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Thread: Fermi Paradox solved?

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    Fermi Paradox solved?

    From the arXiv blog:

    “Assuming the average communicating civilization has a lifetime of 1,000 years, ten times longer than Earth has been broadcasting, and has a signal horizon of 1,000 light-years, you need a minimum of over 300 communicating civilization in the galactic neighborhood to reach a minimum density.”
    My objections to this proposed 'solution':

    a) What is the rationale behind a civilization lifetime of only 1,000 years [Civilization on Earth has been around for more than 4,000 years]. They´re possibly assuming that a technological civilization will somehow kill itself. But that argument is only a conjecture.

    b) Why would a tech civilization be unable to send a detectable signal to over 1,000 light-years? If they are deliberatley trying to establish contact then they could/would make it sure that the signal would be strong enough to be detectable at greater distances.

    c) Why they would limit themselves to EM radiation?
    Last edited by Argos; 2009-Feb-02 at 10:39 PM. Reason: Correct linking

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    Quote Originally Posted by Argos View Post
    a) What is the rationale behind a civilization lifetime of only 1,000 years [Civilization on Earth has been around for more than 4,000 years].
    I assume that the 1000 years is how long that civilization has enough civilization to broadcast a signal ("average communicating civilization") . Sure, there was a civilization on Earth 4000 years ago, but they didn't broadcast anything. Our civilization has been a comunicating civilization for about 90 years (invention of radio).

    By the way, the link doesn't work for me.
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    Quote Originally Posted by Swift View Post
    By the way, the link doesn't work for me.
    Here's the link:

    http://arxivblog.com/?p=1167

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." — Abraham Lincoln

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    Quote Originally Posted by Swift View Post
    By the way, the link doesn't work for me.
    It should work now.

    Well, I´m under the impression it means the span a communicating civilization lasts.

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    My big beef with this "solution" is one that's included in the comments on that page--this "solution" doesn't take into consideration the possibility of space travel. All it takes is for one civilization to figure it out and it's not long before they can expand across the galaxy (not long compared to the age of the galaxy).

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    Quote Originally Posted by Argos View Post
    From the arXiv blog:



    My objections to this proposed 'solution':

    a) What is the rationale behind a civilization lifetime of only 1,000 years [Civilization on Earth has been around for more than 4,000 years]. They´re possibly assuming that a technological civilization will somehow kill itself. But that argument is only a conjecture.

    b) Why would a tech civilization be unable to send a detectable signal to over 1,000 light-years? If they are deliberatley trying to establish contact then they could/would make it sure that the signal would be strong enough to be detectable at greater distances.

    c) Why they would limit themselves to EM radiation?
    I'm not sure what else they would use.

    Your questions should really be addressed to the author. It all depends on the assumptions about the behavior of "communicating civilizations". The time spent attempting to communicate could be 10 years or 10,000,000,000. Similarly the energy they devote to their communications could vary enormously. I agree that communication could potentially occur over much longer distances than 1000 ly using, for example, high power coherent beams. Without physical travel a limiting factor would be their ability to observe suitable targets for communication. Another limitation is interstellar extinction due to dust and gas clouds that could prevent communication between two civs closer than 1000 ly.

    The post doesn't solve the Fermi "Paradox" but it doesn't need to be solved. It's a variation on the natural conclusion that the number of communicating civilizations is small.

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    I think this is the key point of the solution:
    That overlooks one small factor, says Reginald Smith from the Bouchet-Franklin Institute in Rochester, New York state. He says that there is a limit to how far a signal from ET can travel before it becomes too faint to hear. And when you factor that in, everything changes.
    I don't know enough about the details to say if this works, but it does seem like a good point.
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    Quote Originally Posted by Swift View Post
    I think this is the key point of the solution:
    Yeah, but what could make a signal attenuate so drastically in a mere 1000-ly? That´s my question, and it is not well explained. Not all directions are polluted with gas clouds and such.

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    Quote Originally Posted by Argos View Post
    Yeah, but what could make a signal attenuate so drastically in a mere 1000-ly?
    The inverse squared law? :surprised

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    Well, my point is that the inverse square law is not a problem for a 1,000 ly distance.

