ETA: Contest is closed.

Suppose there was a large alien civilization halfway across the observable universe, using a galaxy's resources to try to get our attention. Would we have noticed? What if they were using 0.1% of a galaxy's resources, or 1000 galaxies' resources?

I've argued recently that such an alien civilization is (a) not that unlikely a priori, even given that there aren't any closer aliens, (b) potentially really important to notice.

I believe the answer to my question is probably "definitely." But I can't tell with any confidence, so while it's probably definitely it might be maybe and could be probably not. I'd like to know the answer, but space isn't my thing.

I'm offering a prize for anyone who answers this question. To be a bit more precise:

  • Your goal is to construct a strategy that a technologically mature civilization could use to get our attention, even if they were halfway across the observable universe.
  • The strategy is allowed to use the resources of an average galaxy. Note that they don't know when they are looking, so they need to run the strategy for a few billion years. And they have no idea what direction we are in, so it needs to be visible from any direction (no lasers).
  • By "get our attention" I mean: be interesting enough that we would already have noticed it and devoted some telescope time to looking in more detail at that part of the sky. (Once they have our attention it seems significantly cheaper to send a message.)
  • Alternatively, you can also win by providing an argument for why this isn't likely to be possible. Basically just saying anything that convinces me that the question is no longer open.
  • The second and third parts of the question are the same as the first half, but for 1000x and 1/1000th of an average galaxy's resources.

A simple example of a strategy is to create a really bright beacon somewhere far away from any galaxy, which looks weird in some way. I expect (based mostly on super informal discussions with Anders Sandberg and Jared Kaplan) that this strategy is good enough, i.e. that 0.1% of a galaxy's power is plenty to make a beacon that would be really obvious to us from halfway across the universe. But I'm definitely not sure. The beacon can have a weird spectrum, or flicker in a strange way, or only be active 1% of the time (but be 100x brighter), or whatever.

Note that an answer needs to make reference to the astronomical observations humanity has actually made, e.g. how long telescopes of a particular strength have spent looking at any particular part of the sky, and what kinds of patterns would have been noticed.

With respect to the capabilities of the alien civilization, I'm an unapologetic techno-optimist. If it's within the energy budget, I'm probably willing to believe they can make it happen unless it sounds super crazy. For 1x and 1000x questions, it's fine if they want to grossly disfigure a galaxy if that would be the best way to be noticed. For the 1/1000 question, grossly disfiguring a galaxy isn't allowed unless we can be pretty confident it doesn't reduce the usefulness of that galaxy by >0.1%.

I'm also basically happy to assume that they know exactly what our civilization is looking for and so can optimize their solution to be noticeable to us. (After all, they've run a billion billion simulations of civilizations like ours, they know the distribution, they can spend 5x as much energy to cover the whole thing.)

I don't care about whether we'd notice "things the aliens would want to do anyway," because I have no idea what aliens would want to do and have limited confidence in our ability to make prediction. In particular, it seems plausible that they would blend in with the background by default (e.g. maybe something like aestivation hypothesis is true). I'm much more interested in analyzing deliberate attempts to be observed, since those allow us to argue "If there exists a cheap way to be noticed, and they want to be noticed, they'll do it."


Note: prize is no longer available.

I'm offering a prize for a convincing answer to this question.

Initially the prize is $100. It increases by 10%/day, until capping out at $10,000 in 49 days.

Submit by writing a comment on this post.

The prize starts out low because I think this might be a really easy question. Feel free to try to be strategic if you want. If you get scooped because you are waiting for the prize to grow, I have zero sympathy.

The criterion is "Paul is convinced." Citations and clear explanations are probably helpful. In general sources don't have to be super authoritative; if you cite Wikipedia I'd prefer a citation to a historical version of a page before the contest started, just to rule out hijinks.

You are allowed to just link to an existing analysis that covers this question, or link with a small amount of extra work, if that's convincing. Assuming the linked explanation was written before my blog post, you'll get the prize, not the author of the linked post. The purpose of this prize is to buy information, it's not like the alignment prize.

I expect that winning submissions will be relatively short, probably just a few paragraphs with some links and calculations. You can take longer if you want, but I assume no responsibility for the harm thereby done to the world.

I reserve the right to be arbitrary in evaluating submissions. I am not going to feel guilty about it. If your willingness to participate depends on me feeling guilty about people who spent a bunch of time but who I unfairly rejected, then please don't participate.

I may give partial credit if something seems like a useful contribution but doesn't resolve the question completely (even if it's just a short comment with a pointer to a useful resource).

I may give feedback in the comments.

If you think this isn't the best thing for me to do with my time and are worrying about my life decisions---it was either this or spend my own hours looking into the question. Don't worry too much, this shouldn't take long.

Note: prize is no longer available.


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Your goal is to construct a strategy that a technologically mature civilization could use to get our attention, even if they were halfway across the observable universe.

Launch probes to physically get here, at speeds that are barely slower than light; the amounts of energy/materials needed are so ridiculously small compared with the energy/materials they would wield, that they can easily send reproducing probes to every star in the reachable universe.

