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But that’s the thing, if our galaxy had lots of 20-21st century Earth level civilizations, we would be able to detect them with our existing SETI programmes. P

by davedx 9y ago

But that’s the thing, if our galaxy had lots of 20-21st century Earth level civilizations, we would be able to detect them with our existing SETI programmes. Personally I like the hypothesis that states eventually all comms will be encrypted and therefore indistinguishable from white noise.

teekert 9y ago
Or simply using tech we haven't discovered yet (i.e. some faster than light method).
joe_the_user 9y ago
We could not detect civilizations at our level because standard transmissions decay the square of the distance and so such would be quantum noise by the time they got to the nearest start. And less than that by the time they further.
- davedx 9y ago
  According to the SETI FAQ, it would be possible for alien civilizations to detect our higher powered radar systems with SETI setups similar to our own. Do you dispute this claim from SETI? Can you provide a citation?
  - quickben 9y ago
    He was approaching this from physics and whatever that inverse distance power propagination law is called for 3d space.
- quickben 9y ago
  I am not at a PC right now, but I can see your point. To counter, Seti should catch anything really high powered. Whether or not a civilization can operate something at that level of power, is a thought experiment. But, still, they sift the skies, at least we are trying right ? :D
- WillReplyfFood 9y ago
  You can shine a laser at a place where you expect a planet.
ajuc 9y ago
There's around 512 G-type stars in 100 light years radius around us. That's not many. Milky Way radius is 100 000 light years. If our galaxy had lots of civilizations that just learned radio in last 100 years - odds are quite high that we wouldn't be able to detect them YET, even if we had the technology to do so.
- davedx 9y ago
  I understand the probabilities involved. The OP made a rather extreme claim that 'the universe could full of earth-level societies and we wouldn't know'. If all 512 G-type stars in 100 light years had earth-level societies, we would be able to detect their higher powered radio transmissions. Right?
  - KineticLensman 9y ago
    Only those transmissions that were timed (travelling at the speed of light) to arrive in the small number of decades where we have been listening. Other constraints may also apply
  - joe_the_user 9y ago
    Even Seven light years is a vast, vast distance. The limit isn't 100 light years, it's something like one or less light years. Any ordinary radio transmission dissipates the square of the distance involved. At a certain level, transmissions become so faint they are indistinguishable from quantum noise. You can find the math done on Quora (apologies for semi-walled-garden link). Remember, there's difference between getting a signal whose properties one knows and getting arbitrary signal - if there a strong GPS transmitter or something we knew, we could wait long enough and get micro-data. But arbitrary signal, no, information theory says no. (read all the answers together). https://www.quora.com/How-far-can-radio-transmissions-from-Earth-travel-toward-other-stars-before-they-are-so-weak-as-to-be-practically-undetectable https://www.quora.com/How-far-can-radio-transmissions-from-E...
Chathamization 9y ago
> But that’s the thing, if our galaxy had lots of 20-21st century Earth level civilizations, we would be able to detect them with our existing SETI programmes. But we couldn't. In general, SETI couldn't even detect such a civilization even if it was in the star system closest to earth: > If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth? > In general, no. Most earthly transmissions are too weak to be found by equipment similar to ours at the distance of even the nearest star. But there are some important exceptions. High-powered radars and the Arecibo broadcast of 1974 (which lasted for only three minutes) could be detected at distances of tens to hundreds of light-years with a setup similar to our best SETI experiments. The OP is right, in order for their to be a paradox we have to assume that civilizations much more advanced than ours are using a particular type of tech (the kind we're looking for, like radio broadcasts), that their using much stronger signals that we have ever used, and that the time period they used these hypothetical super powerful radio signals happens to be at the right time for us to encounter them. IE, let's say they used them for 2,000 years before moving on to other tech, but the signals only reached earth between 1,548,000-1,546,000 BC, or 450,000-458,000 BC, etc. People are making a set of very, very specific assumptions based on little evidence. Then they see that the evidence that is available suggests that it's unlikely that every single one of their assumptions is correct, and they respond by calling it a paradox. [1] https://www.seti.org/faq https://www.seti.org/faq
- davedx 9y ago
  OP said: 'the universe could full of earth-level societies and we wouldn't know'. It's a very extreme claim and the SETI FAQ answer directly contradicts it.
  - mannykannot 9y ago
    The SETI FAQ says that in the right circumstances, we might detect some of the closest ones, so the OP's statement is not contradicted.
- pavel_lishin 9y ago
  > But we couldn't. In general, SETI couldn't even detect such a civilization even if it was in the star system closest to earth. Your quote relies on us detecting re-runs of "I Love Lzcwwdrrr", but our telescopes can reliably do spectrographic analysis of faraway stars and planets these days. We'd likely be able to detect things like unnaturally high levels of hydrocarbons in otherwise oxygen-full atmospheres, and other industrial by-products. If Alpha Centauri had a planet with an industrial civilization, we might be able to spot city lights.

