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In a sense, humanity has long been capable of transmitting interstellar messages (METI (messages to extraterrestrial civilizations) - attempts to transmit interstellar messages from humanity to probable intelligent beings outside the solar system.)

The first strong generalization sent into space was probably ... Hitler's speech.

This short television program, literally a few minutes, was broadcast in 1936 from Berlin with the opening of the Olympic Games. Although it was only received in Germany, it was the first, albeit weak, transmission from Earth. Unlike conventional radio broadcasting in the 1930s, TV signals pierced the ionosphere and went into space.

After that, the broadcast of the coronation of George VI took place in England and something else of the same kind. Truly television began only in the late 40s. And all the signals of those years are carried away from the Earth at the speed of light.

However, all of these signals are extremely weak. After passing only a couple of light years, they will fade out and it will be almost impossible to catch them, much less decipher.

Why?

The main problem with sending interstellar messages is that as the distance traveled increases, their power gradually decreases due to gradual scattering (diffraction). Imagine that you have thrown a stone into the calm surface of a lake: from the original stone the circles began to diverge 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 the 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.

After all, as mentioned above, radio waves are not very effective for interstellar communication.

In addition, if Arecibo's message had come to us in 1913, we would not even have been able to receive it.

And here we come to the heart of the question: What kind of the interstellar message can be received by an alien civilization with technology comparable to humanity at the end of the nineteenth century (19th century)?

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    $\begingroup$ Please clarify: the end of nineteenth century is around 1890, while 1990 is the end of the twentieth century. Moreover, is this question related to this other one worldbuilding.stackexchange.com/q/186440/30492 ? $\endgroup$ – L.Dutch - Reinstate Monica Oct 9 at 8:00
  • $\begingroup$ I'm assuming by 19th century you mean 20th century. Ie the year 1999. 1899 we'd detect nothing. $\endgroup$ – Ash Oct 9 at 8:01
  • $\begingroup$ 19th century — 1800s. In this question, I ask how we or some alien civilization can transmit complex information ( interstellar communication ) to a civilization similar in level of development to humanity in the 1890s and 1910s. $\endgroup$ – user79894 Oct 9 at 8:30
  • $\begingroup$ Humanity may not be looking for brothers in mind for very long, but we have been exploring space with telescopes for quite some time. Remember the Martian channels — objects on the surface of Mars, the existence of which was assumed by astronomers from the late seventies of the XIX century to the seventies of the XX century. $\endgroup$ – user79894 Oct 9 at 8:33
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    $\begingroup$ @AlexP Is this the second question you've failed to edit to make that minor correction? $\endgroup$ – rek Oct 9 at 15:28
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The only message the most advanced civilization at the end of 19th century could detect would have been some sort of Morse code by a very bright light source, with apparent luminosity from Earth comparable with that of a star.

At that time there were already astronomers looking at stars, and they could have spotted a "variable star" with a non regular cycle, and maybe attempt to search for a code in the variable signal.

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  • $\begingroup$ How large does this structure have to be, able to block the sunlight, so that the change in the sun's luminosity is noticeable? $\endgroup$ – user79894 Oct 9 at 9:32
  • $\begingroup$ @FrenchThompson, I cannot answer on that. It depends on the size of the light emitting object and where the modulation of the emitted light takes place. $\endgroup$ – L.Dutch - Reinstate Monica Oct 9 at 10:04
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    $\begingroup$ The farther from the star, the smaller the object can be, but the more precise the alignment must be. $\endgroup$ – rek Oct 9 at 15:31
  • $\begingroup$ @FrenchThompson The question sets no limits to the level of the sending civilization. $\endgroup$ – Gangnus Oct 9 at 22:30
  • $\begingroup$ Changing the luminosity of a star, changing the opacity of its photosphere would be enough. But if you want the star to look as a variable star, the info channel is too narrow due to low frequency. And if you speed up the changings, scientists of the end of XIX won't register them by photography. What is funny, they WOULD notice changings before the photo age. $\endgroup$ – Gangnus Oct 9 at 22:34
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With pre 1900's technology on the receiving side...

