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What are the negative results of the SETI project?

For instance, "there is no intelligent civilization within 50 light years from us which tries to broadcast a message to the solar system, assuming they are as developed as our civilization", or "there is no intelligent civilization in our galaxy which produces the same amount of radio waves as we (humans) produce".

I was unable to find any concrete information.

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    $\begingroup$ Lack of evidence is not evidence of lack. $\endgroup$ Commented Dec 21, 2023 at 15:25
  • $\begingroup$ See astronomy.stackexchange.com/questions/8146/… astronomy.stackexchange.com/questions/18059/… $\endgroup$
    – ProfRob
    Commented Dec 22, 2023 at 8:08
  • $\begingroup$ You can see the linked questions for some information. It was the case that SETI wouldn't detect Earth-like emissions, even from a light year away. However, that information might be a bit out of date. $\endgroup$
    – ProfRob
    Commented Dec 22, 2023 at 8:13
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    $\begingroup$ @DavidHammen it is if you've done an experiment where evidence would be expected if the thing you're looking for was there. Which exactly is the point of the question: which experiments have been done that can rule out which specific things? $\endgroup$
    – N. Virgo
    Commented Dec 22, 2023 at 9:47
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    $\begingroup$ This paper states some negative results in its abstract: "No galaxies in our sample host an alien civilization reprocessing more than 85% of its starlight into the [mid-infrared], and only 50 galaxies, including Arp 220, have MIR luminosities consistent with >50% reprocessing". Its predecessor, part II, also seems to mention some negative results, also about waste heat. (Posting as a comment because I haven't read the papers.) $\endgroup$
    – N. Virgo
    Commented Dec 22, 2023 at 9:58

2 Answers 2

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The Arecibo message was intended to be detectable at the globular cluster M13, which is 25,000 light-years away, although the answer to this question suggests that it could only be detected at 5,000 light-years. Other references besides that link are more optimistic, but they don't show their work. In any case, this is assuming a receiver as powerful as Arecibo.

But we haven't searched the whole sky with a telescope as powerful as Arecibo. The largest survey from that telescope I could find on a quick search was ALFALFA, which was to cover about 7000 square degrees of sky out of a total of more than 41,000. And I don't know if it was completed.

If we want more comprehensive coverage, the NRAO VLA Sky Survey has examined the entire sky north of 40° S. The linked site doesn't give the sensitivity in units I know how to translate into distance-at-which-you-could-detect-the-Arecibo-message, but the VLA is Very Large, so I feel like it ought to be able to detect a focused Arecibo message at something like 50 light years. This makes your first suggested negative result at least plausible, given that there have been other surveys.

But it would be unlikely that someone would be steadily beaming a message to us using their most powerful radio antenna for a long enough time that we would be sure to catch it on a sky survey, so it's still a pretty weak negative result.

As for the other version of the question, of whether we can rule out nearby aliens based on their incidental broadcasts, the general consensus is that we cannot so far. The answers to this question discuss this, with one giving an FAQ from the SETI Institute saying that, based on observations we've done, we would not yet have detected Alpha Centauri's TV broadcasts. (The other answer to that question is interesting but really addresses absolute detectability rather than what has been ruled out so far.)

My take, having written this up, is that the lack of obvious radio signals so far observed doesn't tell us much about whether extraterrestrial civilizations exist.

[Note - this was substantially revised after the first two comments.]

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  • $\begingroup$ But how is the optimism of a particular area of space determined in terms of the detectability of a message sent from the earth? $\endgroup$
    – ayr
    Commented Dec 22, 2023 at 6:20
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Hard to say so specifically. Human radio emissions change rapidly. A quarter century ago, the framing signals of analog television were our most conspicuous radio product, but 21st century radio communication is much more like random noise.

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  • $\begingroup$ Certainly there are estimates of what SKA can do in exactly the terms suggested by the OP, so I imagine SETI searches also have those numbers. $\endgroup$
    – ProfRob
    Commented Dec 22, 2023 at 8:33
  • $\begingroup$ @ProfRob Estimates for proposals are oversimplifications. And here, the "amount of radio emissions" is not a good metric. Different sorts of emissions have very different detectability, and the human mix is rapidly changing, so there's no solid foundation for such an estimate. $\endgroup$
    – John Doty
    Commented Dec 22, 2023 at 14:21
  • $\begingroup$ It should be noted that any actual data transmission will likely (just because I hate absolutes) be discernible from noise. For example, modern day QAM may appear like noise to a random observer, but there is an underlying structure that hints it is carrying non-trivial, organized, information. This type of transmission makes it more difficult to decode, but doesn’t hide that it’s a transmission from an intelligence. $\endgroup$ Commented Dec 24, 2023 at 11:05
  • $\begingroup$ @JarrodChristman Yes, "hints" is the right word. The difference is that something like analog television yells that it's artificial by putting the bulk of its energy into narrowband carrier and sync signals. Detecting a carrier is relatively easy, while even detecting that a scrambled QAM signal is present is much harder. Determining that it's QAM is much harder still, requiring a much larger signal to noise ratio. $\endgroup$
    – John Doty
    Commented Dec 24, 2023 at 13:34

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