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    The inverse square law doesn't apply to collimated (focused) beams, even collimated beams of radio waves. The emissions which our civilisation sends out which are most detectable at a distance are collimated beams; ether deliberately sent METI messages, or radar scans of asteroids and planets. There have been plenty of both, but the beams have only covered a fraction of the sky.

    So unless a civilisation lies directly in the path of one of these beams, they'll not get to know that we are here (if they rely on radio).

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    Quote Originally Posted by Argos View Post
    Well, my point is that the inverse square law is not a problem for a 1,000 ly distance.
    Think about it this way: a standard Mexican AM radio station will broadcast at 105 Watts. A light-year is 1016 m. 1,000 of them is 1019 m. The area of a sphere is 4 * π * r2. Thus the area of a 1,000 ly sphere is 1039 m2. Divide the power by the area, and you have an energy flux of 10-34 W m-2 (that's ~10-15 eV m-2). I'm not saying it's impossible to detect, but that it would take extraordinarily expensive efforts to build an antenna capable of making out such tiny signals.

    Thus, as eburacum45 says, the only hope is focused beams. But why would a civilization 1,000 ly from here send a focused beam right at us continuously for thousands of years?

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    Maybe because they have telescope technology a little bit better than ours and have detected Earth and Earth's oxygen atmosphere.

    If we make a few assumptions about alien technology and thinking, this means that Earth is transiting--so the aliens are along the plane of Earth's orbit.

    And if we look at it from our direction, the place where most potential alien stars would be is along the galactic plane. That means we're going to be looking along the galactic plane. The aliens know this, so let's imagine they happen to be exactly along the intersection of Earth's orbital plane and the galactic plane?

    Well, that could be a compelling reason for the aliens to send a focused beam right at us continuously for hundreds of thousands of years. They know that if anyone is going to be looking for the signal, it'll be us.

    That assumes we humans are intelligent enough to figure this out, of course, rather than randomly looking for signals in every which direction.

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    Quote Originally Posted by Warren Platts View Post
    Thus, as eburacum45 says, the only hope is focused beams. But why would a civilization 1,000 ly from here send a focused beam right at us continuously for thousands of years?
    because they're a communicating civilization

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    Quote Originally Posted by IsaacKuo View Post
    Maybe because they have telescope technology a little bit better than ours and have detected Earth and Earth's oxygen atmosphere.

    If we make a few assumptions about alien technology and thinking, this means that Earth is transiting--so the aliens are along the plane of Earth's orbit.

    And if we look at it from our direction, the place where most potential alien stars would be is along the galactic plane. That means we're going to be looking along the galactic plane. The aliens know this, so let's imagine they happen to be exactly along the intersection of Earth's orbital plane and the galactic plane?
    That's an intriguing idea and worth a try, I would guess. However, correct me if I'm wrong, but I think that our own attempts to detect oxygen atmospheres elsewhere don't depend the transiting technique. In which case, only a tiny minority of observed oxygen atmospheres would be also be observed transiting their star.

    Also, there's little hope for detecting such small planets much further than 1,000 ly away.

    Quote Originally Posted by timb
    because they're a communicating civilization
    Even if they wanted to be a communicating civilization (do we really want to be a communicating civilization? Maybe so, but the answer is not obviously yes), think about what's involved. How many teraWatts can a civilization reasonably devote to communicate with someone who may not even be out there? Laser links are a great idea, but both parties have to know about each other first.

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    Quote Originally Posted by Warren Platts View Post
    That's an intriguing idea and worth a try, I would guess. However, correct me if I'm wrong, but I think that our own attempts to detect oxygen atmospheres elsewhere don't depend the transiting technique. In which case, only a tiny minority of observed oxygen atmospheres would be also be observed transiting their star.

    Also, there's little hope for detecting such small planets much further than 1,000 ly away.


