# Communicating the idea of danger to an unknown alien civilization

An alien automatic "beacon" tries to warn us about some imminent danger. It does so with launching a succession of large "veils" in front of their star so we can detect them via the transit method. This succession of dips in the star luminosity forms a binary message.
The first part of the message conveys some cosmological and mathematical constants so we can agree on a set of SI units and know which branch of physics we should stop tamper with.

And here comes the second part of the message, the one wich actively tries to warn us of some danger. No need to be explicit, a simple "DANGER" sign would be sufficient. The problem is, the beacon doesn't know us. It knows our relative position (and maybe can scan our solar system as a whole), that we're a rather intelligent species having tinkered with the wrong physics, but that's about it.
The aliens don't know our language, our appearance, or even what biological process Earth's lifeforms are based on (or maybe just that we're carbon-based). They can't wait for a response and subsequent dialog to slowly build up comprehension: the message should be clear : "YOU ARE IN DANGER".
Also, please note that their goal is to eventually make us come to them for a solution, so no "warning shots" strategy. They must not be considered as the danger themselves.

In short:
How would aliens convey the sense of danger without knowing us beforehand ?

A bit of context:
- The aliens are about 40 light years from us
- The setting is 150 years from now, but they don't know our exact technological advancement (only that we should probably have colonized our own star system and are able to pick up their message)
- No FTL
- The shorter the message, the better (they don't have an infinite supply of stellar veils)
- The danger in question is of lovecraftian nature, and as such can't be properly explained with such "low bandwidth", hence the more universal danger sign sought after.

• Comments are not for extended discussion; this conversation has been moved to chat. – Monty Wild Jan 12 '20 at 0:56

Some thought has gone into this issue with regards to deterring future humans (over a 10000 year timescale) from radioactive waste dumps. Obviously this isn't quite the same as your situation, because although future humans may well be completely different to us socially and linguistically, they're still likely to share quite a lot of traits with us.

There's a big (~350 page) document from Sandia which you can find here: Expert Judgment on Markers to Deter Inadvertent Human Intrusion into the Waste Isolation Pilot Plant which has all sorts of interesting and useful things in it.

They have this notion of "levels" of information... don't expect anyone to understand level "N" if they haven't understood level "N-1", for example. Here are some of the levels they considered:

Level I: Rudimentary Information: "Something manmade is here,"

Level II: Cautionary Information: "Something manmade is here and it is dangerous,"

Level III: Basic Information: Tells what, why, when, where, who, and how (in terms of information relay, not how the site was constructed), and

Level IV: Complex Information: Highly detailed, written records, tables, figures, graphs, maps, and diagrams.

Of course, the marker systems they used to communicate eg. the "danger" referenced at Level II were designed by humans to communicate concepts to other humans, using a basic understanding of human psychology. They know what a human skeleton looks like, and how to draw facial expressions that more or less approximate fear or sadness. Your aliens simply don't have that advantage.

The critical issue for you is that "danger" is an almost entirely nebulous concept. What's dangerous to me might be quite different to what's dangerous to you (you might consider peanuts to be a pretty serious and deadly threat, but I certainly don't). The aliens presumably know that we are a technological species, and are assuming that we will be able to see their signal and interpret it somehow, but beyond that they know basically nothing about us. There's no universal notion of "danger" or even "death" that they can share, in the same way that they might be able to share some knowledge of mathematics or physics.

If the danger were something specific and concrete, like "this start is gonna gamma-ray-burst you in a few hundred years" or "the aliens who live over there will destroy you if they think you're a threat" that is something they may be able to communicate, but doing so is going to require sending somewhat more information than a simple short message.

Basically, communicating even moderately complex but concrete concepts is already hard enough. Communicating abstract concepts may as well be impossible. The best you'll really manage is something like a combination of the Arecibo message and the Pioneer plaque. The latter shows a way to identify locations in the galaxy via pulsar timing, and in the event of the danger being astronomical in origin it would be possible to indicate where it came from, and if it were (or mimicked) a natural occurrence an example of such a thing might be sent, too. A supernova might not communicate "danger!" by itself, but listing a bunch of supernovas or gamma ray burst sources and then a currently-intact ageing star near us should send a fairly obvious message.