This is how future humanity could do it:

If you can convince me the colonization speed is definitely >0.9c, I agree this question is moot. I'm currently putting significantly probability on <2/3 c speed.
The numbers are in the paper (including intergalactic dust). Allowing nuclear fusion reaches 80%c no problem. The most distant galaxies can be colonised at 99%c, because the Hubble drag means you don't have to decelerate much - it's deceleration that's the problem. Now, if you're allowed to go beyond the very conservative assumptions of our paper, you can do a lot more to decelerate - for example, sucking in hydrogen from space to fuel your deceleration. Or you could use more way-points: not aim directly for each galaxy, but for one galaxy in each super-cluster. Or aim for nearby galaxies to construct a massive second wave. Do you want me to summarise the paper here, or do you prefer to read it? PS: the article was peer reviewed and published in Acta Astronautica, if that's relevant to your assessment.
I like the calculations in the paper. I don't see how to get high confidence about colonization speeds getting close to c, rather than e.g. retaining 10% on colonization at <2/3c and 30% on <0.9c. It seems to me like a priori we have a reasonable chance that colonization occurs near c. The calculation in the paper pushes it further, by addressing a few possible defeaters (esp. slowing down, dust), but doesn't seem decisive since there are likely unanticipated difficulties. (Anders also gave guesstimates in line with this intuition in private correspondence, so it's not just me here.) I believe you can get better-than-fusion densities, so that slowing down probably isn't a bottleneck. (I'm not sure I understand your remarks about hubble drag though. The goal is getting to the destination quickly, doesn't that mean we need to be traveling near c for the entire trip? Can't afford to slow down to 0.5c for the second half, or else your average speed is < 2/3c...)
The Hubble drag means that for the most distant galaxies, you can launch at 99%c and arrive with almost null velocity. If you prioritise speed (rather than distance) the best strategy would be to wait till the Hubble drag has reduced (co-moving) velocity to about 80%c, decelerate, Dyson a star, and then re-launch at 99%c. Dysoning and re-launch take a decade or two at most, so that barely changes the average speed. The reason I feel that defeaters will not be an issue (apart from dust), is because of the huge margin this method has. Launching ten thousand times more probes is very doable. Operating in a series of short hops from galaxy to galaxy, is also doable. Sending ten thousand mini probes to accompany the payload probe is also doable (these mini probes would not decelerate, they would just backup the payload's data and check it as we arrived to a destination; or they might replace the payload probe if this one had been damaged). Eric Drexler has many ideas to make the process much more efficient; it seems that using a gun rather than a rocket to decelerate is a better idea. But the whole setup does require some form of automation/weak AI assumptions. Without those, then this become slower/less likely. I need to talk with Anders about these other defeaters :-)
Overall I still think that you can't get to >90% confidence of >0.9c colonization speed (our understanding of physics/cosmology just doesn't seem high enough to get to those confidences), but I agree with you that my initial estimate was too pessimistic about fast colonization and it's pretty unlikely that colonization is slow enough for this question to matter.
If you assume that Dysoning and re-launch take 500 years, this barely changes the speed either, so you are very robust. I'd be interested in more exploration of deceleration strategies. It seems obvious that braking against the interstellar medium (either dust or magnetic field) is viable to some large degree; at the very least if you are willing to eat a 10k year deceleration phase. I have taken a look at the two papers you linked in your bibliography, but would prefer a more systematic study. Important is: Do we know ways that are definitely not harder than building a dyson swarm, and is one galaxy's width (along smallest dimension) enough to decelerate? Or is the intergalactic medium dense enough for meaningful deceleration? I would also be interested in a more systematic study of acceleration strategies. Your arguments absolutely rely on circumventing the rocket equation for acceleration; break this assumption, and your argument dies. It does not appear obvious to me that this is possible: Say, coil guns would need a ridiculously long barrel and mass, or would be difficult to maneuver (you want to point the coil gun at all parts of the sky). Or, say, laser acceleration turns out to be very hard because of (1) lasers are fundamentally inefficient (high thermal losses), and cannot be made efficient if you want very tight beams and (2) cooling requirement for the probes during acceleration turn out to be unreasonable. [*] I could imagine a world where you need to fall back to the rocket equation for a large part of the acceleration delta-v, even if you are a technologically mature superintelligence with dyson swarm. Your paper does not convince me that such a world is impossble (and it tries to convince me that hypothetical worlds are impossible, where it would be hard to rapidly colonize the entire universe if you have reasonably-general AI). Obviously both points are running counter to each other: If braking against the interstellar medium allows you
Thanks, those are some good points. I feel that the laser acceleration option is the most viable in theory, because the solar sail or whatever is used does not need to be connected to the probe via something that transmits a lot of heat. I remember Anders vaguely calculating the amount of dispersion of a laser up to half a light-year, and finding it acceptable, but we'll probably have to do the exercise again.
I would not fret too much about slight overheating of the payload; most of the launch mass is propulsion fuel anyway, and in worst-case the payload can rendezvous with the fuel in-flight, after the fuel has cooled down. I would be very afraid of the launch mass, including solar sail / reflector loosing (1) reflectivity (you need a very good mirror that continues to be a good mirror when hot; imperfections will heat it) and (2) structural integrity. I would guess that, even assuming technological maturity (can do anything that physics permits), you cannot keep structural integrity above, say, 3000K, for a launch mass that is mostly hydrogen. I think that this is still icy cold, compared to the power output you want. So someone would need to come up with either 1. amazing schemes for radiative heat-dissipation and heat pumping (cannot use evaporative cooling, would cost mass), 2. something weird like a plasma mirror (very hot plasma contained by magnetic fields; this would be hit by the laser, which pushes it via radiation pressure and heats it; momentum is transferred from plasma to launch probe via magnetic field; must not loose too many particles, and might need to maintain a temperature gradient so that most radiation is emitted away from the probe; not sure whether you can use dynamo flow to extract energy from the plasma in order to run heat pumps, because the plasma will radiate a lot of energy in direction of the probe), 3. show that limiting the power so that the sail has relatively low equilibrium temperature allows for enough transmission of momentum. No 3 would be the simplest and most convincing answer. I am not sure whether a plasma mirror is even thermo-dynamically possible. I am not sure whether sufficient heat-pumps plus radiators are "speculative engineering"-possible, if you have a contraption where your laser pushes against a shiny surface (necessitating very good focus of the laser). If you have a large solar sail (high surface, low
