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 (fr < 100 μJy 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 1234. 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 1⁄2 credit if unrebutted.
For 1/1000, you have about the same amount of power as a galaxy, and you could only make a very dim quasar, so it seems like you’d need a different line of analysis. (E.g. that we’d notice something as bright as a galaxy with a weird spectrum.)
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?
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.
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).
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.
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.
I’m fine with them converting {1/1000, 1, 1000}x of a galaxy’s matter into energy.
Main question is: do we see all the quasars at that distance, or do we see only a small fraction of them? Is whether we see them a simple function of power, in which case what is the cutoff?
If we see all of them, seems like it would answer the 1x and 1000x questions. Smaller questions:
Checking the mass vs. energy calculation (for the average over the average galaxy—if anything in the galaxy emits faster, then that would dominate and you won’t get the 1000x ratio).
Checking the 1000x brighter claim, probably just with a citation. But it’s a bit tricky since it’s mostly about which quasars we see.
Check that it’s easy to make the quasar noticeable.
I think yes, but it’s a little hard to find a source that says this clearly. Basically modern surveys are now trying to survey high redshift quasars which are all the way across the universe rather than half way across the universe. Also if the aliens used their power to simulate a radio-loud quasar that should be even easier to see. From page 539 of https://www.springer.com/us/book/9783642275630:
(My interpretation here is that all classical radio-loud quasars are above 1 mJy which is easily above detection limits of less than 100 μJy.)
See 1 2 3 4. Note that the last link says it’s 75 times typical quasar power.
Do you mean the claim that quasars are 100x brighter than a galaxy? It’s in the quasar Wikipedia article.
Simply making it 75 times the brightness of a typical quasar might be enough, or use color/spectrum.
Don’t these numbers not add up? If mass is 1000x luminosity, and quasars are 100x galaxy, then how is the ratio 75x? Seems like a random order of magnitude missing.
I tentatively think this resolves the 1 and 1000x questions, but leaves open the 1/1000 question. Will leave this up for rebuttal for a week before concluding that. By default it probably gets 1⁄2 credit if unrebutted.
For 1/1000, you have about the same amount of power as a galaxy, and you could only make a very dim quasar, so it seems like you’d need a different line of analysis. (E.g. that we’d notice something as bright as a galaxy with a weird spectrum.)
The ratio for the sun is actually 1480 to be exact, plus the rest of the galaxy is apparently dimmer per unit mass than the sun is.
For 1/1000x, I think if you put most of the energy into the radio spectrum, perhaps a single frequency or a few frequencies that you predict others will survey for, it should be easily noticeable. I’ll look for details when I get home, unless someone beats me to it.
If you put 1/1000 the mass of a galaxy into radio signals over 10 GHz bandwidth over 10 billion years, you get 2.7e28 W/Hz power spectral density. According to this paper table 2, at redshift z=2.083 (about 10 billion light years away) a radio source of 10^25.78 W/Hz was detected on Earth at a flux density of 3.54 mJy so 2.7e28 W/Hz should translate to 1580 mJy on Earth. According to this paper, NVSS has cataloged all objects of flux density >2.5 mJy over 82% of the sky so it likely has detected and cataloged the alien beacon. Unfortunately according to section 2.1.1 of this paper, “However, the large beam size does not allow one to determine precise structure of sources or to determine positions accurate enough to establish optical counterparts.” so we may not have noticed it as an anomalous object.
Back to the visible spectrum, according to this article:
So if the alien beacon is brighter than a major galaxy (not sure what that means exactly) and within 7 billion LY, then it would have been cataloged, and SDSS captures images at 5 color bands so it would be easy to use color to stand out. (SDSS runs a bunch of algorithmic filters to try to classify each light source based on color, and if none of the filters fit, the source is classified as OTHER and a human looks at it.) 1/1000 the mass of Milky Way over 10 billion years translates to 54 times the luminosity of Milky Way so it should have been noticed by SDSS. But SDSS only 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?
No, because small black holes evaporate too quickly and natural ones would have disappeared long ago, and large black holes evaporate too slowly to be used as an energy source (well technically you can use their accretion discs for matter-energy conversion at 10% efficiency, which is essentially what quasars are, but that’s not as good as using the evaporation of small black holes for 100% efficiency). The aliens would have to constantly form small black holes and let them evaporate.
They would give the beacon a distinct/unnatural color/spectrum. EDIT: For example astronomers have been looking for quasars with especially high redshifts by searching the survey photographs for light in a certain color range, and then doing spectrography on the candidates for more detailed investigations. If the aliens can predict the color filter being used, they can give their beacon that color and then an unnatural spectrum would alert the astronomers. Or the aliens can give the beacon a totally anomalous color like pure blue, which would probably trigger some kind of anomaly detector in the astronomical surveys.
I guess just more energy but I’m not sure how much more.
Are you implying that small black holes have ever formed naturally at all? If there is some process that formed random size black holes long time ago, the small ones might have already evaporated, but the medium ones might be just finishing their evaporation right now. Of course, such a process might not have occurred, ever.
Efficiency isn’t quite the right metric here. I think we need “power”? So, how much power does the small black hole produce? It’s my naive understanding that this power only depends on the radius of the hole, not on how much matter you’re throwing into it. Though I guess you could just have several black holes, if one isn’t bright enough?
See https://en.wikipedia.org/wiki/Primordial_black_hole.
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.