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.
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.