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An alien civilization able to get spectra from Earth's atmosphere would detect the presence of lots of biosignatures including but not limited to chlorophyl, at

An alien civilization able to get spectra from Earth's atmosphere would detect the presence of lots of biosignatures including but not limited to chlorophyl, atmospheric oxygen, and CO2. These are either very reactive or break down readily from UV radiation if not replenished constantly. So there's no hiding life on Earth for any civilization with advanced enough telescopes. Our industrial development is likewise going to be pretty obvious.

kadoban 4y ago
CO2 is pretty stable and unreactive, isn't it? Doesn't change your overall point of course, just curious if I'm wrong about that.
- lazide 4y ago
  It's getting constantly broken down by plants. Also, concentration has been changing pretty rapidly. Those don't say 'dead' or 'stable'.
- giantrobot 4y ago
  You're right that CO2 is not very reactive. But the fact Earth has a lot of O2 says there's some process replenishing it as it's very reactive. The amounts of CO2 changing in proportion to the amount of O2 would point to something processing the CO2 into O2. As long as our theoretical aliens are aware of photosynthesis, which can use a number of pigment chemicals, they would likely settle on photosynthesis. The longer they watch Earth they'll see seasonal variations in CO2/O2 levels.
echelon 4y ago
If there's life / alien AI in the neighborhood, they already know about us. That's relatively frightening.
- giantrobot 4y ago
  I don't think there's any reason for that fact to be frightening. Interstellar shipping rates are astronomical (pun shamelessly intended). Even sending a teeny tiny package to the next solar system over costs a lot and takes a long time. The fastest it's ever going to go is the speed of light. That fact we haven't been obliterated by an alien invasion fleet tells us that there's probably not a hyper advanced alien species that is big mad about us within 100ly. Thats the threshold where a hyper advanced alien civilization could have seen the Industrial Revolution starting, deduced there's a technological civilization here, developed a big mad about the fact, and launched a destruct-o fleet or super weapon at us with hyper advanced speed of light propulsion that is so advanced the trip is practical in terms of expense (practicality factor). There's a "probability of mean hyper advanced aliens" value that's like the Lindy Effect. The longer we go without being obliterated by an alien space fleet the further away we can assume mean hyper advanced aliens might live. For mean aliens that are less advanced the less we need to worry about them because the practicality factor of their propulsion system is lower. An alien invasion fleet sent here at at Parker Solar Probe velocities (0.064c at closest approach to the Sun) are less worrying because the likelihood of them arriving at a point where we personally need to worry about them is pretty low. For instance our practicality factor for building an interstellar invasion fleet is basically zero. Even building a von Neumann probe has a so close to zero it might as well be zero practicality factor. I'd posit that there's a minimum practicality factor a civilization needs to go around destroying other ones. If your best propulsion is below that threshold you're just not going to ever build an invasion fleet. You might build interstellar probes but an invasion fleet is impractical. So I'm not worried about the hyper advanced aliens watching me shower. And I'm also not worried about the less advanced ones either. Space is really big and traveling between stars is really hard. If my existence made the hyper advanced ones so upset they'd come blow me up there's not a lot I could do about it anyways.
  - echelon 4y ago
    > Even building a von Neumann probe has a so close to zero Why do you say this? Could you expand on it? It seems to me like the cost of these sentient, self-replicating factories would be marginal. I'm worried about them sending a von Neumann probe that constructs a series of asteroid deflection ships that would put impactors on a direct collision course with us. Or some smart, self-replicating nanobot that can out compete our biological systems for energy while being metabolically undigestable. Or some kind of energy weapon.

kadoban 4y ago

CO2 is pretty stable and unreactive, isn't it? Doesn't change your overall point of course, just curious if I'm wrong about that.

lazide 4y ago
It's getting constantly broken down by plants. Also, concentration has been changing pretty rapidly. Those don't say 'dead' or 'stable'.
giantrobot 4y ago
You're right that CO2 is not very reactive. But the fact Earth has a lot of O2 says there's some process replenishing it as it's very reactive. The amounts of CO2 changing in proportion to the amount of O2 would point to something processing the CO2 into O2. As long as our theoretical aliens are aware of photosynthesis, which can use a number of pigment chemicals, they would likely settle on photosynthesis. The longer they watch Earth they'll see seasonal variations in CO2/O2 levels.

echelon 4y ago

If there's life / alien AI in the neighborhood, they already know about us. That's relatively frightening.

giantrobot 4y ago
I don't think there's any reason for that fact to be frightening. Interstellar shipping rates are astronomical (pun shamelessly intended). Even sending a teeny tiny package to the next solar system over costs a lot and takes a long time. The fastest it's ever going to go is the speed of light. That fact we haven't been obliterated by an alien invasion fleet tells us that there's probably not a hyper advanced alien species that is big mad about us within 100ly. Thats the threshold where a hyper advanced alien civilization could have seen the Industrial Revolution starting, deduced there's a technological civilization here, developed a big mad about the fact, and launched a destruct-o fleet or super weapon at us with hyper advanced speed of light propulsion that is so advanced the trip is practical in terms of expense (practicality factor). There's a "probability of mean hyper advanced aliens" value that's like the Lindy Effect. The longer we go without being obliterated by an alien space fleet the further away we can assume mean hyper advanced aliens might live. For mean aliens that are less advanced the less we need to worry about them because the practicality factor of their propulsion system is lower. An alien invasion fleet sent here at at Parker Solar Probe velocities (0.064c at closest approach to the Sun) are less worrying because the likelihood of them arriving at a point where we personally need to worry about them is pretty low. For instance our practicality factor for building an interstellar invasion fleet is basically zero. Even building a von Neumann probe has a so close to zero it might as well be zero practicality factor. I'd posit that there's a minimum practicality factor a civilization needs to go around destroying other ones. If your best propulsion is below that threshold you're just not going to ever build an invasion fleet. You might build interstellar probes but an invasion fleet is impractical. So I'm not worried about the hyper advanced aliens watching me shower. And I'm also not worried about the less advanced ones either. Space is really big and traveling between stars is really hard. If my existence made the hyper advanced ones so upset they'd come blow me up there's not a lot I could do about it anyways.
- echelon 4y ago
  > Even building a von Neumann probe has a so close to zero Why do you say this? Could you expand on it? It seems to me like the cost of these sentient, self-replicating factories would be marginal. I'm worried about them sending a von Neumann probe that constructs a series of asteroid deflection ships that would put impactors on a direct collision course with us. Or some smart, self-replicating nanobot that can out compete our biological systems for energy while being metabolically undigestable. Or some kind of energy weapon.