    Even if they wanted to be a communicating civilization (do we really want to be a communicating civilization? Maybe so, but the answer is not obviously yes), think about what's involved. How many teraWatts can a civilization reasonably devote to communicate with someone who may not even be out there? Laser links are a great idea, but both parties have to know about each other first.
    Me no, we, probably. You're making my point. With foreseeable technology we can detect terrestrial planets within a 100 pc or so that are likely to harbor life (eg oxygen signature), and then send them signals. You don't have to beam continuously, just regular short pulses that are obviously not natural and listen for a response (after a suitable delay). Deliberate collimated signals are going to be many orders of magnitude stronger at the receiver than are random RF broadcast leaks so the maximum communication distance is limited by the sender's ability to detect suitable targets, if deliberate communication is common.

    OTOH deliberate communication is limited by the returns expected by the party that sends the signal. If we discovered an Earth twin 25 ly away with a vegetation red edge in its reflection spectrum we'd probably be willing to invest quite a lot in sending signals and listening for answers. If it were a 500 ly away? what would be the point? we'd never hear the reply, nor would our children, nor our children's children, nor.... It's a bit difficult to sell. It's easy to imagine similar considerations would discourage aliens from sending deliberate signals at targets more than a few tens of light years away.

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    Quote Originally Posted by Warren Platts View Post
    I'm not saying it's impossible to detect, but that it would take extraordinarily expensive efforts to build an antenna capable of making out such tiny signals.
    It doesn´t have to be collimated beams. Not sure about you, but I am talking of a superior civilization employing a powerful signal in a deliberate transmission. I´m not talking about standard Mexican AM. The receiving problem is ours, not theirs.

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    Quote Originally Posted by Warren Platts View Post
    That's an intriguing idea and worth a try, I would guess. However, correct me if I'm wrong, but I think that our own attempts to detect oxygen atmospheres elsewhere don't depend the transiting technique. In which case, only a tiny minority of observed oxygen atmospheres would be also be observed transiting their star.

    Also, there's little hope for detecting such small planets much further than 1,000 ly away.
    If it's transiting, then the dip in the star's light could be detectable as long as the star itself were resolvable (you may need a large space telescope for the required signal/noise ratio). That gives the planet's size and mass, indicating it's a terrestrial in the habitable zone.

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    Quote Originally Posted by IsaacKuo View Post
    If it's transiting, then the dip in the star's light could be detectable as long as the star itself were resolvable (you may need a large space telescope for the required signal/noise ratio). That gives the planet's size and mass, indicating it's a terrestrial in the habitable zone.
    OK, I see what you're saying. We can get the mass of a planet from radial doppler shifts, but not the diameter of the planet. But still the Holy Grail is going to be an oxygen signature: it's pretty hard to imagine a planet of any size with an oxygen atmosphere without life. Once you have the oxygen signature in hand, you know you're looking at a habitable planet, at which point the diameter of the planet is of secondary interest.

    Quote Originally Posted by Argos
    It doesn´t have to be collimated beams. Not sure about you, but I am talking of a superior civilization employing a powerful signal in a deliberate transmission. I´m not talking about standard Mexican AM. The receiving problem is ours, not theirs.
    Still, even if you want an isotropic radio station beaming to the stars, how many Watts will you need? For an alien at 1000 ly to be able to pick up a 1 Jy strength signal from Earth (1 Jy = 10-26 W m-2 Hz-1 -- since we're sending a narrow band broadcast, the Hz=1), that's 10-26 W m-2 x 1039 = 1013 W = 10 TW, which is comparable to the total power consumed by the USA. To reach 1,000,000 ly with a 1 Jy flux, it would take 10 million TW (that's 1019 W or 10 exaWatts). In other words, in order to broadcast much farther than 1,000 ly, it would take a Type II civilization to be able to afford such an extravagance. At the rate we're going, I find it hard to believe that any civilization can reach Type II status. . . .

