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The SETI project is trying to detect radio transmissions of aliens.

Assuming the ET have their own SETI using very similar technology to ours, are we sending data for them to receive?

If we are not sending data then it is very likely they are not sending either.

It is unrealistic to not send signals but to expect them to do the opposite.

Amateur radio can not exist when everyone is listening but nobody bother to transmit.

The question is about present and past attempts

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    $\begingroup$ The meti tag is for sending messages; SETI is receive only. Are you specifically asking about the present time, or do you also want past attempts included? $\endgroup$
    – DrSheldon
    Commented Nov 23, 2020 at 19:02
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    $\begingroup$ Its about all history $\endgroup$
    – Joe Jobs
    Commented Nov 23, 2020 at 19:16

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Arguably, we don't have to make an explicit effort to broadcast an intersteller "hello"; we have been leaking radio signals into space for decades. Anyone with a sufficiently sensitive receiver could detect our leaked signals and know we are a civilization at least technologically advanced enough to do that much. However, these signals aren't aimed anywhere in particular in space (they are intended for terrestrial reception) and would be extremely weak at the distance of even our nearest neighbor star. It would take a much more advanced civilization and/or a rather substantial effort to detect our emissions at the likely much greater distances of a possible listener.

There have been efforts to send a deliberate signal (see Active SETI), but doing so raises a number of questions:

  • Where do you aim it? Broadcasting in every direction would require impractical amounts of power in order to ensure a detectable signal at interstellar distances. To make a signal easier to detect using practical power levels, you need to transmit a narrowly focused beam, which means you need to choose a target. How do you decide on an appropriate target?
  • What do you say? Attempts to-date have constructed messages based on certain assumptions about how a message might be decoded and what might be a worthwhile thing to say.
  • What if someone actually receives it? What might that recipient do? Have we endangered ourselves by advertising our presence?
  • Assuming a benign alien at the other end, how do you converse when decades, if not centuries, elapse while each message is in transit?

Without good answers to these questions, it's difficult to justify a concerted METI effort. On the other hand SETI listening is just radio astronomy analyzing for signatures of artificial rather than natural causes.

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  • $\begingroup$ So instead of sending messages and risk to be attacked by those who receive them, we better wait for others to send us messages so we know who we can attack :) $\endgroup$
    – Joe Jobs
    Commented Nov 23, 2020 at 23:45
  • $\begingroup$ Common sense says if we dont want or we cant send messages then others also dont want and cant send messages $\endgroup$
    – Joe Jobs
    Commented Nov 23, 2020 at 23:47
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    $\begingroup$ @JoeJobs You are projecting human values, perspectives and motives onto alien cultures - or vice-versa. While doing so may be helpful to temporarily fill in some blanks and serve as a starting point, it brings biases which could lead to faulty conclusions. The key points behind my statements are that there are both technical and non-technical considerations to address when you think about sending messages into space. $\endgroup$
    – Anthony X
    Commented Nov 24, 2020 at 1:42
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Consider the millions of short wave radio listeners sitting safely and quietly at home by the table late into the night trying to pull in broadcasts from around the world.

The question imagines approximate parity in levels of development between civilizations, but I think the premise of SETI activities are usually that there are significant disparities in development. We listen for signals we couldn't possibly produce.

This is what we'll be like if/once we can start receiving anything at all:

enter image description here

dated 1928. Source: Library of Congress https://www.loc.gov/item/2017680266/ Rights Advisory: No known restrictions on publication. For information, see "American National Red Cross photograph collection," https://www.loc.gov/rr/print/res/717_anrc.html

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  • $\begingroup$ Maybe they cant produce much more than us because of limitations. Cars today are not much faster than 100 years ago. Airplanes are not much faster than 60 years ago. You still need a lot of power to transmit radio signal that far. $\endgroup$
    – Joe Jobs
    Commented Nov 23, 2020 at 23:03
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    $\begingroup$ @JoeJobs the highest power laser pulses have steadily increased and there's no limit in sight (see Petawatts to Exawatts) Physical object analogies don't apply. It's impossible to extrapolate another century or two into the future, but I think that "the future is bright" (pun intended) and could even be X-rays rather than light. I know there is scholarly work on the best wavelengths for interstellar communications, so a separate question about that may receive some "enlightening" answers. :-) $\endgroup$
    – uhoh
    Commented Nov 23, 2020 at 23:12
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    $\begingroup$ Ok asked here - space.stackexchange.com/questions/48733/… $\endgroup$
    – Joe Jobs
    Commented Nov 23, 2020 at 23:52

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