No radio signal whatsoever. The energy needs to make even a single beep audible on receipt would exhaust a supernova.

No infrared or longer, no ultraviolet or shorter. There simply were not receivers for those frequencies.

No neutrino stream, gravity pulses or anything comparably high tech. There simply are no receiving mechanisms built, or even conceived yet.

That leaves visible light. A well-focused laser light, with a slow pulsing visible to human eye would suffice. The laser itself would need to be both enormous in power AND enormous in aperture (optics laws require larger aperture, to allow better focus). We are talking much bigger than planetary scale here, and exawatt levels of power, to reach even from the nearest star. No non-laser artificial light source could even conceivably be bright enough. Even using a stream of Tsar Bomba 50MT nuclear bombs would be totally invisible to the human eye at just 1 light year distance.

Alternately, put a blinker between the star and the target. A star-sized, exactly positioned blinker.

Alternately, reposition the star itself. Multiple stars, actually. This is much harder than you expect. Much!

Really, for a 19th century tech receiving civilization? The easiest way would be to actually travel to them. Yes, even with all the problems of lightspeed and/or longduration interstellar travel.

The FASTEST way would be to.... wait 100-200 years, and send a radio signal using Arecibo-or-larger class transmitter.

So... realworld science is not cooperating with you!

If you want a FICTIONAL but plausible way... make something up like 'natural dilithium crystals that glow when struck by a tachyon beam from outer space". This will make a usable communications media, plausible without needing to explain the details of how and why, and allows for faster-than-lightspeed communications. Still oneway, though.

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  • $\begingroup$ "With pre 1900's technology on the receiving side..." 19th century = 1800-1899, not 1900s. $\endgroup$ – T.J. Crowder Oct 10 at 10:49
  • $\begingroup$ "Alternately, reposition the star itself. Multiple stars, actually. This is much harder than you expect." LOL $\endgroup$ – T.J. Crowder Oct 10 at 10:50
  • $\begingroup$ Who said the signal is not sent from a neighbour planet or moon? $\endgroup$ – Gangnus Oct 13 at 14:05
  • $\begingroup$ "Who said the signal is not sent from a neighbour planet or moon?".. the OP, when he said "Interstellar message" $\endgroup$ – user79911 Oct 13 at 23:04
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Any large enough, optically visible structure. That was already used by Stanislaw Lem in the second story of Cyberiad.

Trurl and Clapautsius there pulled stars into a giant interstellar message, and to attract more attention, the first word was made of supergiants only.

"Two famous constructors look for a job and the appropriate reward at a court of a star monarch." (sorry, the translation is mine. Better read the original)

That message would be clearly seen and understood as a message even by Sumerians.

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There are three options I can think of, but only one (the third in my list) that doesn't rely on the sender knowing the receiver's location.

1 - the Voyager method. Throw something with data on it at them.

2 - Infrared radiation. There actually were methods to detect infrared before 1900 - Samuel Langley invented the bolometer in 1878, which could essentially be used for that purpose. Still, those were roughly prototypes. This would require something akin to a massive nuclear reactor that turns on and off close to the receiver. And the sender would probably not do this - that relies on them having a bolometer that they're watching constantly, which is unlikely.

3 - Visual light. This is more reliable than the second above, and it'd make sense if the sender didn't know where the receiver was. An object akin to a Dyson sphere/swarm with variable opacity could surround the sender's star and be toggled on and off in a pattern that transmits data. This only transmits one bit at a time, so the message would have to be fairly simple so they could decipher the meaning. Computers wouldn't exist to decipher it with.

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  • $\begingroup$ AFAIK, infrared rays don't pass the atmosphere. So, they simply won't see your second signal. And what is the use to send something in a more difficult way when you can send it in an easier way? ... As for throwing - if you throw slowly, your message will go there in hundreds thousands of years. If you do it fast, the message will burn in the atmosphere. $\endgroup$ – Gangnus Oct 17 at 11:25

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