# Weird question: could we see distant aliens?

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.”

## 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.

• 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.

No reviews.
• 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 to get the delta-v for de­cel­er­a­tion down to 0.05 c from, say 0.9 c, but ac­cel­er­a­tion turns out to be so hard that you need to get 0.8 c with rock­ets (you can only do 0.1c with coil guns /​ lasers, in­stead of 0.9 c), then we have not re­ally changed the delta-v calcu­lus; but we have sig­nifi­cantly changed the amount of available mat­ter for shield­ing dur­ing the voy­age (we now need to burn most of the mass dur­ing ac­cel­er­a­tion in­stead of de­cel­er­a­tion, which means that we are lighter dur­ing the long voy­age).

[*] Su­per­con­duc­tors can only sup­port a limited amount of cur­rent /​ field-strength. This limits the ac­cel­er­a­tion. Hence, if you want larger delta-v, you need a longer bar­rel. How long, if you take the best known su­per­con­duc­tors? At which frac­tion of your launch probe con­sist­ing of su­per­con­duct­ing coils, in­stead of fu­sion fuel? Some­one must do all these calcu­la­tions, and then dis­cuss how the re­sult­ing coil gun is still low-enough mass com­pared to the mass of a dyson swarm, and how to sta­bi­lize, power, cool and ma­neu­ver this gun. Other­wise, the ar­gu­ment is not con­vinc­ing.

edit: If some­one pro­poses a rigid bar­rel that is one light-hour long then I will call BS.

• 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 mass) con­nected by teth­ers, then you prob­a­bly can­not use ac­tive cool­ing on the sail; there­fore there is limited room for fancy fu­ture-tech en­g­ineer­ing, and we should be able to com­pute some limits now.

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Since I already started rais­ing ob­jec­tions to your pa­per, I’ll raise a sec­ond point: You com­pute the re­quired launch mass from rocket-equa­tion times fi­nal pay­load, with the fi­nal pay­load hav­ing very low weight. This as­sumes that you can ac­tu­ally build such a tiny rocket! While I am will­ing to sus­pend dis­be­lieve and as­sume that a su­per effi­cient fu­sion-pow­ered rocket of 500 tons might be built, I am more skep­ti­cal if your rocket, in­clud­ing fu­sion re­ac­tor but ex­clud­ing fuel, is limited to 30 gram of weight.

Or did I miss some­thing? While this would af­fect your ar­gu­ment, my heart is not re­ally in it: Brak­ing against the in­ter­stel­lar medium ap­pears, to me, to cir­cum­vent a lot of prob­lems.

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Be­cause I for­got and you know your pa­per bet­ter than me: Do any im­plicit or ex­plicit as­sump­tions break if we lose ac­cess to most of the fuel mass for shield­ing dur­ing the long voy­age?

If you could an­swer with a con­fi­dent “no, our as­sump­tions do not beak when can­not use the de­cel­er­a­tion fuel as shield­ing”, then we can re­ally trade-off ac­cel­er­a­tion delta-v against de­cel­er­a­tion delta-v, and I stay much more con­vinced about your greater point about the Fermi para­dox.

• 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, you have three choices: Either you want to in­crease rel­a­tive ve­loc­ity and ex­pend en­ergy (air­plane, ram­jet), or you want to de­crease rel­a­tive ve­loc­i­ties (air-brak­ing, use of drag/​fric­tion, so­lar sails when try­ing to go with the so­lar wind, brak­ing against the in­ter­stel­lar medium, etc), or you want an elas­tic col­li­sion, e.g. keep ab­solute rel­a­tive ve­loc­ity the same but re­verse di­rec­tion (grav­i­ta­tional sling­shot).

Sling­shots are cool be­cause you ex­tract en­ergy from the fact that the planets have differ­ent ve­loc­i­ties: Hav­ing mul­ti­ple planets is not ther­mo­dy­namic ground state, so you steal from the po­ten­tial en­ergy /​ nega­tive en­tropy left over from the for­ma­tion of the so­lar sys­tem. Alas, sling­shots can’t bring you too much above es­cape ve­loc­ity, nor slow you down to be­low es­cape if you are sig­nifi­cantly faster.

Edit: prob­a­bly stupid idea, wasn’t think­ing straight <strike> Some­one should tell me whether you can reach rel­a­tivis­tic speeds by sling­shot­ting in a bi­nary or tri­nary of black holes. That would be quite el­e­gant (un­bounded es­cape ve­loc­ity, yay! But you have time di­la­tion when close to the hori­zon, so un­clear whether this takes too long from the view­point of out­side ob­servers; also, too large shear will pull you apart).</​strike>

edit2: You can afaik also push against a curved back­ground space-time, if you have one. Grav­ity waves tech­ni­cally count as vac­uum, but not for the pur­pose of the rocket equa­tion. Doesn’t help, though, be­cause space-time is pretty flat out there, not just Ricci-flat (=vac­uum).