Thanks for these critiques! They are useful to hear and think about. > I think that this is still icy cold, compared to the power output you want. I think it's not so much the power, but the range of the laser. If the target is large enough that a laser can hit it over distance of light years, for example, then we can get away with mild radiation pressure for a long time (eg a few years). But I haven't run the numbers yet. >I am more skeptical if your rocket, including fusion reactor but excluding fuel, is limited to 30 gram of weight. I was imagining a sort of staged rocket, where you ejected the casing of the previous rockets as you slow, so that the mass of the rocket was always a small fraction of the mass of the fuel. But Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the "ship", then this doesn't obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles - though the (tiny) acceleration of the laser/ship is). I'll have to crunch the number on it. >Do any implicit or explicit assumptions break if we lose access to most of the fuel mass for shielding during the long voyage? We didn't do the shielding very well, just arbitrarily assumed that impacts less energetic than a grenade could be repaired/ignored, and that anything larger would destroy the probe entirely. As usual, Eric Drexler had a lot of fun shielding ideas (eg large masses ahead of the probe to inonise incoming matter and permanent electromagnetic fields to deflect them), but these were too "speculative" to include in our "conservative" paper.
>I was imagining a sort of staged rocket, where you ejected the casing of the previous rockets as you slow, so that the mass of the rocket was always a small fraction of the mass of the fuel. Of course, but your very last stage is still a rocket with a reactor. And if you cannot build a rocket with 30g motor+reactor weight, then you cannot go to such small stages and your final mass on arrival includes the smallest efficient rocket motor / reactor you can build, zero fuel, and a velocity that is below escape velocity of your target solar system (once you are below escape velocity I'll grant you maneuvers with zero mass cost, using solar sails; regardless, tiny solar-powered ion-drives appear reasonable, but generate not enough thrust to slow down from relativistic to below-escape in the time-frame before you have passed though your target system). >But Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the "ship", then this doesn't obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles - though the (tiny) acceleration of the laser/ship is). I'll have to crunch the number on it. That does solve the "cannot build a small motor" argument, potentially at the cost of some inefficiency. It still obeys the rocket equation. The rocket equation is like the 2nd law of thermodynamics: It is not something you can trick by clever calculations. It applies for all propulsion systems that work in a vacuum. You can only evade the rocket equation by finding (non-vacuum) stuff to push against; whether it be the air in the atmosphere (airplane or ramjet is more efficient than rocket!), the solar system (gigantic launch contraption), various planets (gravitational slingshot), the cosmic microwave background, the solar wind, or the interstellar medium. Once you have found something, yo
>It still obeys the rocket equation. That's what I used to believe. But now, on closer analysis, it seems that it doesn't. The rocket equation holds when you are continuously ejecting a thin stream of mass; it doesn't hold when you are ejecting a large amount of mass all at once, or transferring energy to a large amount of mass. The thought experiment that convinced me of this: assume you have a gun with two barrels; you start at rest, and use the gun to propel yourself (ignore issues of torque and tumble). If you shoot both barrels at once, that's two bullets, each of mass m, and each of velocity v. But now assume that you shoot one bullet, then the other. The first is of mass m and velocity v, as before. But now the gun is moving at some velocity v'. The second bullet will have mass m, but will be shot with velocity v-v'. Thus the momentum of the two bullets is lower in the second case; thus the forward momentum of the gun is also lower in that case. (The more bullets you shoot, and the smaller they are, the more the gun equations start to resemble the rocket equation). But when you eject the payload and blast it with a laser beam, you're essentially just doing one shot (though one extended over a long time, so that the payload doesn't have huge acceleration). It's not *exactly* the same as a one shot, because the laser itself will accelerate a bit, because of the beam. But it you assume that, say, the laser is a 100 times more massive than the payload, then the gain in velocity of the laser will be insignificant compared with the deceleration of the payload - it's essentially a single shot, extended over a period of time. And a laser/payload ratio of 100 is way below what the rocket equation would imply.
How much does the argument break down if we use the rocket equation? I apologize for being a lazy reader. I assume that if you are using a galaxy's power for colonization, then it doesn't matter at all. In that case contacting us would still be mostly-useless.
If you have to use the rocket equation twice, then you effectively double delta-v requirements and square the launch-mass / payload-mass factor. Using Stuart's numbers, this makes colonization more expensive by the following factors: 0.5 c: Antimatter 2.6 / fusion 660 / fission 1e6 0.8 c: Antimatter 7 / fusion 4.5e5 / fission 1e12 0.99c Antimatter 100 / fusion 4.3e12 / fission 1e29 If you disbelieve in 30g fusion reactors and set a minimum viable weight of 500t for an efficient propulsion system (plus negligible weight for replicators) then you get an additional factor of 1e7. Combining both for fusion at 0.8c would give you a factor of 5e12, which is significantly larger than the factor between "single solar system" and "entire galaxy". These are totally pessimistic assumptions, though: Deceleration probably can be done cheaper, and with lower minimal mass for efficient propulsion systems. And you almost surely can cut off quite a bit of rocket-delta-v on acceleration (Stuart assumed you can cut 100% on acceleration and 0% on deceleration; the above numbers assumed you can cut 0% on acceleration and 0% on deceleration). Also, as Stuart noted, you don't need to aim at every reachable galaxy, you can aim at every cluster and spread from there. So, I'm not arguing with Stuart's greater claim (which is a really nice point!), I'm just arguing about local validity of his arguments and assumptions.
Eric Drexler is making some strong arguments that if you eject the payload and then decelerate the payload with a laser fired from the rest of the "ship", then this doesn't obey the rocket equation. The argument seems very plausible (the deceleration of the payload is *not* akin to ejecting a continuous stream of small particles - though the (tiny) acceleration of the laser/ship is). I'll have to crunch the number on it. (if the probe is very robust, we might be able to railgun it instead of using a laser - and railgunning a single mass, once is clearly not subject to the rocket equation).
I think communicating without essentially conquering the Hubble volume is still an interesting question. I would not rule out a future human ethical system that restricts expansion to some limited volume, but does not restrict this kind of omnidirectional communication. Aliens being alien, we should not rule out them having such a value system either. That being said, your article was really nice. Send multiplying probes everywhere, watch the solar system form and wait for humans to evolve in order to say "hi" is likely to be amazingly cheap.

