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Thread: Habitability in Avatar -- greenhouse effect and disallowed orbits

  1. #31
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    Quote Originally Posted by Van Rijn View Post
    People can fly on ultralight airplanes on Earth, so I don't see a physical reason why there would need to be even lower gravity or thicker atmosphere than is assumed for Pandora for there to be flying animals that could be ridden there.
    There's never been anything remotely close in Earth's history. Flying is a difficult thing and I still doubt that any flying animal in an Earth-like environment has so much lift and stability to spare that it can afford to have about 100kg of human on the back of its neck.

    Quote Originally Posted by Van Rijn View Post
    These are not Earth species,
    The same power/weight considerations apply anywhere. Their insolation is comparable to or less than Earth's. That's the source of free energy that powers everything.

    Quote Originally Posted by Van Rijn View Post
    they are stated to be built differently (carbon fiber bones, for instance),
    It's not obvious how that would make them fly better. It might be a little lighter.

    Quote Originally Posted by Van Rijn View Post
    and though it's not discussed in the movie, the flying animals seem to have air intakes on their wings similar to one of Poul Anderson's fictional flying species (in Anderson's stories, it allows them to have a very high energy metabolism).
    Well if a sci-fi writer said so, that's it, I give up. I wont contest this point.

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    Gee you are a lazy bunch. Real space cadets do math!

    Because of its high axial tilt (29), Pandora exhibits considerable annual variation in the day-to-night ratio. In addition, its elliptical orbit produces seasonal temperature variations and a range in daytime illumination of about ten percent.
    If this means S(max)/S(min)=1.1, that implies Polyphemus' e=(sqrt(1.1)-1)/(sqrt(1.1)+1)=0.0238. How does it maintain that axial tilt against Polyphemus' tidal torque? Its orbit could be inclined at 29o to Polyphemus equator, but that has its own problems. Simplest resolution might be to assume Polyphemus is quite inclined itself and Pandora's is roughly equatorial and co-inclined wrt to it.

    At the closest point in its orbit, ACB is about 2,300 times as bright as Earth's full moon; at its furthest point, it is still one hundred and seventy times as bright.
    Let d1 and d2 be ACB's nearest and farthest distances from Pandora. Assume the Pandora-Polyphemus distance makes a negligible difference. Then this says d2/d1=3.678. Let r be Polyphemus' semimajor axis and B1 and B2 be ACB's periastron and apastron. Then, ignoring Polyphemus' small eccentricity,
    r = B1-d1 and r = d2-B2, so
    d1+d2=B1+B2=46.8au
    -> d1=10au
    -> r = 1.2au
    -> S = 1.042

    There's the first shock, Pandora's insolation is greater than Earth's. Maybe Polyphemus' shading needs to be taken into account... (to be continued)

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    Quote Originally Posted by whimsyfree View Post
    There's never been anything remotely close in Earth's history.
    So what?

    The issue is if it is physically possible, and I see nothing that would make it physically impossible.

    Well if a sci-fi writer said so, that's it, I give up.
    Yes, a science fiction writer known for his hard science fiction.

    I wont contest this point.
    Well, naturally you won't, since there's nothing to contest.

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    Quote Originally Posted by whimsyfree View Post
    There's never been anything remotely close in Earth's history. Flying is a difficult thing and I still doubt that any flying animal in an Earth-like environment has so much lift and stability to spare that it can afford to have about 100kg of human on the back of its neck.
    I've read that the Great Horned Owl can carry up to 3x their own weight, while many birds can carry up to half. The numbers on Pteranadons and Hatzegopteryx are varied and speculative, but if they could also carry a significant fraction of their body weight, then you're getting into fairly large payloads. Then realize that the "Ikran" in the movie were several times larger in an atmosphere that would support more weight and the possibility is there.[/QUOTE]

    It's not obvious how that would make them fly better. It might be a little lighter.
    It would also make the wings stronger, allowing for more powerful wing beats.
    Et tu BAUT? Quantum mutatus ab illo.

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    I wonder if Ikran was from Ekranoplan. Cameron is quite familiar with Russian equipment having spent much time on the ship Acadamician Mistislav Keldysh for Titanic
    http://en.wikipedia.org/wiki/Mstislav_Keldysh

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    Quote Originally Posted by publiusr View Post
    One way or another, Pandora is an engineered world. Existing only in CGI for real, one could say the fictional world was made by some super entity that took lfe forms from all over and made them work by introducing anti-gravity elements that in trace amounts allow large creatures and slightly different gravity while not losing a lot of atmosphere. Here there is no need for the internet, just that tree. No need for fuel burning aircraft or ground mobility, there are animals easily ridden for that. We can't ride on flying animals--we have to have engines.

