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The main problem with sending interstellar messages is that with the distance traveled, their power gradually decreases due to gradual scattering ( diffraction). Imagine that you threw a stone into the quiet surface of a lake: from the source stone, circles started to go in all directions, but the further they diverge, the less noticeable they become. The same thing happens with radio waves: if you calculate approximately, then twice the distance from the communication source will reduce the power of radio waves by four times. As you can see, detecting such radio waves at a distance of several hundred light-years from Earth will be a difficult task for space civilizations.

And here is the whole question: how can we send an interstellar message in all directions ( since we want to make ourselves known to everyone ) so that we can be heard?

Note: please describe the technical features in as much detail as possible and give the appropriate calculations, as if you really want to make yourself known as loudly and further as possible.

Also, please do not offer answers related to really huge energy costs or the use of astroengineering, such as the construction of the Dyson sphere.

The signal transmitted by our civilization should be focused on civilizations of technological development similar to us ( starting from the mid-20th century ), but if you know how to make the signal available to less developed civilizations, for example, at the end of the 19th century, I will be happy to hear from you.

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    $\begingroup$ You do not need to send it in "all directions". There are only a small-ish number of star within 50, 100, 200, 1000 light years. Set your goal, and then send pulses towards the stars within your pre-set radius. Adapting a powerful military radar for this purpose would be rather easy. $\endgroup$
    – AlexP
    Commented Sep 29, 2020 at 16:45
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    $\begingroup$ We do know exactly where are the stars within the given radius of "several hundred light years". And there are not all that many of them. $\endgroup$
    – AlexP
    Commented Sep 29, 2020 at 16:49
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    $\begingroup$ @FrenchThompson Hence why you laser your message in tight, focused beams at target stars. Granted, lasers aren't perfect cylinders, more like cones, but the energy is much more focused than an omnidirectional beacon and can thus travel much farther for that same energy. $\endgroup$
    – BMF
    Commented Sep 29, 2020 at 16:50
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    $\begingroup$ "The visible-range laser beam expands to the size of Jupiter's orbit": that depends on how well it is collimated, doesn't it? And anyway, this only gives a nice firm design goal. Engineers love those. Design a laser / maser powerful enough to that its signal will be easily distinguishable from noise when the width of the beam equals the major axis of the orbit of Jupiter. Many companies will bid to get the contract. $\endgroup$
    – AlexP
    Commented Sep 29, 2020 at 16:51
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    $\begingroup$ @FrenchThompson following Emilio, what is the threshold of detection? Who is our target audience? Primitive aliens who might see our signal in their skies? Or aliens millions of years our elder? $\endgroup$
    – BMF
    Commented Sep 29, 2020 at 17:50

9 Answers 9

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Use the sun.

Stars can be detected at a distance. They are hot! Exoplanets are detected because of the interruption they produce in the output of their suns as they pass between the distant viewer and the star.

You do not need to have planetary mass to interrupt the output of a star - a huge flat object would do just fine. A series of these objects spaced around our sun would interrupt its (very energetic) output to a distant observer. They could be spaced so that their transits could be interpreted as a series of prime numbers.

That should be adequate to demonstrate that intelligent life is here, ready to welcome our new alien overlords.

Prior art: https://en.wikipedia.org/wiki/Tabby%27s_Star. - a star which attracted attention because of periodic dimming thought to be from something in orbit.

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  • $\begingroup$ It is advisable to use not so large objects, because we will not build this for many centuries. $\endgroup$
    – French Thompson
    Commented Sep 29, 2020 at 16:42
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    $\begingroup$ @FrenchThompson the technology needed to manipulate the solar light curve is not advanced at all. Not big sheets of solid metal or whatever, mere dust will do the trick. $\endgroup$
    – BMF
    Commented Sep 29, 2020 at 16:48
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    $\begingroup$ Which is sort-of like the starting point of Jack McDevitt's The Hercules Text. (Aliens send an encyclopedia using an artifical pulsar. Much merriment ensues.) $\endgroup$
    – AlexP
    Commented Sep 29, 2020 at 19:09
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    $\begingroup$ @FrenchThompson - I am going to interpret "leaf" as "thing in the way of starlight". The leaves will move because they are in orbit and if they did not they would fall into the star. Ideally the shape will be fixed. Just as exoplanets are now discovered routinely because of improved telescopes, the ability to detect the transit of a leaf across the face of the sun will depend on the telescopes of aliens viewing our star. The message they send will be by virtue of their arrangement, which will be patently artificial - the prime numbers are standard stuff for this context. $\endgroup$
    – Willk
    Commented Sep 29, 2020 at 19:46
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    $\begingroup$ @FrenchThompson You don't want to use large objects, because we will not build this for many centuries. My counterargument is that we shouldn't send a "we are here!" message to the galaxy until we are capable of defending ourselves, just in case somebody out there misreads the message as "free food here!". $\endgroup$
    – NomadMaker
    Commented Sep 30, 2020 at 4:31
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A Self-Reproducing Interstellar Probe

http://www.rfreitas.com/Astro/ReproJBISJuly1980.htm

It expands exponentially by converting parts of each solar system reached into copies of itself. It will eventually cover the entire galaxy.

