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This question describes the kinds of signals sentient beings would transmit to let others know of their existence. The methods described are geared towards communicating the mere existence of intelligent life. But what if an alien civilization wanted to send an SOS signal, asking for help? What kind of message would they transmit to have the highest chance of someone hearing and understanding it?

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    $\begingroup$ I'm tempted to VTC as too opinion-based, but I'm going to let it roll and see what others have to say. The problem I'm having is that how creatures express alarm is enormously species-dependent. The average human often can't tell when a bird, or a cat, or any other species is making a sound that means "I need help!" We figure it out over time and with research... but I personally can't imagine any SOS an alien race could send that would mean beans to someone who knew nothing else about the race. There's simply no context to judge the message content. Hence my worry that this is opinion-based. $\endgroup$
    – JBH
    Jun 21, 2020 at 17:22
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    $\begingroup$ What sort of problem would be both so threatening that they would send such an SOS and so slow that they could expect a timely response? $\endgroup$
    – Mary
    Jun 21, 2020 at 22:18
  • $\begingroup$ @Mary that's an interesting question, but it can be ignored as irrelevant. For example, "what happens when SETI detects its first extraterrestrial message - and it's a call for help?" The civilization may be eons dead ... but that doesn't irrationalize the question. Indeed, many questions are asked on this site without a rational background. That's OK, so long as the question can be answered without it. Though I have other issues with this question, the lack of background is not among them. $\endgroup$
    – JBH
    Jun 21, 2020 at 22:28
  • $\begingroup$ @JBH Actually, it would determine a lot. For instance, if it were slow enough, you could use slower than light means to transmit it. $\endgroup$
    – Mary
    Jun 21, 2020 at 22:33
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    $\begingroup$ @Mary "Our sun is going to explode in 100 years, but we can't get our civilisation off-planet because our anti-gravity drive needs helium to work and we wasted it all on balloons for children" $\endgroup$ Jun 24, 2020 at 10:13

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Really, there are three parts to your question:

  1. Transmitting the message
  2. Translating the message
  3. Translating the meaning

Number 1 is rather straightforward and there are tons of resources and other questions where you can read about sending messages at interstellar distances. Whether the signal is via radio, smoke signal, enormous flash light, or something else, making a signal receivable across interstellar distances is well within the capabilities of your standard dedicated multi-billionaire/small country today. A bigger problem would probably be that with current technology all messages would be limited to light speed and would probably end up arriving hundreds if not tens of thousands of years too late for anyone to reasonably help.

Number 2 is a bit more esoteric, but still possible in my opinion. Smarter people than I have worked at the problem of making a message decipherable to most conceivable alien intelligences. Usually, these approaches start with the understanding of a fundamental concept, such as math or elemental composition, and work upwards. I think it's reasonable to assume that through work and sufficient complexity, a language instruction system could be built which works its way up to something like English eventually.

Number 3 is a complete shot in the dark. Currently, we have no evidence of alien intelligences past or present. All we have is a couple hundred years of science-fiction writers and scientists taking their best (wild) guesses, but really, we have no clue at all what an alien might be like. Maybe, bipedal beings who navigate primarily by observing visual light are the standard, but maybe they're not. Additionally, there are meanings which may be impossible to convey to an alien mind. For example, take a core human emotion: jealousy. What if the aliens have no sense of self? No I? Yes, distress, fear, and asking for help all seem like rather fundamental animal drives and emotions, but as our sample size of "life" is one (earth), we can't really do more than speculate wildly on what aliens might be like.

If I had to do this my message would be (in English):

"Please send help, we're in trouble! Our problem is [...]"

And then I'd transmit the entirety of human knowledge (all literature, media, internet, etc) along with the message so that they can figure it out themselves.

