Weird question: could we see distant aliens?

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 superintelligence and had solved metaphilosophy, which confirms that UDT (or something like it) is correct, and I could predict what decision theory the aliens are probably using, and calculate "logical correlation" using some sort of philosophically sound method, etc.

The preceding analysis also implicitly presumes aggregative values—for which “twice as big is twice as good.” With respect to more easily-satiable values, like the desire to live out a happy life, I suspect that SETI is an even better deal. I’d expect the message to be something that allows humanity to keep living a happy life and to benefit from significant technological acceleration, since the benefits of killing us are de minimis and we’d have been willing to pay a lot to prevent it.

A CDT agent wouldn't care that we’d have been willing to pay a lot to prevent it, right? I don't think the benefits of killing us are necessarily de minimis, either. It's only that if the aliens or the message (seed AI) can very confidently predict that we won't ever be a threat to its plans. If we might be a threat (for example if there's a risk that some faction of humans might disagree with letting the seed AI loose, and try to stop it before it becomes too powerful), it could easily be safer to kill us (for example by using a biological weapon) and then proceed without any chance of interference.

Overall I think that a 1% absolute reduction in extinction risk is a not-crazy estimate for the value of successful SETI in a sparsely populated universe.

I'm very confused that you'd even consider taking this gamble, if winning gives 1% reduction in extinction risk but losing gives 100% extinction risk. It seems implausible that we could reduce our uncertainty about the alien AI being unfriendly to <\1%, at least before we become superintelligent, etc. What am I missing here?

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 telescope time, and to deserve its own feed. I think people are pretty sure it is natural at this point, with a probable case of "indigestion" from the star colliding with a metallic planet in the last 10k years or so.

However, the fact that it got our attention means someone might do that to one planet/star combo like clockwork, every 1000 years in a regularly spaced line of stars.

It could work as a local "we exist" signal whose clocklike timing would count as the signature of intentional planning and sort of function like an invitation to show up at the logical NEXT star in the timed "indigestion collision" sequence to watch the collision and parley with whoever else showed up...

However, I don't think these events would be bright enough for the weird question?

(This does raise the question as to what counts as a "message" and what the bitrate of said message is allowed to be? Is a valid message just "this was intentionally created", or "this was intentionally sent", or "here is a place that will be interesting at a future time" or something even more than that? Also, what if the evidence of intentionality comes from a coincidence of timing spread across spans of time that requires detailed astronomical records for longer than humans seem to be able to maintain political or cultural or linguistic institutions?)

2. In 1967 Pulsars caused people to be very excited for a short period of time, thinking that such regularity must be intentional. However then it was worked out that pulsars were just spinning charged neutron star remnants leftover from supernovas. Still, they are pretty great natural clocks ;-)

This might make them a great "medium" in which to encode intentionality, but it means you have to modulate or sculpt them somehow so that when alien astronomers get interested they can see a deviation from what's natural.

Another problem is that they are highly directional, with most of the energy going out of their wobbling north and south poles (which when they wobble across your telescope is one of the pulses), so they don't signal very widely.

Another problem is that they aren't actually very bright. We see them in the Milky Way, and in our galactic neighbor the Large Magellanic Cloud, but finding an unusually bright pulsar 2 million light years away in Andromeda was newsworthy. In 2003 McLaughlin and Cordes tried to find very bright pulsars further afield and maaaaybe got a hit in M33 (aka "The Triangulum Galaxy") which is only 3M light years away. But seeing these things from 8000M light years away is highly questionable.

Binary pulsars are more rare and more likely to get scientific attention.

The first binary pulsar, discovered in 1974, won the 1993 Nobel in physics for Taylor and Hulse. By 2005 there were 113 discovered. They are interesting because they modulate the "clock" dynamics inherent to singleton pulsars.

Binary pulsars tick faster when coming towards you and tick slower when moving away, so the orbital parameters of the system can be characterized precisely just from the timing of the ticks. These orbital parameters measurably changes on the timescale of human lives, slowing down in a way that can be naturally interpreted as indirect proof that gravity waves exist and are pulling energy out of such massive systems :-)

If you wanted to catch someone's attention you might construct or find a three star system that included a pulsar aimed the way you wanted to send a message, and then mess with the orbital parameters intentionally.

Non hierarchical three star systems are chaotic by default and well understood chaotic systems can be controlled with surprisingly little energy which might make something like this attractive.

A probable hierarchical trinary-with-a-pulsar (and so not necessarily chaotic) that includes a sun-like star was surveyed in 2006. The third star is not totally confirmed, and even if it exists the arrangement here is more like a binary system, where one of the binaries has a large planet/star/thing orbiting it alone (hence "hierarchical" and hence probably not chaotic).