    It is if some Q level entity wanted a race of creatures that had all the modern conveniences, but in a greener, animal based form not available to us here.
    I think one of the dead giveaways of Pandora being engineered are the half dozen arches made entirely of rock surrounding their holy place.

    Granted, the Na'vi might have built them themselves, but I didn't see any indication of that.

    Quote Originally Posted by whimsyfree View Post
    They seem shy about its orbit, but

    seems to confuse Pandora's orbit with Polyphemus'. If anyone can be bothered they can get a handle on the Polyphemus' apastron and periastron from

    and thence Pandora's semimajor axis (its eccentricity should be negligible) from

    (The last bit makes me think Pandora's rotation is not synchronous, which is very unlikely).
    Perhaps the clue might be in Pandora's possible axial tilt: 29 seems to suggest that since it is most likely to have synchronous rotation, Pandora's orbit might be inclined about 29 to Polyphemus' ecliptic. As far as I know, the rotation of a parent planet should pull its satellites into equatorial orbits if they're close enough, meaning that Polyphemus' axial tilt should also be about 29, but I think in most scenes where you see Polyphemus, the cloud bands are at 90 angles to the terminator, which shouldn't be like that in the time frame of the movie, if Polyphemus indeed has a 29 obliquity.

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    Quote Originally Posted by PlutonianEmpire View Post
    I think one of the dead giveaways of Pandora being engineered are the half dozen arches made entirely of rock surrounding their holy place.

    Granted, the Na'vi might have built them themselves, but I didn't see any indication of that.
    Of course Pandora is a figment of the imagination. I treat things like the arches as cinematic eye candy and explore the physical aspects of the setting from the perspective of modern science. If you conclude the world was built by super-beings using their super-science then there's not much to discuss. Maybe their super-tech keeps Pandora in an impossible orbit, etc.

    Quote Originally Posted by PlutonianEmpire View Post
    Perhaps the clue might be in Pandora's possible axial tilt: 29 seems to suggest that since it is most likely to have synchronous rotation, Pandora's orbit might be inclined about 29 to Polyphemus' ecliptic. As far as I know, the rotation of a parent planet should pull its satellites into equatorial orbits if they're close enough, meaning that Polyphemus' axial tilt should also be about 29,
    That's what I was alluding to. To keep Pandora inclined to Polyphemus' ecliptic Polyphemus has to be similarly inclined to Polyphemus' ecliptic, unless Pandora is in an implausibly wide orbit. So I was plugging for this idea but...

    Quote Originally Posted by PlutonianEmpire View Post
    but I think in most scenes where you see Polyphemus, the cloud bands are at 90 angles to the terminator, which shouldn't be like that in the time frame of the movie, if Polyphemus indeed has a 29 obliquity.
    That's a good observation that I hadn't noticed. Would that always be true? what if Polyphemus was at equinox?

    The only picture I could find isn't very helpful because clouds obscure the most informative part. Maybe I'll <cough> get my own copy <cough> of the film and go through it with a fine tooth comb looking for scenes with gas giants.

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    I made the following assumptions. (1) ACB is at infinity and has negligible angular diameter, (2) Pandora is far enough from Polyphemus that the lengths of the arc and the chord marking eclipse ingress and egress are close, (3) Polyphemus' mass is 0.5 Jupiter's, (4) Polyphemus' radius is 0.75 Jupiter's, (5) the contribution of Polyphemus' orbit to the eclipse duration is small. Then, using Io as a reference, I tried to compute Pandora's sma from the given 100 min transit time of ACB. I got 0.47 times Io's, or 200,000 km. Seems too small.

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    Quote Originally Posted by Van Rijn View Post
    People can fly on ultralight airplanes on Earth, so I don't see a physical reason why there would need to be even lower gravity or thicker atmosphere than is assumed for Pandora for there to be flying animals that could be ridden there. These are not Earth species, they are stated to be built differently (carbon fiber bones, for instance), and though it's not discussed in the movie, the flying animals seem to have air intakes on their wings similar to one of Poul Anderson's fictional flying species (in Anderson's stories, it allows them to have a very high energy metabolism).
    Any flying animal which wastes mass and energy by making itself excessively strong will be out-competed by flying animals which put the energy into reproductive effort. It's irrelevant, for example, that an owl can lift 3 times its body weight for a few seconds to get a prey animal to its nest when arguing that a flying animal can be used as a beast of burden for massive payloads for hours.