The shorter the message, the better (they don't have an infinite supply of stellar veils)

How would one "use up" such a thing? just orbit em back round again, or fold em up and redeploy. It does sound hideously low frequency though, which suggests that it'll take a really, really long time to send even quite a simple message.

Realistically, they're gonna have to keep sending the message on repeat for years in the hopes that we'll eventually notice, and then note down the whole thing. If the veils get used up in weeks or months or even years, we might just never notice, or if we did the message that could be transmitted would be so short and simple that it couldn't convey anything as nuanced as "danger".

I'm not sure what the best alternative is, of course, but if they're sending a message to us in particular then I'd consider investigating the possibilities of setting up a huge nuclear-powered laser in their local stellar gravitational lensing point. That has the benefit that unlike the veils, people who aren't on the axis of the beam will find it quite hard to see, which would be important if the threat was a hostile intelligence.

• I see it will be quite dependant of the nature of the danger, indeed. Maybe two simple images could be enough: one with a simplistic view of the solar system, and another without the Earth. As for "How would one "use up" such a thing?", I've pictured that the veils are just railgunned and fined placed on a fixed stellar orbit, then left untouched (that's hard a feat enough), repeating their message on each revolution. But replacing is indeed a good idea if more bandwidth is needed – Keelhaul Jan 9 '20 at 16:14
• "Eyes in the dark, one moon circles." - How would we communicate with Aliens if we meet them in person? – Mazura Jan 10 '20 at 15:00
• @Keelhaul I think we might interpret that as a threat, like "Nice planet you've got there. Here's what we're going to do to it..." If the aliens are trying not to look like bad guys, that might not be the best way to go... – Darrel Hoffman Jan 10 '20 at 20:11
• @DarrelHoffman Knowing that the communication form may be limited, I wouldn't try and assume intent with the message itself, and would just take the hint that something could happen. – MartinArrJay Jan 14 '20 at 13:48

Take something that we know (well, assume) conveys "no life" to everyone: The idea of 0 K temperatures. At 0 K, there is no vibration in an atom, which is (in theory) pretty much certain death for anything cooled down that low. If Earth receives a message that says "0K, 10,000,972 seconds" they might interpret that as "something will hit absolute 0 in 4 months." There would be a whole lot of debate among the scientific community, obviously, as they tried to figure out just what was intended by this, but I think that at least a few people could interpret it appropriately.

Depending on what the danger they're attempting to convey is, they could go about it in different ways. Maybe the Earth will get hit by a giant meteor that will destroy all life? Give us a number that we can associate with Earth-- say, its orbital velocity around the sun-- and its position in three space relative to a point like the sun (I assume that they'd think our sun was our point of reference. Maybe, if we want to make it harder for the earthlings, we use Jupiter as a point of reference, and it takes some brilliant lateral thinking to interpret it correctly). We also give the present time in whatever units we've agreed upon.

Then, they transmit a new time in the future and a new set of velocities and positions-- velocities and positions that don't match up with our predicted orbit. They also send out a 3 space location of something outside of our solar system, with a massive speed (maybe even a massive mass or gravitational pull) that we can interpolate into something that will hit us on the same day as the earth's orbit is supposed to shift!

For better or for worse, communicating a vague sense of danger is difficult; conveying a somewhat ambiguous message of a specific danger seems to be much easier.