by davedx 9y ago

teekert 9y ago

Or simply using tech we haven't discovered yet (i.e. some faster than light method).

joe_the_user 9y ago

We could not detect civilizations at our level because standard transmissions decay the square of the distance and so such would be quantum noise by the time they got to the nearest start. And less than that by the time they further.

davedx 9y ago
According to the SETI FAQ, it would be possible for alien civilizations to detect our higher powered radar systems with SETI setups similar to our own. Do you dispute this claim from SETI? Can you provide a citation?
- quickben 9y ago
  He was approaching this from physics and whatever that inverse distance power propagination law is called for 3d space.
quickben 9y ago
I am not at a PC right now, but I can see your point. To counter, Seti should catch anything really high powered. Whether or not a civilization can operate something at that level of power, is a thought experiment. But, still, they sift the skies, at least we are trying right ? :D
WillReplyfFood 9y ago
You can shine a laser at a place where you expect a planet.

ajuc 9y ago

There's around 512 G-type stars in 100 light years radius around us. That's not many. Milky Way radius is 100 000 light years. If our galaxy had lots of civilizations that just learned radio in last 100 years - odds are quite high that we wouldn't be able to detect them YET, even if we had the technology to do so.

davedx 9y ago
I understand the probabilities involved. The OP made a rather extreme claim that 'the universe could full of earth-level societies and we wouldn't know'. If all 512 G-type stars in 100 light years had earth-level societies, we would be able to detect their higher powered radio transmissions. Right?
- KineticLensman 9y ago
  Only those transmissions that were timed (travelling at the speed of light) to arrive in the small number of decades where we have been listening. Other constraints may also apply
- joe_the_user 9y ago
  Even Seven light years is a vast, vast distance. The limit isn't 100 light years, it's something like one or less light years. Any ordinary radio transmission dissipates the square of the distance involved. At a certain level, transmissions become so faint they are indistinguishable from quantum noise. You can find the math done on Quora (apologies for semi-walled-garden link). Remember, there's difference between getting a signal whose properties one knows and getting arbitrary signal - if there a strong GPS transmitter or something we knew, we could wait long enough and get micro-data. But arbitrary signal, no, information theory says no. (read all the answers together). https://www.quora.com/How-far-can-radio-transmissions-from-Earth-travel-toward-other-stars-before-they-are-so-weak-as-to-be-practically-undetectable https://www.quora.com/How-far-can-radio-transmissions-from-E...

Chathamization 9y ago

> But that’s the thing, if our galaxy had lots of 20-21st century Earth level civilizations, we would be able to detect them with our existing SETI programmes. But we couldn't. In general, SETI couldn't even detect such a civilization even if it was in the star system closest to earth: > If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth? > In general, no. Most earthly transmissions are too weak to be found by equipment similar to ours at the distance of even the nearest star. But there are some important exceptions. High-powered radars and the Arecibo broadcast of 1974 (which lasted for only three minutes) could be detected at distances of tens to hundreds of light-years with a setup similar to our best SETI experiments. The OP is right, in order for their to be a paradox we have to assume that civilizations much more advanced than ours are using a particular type of tech (the kind we're looking for, like radio broadcasts), that their using much stronger signals that we have ever used, and that the time period they used these hypothetical super powerful radio signals happens to be at the right time for us to encounter them. IE, let's say they used them for 2,000 years before moving on to other tech, but the signals only reached earth between 1,548,000-1,546,000 BC, or 450,000-458,000 BC, etc. People are making a set of very, very specific assumptions based on little evidence. Then they see that the evidence that is available suggests that it's unlikely that every single one of their assumptions is correct, and they respond by calling it a paradox. [1] https://www.seti.org/faq https://www.seti.org/faq

davedx 9y ago
OP said: 'the universe could full of earth-level societies and we wouldn't know'. It's a very extreme claim and the SETI FAQ answer directly contradicts it.
- mannykannot 9y ago
  The SETI FAQ says that in the right circumstances, we might detect some of the closest ones, so the OP's statement is not contradicted.
pavel_lishin 9y ago
> But we couldn't. In general, SETI couldn't even detect such a civilization even if it was in the star system closest to earth. Your quote relies on us detecting re-runs of "I Love Lzcwwdrrr", but our telescopes can reliably do spectrographic analysis of faraway stars and planets these days. We'd likely be able to detect things like unnaturally high levels of hydrocarbons in otherwise oxygen-full atmospheres, and other industrial by-products. If Alpha Centauri had a planet with an industrial civilization, we might be able to spot city lights.