Can the aliens convert matter completely into energy (for example by forming small black holes and letting them evaporate) or can they only use energy from fusion in stars? This makes about a 1000x difference.

If matter-energy conversion is allowed, then an alien beacon should have been found easily through astronomical surveys (which photograph large fractions of the sky and then search for interesting objects) like the SDSS, since quasars can be found that way from across the universe (see following quote from Wikipedia), and quasars are only about 100x the luminosity of a galaxy. However this probability isn't 100% due to extinction and the fact that surveys may not cover the whole sky.

Quasars are found over a very broad range of distances (corresponding to redshifts ranging from z < 0.1 for the nearest quasars to z > 7 for the most distant known quasars), and quasar discovery surveys have demonstrated that quasar activity was more common in the distant past. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, or approximately 10 billion years ago.[4]

I'm fine with them converting {1/1000, 1, 1000}x of a galaxy's matter into energy. Main question is: do we see all the quasars at that distance, or do we see only a small fraction of them? Is whether we see them a simple function of power, in which case what is the cutoff? If we see all of them, seems like it would answer the 1x and 1000x questions. Smaller questions: * Checking the mass vs. energy calculation (for the average over the average galaxy---if anything in the galaxy emits faster, then that would dominate and you won't get the 1000x ratio). * Checking the 1000x brighter claim, probably just with a citation. But it's a bit tricky since it's mostly about which quasars we see. * Check that it's easy to make the quasar noticeable.
I think yes, but it's a little hard to find a source that says this clearly. Basically modern surveys are now trying to survey high redshift quasars which are all the way across the universe rather than half way across the universe. Also if the aliens used their power to simulate a radio-loud quasar that should be even easier to see. From page 539 of (My interpretation here is that all classical radio-loud quasars are above 1 mJy which is easily above detection limits of less than 100 μJy.) See 1 [] 2 [] 3 [] 4 [] . Note that the last link says it's 75 times typical quasar power. Do you mean the claim that quasars are 100x brighter than a galaxy? It's in the quasar Wikipedia article. Simply making it 75 times the brightness of a typical quasar might be enough, or use color/spectrum.
Don't these numbers not add up? If mass is 1000x luminosity, and quasars are 100x galaxy, then how is the ratio 75x? Seems like a random order of magnitude missing. I tentatively think this resolves the 1 and 1000x questions, but leaves open the 1/1000 question. Will leave this up for rebuttal for a week before concluding that. By default it probably gets 1/2 credit if unrebutted. For 1/1000, you have about the same amount of power as a galaxy, and you could only make a very dim quasar, so it seems like you'd need a different line of analysis. (E.g. that we'd notice something as bright as a galaxy with a weird spectrum.)
The ratio for the sun is actually 1480 to be exact, plus the rest of the galaxy is apparently dimmer per unit mass than the sun is. For 1/1000x, I think if you put most of the energy into the radio spectrum, perhaps a single frequency or a few frequencies that you predict others will survey for, it should be easily noticeable. I'll look for details when I get home, unless someone beats me to it.
If you put 1/1000 the mass of a galaxy into radio signals over 10 GHz bandwidth over 10 billion years, you get 2.7e28 W/Hz [] power spectral density. According to this paper [] table 2, at redshift z=2.083 (about 10 billion light years away) a radio source of 10^25.78 W/Hz was detected on Earth at a flux density of 3.54 mJy so 2.7e28 W/Hz should translate to 1580 mJy [*+3.54] on Earth. According to this paper [], NVSS has cataloged all objects of flux density >2.5 mJy over 82% of the sky so it likely has detected and cataloged the alien beacon. Unfortunately according to section 2.1.1 of this paper [], "However, the large beam size does not allow one to determine precise structure of sources or to determine positions accurate enough to establish optical counterparts." so we may not have noticed it as an anomalous object. Back to the visible spectrum, according to this article []: So if the alien beacon is brighter than a major galaxy (not sure what that means exactly) and within 7 billion LY, then it would have been cataloged, and SDSS captures images at 5 color bands so it would be easy to use color to stand out. (SDSS runs a bunch of algorithmic filters to try to classify each light source based on color, and if none of the filters fit, the source is classified as OTHER and a human looks at it.) 1/1000 the mass of Milky Way over 10 billion years translates to 54 times [] the luminosity of Milky Way so it should have been noticed by SDSS. But SDSS only
1580 is much more than 2.5, and even there are only a million things in their survey, surely we would notice such a bright source and inspect it in detail? It seems like there is basically nothing in the sky that bright at that redshift.
Just realized, if you combine colonization and radio beacons, 1/1000x galaxy mass would be enough to make an artificial pattern of >2.5mJy sources over an area of the sky that's bigger than NVSS's beam size, and that may have been noticed by someone as an anomalous cluster/pattern of radio sources.
Between the analysis we've done so far and revisiting Anders and Stuart's colonization analysis, I think it's unlikely that there are unobserved aliens who are worth looking for. Especially given that 1/1000 of a galaxy is a pretty negligible budget, I expect someone would have been willing to spend >1 galaxy on this project if it makes sense and that's a key margin. My current plan is to award you and Stuart each $100 prizes and declare the contest closed.
It could be a drawing, but consisting of quasars, not from individual stars. A cube with a side of 1 billion ly could have a few million galaxies in it, so the drawing's patter could be rather complex and provide tens or hundred kilobytes of information. Or else, the drawing could be rather simple beacon like a circle.