• I love the idea of "0 K = death". That's really clever. – Cobbington Jan 10 '20 at 1:35
• I agree. However, as @user71425 pointed out, the aliens probably wouldn't be using Kelvin or seconds, since these are arbitrary units us humans made up. – ReinstateMonica3167040 Jan 10 '20 at 1:58
• @Userthatisnotauser: The question states that the initial part of the message will establish "a set of SI units". So while the aliens would not be using Kelvin or seconds, they would (theoretically) have given us the information we need to convert their units to ours. (I imagine they'd probably use something similar to Planck units for this communication, so the hardest bit might well be to identify "time" or "temperature".) – Charles Staats Jan 10 '20 at 3:49
• Instead sending only single metaphor for danger, try to send as many as possible. Science is all about finding patterns. And the more examples you have, the easier it is to find what is common among them. Sending things like "absolute zero temperature", "close to black hole", "runaway chain reaction", don't need a scientist to figure out that the theme is "danger". And even better, include the thing you are warning about into this list. – Euphoric Jan 10 '20 at 8:06
•  If Earth receives a message that says "0K, 10,000,972 seconds" they might interpret that as "something will hit absolute 0 in 4 months." — Or as "You gotta keep something at 0K for 4 month and something interesting will happen". And Earth will waste countless resources trying to get 0K for so long span of time. – user28434 Jan 10 '20 at 9:13

# Codification

Instead of a message of two blocks I suggest one of five separated by a common pattern that is never used anywhere else in the message. It could just be decided after the rest of the message. In this answer I'll assume "010101".

## Preamble

The first block would be a patter that would be very unlikely to happen in nature, to increase the probability of being detected without complicated analysis. My ignorant guess would be "010101000001010011100101110111010101" which at first may seem just irregular, but if split in groups of three is the delimiter 010101 followed by 000 001 010 011 100 101 110 111 followed by an other delimiter. I proposed this as I never heard of a star counting in binary. And counting is usually associated with intelligence. It would be a message that there are actually intelligent creatures there. Not to mention that it would set the bitrate frequency for the rest of the message.

## Multi-valued intensity

From here on I suggest using different veils that are able to let though different amounts of light (For example by making them smaller or with some holes). This may allow you to use less material for some veils and also to shorten the message. For example if their star is particularly bright they could use 8 different levels and codify three bits in one veil. This is a particularly basic way of codifying signals and it would be made clear by the second block.

The second block contains a rising scale of the different possible intensities repeated two times. Two times to make sure the instant of start and the end of the scale are not mistaken with the delimiter. The delimiter is codified in normal binary and never in the compressed way.

I also want to point out that this codification and the delimiter are basic patterns that would surely be known by any culture with a basic knowledge of information theory. They are like one of the first strategies we developed to store information and send signals.

## Math

This block is made by a big image containing various things. The first row has the opposite value of the background and the next one is empty. This both to make sure they get the size of the image and to ease the understanding of it being an image (They'd just need to break the lines at the same point of the first one and they'd see the image).

The upper part of the image is used to show some mathematical symbols. In particular it's possible to use sticks for numbers and some small combination of pixels for the symbol. In the next part the numbers are codified in sticks and any letter is a symbol. For example "1 2 3 1A1B2 2A3B5 3A3B1A5 2C3B6 D2EB2 D1A1EC3B6 XB10" could be used to define addition, multiplication, parenthesis and the symbol X to be the same as 10. More symbols and example should be added as necessary. In particular a better way to represent numbers is essential, but it's done in the same way.

## Branch of physics.

To represent the branch of physics I suggest using an image like the previews block, just filled with equations, images or numeric examples of the equations with relative image to show the branch of physics.

For example you you could use some drawing of different atoms with next to them three numbers with two fixed symbol in the middle. For example 1F1 with hydrogen, 2F4 with helium, 1F2 for deuterium and 1F1G1F1 for a molecular hydrogen. Then you could use simple chemical equations like "1F2 + 1F2 = 4 * 2F4" which with our conventions is "D+D = 4He", an approximation of the nuclear fusion equation "D + D → 4He* + 24 MeV".

## Danger

As per the danger I'd use multiple examples represented in an image. The first is an pair of drawings, the ones suggested by OP in a comment: A pair of images of the solar system, one with and one without our planet.