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    $\begingroup$ Disclosure: I just searched the web for "Von Newmann Probe" and got the first paper linked from Wikipedia. $\endgroup$
    – Emilio M Bumachar
    Commented Sep 29, 2020 at 17:34
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    $\begingroup$ This is the only civilized way to do it. The alternative, sending radiation in all directions, means we fry half of the galaxy. $\endgroup$
    – The Square-Cube Law
    Commented Sep 29, 2020 at 17:41
  • $\begingroup$ This was my first thought. Once we build a machine that builds the same machine that builds the same machine ad infinitum, there's almost nothing in our way to astroengineering megastructures. $\endgroup$
    – BMF
    Commented Sep 29, 2020 at 17:47
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    $\begingroup$ Unless you have FTL travel, odds are people 1000 ly away are only going to receive our self-reproducing ambassador long after our species is extinct, but this is otherwise a reasonable answer. $\endgroup$
    – jdunlop
    Commented Sep 30, 2020 at 0:36
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    $\begingroup$ A paperclip maximiser, that'll go down well with everyone! $\endgroup$
    – Separatrix
    Commented Sep 30, 2020 at 9:08
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Use a Bomb

To be more precise, use several many.

If you intend to send a message everywhere, attenuation is a certainty. It is unavoidable. So you have to up your transmission power. The sun is an excellent idea, as suggested by Willk, but you appear to be down on that, so another option is popping a gigaton-range device, ideally as far outside the solar system as we can afford to drop it.

Attenuation will still be a problem, but anyone with a (powerful enough) radio telescope pointed in our direction is going to see a sudden, inexplicable bounce in power. Set them off at irregular (ideally mathematically patterned) intervals, and that should be enough to let anyone looking for intelligent life know that we're here.

It will be monumentally expensive, of course, but any plan to let the galaxy know we're here is going to be expensive.

Edit: Clarification in light of comments.

Even enormous bombs are going to require that other civilizations be looking right at us when the radiation from the bombs' detonation reaches them. A gigaton-yield warhead produces ~ 4.18 * 10E18 J of energy. By comparison, the sun produces 3.8 * 10E26 J per second. So for an observer, this would be an apparent magnitude 40 higher than the sun at the same distance. This is why you'd want it out of the solar system, ideally out of the plane of the ecliptic, in a place where that amount of energy wouldn't be expected to come from.

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  • $\begingroup$ will they though, even our most powerful nuclear weapons would not even be a blib at that range more importantly how to make it distinguishable from an impact event which can release far more energy. $\endgroup$
    – John
    Commented Sep 29, 2020 at 20:08
  • $\begingroup$ @John The first point can be amended with some scifi technology; maybe future humanity develops a super-powerful weapon. The second is the reason jdunlop suggested setting them off in mathematically patterned intervals. One bright flash is indistinguishable from an impact, sure, but what if they went in groups of, say, 2-3-5-7-11-13? $\endgroup$
    – parasoup
    Commented Sep 29, 2020 at 21:15
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    $\begingroup$ @parasoup groups would be less effective then say, time intervals, groups will wash out to indistinct variation in intensity at distance but time is transmitted with high fidelity. $\endgroup$
    – John
    Commented Sep 29, 2020 at 23:36
  • $\begingroup$ @John - the signature of an impact is going to be different than a nuclear weapon going off. Most notably, the gamma radiation from a nuke would be visibly different from normal background events. A paper was written describing recognizing alien nuclear weapons; the reverse would be true for us, and if we pop off a big one every <significant interval> it will be far more noticeable. Still monumentally expensive, particularly since you'd have to keep doing it. $\endgroup$
    – jdunlop
    Commented Sep 30, 2020 at 0:35
  • $\begingroup$ Do you have some numbers on how bright a gigaton-range bomb would be at a distance of, say, a hundred light-years? Or a hundred thousand? $\endgroup$
    – Mark
    Commented Sep 30, 2020 at 1:23
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Don't send the signal to all stars at once. Instead send a focused message with a very narrow spread rather than an omni-directional transmission. Focus the spread such that it only spreads to fill the area of the goldilocks zone of each star you target; so, by the time it reaches any civilization there it will have the same level of dissipation as an omni-directional signal traveling between two inner planets within our own solar system.