According to this paper [] (which I linked to), it looked in detail at a set of S > 1.3 Jy radio sources (274 of them), in a small patch of the sky, which makes me think that there are enough bright radio sources that 1.5 Jy wouldn't stand out that much. EDIT: Oh you can't tell the redshift of a radio source without looking at it optically, but that requires "determine positions accurate enough to establish optical counterparts" which can't be done with the NVSS survey data. The paper linked above did it by using another more accurate radio survey to establish optical counterparts but that survey only covered a small patch of the sky.
First, are there no naturally evaporating black holes? Would we be able to tell them apart from other light sources? Second, what happens if, by chance, the alien galaxy is exactly on the other side of the center of the Milky Way. Does their light even reach us then? Or is is just an issue of needing more energy to make it noticeable?
No, because small black holes evaporate too quickly and natural ones would have disappeared long ago, and large black holes evaporate too slowly to be used as an energy source (well technically you can use their accretion discs for matter-energy conversion at 10% efficiency, which is essentially what quasars are, but that's not as good as using the evaporation of small black holes for 100% efficiency). The aliens would have to constantly form small black holes and let them evaporate. They would give the beacon a distinct/unnatural color/spectrum. EDIT: For example astronomers have been looking for quasars with especially high redshifts by searching the survey photographs for light in a certain color range, and then doing spectrography on the candidates for more detailed investigations. If the aliens can predict the color filter being used, they can give their beacon that color and then an unnatural spectrum would alert the astronomers. Or the aliens can give the beacon a totally anomalous color like pure blue, which would probably trigger some kind of anomaly detector in the astronomical surveys. I guess just more energy but I'm not sure how much more.
Are you implying that small black holes have ever formed naturally at all? If there is some process that formed random size black holes long time ago, the small ones might have already evaporated, but the medium ones might be just finishing their evaporation right now. Of course, such a process might not have occurred, ever. Efficiency isn't quite the right metric here. I think we need "power"? So, how much power does the small black hole produce? It's my naive understanding that this power only depends on the radius of the hole, not on how much matter you're throwing into it. Though I guess you could just have several black holes, if one isn't bright enough?
See []. Exactly, you use as many as needed to reach the power you want.
Yes, there are primordial black holes, I'm just not certain exactly how dubious their existence is. Anyway, the point is that if there might be currently evaporating black holes, but we don't see them, then maybe that's because they're not all that bright. Then, despite their high efficiency, they may not be a viable tool for signaling.
Would be interesting to know: Suppose we have a ~1 billion year old civilization a third of the way across the universe, occupying a 0.5 billion light year sphere. What fraction of the sky is that? Is there some fraction of the sky that happens to be especially difficult to see (e.g. because it's on the other side of the milky way), and how much harder is it to see? My guess would be that there is at most a negligible probability of this making it really hard for us to see a large alien civilization (if e.g. they had 3 beacons scattered randomly over their territory).
See zone of avoidance []. At 7b ly, alien civilization would take up 4 degrees in the sky, and it seems that Milk Way makes more than that hard to see (not impossible though).
My impression from wikipedia is that radio transmission is still fine, so radio loud quasars are still easy to detect. Does that sound right?
It seems that there are definitely some extragalactic objects known in the zone of avoidance, however I haven't been able to find how far the farthest of them are, or how close to the center they appear. Radio waves pass through dust more easily than visible light, but I don't think they are entirely unhindered. I have no idea, you might want to ask these questions somewhere like physics.stackexchange, where somebody knows something.
I wanted to comment that creating quasars may be difficult, but found that it may be done relatively simple. Let's assume that aliens don't have any magical technology to move stars or convert energy in matter. In that case, they could create a quasar by directing many stars to the center of the galaxy: falling stars will increase accretion rate in the central black hole and thus its luminosity (note that too heavy black holes may be not luminous, as they will eat stars without destroying them), and by regulating the rate and types of falling stars the quasar spectrum could be manipulated. But how to move stars? One idea is that if aliens could change a trajectory of a star slightly, it will eventually pass near another star, make a "gravitational manoeuvre" and fall to the center to the galaxy. Falling to the center of the galaxy would probably require tens of millions years (based on Sun's rotation period of 250 mln years). Finding an appropriate star and changing the star's trajectory to pass near it will require probably also at least millions years. But how to change the trajectory of a star? One idea is to organise impacts of the star with large comets. It is not difficult, as remote Oort cloud objects (or better wandering small planets, as they are not part of already established orbital movement of the star) need only small perturbations to start falling down on the central star, which could be done via nuclear explosions or even smaller impacts. The impacts with comets will have very small effects on the star's trajectory. For example, Pluto's mass is 100 million times less than Sun's mass and impact with Pluto-size object will probably change Sun's trajectory only on 1 mm/sec, but it will be like 1 billion km difference in 20 million years. Close flyby by stars are very rare, so may take tens of million of years of very complex space billiard to organise need flyby. All this suggests that creating an artificial quasar is possible, but may take up to

Paul, I love what you're doing here, have been thinking about this a long time. I look forward to seeing an answer and would like to write a clarifying essay full of non answers :-)

By "get our attention" I mean: be interesting enough that we would already have noticed it and devoted some telescope time to looking in more detail at that part of the sky. (Once they have our attention it seems significantly cheaper to send a message.)