There is another pulsar trinary that might be chaotic found in 2014. These things tend not to last however, because "chaos".

Those are the only two I know of. I'm pretty sure the trinaries are being examined "because physics" but I've heard no peeps about unusual patterns of timing from them. But still, no matter how many neighbors pulsars have, they are fundamentally too dim and too directional to count as part of an answer to the weird question here I think...

3. The 234 star's that might be called "Borra's Hundreds" can probably also be discounted directly because at best, if these are signaling extraterrestrials, then they are just using puny pulsed lasers with roughly our own planet's industrial energy outputs, in more or less the visible spectrum (blockable by dust), which probably doesn't count because it obviously can't be seen from somewhere far away like the Sloan Great Wall.

The idea, initially articulated by Ermanno Borra in 2010 as I minimally understand it, is that a laser could shoot out light of nearly any frequency (frequency as given by the wavelength of individual photons), but if we or aliens could pulse the quantity of photons sent out fast enough, this would be visible to typical methods for measuring the "frequency of light from a star" in standard spectrographic surveys whose intentional goal is to figure out the atomic constituents of those stars from the wavelengths (and hence the frequencies) of the specific photons they emit. The methods aren't looking for very fast pulses of more and then less photons, but they could nonetheless see them by "accident".

In 2012, Borra tried to explain it again and spelled out more of the connections to SETI, basically saying that formal SETI was doing one thing, but spectrographic star surveys were better funded and you could do SETI there too just by processing the exact same data through another filter to make the possible injected signals pop out.

Aliens seeking to be discovered would know anyone smart would do spectrographic surveys of the stars, so that would be an obvious place to try to put a signal.

Then in 2016 Borra published again, now with Trottier as a coauthor, saying that he'd gone ahead and looked at archival spectral data, and found 234 stars that seemed to be sending out "peculiar periodic spectral modulations" of the sort that he predicted... unless the recorded version of the data had frequency artifacts in it?

As summarized by Snopes (normally a good source) the claim is disregarded but all the criticisms are status attacks rather than attending to any kind of object level analysis of the math, the physics, or the collected data.

The BEST argument against Borra is one I've almost never seen leveled, which is that the data processing method involved complex math, and had error bars, and they analyzed 2.5 million stars and only found 234 results. This makes me instantly wonder: data mining artifact?

But in that case you'd expect someone to make this argument seriously and explain in detail how the math went wrong somewhere? I don't get it.

Maybe people think that lasers that blink with a terahertz frequency are impossible because of "laser physics" or something? But no one seems to have raised this objection. And it seems to me like it might be possible to do this just from having a normal continuous laser and then spin something very very fast that periodically blocks the light coming out of the laser? I'm not a laser engineer, I don't know, it just seems weird to me that I've seen no speculation one way or another.

I've tried googling the coordinates of the stars Borra found and none of them have wikipedia pages, Google sends all the searches for the stellar coordinates back to Borra's own paper. I don't know how many light years away any of them are.

There's no kickstarter. The normal SETI people at UC Berkeley eventually, in October of 2016, agreed to look at a few of Borra's stars but you could see their heart wasn't in it. There's been no word since then.

However, despite humans being boring and uninterested in important things, what about a generalization of this method! :-)

(EDIT NOTE: In the first draft I had text here where I imagined Niven's fictional Ringworld made out of an impossible super material and then suggested modifications to create a "flicker ring" that could spin around a star and make the star appear to blink at spectral frequencies from certain perspectives. My optical reasoning was ludicrously wrong in the first draft, built around how things would be seen from very close rather than very far. Even with the hypothetical magic substance "scrith" a flicker ring big enough and fast enough to look right at a vast distance would be impossible. The material would have to be many orders of magnitude more magical than scrith to work in this capacity.)

4. Hoag's Object is pretty fascinating and fascinatingly pretty.

Sometimes I wonder if the only reason we don't believe in aliens yet is some kind of social signaling equilibrium similar to plate tectonics.

In 1915 Wegener was like "Duh, the continents obviously line up like a jigsaw puzzle" and people were like "No way!" and then 50 years later they were like "Oh, yeah, I guess so, funny how this is obvious to kids now but wasn't obvious to fancy scientists in 1890..."

If there are "Hoagians" shepherding all the stars in their galaxy into a pretty ring as a collective art project (or maybe just to prevent expensive damaging collisions?), that would be pretty epic.

In terms of the weird question however, the problem is that Hoag's Object is only 9M light years away (vs Andromeda's 2M, and that's part of why we easily see it. Picking it out uniquely from 8000M light years away would be a totally other thing. Also, it is only visible if you see it from the poles rather than the edges, which is another reason it isn't a very good universal signal.