Then I'd use a similar structure of my example on nuclear fusion to represent a couple of chemical equations. One could be of a highly corrosive substance like HF (Hydrogen fluoride). If HF is not considered to be enough, we could use Fluoroantimonic acid, which can only be stored in teflon due to its exceptional corrosiveness (which I assume to mean that it corrodes practically anything else and is thus dangerous to any kind of alien and their buildings).

Additional we could use and other things like represent spontaneous explosive reactions to show that we are talking about a more generic thing. Explosives simply because they create damage and while different materials are able to withstand different amount of energies, I doubt that there are civilizations with the assumed knowledge of physics without have ever experienced an explosion.

# Other relevant things

One potential problem is that images need eyes to be seen and we can't be sure an aliens can see them.

In case this message is received by a race that can't see, I think they would still be able to perceive this as a surface. The point is if their technology has output interfaces braille-style that can be used for this task.

I think that they would: a 2D interface would be so much better to use then a 1D one in so many applications. For example if one wants to use a microscopes, consider something about the positions of the stars or to see the result of an experiment with a particle collider. I'm not saying you can't do all those things with sounds, it's just much easier with a 2D interface.

## Number of veils

If we leave out the content of the last two blocks then we have a bare minimum of 209 veils, with the following assumptions:

• background with high luminosity (no veil)
• the material used by a veil is proportional to the amount of light they block
• we have 8 levels of luminosity
• math has 5 symbols +*=() and we use the example sequence.
• formulas written rotated 90° clockwise.
• a bigger delimiter of 7 veils that is probably enough for any content.

And if we codify the math symbols in this way. 1 for each row separated by an empty line. Each equation have 4 empty lines above and below. X is low luminosity.

... ... ...
.XX XXX XXX 1    2+3/7
... ... ...
... ..X .X. B=   3/7
... ... ...
... ..X ..X A+   2/7
... ... ...
... ..X .XX D(   4/7
... ... ...
... .XX ..X E)   4/7
... ... ...
... .XX .X. C*   5/7
... ... ...


The 209 veils can be broken down in this way:

• 42 = 7*6 for the delimiters.
• 12 for first block.
• 8 = 4*2 for 2° block.
• 3 = 9/3 for the first row of first image.
• 136 = (2+3/7)*56 for each stick of the 3° block.
• 8 = 5*2/7 + 6*3/7 + 2*5/7 + 2*4/7 + 2*4/7 to explain math symbols

If the danger message was nuclear fusion, then an estimation of the additional size would be two times that (additional two for reactions, their syntax, -1 and 0). I'd guess an helium atom is way smaller than a recognizable solar system in somewhat right proportions. Therefore I think that the two images of with/without earth are the most heavy ones.

# Recap

The message is a set of five blocks, the first two to detect and decode the message. Same for the third that represent how to decode those equations. The remaining two form the actual message: one contains things related to a certain field of knowledge while the other contains dangerous things and images of bad events.

# EDIT:

• reshuffling and minor improvements.
• improved Branch of physics
• fixed/improved danger section.
• removed part about SI constants as I made a wrong assumption.
• What if the aliens breathe fluorine the way we breathe oxygen? They might not think fluorine is dangerous. Maybe their biology isn't affected much by HF but dissolves easier in water. They could send a message with 2H2 + O2 and we think "oh, you found another planet we could live on?" – user253751 Jan 10 '20 at 11:24
• @user253751 HF is highly corrosive to lots of materials, regardless of the chemist's biology. Even if aliens have some special biology that makes them impervious to HF, it could still be recognized as a substance which destroys almost anything else it touches. – Nuclear Hoagie Jan 10 '20 at 14:37
• @NuclearWang You mean like oxygen on Earth is recognized as a destructive substance? – user253751 Jan 10 '20 at 14:58
• @user253751 I don't see what you're getting at... exposing materials to oxygen is not as destructive as exposing them to HF in almost any case. Watching a piece of metal rust over the course of years isn't quite as dramatic a show of danger as seeing it dissolve in a vat of HF. – Nuclear Hoagie Jan 10 '20 at 15:16
• @NuclearWang Oxygen is a dangerous chemical but we don't see it that way because we're used to it. That's because of the particular chemical environment we have on Earth. The aliens might see oxygen as dangerous, and they might also see fluorine or HF as less than dangerous. – user253751 Jan 10 '20 at 15:27

You catch more flies with sugar.