    Flight is hard, and flying animals readily give it up when it's not necessary: just look at how many birds on small, predator-less islands have become flightless. There are a lot of creature features I find incredibly unlikely, and therefore jarring, in sf: warm blooded animals with an IR sense (sensor self-noise), LTA creatures on planets with Earth-like atmospheres, ramjet powered flying beasts, etc. The flying creatures in Andersen's stories fall into that category, as do the dragons of Pern: so improbable as to be break the necessary suspension of disbelief.
    Last edited by swampyankee; 2012-May-27 at 04:12 PM.
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    Quote Originally Posted by swampyankee View Post
    Any flying animal which wastes mass and energy by making itself excessively strong will be out-competed by flying animals which put the energy into reproductive effort. It's irrelevant, for example, that an owl can lift 3 times its body weight for a few seconds to get a prey animal to its nest when arguing that a flying animal can be used as a beast of burden for massive payloads for hours.

    Flight is hard, and flying animals readily give it up when it's not necessary: just look at how many birds on small, predator-less islands have become flightless. There are a lot of creature features I find incredibly unlikely, and therefore jarring, in sf: warm blooded animals with an IR sense (sensor self-noise), LTA creatures on planets with Earth-like atmospheres, ramjet powered flying beasts, etc. The flying creatures in Andersen's stories fall into that category, as do the dragons of Pern: so improbable as to be break the necessary suspension of disbelief.
    A lot of things like that are old myths about dragons and flying horses transposed into science fiction. That combined with the limitations of the imagination: can't make them too much like Earth animals so they model them on machines. OTOH intelligent aliens are often implausibly humanoid (such as the Navi).

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    Quote Originally Posted by whimsyfree View Post
    Of course Pandora is a figment of the imagination. I treat things like the arches as cinematic eye candy and explore the physical aspects of the setting from the perspective of modern science. If you conclude the world was built by super-beings using their super-science then there's not much to discuss. Maybe their super-tech keeps Pandora in an impossible orbit, etc.



    That's what I was alluding to. To keep Pandora inclined to Polyphemus' ecliptic Polyphemus has to be similarly inclined to Polyphemus' ecliptic, unless Pandora is in an implausibly wide orbit. So I was plugging for this idea but...



    That's a good observation that I hadn't noticed. Would that always be true? what if Polyphemus was at equinox?

    The only picture I could find isn't very helpful because clouds obscure the most informative part. Maybe I'll <cough> get my own copy <cough> of the film and go through it with a fine tooth comb looking for scenes with gas giants.
    Yeah, in that pic, I myself can tell Polyphemus is indeed inclined to its orbit.

    Granted, some people may not be able to tell from that pic, but, spend a whole lot of time fiddling around with open source space programs, and you'll wind up learning a few skills here and there. (EDIT: It's easier to tell if the pic appears small/zoomed out, at least for me.)

    At the beginning of the movie, Polyphemus is exactly like I described it earlier, with Polyphemus possibly being at equinox, and I'm fairly sure the movie itself shows a time span of about three months. If you look at the numbers on the screen when Jake is talking into the camera, you can see the date on the bottom right (it might be easier to see it if you have a high-def version, such as blu-ray). I'm guessing the pic you provided is maybe two months in or so, perhaps? At one point, Jake himself says he's been there about three months, IIRC.

    My only beef is how they show the planet through the clouds on Pandora; Polyphemus' edges appear blurred when seen through the clouds. As far as I know, they shouldn't be. I've seen the sun and the moon on heavy cloud cover days and I could still see their clear outlines, during the times the clouds would try to break. Even on a day like the one in the pic, I've seen the sun and moon's clear outline through similar clouds. It's possible they knew that but had technical difficulties rendering it, perhaps?

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    Quote Originally Posted by PlutonianEmpire View Post
    Yeah, in that pic, I myself can tell Polyphemus is indeed inclined to its orbit.

    Granted, some people may not be able to tell from that pic, but, spend a whole lot of time fiddling around with open source space programs, and you'll wind up learning a few skills here and there. (EDIT: It's easier to tell if the pic appears small/zoomed out, at least for me.)
    You've gone 180o on this but I won't argue. Inclined Polyphemus works much better.

