fraction of this haystack that several large radio SETI programs have collectively examined,” they say.
This parameter space is vast. The relevant dimensions include the three dimensions of space, the frequency range of potential signals, their repetition rate, polarization, and modulation, the transmission bandwidth, and the sensitivity of searches to this transmitted power. The volume of three-dimensional space that can be searched is the volume of the universe centered on our solar system out to a specific distance.
[Jason] Wright and co define this as 10 kiloparsecs—about 30,000 light-years, or roughly the distance to the globular clusters that orbit the Milky Way galaxy.
Most radio telescopes are able to observe signals in both polarizations at the same time, but this has not always been true in the past. So this is a dimension that limits the exploration of the parameter space.
Other dimensions are complex to characterize. The signal repetition rate, for example, is tricky to handle in the model. Continuous signals are easy to deal with, but signals that repeat rarely are difficult.
Defining the size of this cosmic haystack is then the task of adding all these spaces together. As Wright and co put it: “The volume of the haystack is then a definite volume integral in this 8D space, and the fraction searched can be calculated given the sensitivity function for a given survey.” The result is a space of truly gargantuan proportions. “This leads to a total 8D haystack volume of 6.4 × 10^116 m5Hz2 s/W,” say Wright and co.
But how much of this have astronomers explored? Wright and co say that the searches to date have covered just 5.8 x 10^-18 of this volume. That’s a tiny fraction....
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https://www.technologyreview.com/s/61223...lien-life/