/Also, please note that their goal is to eventually make us come to them for a solution, so no "warning shots" strategy. They must not be considered as the danger themselves/

The problem with warning someone of danger is that you yourself might be perceived as the danger. If I tell someone he is in trouble it might be because I hear the cops coming for him, or because I have caught him wearing my fuzzy velvet pants without permission and I am going to whup him. Either way he will run and a safe bet is to run away from me.

No, if the aliens want to come they need to entice us. Tell us "COME VISIT, WE HAVE BEER" or "WE ARE XENOCURIOUS HOTTIES IN SILVER JUMPSUITS AND WE ARE SO LONELY". Once we show up we will find out that they are actually a bunch of teetotalling monks, but we will then get a chance to hear their warnings about danger and also if they actually know any xenocurious hotties.

• The thing is, they want us to be aware of the danger as soon as possible, waiting more than 80 years to get there and back is out of the question – Keelhaul Jan 10 '20 at 8:46
• I really like your message templates :P however you don't explain how to convey these in understandable form, taking in account they don't know details about us, so no language knowledge (dang, we barely understand ourselves on earth). Also, considering OP said there's no FTL, it's rather centuries needed to come to them.. – Kaddath Jan 10 '20 at 10:13
• How do they communicate that if they don't know our language, let alone our binary encoding? They wouldn't know anything about our culture or what entices us, so they'd be just as likely to send a message like "ARSENIC BONBON CACHE THIS WAY. ONLY 40 UNITS LEFT!" as one that would entice us with sex appeal and alcohol. Even still, the party signal is not likely to attract frat boys if the journey takes more than 40 years. – Beefster Jan 10 '20 at 23:28

If you can convey a set of constants, you can then transmit any image you like in the 3 spatial dimensions + time animated fashion which is independent of any specific sense system an alien civilization may have.

Then you can just transmit a simulated "video" of a supernova as a generic "danger" sign.

You can then add some specific "video" illustrating the problem in question. Please note that as soon as it shows something astronomically macroscopic (a planet) you may stop worrying about interpretation since we share astronomy with any alien species.

• Slow-scan TV is still quite bandwidth intensive, making it potentially unsuitable when signalling involves a finite number of stellar-sized objects... – Starfish Prime Jan 9 '20 at 15:58
• Using astronomical common references is a good idea, but anything too specific has the risk to convey the wrong message. "A supernova? Though luck for these guys, but we can't help them". And there is the bandwidth issue. As for picturing the actual danger, I've added a clarification in the context. – Keelhaul Jan 9 '20 at 16:04
• It does not have slow-scan, it's basically 3D animated vector graphics which is pretty efficient. However, you can surely work on that message. – alamar Jan 9 '20 at 16:08
• @alamar I've a sneaking suspicion that communicating a protocol for animated 3d vector graphics is going to be harder (and hence ultimately more bandwidth intensive) than SSTV. It'll eventually become efficient as more data is sent, of course, but brevity seems to be a goal here. – Starfish Prime Jan 9 '20 at 17:22
• SSTV is only relevant to species with visual or planar thinking, but 3d vector image is relevant to any species who dwell in material world. Also, I think a few kb will get you there. – alamar Jan 9 '20 at 17:47

It is not enough to communicate the concept of danger alone:

1. With no context of where the danger is coming from, your message may be perceived as the threat, warning, distress call, etc. All of which illicit very different responses.
2. With no context of what is dangerous, you have no way of determining if it is dangerous to you.
3. If you are unfamiliar with the danger, you need explicit instructions about how to respond to it.

So a minimally useful warning to someone who has no context for what you are talking about really take more the form of "X is dangerous, do Y".