    Quote Originally Posted by timb
    Deliberate collimated signals are going to be many orders of magnitude stronger at the receiver than are random RF broadcast leaks so the maximum communication distance is limited by the sender's ability to detect suitable targets, if deliberate communication is common.
    Exactly, and the only way to find targets is to identify oxygenated planets, as Mr. Kuo suggests. That's going to be real difficult to do for distances >> 1,000 ly.
    Quote Originally Posted by timb
    OTOH deliberate communication is limited by the returns expected by the party that sends the signal. If we discovered an Earth twin 25 ly away with a vegetation red edge in its reflection spectrum we'd probably be willing to invest quite a lot in sending signals and listening for answers. If it were a 500 ly away? what would be the point? we'd never hear the reply, nor would our children, nor our children's children, nor.... It's a bit difficult to sell.
    Another good point. It just goes to show that Reggie Smith's article makes good sense. His main point is that it will be difficult for space civilizations separated from each other by more than 1,000 ly to find each other even if they want to--thus allowing us to set a reasonable upper bound to the density of alien civilizations. If anything, his 1,000 ly detection radius is way overly generous.

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    Quote Originally Posted by Warren Platts View Post
    OK, I see what you're saying. We can get the mass of a planet from radial doppler shifts, but not the diameter of the planet. But still the Holy Grail is going to be an oxygen signature: it's pretty hard to imagine a planet of any size with an oxygen atmosphere without life. Once you have the oxygen signature in hand, you know you're looking at a habitable planet, at which point the diameter of the planet is of secondary interest.
    How do you know which planets to test for the oxygen signature?
    Exactly, and the only way to find targets is to identify oxygenated planets, as Mr. Kuo suggests. That's going to be real difficult to do for distances >> 1,000 ly.
    You can find candidate terrestrial planets in the habitable zone millions of light years away using a transit method. At that point, you can look closer using a large space telescope over a long period of time to collect enough photons to look for an oxygen atmosphere.

    And as always, the entire argument is silly because it presumes no space travel.

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    Quote Originally Posted by timb View Post
    If we discovered an Earth twin 25 ly away with a vegetation red edge in its reflection spectrum we'd probably be willing to invest quite a lot in sending signals and listening for answers. If it were a 500 ly away? what would be the point? we'd never hear the reply, nor would our children, nor our children's children, nor.... It's a bit difficult to sell.
    I actually see it the other way. We sent the Arecibo message to a destination some 25,000 light years away partly BECAUSE it's so far away. If the aliens hear our message and decide to attack us because of it, we have some 50,000 years to improve technology and colonize star systems to prepare for it.

    If we discovered an Earth twin 25ly away, I think we'd stay silent and got a hurry on developing space colonization and space warships. We wouldn't want to draw attention to ourselves before we were prepared for the consequences.

    But 500ly away? I could see someone sending a message and then telling the world--"People of Earth--the aliens have been alerted to our presence. We have one millenium to prepare for their possibly genocidal response." That would put a kick start into space technology development!

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    Quote Originally Posted by Warren Platts View Post
    Still, even if you want an isotropic radio station beaming to the stars, how many Watts will you need? For an alien at 1000 ly to be able to pick up a 1 Jy strength signal from Earth (1 Jy = 10-26 W m-2 Hz-1 -- since we're sending a narrow band broadcast, the Hz=1), that's 10-26 W m-2 x 1039 = 1013 W = 10 TW, which is comparable to the total power consumed by the USA. To reach 1,000,000 ly with a 1 Jy flux, it would take 10 million TW (that's 1019 W or 10 exaWatts). In other words, in order to broadcast much farther than 1,000 ly, it would take a Type II civilization to be able to afford such an extravagance. At the rate we're going, I find it hard to believe that any civilization can reach Type II status. . . .
    This SETI spreadsheet tells me that a 1 GW hydrogen-line signal [10HZ bandwidth] can be detected at almost 1,000 ly with a 100-meter dish at both ends. Does not seem unachievable by a type II civilization, the kind I expect will be transmitting or receiving [since the probability that we´re at the same technological level is negligible]. In fact it is doable by Earthlings in the present time.

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    Quote Originally Posted by Warren Platts View Post
    But still the Holy Grail is going to be an oxygen signature: it's pretty hard to imagine a planet of any size with an oxygen atmosphere without life.
    You want to look for ozone, actually; oxygen might occur frequently on water-covered worlds, because water vapour can be converted to oxygen by sunlight (starlight). But this process destroys ozone, according to A Leger here
    http://arxiv.org/ftp/astro-ph/papers/0308/0308324.pdf
    so you want to look for oxygen and ozone together, which can probably only happen on a life-bearing world.