    Quote Originally Posted by PlutonianEmpire View Post
    At the beginning of the movie, Polyphemus is exactly like I described it earlier, with Polyphemus possibly being at equinox, and I'm fairly sure the movie itself shows a time span of about three months. If you look at the numbers on the screen when Jake is talking into the camera, you can see the date on the bottom right (it might be easier to see it if you have a high-def version, such as blu-ray). I'm guessing the pic you provided is maybe two months in or so, perhaps? At one point, Jake himself says he's been there about three months, IIRC.

    My only beef is how they show the planet through the clouds on Pandora; Polyphemus' edges appear blurred when seen through the clouds. As far as I know, they shouldn't be. I've seen the sun and the moon on heavy cloud cover days and I could still see their clear outlines, during the times the clouds would try to break. Even on a day like the one in the pic, I've seen the sun and moon's clear outline through similar clouds. It's possible they knew that but had technical difficulties rendering it, perhaps?
    The apparent sharpness of the moon might be partly an illusion caused by its small size. Polyphemus would fill a large fraction of the sky if it were only 200k km away. Plus the moon really does have a sharp edge whereas Polyphemus just has increasingly thin cloud layers.

    I don't have time to find references but I think putting it so close to Polyphemus was a mistake. They probably did it to give it an Earth-like day length but tidal dissipation would be much greater than in Io. I think Pandora would be a volcanic hell.

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    Quote Originally Posted by whimsyfree View Post
    You've gone 180o on this but I won't argue. Inclined Polyphemus works much better.
    Yeah, I guess I was a bit quick to judge in the first place.

    I'll be sure to look more carefully next time I watch it.
    The apparent sharpness of the moon might be partly an illusion caused by its small size. Polyphemus would fill a large fraction of the sky if it were only 200k km away. Plus the moon really does have a sharp edge whereas Polyphemus just has increasingly thin cloud layers.

    I don't have time to find references but I think putting it so close to Polyphemus was a mistake. They probably did it to give it an Earth-like day length but tidal dissipation would be much greater than in Io. I think Pandora would be a volcanic hell.
    Not to mention being quite close to the roche limit. Which is where the bad astronomy comes in: http://2.bp.blogspot.com/_9mpFUAnTXJ...Avatar%202.png

    There's large moons even closer to Polyphemus than Pandora!

    I'm fairly sure the moon behind the tree to the left casting its shadow on Polyphemus is a bit too close to the planet.

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    Quote Originally Posted by PlutonianEmpire View Post
    Yeah, I guess I was a bit quick to judge in the first place.

    I'll be sure to look more carefully next time I watch it.

    Not to mention being quite close to the roche limit. Which is where the bad astronomy comes in: http://2.bp.blogspot.com/_9mpFUAnTXJ...Avatar%202.png

    There's large moons even closer to Polyphemus than Pandora!
    All three appear spheroidal and in hydrostatic equilibrium, which would imply a mass around Ceres' at least. All in a band between Pandora and the Roche limit, say 100k wide. That's bad astronomy, and gratuitous too. The other moons play no role whatsoever in the story other than as eye candy.

    Googling (avatar astronomy, avatar pandora) I found this is just about the only place on the web that has discussed Pandora's feasibility at all critically. Got lots of "Avatar moon could be real, astronomers say".

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    Quote Originally Posted by swampyankee View Post
    Any flying animal which wastes mass and energy by making itself excessively strong will be out-competed by flying animals which put the energy into reproductive effort.
    Possibly, or possibly there could be an evolutionary purpose for being able to carry the extra weight. In any event, that's a much fuzzier issue than whether or not it is physically possible. In the case of Pandora, we don't even know that unfettered evolution was responsible for those species, and some evidence that it might not be.

    The flying creatures in Andersen's stories fall into that category, as do the dragons of Pern: so improbable as to be break the necessary suspension of disbelief.
    I'd agree with the dragons of Pern, but Anderson worked a good bit harder on the plausibility of his species.

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    Quote Originally Posted by whimsyfree View Post
    A lot of things like that are old myths about dragons and flying horses transposed into science fiction. That combined with the limitations of the imagination: can't make them too much like Earth animals so they model them on machines. OTOH intelligent aliens are often implausibly humanoid (such as the Navi).
    I think a lot of the dragons and flying beasts of burden that show up in sf because a) a lot of people think "hey, that'd be cool!" and b) the border between sf and fantasy is very fuzzy: "hard" sf uses a significant amounts of handwavium, even if they don't have ftl and c) having unscientific biology doesn't push something out of hard sf.