This suggests that we can list various anomalies that might have been thought to be extraterrestrials and already received attention, and then exclude them for various reasons.

1. For example, Tabby's Star recently had me wondering/hoping/worrying for a good year or two.

It is only 1,280 light years from Earth and I think it is plausible that we wouldn't even be able to see similar stars on the far side of our own galaxy which is mere ~100k light years in diameter... it can't count for this exercise because seeing it from other galaxies would be quite a trick.

HOWEVER, despite being an F type star (that shouldn't be variable (that varies in very irregular ways)) it was interesting enough raise $100k on Kickstarter for tele... (read more)

I'm thinking large numbers of synchronized reusable beacons - either recurrent novas or black holes - where a flash is produced by feeding the beacon with gas in a controlled way. For rapid reuse, you want local byproducts of the flash to get out of the way quickly, so the next batch of gas can be introduced. That could mean dwarf novas, or black hole processes in which the waste comes out in tightly focused jets.

There is a "remarkable recurrent nova" in the Andromeda Galaxy, which repeats on a timescale of months.

To respond to your thinking (in the linked blog post) that, to a first order approximation, if we find an AI in the alien message we should run it:

The preceding analysis takes a cooperative stance towards aliens. Whether that’s correct or not is a complicated question. For the most part, I think growing the pie by enabling intelligence to access more of the universe, is probably the first order term here. That might be justified by either moral arguments (from behind the veil of ignorance we’re as likely to be them as us) or some weird thing with acausa

... (read more)
If it seems plausible that there are aliens, I think "figure out what to do" would become a high-priority item, and I think there is a very significant chance "definitely don't run it" would be the right answer and that the main resulting intervention would be to push hard against passive SETI (about which people are horrifyingly unconcerned). Unless its predecessor entertained the possibility of being in a simulation run by a civilization like ours that made it to technological maturity. This suggests a similar disagreement w.r.t. the expected moral value of unaligned AGI, which seems way more interesting and important. Only seems to require EDT. But I agree the question "can you trade with primates" is very open, and that the other routes to trade would also be quite speculative. We just care about the difference P(alien is friendly) - P(we are friendly). We don't seem to be in an especially good situation to me, so I'm not as concerned. (Actually, I'm not sure whether you mean "friendly" in the sense of FAI or the conventional usage.) I think the main question is how we feel about handing our planet to a random alien who happened to evolve first. If you are neutral about that, and think that we are in a "generic" situation w.r.t. alignment, then it seems like contact is a significant plus due to avoiding other risks. That's where I'm at. But I can understand the case for concern.
I don't see how. I think an EDT agent would make the decision by simulating (or doing some analysis that's equivalent to this) a bunch of worlds, then look at the worlds where it or agents like it happened to make the message benign/malign to see what the humans do in those worlds, and it would see no correlation between its decision and what the humans do and therefore end up making the message malign. By "unfriendly" I meant that running the alien AI results in something as bad as extinction. So my point was that if P(running alien AI results in something as bad as extinction) > 1% then this risk would more than cancel out the expected gain of 1% of our future light cone from running the alien AI (conditional on alien colonization being as good as human colonization), and I don't see how we can get this probability to be less than 1%.

Minor issue - for us to see a signal from "far away", the signal needs to have been sent "long time ago" (naively you'd say that a signal from 7 billion light years away needs to be sent 7 billion years ago, but with expansion that's not quite true, so I'll just stick with "far away" and "long ago").

Now, the probability of new intelligent life evolving should be smaller "long ago". At least, there used to be fewer metals, so, fewer rocky planets, fewer possible chemical compounds (and before ... (read more)

I'm aware of this. I agree that very old life is less likely (I'm a bit skeptical about our a priori ability to judge the relative merit of different conditions to form life, but the anthropic argument is pretty simple and seems solid). I'm still happy to start with "halfway across the universe."

My first idea is to make two really big black holes and then make them merge. We observed gravitational waves from two black holes with solar masses of around 25 solar masses each located 1.8 billion light years away. Presumably this force decreases as an inverse square times exponential decay; ignoring the exponential decay this suggests to me that we need 100 times as much mass to be as prominent from 18 billion light years. A galaxy mass is around 10^12 solar masses. So if we spent 2500 solar masses on this each year, it would be at least as prominent a... (read more)

4Donald Hobson5y
The first merger event that Ligo detected was 1 billion ly away and turned 1 solar mass into gravitational waves. 1030kg=1047J at a distance of 109×1016=1025 m so energy flux received is approx 1047×(1025)−2=10−3J/m2 The main peak power output from the merging black holes lasted around one second. A full moon illuminates earth with around 10−3W. So even if the aliens are great at making gravitational waves, they aren't a good way to communicate. If they send a gravitational wave signal just powerful enough for us to detect with our most sensitive instruments, with the same power as light they could outshine the moon. Light is just more easily detected.
My main concern with this is the same as the problem listed on Wei Dai's answer: whether a star near us is likely to block out this light. The sun is about 10^9m across. A star that's 10 thousand light years away (this is 10% of the diameter of the Milky Way) occupies about (1e9m / (10000 lightyears * 2 * pi))**2 = 10^-24 of the night sky. A galaxy that's 20 billion light years away occupies something like (100000 lightyears / 20 billion lightyears) ** 2 ~= 2.5e-11. So galaxies occupy more space than stars. So it would be weird if individual stars blocked out a whole galaxy.
Another piece of idea: If you're extremely techno-optimistic, then I think it would be better to emit light at weird wavelengths than to just emit a lot of light. Eg emitting light at two wavelengths with ratio pi or something. This seems much more unmistakably intelligence-caused than an extremely bright light.
Same question as Michael: if there were a point source with weird spectrum outside of any galaxy, about as bright as the average galaxy, would we reliably notice it?
I'm a bit confused if you already read my comment [] . If you fake extremely high red-shift, it would likely be noticed. Radio galaxies were systematically used when looking for distant objects [] so if someone at the same time created very bright radio source & optical counterpart with impossibly high redshift, it would grab attention to the source, and you can signal intelligence using spectra later.