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    Quote Originally Posted by IsaacKuo View Post
    I actually see it the other way. We sent the Arecibo message to a destination some 25,000 light years away partly BECAUSE it's so far away. If the aliens hear our message and decide to attack us because of it, we have some 50,000 years to improve technology and colonize star systems to prepare for it.

    If we discovered an Earth twin 25ly away, I think we'd stay silent and got a hurry on developing space colonization and space warships. We wouldn't want to draw attention to ourselves before we were prepared for the consequences.

    But 500ly away? I could see someone sending a message and then telling the world--"People of Earth--the aliens have been alerted to our presence. We have one millenium to prepare for their possibly genocidal response." That would put a kick start into space technology development!
    I wasn't trying to predict what you would do. I was suggesting constraints on the behaviour of rational entities. If you think the likely response to signals is a genocidal one, why would you send signals at all? If that reasoning is common then interstellar communication is even less likely than KFC's blog post says.

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    Since we humans only have access to a data point of ONE technological species, we may as well use that data point. Historically, we did, in fact, send the Arecibo message. It was NOT sent to a destination a mere 25 or 500 light years away. It WAS sent to a destination 25,000 light years away, and part of the reason was fear of a hostile alien response.

    Was this a "rational" thing to do? Maybe, maybe not. But it's something that actually happened, which counts for more than mere speculation.

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    Quote Originally Posted by eburacum45 View Post
    You want to look for ozone, actually; oxygen might occur frequently on water-covered worlds, because water vapour can be converted to oxygen by sunlight (starlight). But this process destroys ozone, according to A Leger here
    http://arxiv.org/ftp/astro-ph/papers/0308/0308324.pdf
    so you want to look for oxygen and ozone together, which can probably only happen on a life-bearing world.
    I wouldn't put much reliance in that paper. It's reasoning seems loose; in particular their argument that only oxygen produced by photosynthesis can be converted to ozone.

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    I think the chain of reasoning is that ozone would be produced on a waterworld as well as on a world with a biosphere.
    However the upper atmosphere of a waterworld would hold more water, which would be converted to H, HO and of course oxygen. Given a damp upper atmosphere (which is likely if an oxygen atmosphere is being produced by photolysis) then the persistence of ozone is unlikely.

    If for some reason the upper atmosphere of a waterworld is as dry as the upper atmosphere of a world with a biosphere, then ozone will persist, and we've lost another biomarker.

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    Quote Originally Posted by eburacum45 View Post
    I think the chain of reasoning is that ozone would be produced on a waterworld as well as on a world with a biosphere.
    However the upper atmosphere of a waterworld would hold more water, which would be converted to H, HO and of course oxygen. Given a damp upper atmosphere (which is likely if an oxygen atmosphere is being produced by photolysis) then the persistence of ozone is unlikely.
    Yeah but why can't a water world have a stratospheric water trap too? The photolysis could take place in the lower atmosphere and the oxygen spread by diffusion to the dry upper atmosphere. It seems more like a test for a stratospheric water trap than for life. Ice worlds can have a (thin) oxygen atmosphere with ozone in it too. eg Ganymede Probably a combination of physical and chemical indicators will be used to identify life-bearing worlds rather than one "smoking gun".

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    And any test you might care to name (the red edge, oxygen, ozone) all depend on the existence of Earth-like life. Life might use quite different biochemistries on other worlds, especially if the temperature , pressure and local abundance of elements is different.

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    Quote Originally Posted by eburacum45 View Post
    And any test you might care to name (the red edge, oxygen, ozone) all depend on the existence of Earth-like life. Life might use quite different biochemistries on other worlds, especially if the temperature , pressure and local abundance of elements is different.
    It's kind of tautologous to say that we will only recognize the life that we can recognize. I'm doubtful about the existence of these "different biochemistries". AFAIK nothing comes close to carbon chain compounds for complexity.

    That's why I think an important and achievable early step in identifying worlds likely to bear life is the identification of terrestrial planets with optically thin atmospheres and surface water.

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