Our technological society has been emitting signals continuously that could be received by other extraterrestrial civilizations. But, the question is, when will they give response to our signals? Now, Amir Siraj and Abraham Loeb explored the chance of detecting a response in the future, and shows that a response should only be expected to arrive after a few millennia. Their study recently appeared in Arxiv.
The search for extraterrestrial intelligence (SETI) can be divided into two broad categories: searches for generic signals, and searches for signals intended specifically for Earth. Siraj and Loeb considered the latter category, specifically the case of an ETI responding to a technosignature from Earth, which they dub the search for extraterrestrial responding intelligence (SETRI). In other words, they focus on humanity ‘starting a conversation’ with an ETI.
They first considered electromagnetic (including radio) responses to radio signals. Since radio signals have been produce that could be received by other ETIs, within a sphere around us with a radius of 132 light years, the location of ETRIs at present day is limited to a sphere of radius 66 light years. Then, they derived the probability distribution for Nresponse, the upper limit on the number of responses we could expect to receive to our technological communications from civilizations on Earth-like planets orbiting Sun-like stars at the present time, as well as for the 50 and 95 percentile values for Nresponse being of order unity.
“The latter corresponds to a conservative estimate on the timescale over which we should not expect responses of any sort to our technological signals, since Nresponse is already an upper limit.”
They found the 95% lower limit for τobs to be 2.6 × 10³ yr, and that the median of the upper limit Nresponse ∼ 1 for τobs ∼ 9.2 × 10³ yr. (probability distributions displayed in Figure 1).
They next considered physical probe responses to radio signals.
“The number of ETRIs from which we could receive such responses is dependent on the propulsion speed, and is necessarily smaller than the number of ETRIs we could receive electromagnetic responses from.”
By applying the methods, they found that, for chemical rockets with v ∼ 30 km s¯1 = 10¯4 c the median of Nresponse is of order unity for τobs = 2.8 × 106 yr and the 95% upper limit of Nresponse is of order unity for 1.1 × 106 yr, implying that we should not expect a response from a chemically propelled probe before then (probability distributions displayed in Figure 2).
Finally, for a probe traveling at v ∼ 0.1c, such as the proposed Breakthrough Starshot 1 initiative, the median of Nresponse is of order unity for τobs = 2.9 × 10⁴ yr and the 95% upper limit of Nresponse is of order unity for 8.1 × 10³ yr, implying that we should not expect a response from a sub-relativistic probe before then (probability distributions displayed in Figure 3).
From all these results, they suggested that, for the next few millennia, we should not expect to receive signals or probes from any ETRI on an Earth-like planet orbiting a Sun-like star. In other words, if we receive communication from any such ETI before then, the communicating ETI would most likely have been unaware of humanity’s existence as a technological society.
“The number of expected responses to our radio communications is dependent on time since both the distance out to which our radio transmissions reach and the prior for the typical lifetime of an radio-communicating civilization increase with time.”— they concluded.
All images credit: Siraj and Loeb
Reference: Siraj, A., & Loeb, A. “Intelligent Responses to Our Technological Signals Will Not Arrive In Fewer Than Three Millennia,” submitted for publication. [arXiv:2108.01690]
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