This clearly fits into “Things we learned on LW in 2018”.

This needs comments to be nominated too. It would be really awesome if someone could write a straightforward distillation of the arguments that lead to consensus on this issue between many of the commenters.

This example discusses how a type III civilization could signal its existence to a technological civilization halfway across the visible universe (~7 billion light years) over a time span of 5 billion years. Constraints: It should use a relatively small percent of its available resources, and the methods should not rely on unproven physics.

In the nearest 100 star systems (which include ~150 stars), there are 8 white dwarfs (5% of the stars). There is a distribution of masses, but most white dwarfs are between 0.5 and 0.7 (average ~ 0.6) times the mass of ... (read more)

Same question as to Wei Dai: do we notice all type 1A supernovaea that occur, or just some of them? The fact that we've only noticed out to 10 billion light years suggests we probably can't see all of them?
I expect we don't notice most of them. We may notice a lot more the next few decades though. Some would still probably be hidden behind dust.
If we only notice 10% (say), then that seems to increase the cost of being noticed by 10x, so wouldn't yet be above the bar.
Some more thoughts pertaining to limits of detection: The Milky Way weighs 5.8e11 times M*, which itself is 2e30kg. Total mass of the galaxy = 1.2e42kg. If all that mass were converted to energy with perfect efficiency, say via black hole evaporation, or annihilation with antimatter, then that's a total of 1.0e59 joules. That many joules over 5 billion years (1.5e17 s) is a power of 7e41 watts. At a radius of 7 billion light years (6.6e25m), that's an energy flux of 1.3e-11 W/(m*m). The sun puts out about 1400 W/(m*m)at our distance. So the sun would be about 1e14 times brighter than this distant galaxy trying to get our attention. Move the sun 1e7 x farther away to about 158 light years to match this brightness, and you get a ~8.5 magnitude star, never visible without aid. (Note: If using 1000x as much energy it becomes a clearly visible star and among our top 20 or so.) So, if a type III civilization were using the entire mass-energy of 1 galaxy with 100% efficiency and used this resource to signal continuously for 5 billion years, they would not be bright enough to see unaided. We would still probably notice the light as a third-rate star if it wasn't blocked by dust. How could they make it unusual enough to be noticed as a signal? Perhaps the signal has a complete blackbody spectrum, but they surround the galaxy with an unusual spectral absorption signature. Example: Surrounding the galaxy they could have concentric clouds of He, Li, B, N, Na, Al, etc. The elements with a prime atomic number. That's unusual enough to draw attention. Maybe they could even encode a message in the degree of absorption.

Why doesn't any monochromatic light not on the natural spectrum of an element do it? Or rather, any cluster of nearby frequencies to accommodate redshift.

Just needs to be bright enough to see. I think I'm convinced that at ~1x galaxy you can do it easily, owing to the 1000x factor from using the mass of the stars rather than letting them burn. But not as clear for 1/1000. If there were a single point source with weird spectrum, halfway across the universe and outside of any galaxy, about as bright as a galaxy, would we reliably notice it?
1Charlie Steiner5y
Sure, you could try to cover the sky with lasers whose frequencies encode some mathematical fact. I think we might notice such a thing in the course of doing regular redshift measurements.

I think first we have to agree that a) aliens and humans are similar enough to even recognize the other as both life and intelligence and b) the alien must have some existence that experiences the physical universe in a way that is consistent with how humans do.

I think given these two (very general) requirements the clear way for that alien civilization to get our notice would be to modulate the emissions from their galaxy in a way that cannot be due to a natural state or natural process.

I think it would be necessary that the two civilizations have some s... (read more)

OK, what is the modulation, and when would we have seen it? Would we notice if a galaxy had an unusual spectrum? (I think you can't make a galaxy flicker or anything like that because it is too big. Though seems fine to concentrate the power and then have it flicker.)

Clarifying question - how much can these aliens move? You talked about visual signals, but is that necessary? If they're allowed to move as much as the want, what's wrong with a plain old von Neumann probe? Too slow? Too expensive? But if they're not allowed to move from their galaxy, then I'm afraid any galaxies between them and us might make their efforts useless.

They are allowed to move as fast as they can wherever they want. Seeing them is only interesting if travel is significantly slower than the speed of light (e.g. only 0.6c), which I think is an open possibility.
It depends on how long the alien civilization is allowed to last. If it poofed into existence 1b years after big bang and then spread at 0.6c for 7b years (leaving ~7 more billion years for their light to reach us), then they might occupy a big enough fraction of the sky, that it wouldn't be entirely obscured by the milky way or any other single galaxy (not that I checked the math). But that's very generous. Otherwise, we may as well consider them stationary. In that case, if their light really couldn't pass through the denser parts of galaxies, they could use some other signal, like neutrinos or gravitational waves. Not sure how to make that many neutrinos. For the latter, I suspect making two massive black holes and making them merge might not be that hard. I imagine you could even do it without moving solar-mass objects - just build two small-ish black holes and launch them on precise trajectories such that they would eventually collide, while eating up many smaller objects and gaining mass along the way. This assumes that the aliens have perfect information about their own galaxy. Each trajectory would of course take a very long time, but if you launch many, you could produce a repeating signal. Unless they run out of stars to use.