5. Black hole collisions have never been attributed to aliens, to my knowledge. However, they are obviously big and awesome and get a lot of news. If you could survey moderately sized black holes in your galaxy and nudge them around in a controlled way you might have a partial solution? Timed collisions would be hard to deny were aliens I think. Imagine:

Chirp! (then wait 16.30 days)

Chirp! (2.32 days) Chirp! (then wait another 16.30 days)

Chirp! (2.32 days) Chirp! (2.32 days) Chirp!

You going to tell me that's not an intentional "here I am!" signal? You can't! :-P

From a long term signaling perspective (like to break through the Fermi Paradox by visibly declaring once and for all "intelligence existed!" before the Great Filter gets you) the problem here would be that this would be a one time signal that only communicates to a small shell of stars a precise distance away.

Many such events could have occurred before humans could hear them, and many might exist after we go extinct, with us none the wiser :-/

6. Gamma Ray Bursts are more usually associated with death and life. Basically they are so bright that they would probably cause mass extinctions in their home galaxies.

However, if you could figure out a way to cause them (not that hard? just crash neutron stars into each other in head on collisions?) and somehow survive a series of six-ish closely timed blasts then it could work like black holes, but way more obvious. No theory of relativity is even required to know to build a gravity wave detector! Black holes are still probably better in terms of style points, because their collisions don't seem to cause mass extinctions :-P

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Anyway, my point is that all of these are thing that have already come to mainstream scientific human attention and caused lots of exploratory interest and analysis.

ALSO, all of them have been more or less dismissed by mainstream astronomers as being conclusive evidence of extraterrestrial civilizations.

ALSO, I don't instantly see super obvious ways to twist any of these things around to function as a clean cut answer to the weird question where a short-lived Kardashev Type III species with our physics and material science (but better and more manufacturing capacity) could set something up, have it persist after the Great Filter gets them, and signal to everyone forever.

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 that there were no planets at all). No idea what that probability distribution looks like and how it would be corrected for the fact that there is "more" space "far away". My point is that signals from halfway across the observable universe could be very unlikely.

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 as the gravitational wave that we detected, and we could do this a billion times with a galaxy. To be safe, I'd 10x the strength of the waves, so that we could do it 100 million times with a galaxy.

Currently our instruments aren't sensitive enough to detect which galaxy was emitting these bizarrely strong gravitational waves. So I'd combine this with Wei Dai's suggestion of making an extremely bright beacon using the accretion disks resulting from the creation of these black holes.

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 the sun (M*). A white dwarf cannot be more than ~1.44 M* because the self gravity becomes too strong to be supported by electron degeneracy pressure.

Type 1A supernovae occur when a white dwarf reaches ~1.44 M* via accretion from a companion star that is expanding. The white dwarf collapses, a large percent of the mass undergoes fusion, and it releases 1e44 to 2e44 joules of energy. Type 1A supernovae have a characteristic brightness profile and spectrum, and are readily identified. They occur naturally at a rate of approximately 1-2 per century (1-2 per ~3e9 seconds) in the Milky Way. They have been detected from as far away as 10 billion light years.

The Milky Way contains 1e11 to 4e11 stars. Therefore, there are up to 2e10 white dwarfs in the Milky Way.

An advanced civilization that wants to send an omnidirectional signal could intentionally induce type 1A supernovae by coalescing white dwarfs or crashing other stars into them. If using only white dwarfs (5% of the stars in the galaxy, maybe ~2-3% of its mass), then 1.44/0.6 = 2.4 average white dwarfs per supernova explosion would be required. This would allow 2e10 / 2.4 = 8e9 type 1A supernovae total in the galaxy.

This could be done by calculated, relatively small shifts in velocity that cause interstellar collisions many years in the future. For example: Two stars are calculated to pass within a light year of each other (1e16 m) in 10 million years (3e14 s). A shift in velocity on the order of 1e16m/3e14s = 33 m/s will instead cause a collision. Acting over 3e13 s (1 million years), this would require a constant acceleration of ~1e-12 m/(s^2). This could be accomplished by light pressure with mirror satellites or other low acceleration means that are a small fraction of a star system's mass. In this example it would require ~10 million years to get signalling started, but that is 0.2% of the timescale under discussion (5 billion years).

If the signal needs to be maintained for 5 billion years (1.5e17 seconds), then the civilization could on average initiate a type 1A supernova every 1.5e17 / 8e9 = 1.9e7 seconds = 217 days, which would be ~50-100x the natural rate. If visible to us, we would notice a galaxy with so many supernovae.

https://en.wikipedia.org/wiki/White_dwarf

http://www.astro.gsu.edu/RECONS/TOP100.posted.htm

https://en.wikipedia.org/wiki/Supernova

https://en.wikipedia.org/wiki/Milky_Way

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 shared understanding of how the universe really works (I don't really think we do yet). If not then all the signally will simply be seen as unusual natural phenomena that needs to be explained. This of course has me wondering just how much of our current cosmology is the equivalent of the alien civilization trying to figure our what natural process produces the radios signals they see which were actually just the broadcast of Two and a Half Men or Married with Children.