    I actually very much like Poul Anderson's writing -- more so than either Asimov's or Heinlein's -- but there are also occasions where I found his alien biologies less than believable.
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    Quote Originally Posted by PlutonianEmpire View Post
    I think one of the dead giveaways of Pandora being engineered are the half dozen arches made entirely of rock surrounding their holy place.
    My assumption was that it was petrified wood from a previous great tree as part of the life cycle of the Eywa entity.

    Quote Originally Posted by swampyankee
    Any flying animal which wastes mass and energy by making itself excessively strong will be out-competed by flying animals which put the energy into reproductive effort. It's irrelevant, for example, that an owl can lift 3 times its body weight for a few seconds to get a prey animal to its nest when arguing that a flying animal can be used as a beast of burden for massive payloads for hours.
    "Out-competing" suggests evolution, which we are lead to believe is not the case, which is intelligent design by the Eywa entity. For all we know, the Ikran have some unobtainium in their system that makes them lighter against gravity. The owl issue, as you point out, is that of lift-off, but soaring may be another issue. There are some scenes where the Ikran take off from the ground with Navi on their backs, but many take-offs are from high perches where it's easier to just fall first.

    Quote Originally Posted by PlutonianEmpire
    My only beef is how they show the planet through the clouds on Pandora; Polyphemus' edges appear blurred when seen through the clouds. As far as I know, they shouldn't be. I've seen the sun and the moon on heavy cloud cover days and I could still see their clear outlines, during the times the clouds would try to break. Even on a day like the one in the pic, I've seen the sun and moon's clear outline through similar clouds. It's possible they knew that but had technical difficulties rendering it, perhaps?
    Don't forget that the atmosphere has a different composition, so there might be a lot of refraction due to density variations.
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by swampyankee View Post
    I think a lot of the dragons and flying beasts of burden that show up in sf because a) a lot of people think "hey, that'd be cool!" and b) the border between sf and fantasy is very fuzzy: "hard" sf uses a significant amounts of handwavium, even if they don't have ftl and c) having unscientific biology doesn't push something out of hard sf.
    What, precisely, is supposed to be wrong about flying beasts of burden? There are birds which nest 80km away from where they fish for food. This requires carrying food over that distance.

    There can be valid evolutionary reasons to nest in places far away from where the food is available, so there can be valid evolutionary reasons for a flying creature to have the capacity to carry a significant load over long distances.

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    Quote Originally Posted by IsaacKuo View Post
    What, precisely, is supposed to be wrong about flying beasts of burden? There are birds which nest 80km away from where they fish for food. This requires carrying food over that distance.
    And what bird would that be?

    Quote Originally Posted by Ara Pacis View Post
    "Out-competing" suggests evolution, which we are lead to believe is not the case, which is intelligent design by the Eywa entity.
    I don't want to go there. It has an political dimension (the superior Pandoran environment is the product of ID...) and it's unscientific. Morever critical discussion of the life forms will almost certainly lead to an irruption of angry fan persons.

    Quote Originally Posted by Ara Pacis View Post
    Don't forget that the atmosphere has a different composition, so there might be a lot of refraction due to density variations.
    The atmosphere of Pandora is another problematic area.

    Nitrogen, Oxygen, Carbon Dioxide (over 18%), Xenon (5.5%), Methane and Hydrogen Sulfide >1%
    A significant percentage hydrogen sulfide is incompatible with an oxidizing atmosphere. In Earth's atmosphere the residence time of H2S is about three days. So to maintain 1% H2S something would have to be pumping out that volume of H2S every three days. The main factor in the oxidization of H2S is the hydroxyl radical. With a warm, wet environment and rather more UV incident than Earth, why would the hydroxyl radical be absent from Pandora's atmosphere?

    Non-trivial amounts of methane are also problematic but I have focussed on the H2S because its residence time is much shorter than methane's. In sum I think they've added too many exotic ingredients to the atmosphere for no good reason (why Xenon? because it's eXotic?).
    Last edited by whimsyfree; 2012-May-30 at 04:46 AM.

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    Quote Originally Posted by whimsyfree View Post
    I don't want to go there. It has an political dimension (the superior Pandoran environment is the product of ID...) and it's unscientific. Morever critical discussion of the life forms will almost certainly lead to an irruption of angry fan persons.
    Political? The movie is fictional and the idea that there was a planet-wide neural network that had independent volition seems rather well supported by the dialogue, unless there is debate somewhere else on the web, but I haven't followed fan discussions and am only getting my info from watching the movie and the one tidbit I googled about the ikran that lead me to their wiki.

