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:
http://www.fhi.ox.ac.uk/wp-content/uploads/intergalactic-spreading.pdf
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 acausal trade (which I think is actually relatively likely).
The moral argument is not very compelling to me, and I think the acausal trade argument depends on the aliens using UDT, and the aliens thinking there's enough logical correlation between them and us (even though they're superintelligent aliens/AIs and we're barely intelligent primates (or rather, a probabilistic mixture of evolved beings that are barely intelligent enough to have built the beginnings of a technological civilization)). If either of these assumptions fail we'd be in trouble. In general I'd only be comfortable with doing anything like acausal trade if I had sup...
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]
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...
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.
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 ...
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...
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 ...
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.
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...
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.
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 ...
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 "...
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...
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...
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 ...
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...
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 ...
,,,,,,,,,
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:
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."
Prize
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.