I think this might not be possible.

Per Wikipedia's list of most distant astronomical objects, the most distant object we've detected is GN-z11 at 13.9Gly. This is slightly greater than the galaxies seen in Hubble's Deep Field images, with max redshifts corresponding to a distance of around 12Gly. The radius of the observable universe is 46 Gly; to see something half-way to that distance would be 23Gly. (A distance which filled half the volume would be a bit farther than that). So we're trying to make a beacon visible at ~2x the maximum ... (read more)

This looks like it's due to a mixup between comoving distance and light travel distance? GN-z11 seems to be 13.9 billion years old, almost as old as the universe itself, and to be at comoving distance 32 billion light years.
I'm also basically happy to assume that they know exactly what our civilization is looking for and so can optimize their solution to be noticeable to us. (After all, they've run a billion billion simulations of civilizations like ours, they know the distribution, they can spend 5x as much energy to cover the whole thing.)

Okay, so a critical response here. Is it just me? The above seems very irrational and illogical to me. Knowledge of any true distribution doesn't say very much about any specific member of the population much less "... (read more)

The question is: how broad is the distribution of stuff that a civilization like ours might look for? If the distribution is extremely broad, then I agree that knowing the distribution isn't that helpful. (For example, they know the distribution of years at which we might be listening, but it doesn't help them since there are lots of years.) Actually trying to make a sign seems out, since it only works from one direction, and I assume that we lack the resolution. Manipulating the frequency of stars is fair game if a small enough manipulation would work, that's the kind of thing I meant by "disfigure."

If aliens are rather remote, they are moving away with large speed because of the universe expansion. Thus any signals they sent will experience Doppler slowdown. Moreover, the time needed to build a beacon will be also (observationally for us) diluted. For example, if they need around 100 mln years to build a quasar (by moving stars as I described in another comment here), it may look like 200 millions years for us.

Such delay may be too long according to their goals and they may try quicker ways to send data. Drawing by the use of Dyson spheres is quicke... (read more)

Probably I am too late, but, anyway, I have been thinking on the topic and even have an article under review where the idea is mentioned.

My idea is that alien supercivilization could use Dyson spheres to make a drawing on the galactic plane. The drawing is stable and the Dyson spheres are its pixels. Given that typical galaxy has 100 billion of stars, the drawing could be used to send large amount of data on billion of light years (most likely it will be description of an AI, I think).

Yes, there are some difficulties, as galactic rotation, limited speed o... (read more)

If you have a dyson swarm around a star, you can temporarily alter how much of the star's light escape in a particular direction by tilting the solar sails on the desired part of the sphere.

If you have dyson swarms around a significant percentage of a galaxy's stars, you can do the same for a galaxy, by timing the directional pulses from the individual stars so they will arrive at the same time, when seen from the desired direction.

It then just becomes a matter of math, to calculate how often such a galaxy could send a distinctive signal in your ... (read more)

One guess for cheap signaling would be to seed stellar atmospheres with stuff that should not belong. Stellar spectra are really good to measure, and very low concentration of are visible (create a spectral line). If you own the galaxy, you can do this at sufficiently many stars to create a spectral line that should not belong. If we observed a galaxy with "impossible" spectrum, we would not immediately know that it's aliens; but we would sure point everything we have at it. And spectral data is routinely collected.

I am not an astronomer, th... (read more)

Milky way

Mass of observable universe

Radius of observable universe

Lets suppose that the milky way has (5% of all stars) stars suitable for life. (because some stars are too small or close to the galactic core.

Scaling up by mass gives around stars of interest.

As planetary location is not known, they must fill the entire habitable zone with energy. Radius of earths orbit so area around

This gives total that it must illuminate to hit us.

If they want to broadcast the galaxies mass-energy over 3billion ... (read more),,,,,,,,,,,

I may be misunderstanding: Are you suggesting a targeted beam to the habitable zone of every star they can see? If so, I don't see how that could work, considering that most stars visible at time of transmission would be dead by the time the transmission reaches them. Also the fact that they have orthogonal velocity that would be difficult or impossible to measure and account for. My apologies if I have misunderstood.
1Donald Hobson5y
That was what I was considering. I was hoping the aliens had telescopes that could see the collapsing cloud of gas and work out where the star would end up.

Simple answer ... make something with the power of a very bright quasar (10^40W), in our distance the energy flux is like 10^-14 W/m^2 ... convert big part of the power to radio at some Mhz-Ghz band or similar , so it is very bright at some specific band, to grab attention.

According to this the flux densities observed are of order 0.1 Jansky (Jy) at 1,400 MHz, where 1 Jy = 5x10-26 W/m^2/Hz, so if you spread that 10^-14 W/m^2 over 100 MHz, the flux will be ~ 10^-21 W/m^2/Hz, likely very bright for an ... (read more)

The problem with quasars is that they only emit that much power along their axes, not in every direction.

One classic way of detecting aliens is by detecting stellar-scale engineering projects. If aliens could spread out 100 million light years and recycle 90% of UV/visible light from those stars into IR in order to power their civilization, we'd probably notice - it would be a mysterious patch in cosmological maps that people would probably stop to think about.

Unless they're in the plane of the Milky Way, of course, then we'd never notice.