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 distance at which we've seen any object so far, so at a minimum it has to be 4x the brightness. But...

GN-z11 came out of an experiment that imaged 0.02 square degrees, and was made visible by lucky gravitational lensing. The Hubble deep field was about 0.04 square degrees. Since we need it to be detected by a broader sky survey, it needs to be significantly brighter.

At these sorts of distances, detecting fluctuations in brightness is mostly out because of the need for long exposures to detect anything at all. And (I could be wrong about this) I don't think we get very much information about an object's spectrum until after we've singled it out as interesting, so objects can't use a weird spectrum to stand out.

That leaves the question of how much you can increase the brightness of a galaxy, if you're willing to disfigure it. That's not something I know much about, but the requirement that it remain visible for a billion years seems like it would be pretty constraining.

Another issue is that, at sufficiently long distances, a large fraction of the sky is blocked by foreground objects and dust.

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 "exactly what" a given member's characteristics are. Simulations, without any underlying empirical basis cannot be anything other than speculative effort and so provide no insight on something like a real distribution.

I think the unrealistic assumption that the alien civilization knows what we would be looking for could be used for simplification, but to my mind also makes the question a technical problem of how to give us what we are looking for and not how to get some external civilization to notice. In that case they simply need to have the technology to put a sign up that says "We are here." Adding the little arrow would be a bonus ;-) That should be simple enough by manipulating the frequency of light from selected stars in the galaxy.

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 quicker than building a quasar.

However, if we look at the convertor of redshift z to distances and ages, link, than z=1 (equal to slowdown of two times) is corresponding to the age of the galaxies there of 6-7 billions years, which is probably the youngest generation capable to support civilizations. (In other words, the biggest part of the observable universe on higher z is too young to support civilisations.)

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 of light and non-perpendicular angle of the galactic observation, but supercivilization could find the ways to overcome them. One is that the drawing could have two levels: the beacon, which attracts attention, like simple, clear artificial geometric figure, and the second level which provide data in form of smaller drawings.

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 direction:

Nm (number of messages)

The surface area of a sphere at 1 AU is about 200,000 times that of the area of the sun's disc as seen from afar.

Lm (bit length of message)

The Aricebo message was 1679 bits in length.

Db (duration per bit)

Let's say a solar sail could send a single bit every hour.

We could expect to see an aricebo length message from such a galaxy once every Db x Lm x Nm = 40 millennia.

Of course messages could be interleaved, and it might be possible to send out messages in multiple directions at once (as long as their penumbra don't overlap). If they sent out pulses at the points of a icosahedron and alternated sending bits from the longer message with just a regular pulse to attract attention, 200 years of observation should be enough to peak astronomer's interest.

But would such a race really be interested in attracting the attention of species who couldn't pay attention for at least a few millennia? It isn't as if they'd be in a rush to get an answer.

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, though. So this is not meant as an answer, but rather as a starting point for others to do more literature research. I think I have seen this discussed somewhere, using technetium; but googling revealed that stars with technetium actually exist!

Milky way ${10}^{42}kg={10}^{59}J$

Mass of observable universe $={10}^{5}3kg$

Radius of observable universe $46Gly$

Lets suppose that the milky way has ${10}^{10}$ (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 ${10}^{21}$ stars of interest.

As planetary location is not known, they must fill the entire habitable zone with energy. Radius of earths orbit ${10}^{11}m$ so area around ${10}^{22}{m}^2$

This gives ${10}^{43}{m}^2$ total that it must illuminate to hit us.

If they want to broadcast the galaxies mass-energy over 3billion years (${10}^{17}s$) then flux is

${10}^{59}/{10}^{43}/{10}^{17}=0.1W/m^2$, 100X a full moons illumination.

If they choose to send a 1 second pulse of energy every 30 years, That is $1:{10}^9$ time ratio so the flux for that second can be $0.1\times {10}^9={10}^{8}W/m^2$, This is comparable to the beam of current laser weapons or being within 1km of the trinity test, and would likely set fire to most exposed organics and heat the surface of black rocks to a red glow. As we can see, the aliens can reduce power usage by orders of magnitude and still be highly noticeable.

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 http://www.pnas.org/content/pnas/96/9/4756.full.pdf 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 extragalactic object.

Once you get the attention on radio, modulate optical spectra so it does not match periodic table, but is e.g. some binary code. First sightoptical attention-grabbing may be also to artificially create some impossibly high red-shift spectra.

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.