• >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.

• 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?

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 https://​​www.springer.com/​​us/​​book/​​9783642275630:

Deep ( < 100 Jy at 1.4 GHz) ra­dio sur­veys are also re­veal­ing large sur­face den­si­ties of AGNs: the VLA Chan­dra deep field south (CDFS) sur­vey has reached 520 ra­dio-quiet AGNs deg2 [48], al­most ex­actly in be­tween op­ti­cal and X-ray sur­veys. Clas­si­cal ra­dio-loud quasars, be­ing in­trin­si­cally ra­dio pow­er­ful, are ba­si­cally nonex­is­tent be­low 1 mJy.

(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.)

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).

See 1 2 3 4. Note that the last link says it’s 75 times typ­i­cal quasar power.

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.

Do you mean the claim that quasars are 100x brighter than a galaxy? It’s in the quasar Wikipe­dia ar­ti­cle.

Check that it’s easy to make the quasar no­tice­able.

Sim­ply mak­ing it 75 times the bright­ness of a typ­i­cal quasar might be enough, or use color/​spec­trum.

• Do you mean the claim that quasars are 100x brighter than a galaxy? It’s in the quasar Wikipe­dia ar­ti­cle.
Note that the last link says it’s 75 times typ­i­cal quasar power.

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 12 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.)

• Don’t these num­bers not add up? If mass is 1000x lu­minos­ity, and quasars are 100x galaxy, then how is the ra­tio 75x?

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 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:

The most re­cent phase, SDSS-III, be­gan in 2008 and in­cludes the Baryon Os­cilla­tion Spec­tro­scopic Sur­vey (BOSS), a part of SDSS-III aimed at map­ping the cos­mos. Its goal is to map the phys­i­cal lo­ca­tions of all ma­jor galax­ies back to seven billion years ago, and bright quasars back to 12 billion years ago – two billion years af­ter the Big Bang.

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 cov­ers 35% of the sky, and it doesn’t seem like there’s an­other sur­vey that’s com­pa­rable, so I guess there’s still a pretty good chance it wouldn’t have been no­ticed af­ter all.

• 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? • First, are there no nat­u­rally evap­o­rat­ing black holes? 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. Would we be able to tell them apart from other light sources? 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. 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? I guess just more en­ergy but I’m not sure how much more. • No, be­cause small black holes evap­o­rate too quickly and nat­u­ral ones would have dis­ap­peared long ago 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. 100% efficiency 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? • Though I guess you could just have sev­eral black holes, if one isn’t bright enough? 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 100 mil­lion years in a typ­i­cal galaxy; chang­ing the galaxy’s lu­minos­ity by tiling it with Dyson Spheres could be prob­a­bly done much quicker, in a less than 1 mil­lion years. Thus, cre­at­ing ar­tifi­cial quasars as bea­cons make sense only if the differ­ence in 100 mln years is not sub­stan­tial, that is on a few billions years dis­tances. • 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. • 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.” • 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.

• 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?

• 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).

A CDT agent wouldn’t care that we’d have been will­ing to pay a lot to pre­vent it, right?

Un­less its pre­de­ces­sor en­ter­tained the pos­si­bil­ity of be­ing in a simu­la­tion run by a civ­i­liza­tion like ours that made it to tech­nolog­i­cal ma­tu­rity.

The moral ar­gu­ment is not very com­pel­ling to me

This sug­gests a similar dis­agree­ment w.r.t. the ex­pected moral value of un­al­igned AGI, which seems way more in­ter­est­ing and im­por­tant.

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

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.

It seems im­plau­si­ble that we could re­duce our un­cer­tainty about the alien AI be­ing un­friendly to <\1%, at least be­fore we be­come superintelligent

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.

• Only seems to re­quire EDT.

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.

Ac­tu­ally, I’m not sure whether you mean “friendly” in the sense of FAI or the con­ven­tional us­age.

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%.

• 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.

• The first merger event that Ligo de­tected was 1 billion ly away and turned 1 so­lar mass into grav­i­ta­tional waves. at a dis­tance of so en­ergy flux re­ceived is ap­prox The main peak power out­put from the merg­ing black holes lasted around one sec­ond. A full moon illu­mi­nates earth with around . 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?

• 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

• 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.

• 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.

• 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?

• 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.

• 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 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.

• mod­u­late the emis­sions from their galaxy in a way that can­not be due to a nat­u­ral state or nat­u­ral pro­cess.

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.)

• 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.

• 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.)

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.

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.”

• 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.

• 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.

• 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.

• 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!

• 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.

• 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.

• 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 years () then flux is

, 100X a full moons illu­mi­na­tion.

If they choose to send a 1 sec­ond pulse of en­ergy ev­ery 30 years, That is time ra­tio so the flux for that sec­ond can be , 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.

• 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.

• 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.

• 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.)