Doing 1 star at a time this way may seem inefficient, but compared to the time it takes light to travel between stars, you could still cycle between all the stars in the galaxy much faster than it would take the signal to reach the most distant of stars. Infact, there are only 133 stars within 50 lightyears of Earth http://www.atlasoftheuniverse.com/50lys.html. Since light is so slow anyway, you should probably just focus on these if you hope to get any response to your message within any humanly meaningful time frame.

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Continuously denote nuclear weapons in the atmosphere of one of the planets (preferably not Earth). The idea is to produce a large number of short lived isotopes of very heavy elements that can not be explained as a natural occurrence.

Then when your aliens are searching for planets and decide to do some transit spectroscopy, by looking at a the Sun through the atmosphere of one of the planets they can then pick out the elements that make up the atmosphere.

So if they start to see lots of Fermium (atomic number 100) they will either assume a Supernovae exploded within the last ~hundred days (as the isotopes are short lived) or someone is blowing up nuclear weapons. Given that supernovae are very bright and easily detected[citation needed], I expect your aliens will suspect the later is occurring.

This is the more explode-ly version behind the recent detection of phosphine in Venus's atmosphere.

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    $\begingroup$ The question involves transmitting information over extremely long distances ( more than 200 light-years ), while a nuclear explosion is unlikely to be visible even from Saturn. $\endgroup$
    – French Thompson
    Commented Sep 30, 2020 at 9:12
  • $\begingroup$ In addition, the signal should be broadcast on a continuous basis ( cyclically ) for as long as possible. $\endgroup$
    – French Thompson
    Commented Sep 30, 2020 at 9:13
  • $\begingroup$ In this regard, obscuring our star with a large but light sail and periodically transmitting light seems more rational. $\endgroup$
    – French Thompson
    Commented Sep 30, 2020 at 9:14
  • $\begingroup$ I'm not saying they see the explosion, instead they see the radioactive fallout. $\endgroup$
    – Rob
    Commented Sep 30, 2020 at 9:44
  • $\begingroup$ I repeat, we need to convey a message, not indicate the existence of intelligent life, and that is quite controversial. $\endgroup$
    – French Thompson
    Commented Sep 30, 2020 at 10:15
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You could build a light array in orbit around the planet which sent extremely large pulses of light in a binary pattern that would be recognized as unnatural by scientific observers on other planets, and make the message a short one, that way increasing the likelihood that the message would reach some destination. You could also use a totally violet light to do this as pretty sure this does not exist as much as one would think given the light is high energy.

You could also convert entire dead planetoids into the signal lights and power them from the local sun ?

Light we see now is sometimes reaching us, supposedly, from distant stars that are long dead.

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  • $\begingroup$ we had the same idea :D $\endgroup$
    – Pica
    Commented Oct 2, 2020 at 9:11
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Supernova

If the requirement is to make every star in the Milky Way see us, there is not another way do the things you ask:

  • "You really want to make yourself known as loudly and further as possible"
  • "Send an interstellar message in all directions"

Your question as posted has no answer at all and simply can not have the "Hard-science" tag, for these reasons:

  • "Please do not offer answers related to really huge energy costs"
  • "Please do not offer answers related to the use of astroengineering"

Because the question creates an unanswerable problem with its constraints and parameters, and the Hard Science tag; this is the answer to the problem in the title:

Q: How do I announce us to the entire galaxy?

Hard science answer:

A: Supernova

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I can't remember who suggested this (possibly Arthur C. Clarke?). Dump a few thousand tons of an isotope of some element that is either very rare to find in a star like the sun or even one that doesn't occur in nature. Done correctly, you've just added a very large "Wow!” factor to anyone checking lines in the sun's spectrum.

The drawback is that the best case scenario is that aliens decide this cannot be natural. It's absolutely not suitable for anything more than "Someone's dumping odd materials into a star for some unknown reason. Maybe it's a signal we should respond to or maybe it's a crazy culture that uses their own star as a trashcan.”

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Build a lighthouse in orbit.

Basically a solid slab of diamond, add solar cells and something similar to hubble, compute the orbits of planets in habitable systems you looked at, compute where they will be dependend on the distance.

Send a laser pulse, with your cultures encoded information.

On the planet it will look like a disc of light, racing at rotation speed against the rotation direction of the planet (40.000 /24h => 1666 kmh).

Dark Forrest theory dictates though, that you should do such a thing only when you go extinct, aka it doesent matter anymore.

Ufos are the gravelights of the stars.

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  • $\begingroup$ Can you describe this technology in more detail? And isn't the light fading? $\endgroup$
    – French Thompson
    Commented Oct 2, 2020 at 11:12
  • $\begingroup$ what-if.xkcd.com/13 The trick is to be really good at interstellar physics. Basically you use gravitational bodies as lenses and fire only if some singularity inbetween allows for a reassembly of light at the right location. $\endgroup$
    – Pica
    Commented Oct 5, 2020 at 8:43

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