I can not find the clip, but there is a show I saw once that featured a brief video of colored dots moving around that people would universally interpret as being friends, fighting, chasing, protecting each other, etc just based on how they move in relationship to each other. The dots had no human characteristics whatsoever, but we could still anthropomorphize them because of how they moved in relation to one another. The important factor here is motion.

If you were to cast a representation of the night sky as the recipient sees it, and start moving stars around in a way that shows the dangerous star "attacking" other stars, then we would know to fear what it at the dangerous star.

To achieve this, you only need to cast 1 "solar screen" but it needs to be sophisticated enough to open and close "pixels" allowing for the sun screen to become a video screen.

For example, if I were an alien trying to warn humans about a world eating monster that is attracted to civilizations emitting radio waves in the direction of the Orion constellation from Earth, my video might look something like this:

• Show a picture of symbol that looks like + being orbited by a symbol that looks like o

• Show picture with a bunch of + symbols in the shape of the constellation Orion, but somewhere in the picture there is symbol that looks like Y that is clearly not a star on our star charts.

• The Y then moves to the closest +, and the + disappears. All the other + run away from the Y.

• Then you see a picture of an o on one side of the screen and a Y on the other. A symbol that looks like ~ then moves from the o to the Y. When it reaches the Y, the Y chases the o and makes it disappear. Then show and o with a line between it and the Y. The ~ moves until it is stopped by the line. The Y then goes the other way.

# Message Formatting

Taking cues from the Arecibo Message, you have to send a 1 bit-per-pixel message with a prime number of pixels in each dimension. This makes it so that the image can only be interpreted one way (though 8 orientations of the picture are possible) and eliminates all possibility of incompatible encodings (There will be. It was hard enough for humans to standardize data encodings and we still suck at it)

As to what you put in this picture? That's the challenging part. There are going to be visual idioms that don't translate and they could be anything as simple as circling things or as complicated as tentacled horrors. Even on earth, we have problems with images and body languages, and this causes a lot of miscommunication between e.g. cats and dogs.

• Fire is pretty dangerous and it likely exists on another planet with life. Draw fire.
• If they know what humans look like, draw some humans on fire. Otherwise, the planet on fire would probably get the message across.
• Be careful to not make it look like a threat.
• Draw a ship leaving the planet
• Draw the solar system that the aliens are from (much like the planet illustration in the Arecibo Message), indicating their planet
• Draw the same ship arriving at their solar system and planet

All of this should fit in under 300x300 pixels, probably far less.

# Be Mindful of Latency

The distance of this alien civilization means you have at least 80 years between when the dangerous thing the aliens detected actually occurred and when humans receive the warning message. It may be too late by the time the message is received and interpreted.

• It should be noted that this particular sequence of pictures could be misinterpreted as "we are coming to your planet to destroy you" if sequenced in reverse, so it would probably be good to put unary dots next to each panel to disambiguate the order in which they should be read. – Beefster Jan 13 '20 at 18:06

There are a multitude of methods they could use to warn us of the "danger."

Binary code, or at least some version of it, could be used to give a message or signal, possibly one that could send data, which has a brief description of the danger.

Audio/Visual messages, possibly an audio message of the danger, or a picture/video of the danger, of some sort, could be sent.

Messages written in some form of DNA, written with the protien patterns of some sort.

EDIT

How to communicate what the danger is? The aliens don't know what we speak, or how to communicate with us, but they can explain the situation in some terms that are common. For example, they could describe the danger as like "We're all getting killed by this big thing. It is coming after you."

The problem is, there is no way to communicate the danger without there being some sort of "universal common" so it's difficult to state the exacts of the alien warning when there is the problem of the recipients not understanding the context.