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    $\begingroup$ What's scary is that it's not unreasonable to believe the aliens would transmit their version of an SOS- or Mayday-style message. So, they're sending SOS or MAYDAY and what we're receiving is Glurk-abbadu-WHUM or Vitleble.. Vitleble... and how on earth do we figure that out? SOP isn't just species-centric, it's society-centric, too. +1 for your conclusion, though (transmit everything). Can you imagine the Contact-style argument that would ensue? "You told the brains-eating aliens what!? What do you mean they don't eat brains? Prove it!" $\endgroup$
    – JBH
    Jun 21, 2020 at 20:22
  • $\begingroup$ I'm rather uncertain that any signal we can transmit from here is above the background noise for any kind of interstellar distance. In the novel "The Three-body problem" they use the Sun as amplifier precisely because the most powerful anthena we could make now would be probably too weak to be received four or five light years from here. $\endgroup$
    – Rekesoft
    Jun 23, 2020 at 8:44
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    $\begingroup$ @Rekesoft "The three body problem" is a work of fiction and in reality, sending messages over very long distances isn't that difficult. For example, we have no problem communicating with Voyager 2 which is about a light-day away only has a has a 22 watt (my smartphone charges at 25 watts for comparison) transmitter and was built half a century ago. Modern radio or laser communication can easily reach nearby stars and building it bigger is an easily accomplishable engineering task. $\endgroup$
    – Dragongeek
    Jun 23, 2020 at 10:17
  • $\begingroup$ @Dragongeek The state-of-the-art long range laser communication is going to be tested in a NASA probe set to be launched in 2022. It will be tested at ranges from 15 to 225 million kilometers. In the future it is expected to work over distances long enough to cover most of the solar system. No one expects to be workable on interstellar distances. Laser decoherence would turn any information into garbage far before the signal is too weak to be detectable. $\endgroup$
    – Rekesoft
    Jun 23, 2020 at 11:51
  • $\begingroup$ @Rekesoft Inter satellite communication is a whole different beast and has it's own host of problems. I'm talking about a transmitter here on Earth, where you could pump however much power you wanted to into a radio or laser. Even if the beam decoheres, it wouldn't really matter once you've got a couple mega or gigawatts of light blasting off into space. $\endgroup$
    – Dragongeek
    Jun 23, 2020 at 11:58
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The language of mathematics

The process for sending an SOS message would be the same as the process for sending any other type of message.

Lambros D. Callimahos of the NSA wrote a technical thought-experiment paper titled "Communication with Extraterrestrial Intelligence", which has now been declassified. You can easily find it on Google.

It explains how the message should establish a mathematical vocabulary and grammar by starting with extremely simple math equations and then increasing in complexity.

This paper quotes British mathematician Lancelot Hogben, who I think put it perfectly: "Number is the most universal concept for establishing communication between intelligent beings; therefore, mathematics forms the basis for the first steps in extraterrestrial communication."

After this, physical values (such as the constants on the periodic table) could be used to define words (e.g. for all the periodic elements).

This form of defining new words continues, using previously defined words, until you have defined all of the vocabulary you need for your message.

After this, an SOS message (or any other message) could be crafted.

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A very long and detailed mathematical algorithm where left side should equal the right side that ends in the middle of

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Chaos is danger. The disruption of order by chaos is a call for help

Intelligent life must rely on patterns and pattern recognition to obtain energy in opposition to the forces of entropy. Intelligence enables pattern recognition in the service of life. Unpredictability and the failure of patterns is dangerous to life.

A sequence of prime numbers is a fine way to convey intelligence - a pattern, and one that would be recognized only by an intelligence and not produced by a natural phenomenon (as something like the Fibonacci series might).

The distress call would be disruption of the sequence of primes by random numbers, which would end the sequence. The sequence of primes would then restart at the beginning and again be disrupted and ended at a different point by random numbers: the disruption of order by chaos, anathema to life.

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  • $\begingroup$ I, as a human, can't decipher your proposed call for help without your explanation and the background provided to me. Still, if we assume the aliens are smarter than I, sending a series of numbers is still highly ambiguous and relies heavily on interpretation. $\endgroup$
    – Dragongeek
    Jun 21, 2020 at 19:26
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    $\begingroup$ @Dragongeek - I will write you a sonnet if you can explain a series of prime numbers using natural phenomena. Disclosure - Carl Sagans idea, not mine. I agree that disrupting a series with random numbers might be interpreted as art, or a mating call, or something other than distress. $\endgroup$
    – Willk
    Jun 21, 2020 at 19:33
  • $\begingroup$ Oh, I don't disagree that prime numbers are a great way to indicate intelligence, but I don't think a series of numbers is enough to express the nuanced idea of "help". For the message to be effective, it needs to have a single, non-misinterpretable meaning $\endgroup$
    – Dragongeek
    Jun 21, 2020 at 19:38
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Show, don't tell.