• This doesn't answer the question. You're basically saying "you could communicate the idea of danger by communicating the idea of danger". Sure, binary code could be used. But how? What would be sent that could be understood as meaning "danger" by an alien civilisation? That's the core of the question. – Starfish Prime Jan 9 '20 at 15:32
• How to you specify the number of bits in a byte? It doesn't have to be 8! – Binary Worrier Jan 10 '20 at 15:20
• The probability that both planets share a common language and binary string encoding is astronomically low. We didn't even have standard encodings for English until ASCII in the 60s, it took until the 80s for Unicode to exist, and it wasn't really adopted widely until the 2010s with the rise of UTF-8. Even today, you'll encounter websites from time to time being interpreted in the wrong character set. – Beefster Jan 13 '20 at 22:19

As you're talking of Lovecraftian danger, I take it you're not after hard sciences and maybe you'll like my idea :

The universal signal of danger is fear. The aliens are thus emitting "fear waves" in addition to their messages. It's possible because most sapient species in the universe have very similar brain waves and you can broadcast directly to it with a good ol' space emitter.

• Sorry, the question is about actual signals, the aliens aren't lovecraftian in nature themselves – Keelhaul Jan 10 '20 at 14:08

How about telegraphing the theme of Jaws?

If your signal is binary (on/off) and expensive (planet-sized shades orbiting your star), you'll want the signal to be brief. One thing you can still vary is the time between each signal. I suggest that, like the iconic theme from Jaws, you use a very few signals but regularly decrease the time between them. The recipients can interpret this as a "countdown", and a countdown to an unknown event from an unknown alien sure seems intimidating. Even if nothing actually happens at the end of the sequence, they should be spooked enough to want to look into the matter further.

The risk is that they might react by accelerating their research into exotic physics, warp drives, and supernova bombs, in order to prepare for whatever might be coming. But it seems like a risk your aliens have to take.

• These aliens have never seen Jaws and their equivalent probably uses a different theme. – Beefster Jan 13 '20 at 22:21

All these aliens need to do is shine some light in our direction. They could do that with immensely less efforts than putting a string of lampshades in orbit around their sun.

• some pretty big laser (the more focussed the beam, the less energy needed to send a given amount of light our way)
• some hefty mirror array reflecting the light of their sun in our direction. The mirror field don't need to be nearly as huge as these sun shades though. A castaway can easily draw the attention of a passing ship with a tiny pocket mirror.

All they need is a satellite that happens not to be between their sun and Earth when the signal is transmitted. That would appear as a light source blinking very close to their home star. Besides, that could hardly be confused with a regular planet orbiting its sun, so it would be even more likely to draw our attention.

Even using a local satellite, they would only need to interrupt the transmission when their planet passed too close to their sun for the light to be perceptible.
If that were not enough, they could put their probe in a carefully designed highly eccentric and distant orbit, so that the giant flash light spent most of its time far away from their sun in a region clearly visible from Earth.

Besides, unless they were fitted with cosmic-sized engines fuelled by Star Wars grade super antimatter, these veils would still be subject to Newton's law and traverse Earth's field of view only periodically. Like a cosmic-sized teletype ribbon of rather short length.

There is also Einstein's speed limit. Assuming good old Albert was right, however advanced their science, all that aliens can see is an image of our more or less distant past. At the very least 4 years if they happen to be our Alpha Centauri neighbours, but more likely decades, centuries or more.
Even assuming the aliens keep a supply of cosmic lampshades handy and can start sending their message immediately, they could notice the problem only decades or centuries after we started playing the silly buggers.
It would then take more decades or centuries for the change in their star luminosity to reach Earth.
Chances are, we would have long noticed and suffered the consequences of whatever stupid thing we were doing before receiving the first "bit" of warning.

I don't think the concept of "danger" itself can be broadcast. It's a relative notion. After all, even the Earth exploding would not necessarily be a mortal danger to a civilization able to colonize other planets.

The aliens might however warn us about potential events like the Earth exploding or even something a bit less obvious (a change in temperature or composition of the oceans or the atmosphere, maybe?) as long as it remains on a planetary level.

Any kind of dialogue with the aliens is out of the question, so the "science preamble" of the message has to be very cleverly designed indeed. To be honest I very doubt many aliens could make heads or tails of the Voyager message, for instance.