It's really unlikely that any detailed explanation of a problem could be transmitted and then translated in a reasonable amount of time. So I'll just focus on sending a generic "We have trouble" signal. If you are lucky, your neighbours would decide to help and then, with a 2-way contact, it would be certainly easier to explain the exact problem.

So, what could be a likely sign of intelligent life in trouble? I'd argue that a few high-altitude nuclear explosions would send the message. Maybe reducing radio transmissions afterwards, to make it clear the explosions caused some damage. Or starting rapidly changing atmosphere composition and albedo.

Coincidentally, humanity has sent such SOS signal already.

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  • $\begingroup$ Please add details and a link to support your last statement. Assuming that everyone will go out and do the research to understand what you're talking about makes for a low-quality answer. Thanks. $\endgroup$
    – JBH
    Jun 22, 2020 at 4:13
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    $\begingroup$ @JBH I'm sorry, the research of what? Are you saying someone may be unaware humans have used nuclear weapons? Or about rapidly increasing CO2 levels? $\endgroup$
    – Alice
    Jun 22, 2020 at 6:42
  • $\begingroup$ I recognize your 2nd paragraph sets up the idea that we've sent a cry for help into space, but that's a metaphor (an artistic or philosophical belief that acting stupidly constitutes a "cry for help"). Global warming happens by both natural and industrial means, as do nuclear explosions (our own sun, for example), making both actions indistinguishable as an SOS. So, that last sentence is intrinsically false - unless you can demonstrate that humanity has sent such a message in the context of the OP's question? $\endgroup$
    – JBH
    Jun 22, 2020 at 15:56
  • $\begingroup$ @JBH Wow, you are so wrong on both counts. But alright, here is an amazing research for you. Sun is nothing alike nukes (it's not even exploding), and CO2 levels never increased that fast by natural processes alone. $\endgroup$
    – Alice
    Jun 23, 2020 at 4:25
  • $\begingroup$ That bought you a down vote. The OP's asking how an SOS could be interpreted if received by us. I'll happily change my down vote to an up vote if you can find any research suggesting rising CO2 (for any reason and no matter how quickly) or nuclear explosions on only a planetary scale could be (a) detected and (b) interpreted as a call for help on another planet in another solar system. Political opinions aren't facts. $\endgroup$
    – JBH
    Jun 23, 2020 at 18:02
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They'd send an SOS

It would be a short repeating pattern to get attention. Needs to be complex enough to be not natural but simple enough to be short and blasted at high power that it will get noticed.

Any alien lifeforms capable of receiving such message won't understand the language but will know someone has sent an alert because it's not background noise and it's too short to be a message.

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  • $\begingroup$ For all we know that could be an alien teenager broadcasting the word "ass", for kicks. And due to dark forrest this might make other civilizations go radio silent. $\endgroup$ Feb 25 at 5:35
  • $\begingroup$ Ummm the Dark Forest theory means everyone stays silent all the time and those who make a noise get hunted $\endgroup$
    – Thorne
    Feb 26 at 1:51
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Send a constant like Pi or e in binary at the hydrogen line, continuously for at least several days, preferably as long as possible. Use a frequency of: $$x\pi\left(1420.40575177\right)\ MHz$$

x = any natural number

This will ensure that the receiving civilization is aware of the sending civilization’s intelligence, and it cannot be a naturally generated harmonic if it is at the frequency shown above.

If the receiving civilization continues to hear the message, they will pinpoint the source of the message. They can then try to contact the civilization out of curiosity, even if they don’t know that they need help.

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  • $\begingroup$ Not necessarily. Dark forest problem and stuff. This might even make some species go completely radio silent. $\endgroup$ Feb 25 at 5:33
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Send sequences, then syntax that defines algorithms to define sequences, work your way up to image processing, then send pictures of the problem

This is homebrew, on the spot, so please be gentle in the comments. I suggest:

  1. Do as everyone else says, and send sequences of primes, which is an obvious sign of intelligence.

  2. Send other number sequences, with a bit of data that represents a simple algorithm for generating that sequence. E.g. send 1,2,4,8,16...and then some data that represents n(x+1) = n(x) * 2. Send a load of these sequences. Alien mathematicians should be able to figure out the syntax rules. Send sequences that use bit shifts / rotations / array operations.

  3. Send the idea of variables using equations that use the syntax rules already sent.

  4. Send the idea of a Turing Machine. This is the trickiest step. Comments welcome.

  5. Send an algorithm for image processing.

  6. Send images.

Possibly deviate at 3.5 and find a way to send bitmaps.

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Extraterrestrials could broadcast their existences through various mediums (such as radio waves, photon waves, xrays, gamma rays, ...) to broadcast maths (such as calculus, or universal constants.)

To signal distress, aliens could add errors (such that it was clear that natural interference has not caused those errors) to those broadcasts.

For instance. you could broadcast the same universal constants (or calculus) multiple times, with errors added to every ith broadcast (such as 1 of 2 broadcasts have the same errors) to signal distress/SOS.

This would signal that, although the species exists long enough to produce tools so complex it is outrageous, that such species has lost all hope except to ask for aliens to assist.

Perhaps humans should broadcast such an SOS

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  • $\begingroup$ Thousands of students put errors in their work through 12 years of school, and nobody takes those as distress signal. What would be different here? $\endgroup$
    – L.Dutch
    Feb 25 at 7:16
  • $\begingroup$ Should a whole species reach consensus to broadcast a space SOS, the messages would not come from gradeschool students but from the top scientists. To broadcast tangible signals across space requires enormous resources, and the use of those resources is more serious, versus the few sheets of paper used at school tests. Broadcasts across space would consume exajoules of power to produce, and the structures required for those broadcasts would require more materials than required to construct all of our schools. Civilizations able to broadcast across space would surpass us. $\endgroup$ Feb 28 at 17:22
  • $\begingroup$ Photons use less power and can encode lots of datums physicsworld.com/a/… whereas particle accelerators consume more power but the waves pass through mass: home.cern/news/news/accelerators/… $\endgroup$ Mar 6 at 16:31
  • $\begingroup$ Could broadcast universal constants -- or, through medium.com/aiskunks/… suchas wikipedia.org/wiki/G%C3%B6del_numbering or Friedberg numberings ( ams.org/journals/proc/1964-015-02/S0002-9939-1964-0174479-5/… ), broadcast calculuses $\endgroup$ Mar 6 at 16:32
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Regardless of language, the simplest signal would be a repeating sequence message or something whose audio interpretation consists of a mathematical pattern; a pattern that doesn't exist in nature. 1 1-2 1-2-4 1-2-4-8 1 1-2 1-2-4 1-2-4-8 or a exponential regression 1 1-3 1-3-9 1-3-9-27 A distress signal cannot be interpreted as "Distress" unless one has prior knowledge of their signals capability to be used in such fashion.

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Send an envoy

I am writing a story where an alien species has this problem. My take on this is that if you are advanced enough, you should be able to accelerate stuff to near light speeds (and hopefully deccelerate it too). Unless the threat against your and others' existence moves at $c$, near $c$ should be good enough for best effort. And then you can send actual people or things to get help.

You can then send an AI that, upon reaching a completely alien species and not being destroyed by them, can learn how to communicate with them. Like really learn their languages, in whatever media they may be communicated (sound, touch, interpretative dance, wifi etc.). Once you are able to dialogue, you can state your problem to the aliens.

If the ones needing help are able to build Von Neumann probes, they will be able to reach someone in the galaxy eventually (as long as there is someone) (and even if the senders have long gone extinct).

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They wouldn't as it would make no difference. By the time the signal reached anyone they would be long gone if they couldn't solve the problem themselves. But if they could then there is no reason for the distress signal in the first place.

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