I would rather have them transmit pictures. That seems like a more or less universal language, since photons are by far the most common source of distant information available in space. A way to draw our attention would be to use a couple of different wavelengths (say some visible light and some X rays repeating the same pattern). The message would be a succession of fixed frames. Maybe simple squares, maybe spirals drawn from the centre outwards, anything regular.
The first images would allow to recognize the format. For instance a few geometric figures would rather easily reveal the way the "pixels" are disposed. Some greyscale pictures could even be possible.

Then you would see our solar system. Quite easy to represent the sun and major planets in a few successive positions, or perhaps draw the orbits. Then a zoom to Earth and the moon. Then an illustration of whatever is coming our way (the Earth exploding, the oceans evaporating, etc...).

The tricky bit is for our benevolent neighbours to tell us what bits of physics are supposed to spell our doom though. I have no idea how an alien mind could come up with an universal enough way of lecturing Earthlings about physics using a mere broadcast with no feedback whatsoever.

BTW. it reminds me of my favourite Arthur C Clarke's short story.
Using stars as semaphores is rather God's business :)

Considering there is no FTL travel, they will take some time to reach the Solar System. Since we are emitting large amounts of electromagnetic radiation that contains actual information (TV, communication etc.) the probe can start recording this information with highly sensitive antennas. They can then use machine learning to figure out patterns that indicate how our species perceives and communicates danger.

Additionally, the probe can send smaller sub-probes ahead with telescopes that scan us in the visible, IR and UV electromagnetic spectrum. That will help - for example - to recognize TV images since they can be compared with images of the real world. Once the main probe arrives at the Solar System it should have gathered and processed enough data to customize the message so that we can understand it. By then it can probably even use our language and units. A short morse-code message in English should be defenitely feasible from the information you get from scanning our electromagnetic emissions.

Actually, you need a good justification why you would convey the message with a heavyweight, high-energy, low bandwidth method like darkening the sun instead of just sending an audio message on an emergency frequency that is guaranteed to be listened to by many stations.

Considering the level of technology required to send such a probe to us and to block out a portion of the sunlight, the observation and data processing technology required to customize the message should be simple in comparison. It is much closer to what we can already do with current technology. There is no major technological obstacle that prevents getting everything you need to compose a message that can be understood by us.

The aliens also seem to know us and know (or suspect) that we are on a dangerous technological path. So they already have some information about us.

• But they just got a signal that we somehow tinkered with the dangerous physics, not knowing about us before that. There are no probes sent before, and the electromagnetic radiation we're suppose to blast into space is largely overestimated. Sure, an outpost dedicated to spy on us since centuries would get some data, but here that's just an automated beacon destined to look out for "wrong physics being used in the vicinity" and sending a "beware of that stuff" message. – Keelhaul Jan 10 '20 at 9:19
• @Keelhaul: They need to send the actual probe/beacon that will look out for the bad physics and that will convey the message. This beacon can also scan the EM spectrum and process it. – Sefe Jan 10 '20 at 9:28
• @Keelhaul: Considering that they would have thought about the problem of how to convey the message, they would have figured out this solution and designed their beacon accordingly. – Sefe Jan 10 '20 at 9:34
• "TV images can be compared with images of the real world" ... only by the species which have the same visual spectrum than humans. Many other species, even on earth, could not make anything out of a human picture or TV snapshot. – Hoki Jan 10 '20 at 17:38
• Actually, comparing TV to real life should not be as hard as you might think... most video is YUV encoded, where the 'Y' is a measure of overall light intensity. They'd have to figure out roughly what wavelengths we are sensitive to, but there aren't a whole lot of possibilities in that respect. From there, they can probably decode the Y component, even if they can't make heads or tails of the U/V, which is good enough to send at least "black and white". The real problem is that this will only work with analog signals; good luck decoding digital without instructions. – Matthew Jan 10 '20 at 17:50

Send two images in a reasonable encoding we can decipher, such as our bitmap. Send a crude simplified image of our Solar System ( think astronomy books for children), followed by an identical image with Earth missing.

We'll get the message.

Alternative: our corner of the galaxy, plus copy with our star missing.