    The atmosphere of Pandora is another problematic area. A significant percentage hydrogen sulfide is incompatible with an oxidizing atmosphere. In Earth's atmosphere the residence time of H2S is about three days. So to maintain 1% H2S something would have to be pumping out that volume of H2S every three days. The main factor in the oxidization of H2S is the hydroxyl radical. With a warm, wet environment and rather more UV incident than Earth, why would the hydroxyl radical be absent from Pandora's atmosphere?
    Dunno, wasn't my point to explain fiction, only to point out that the fiction stated and illustrated density different than Earth. A similar phenomenon happens on earth and causes stars to appear to twinkle.
    Et tu BAUT? Quantum mutatus ab illo.

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    Quote Originally Posted by whimsyfree View Post
    And what bird would that be?
    The first google hit I found was: http://en.wikipedia.org/wiki/Marbled_Murrelet

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    Quote Originally Posted by IsaacKuo View Post
    The first google hit I found was: http://en.wikipedia.org/wiki/Marbled_Murrelet
    You weren't googling after you posted were you?

    The linked article doesn't say anything about carrying fish 80km. It does say it feeds on small fish, which would weigh much less than a human. I don't think the Marbled Murrelet is a good justification for the ikran.

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    Quote Originally Posted by whimsyfree View Post
    You weren't googling after you posted were you?
    Umm...why in the world would I give a specific figure of 80km if I hadn't already read about it?
    The linked article doesn't say anything about carrying fish 80km.
    The fact is that Marbled Murrelets nest as far away as 80km from the sea where they get their food. It's simply a fact that in order to do so, they must carry fish across that distance.

    I don't know why you think it's worth your time to even argue about this. Do you honestly hope to convince anyone that sea birds which nest far from shore don't exist?
    It does say it feeds on small fish, which would weigh much less than a human.
    So what? A Marbled Murrulet is a small bird.

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    Quote Originally Posted by IsaacKuo View Post
    The fact is that Marbled Murrelets nest as far away as 80km from the sea where they get their food. It's simply a fact that in order to do so, they must carry fish across that distance.
    I suspect they eat them first.

    Quote Originally Posted by IsaacKuo View Post
    I don't know why you think it's worth your time to even argue about this. Do you honestly hope to convince anyone that sea birds which nest far from shore don't exist?

    So what? A Marbled Murrulet is a small bird.
    So they are not flying beasts of burden. You said
    What, precisely, is supposed to be wrong about flying beasts of burden? There are birds which nest 80km away from where they fish for food. This requires carrying food over that distance.
    Marbled Murrulet aren't an example of a flying beast of burden.

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    Quote Originally Posted by whimsyfree View Post
    I suspect they eat them first.
    They don't. They carry one fish back to the nest, once or twice per day, to feed to the chick.

    And even if they did, so what? The mass of the food still needs to be carried back to the nest. The bird needs to carry the extra food mass either way.
    So they are not flying beasts of burden.
    Why not? A beast of burden needs to be able to carry a significant load compared to its own mass, it does not need to carry a human. For example, dogs and small ponies can be beasts of burden, even though they are too small to carry a human.

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    Quote Originally Posted by IsaacKuo View Post
    Why not? A beast of burden needs to be able to carry a significant load compared to its own mass, it does not need to carry a human. For example, dogs and small ponies can be beasts of burden, even though they are too small to carry a human.
    You don't know what a beast of burden is. Wikipedia gives a reasonable definition

    A pack animal or beast of burden is a working animal used by humans as means of transporting materials by attaching them so their weight bears on the animal's back
    Small wild birds are not beasts of burden. They are not used by humans as a means of transporting materials. They don't carry loads on their back.

    If you don't have anything of relevance to the topic to say I will ignore your posts.

  27. #57
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    Now, you've got me wondering... what's the airspeed velocity of a laden swallow?
    Et tu BAUT? Quantum mutatus ab illo.

  28. #58
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    Quote Originally Posted by whimsyfree View Post
    You don't know what a beast of burden is. Wikipedia gives a reasonable definition
    Which does not require the ability to carry a human, of course.

    Small wild birds are not beasts of burden. They are not used by humans as a means of transporting materials. They don't carry loads on their back.
    They certainly can carry loads on their back.

    But the location of the load is irrelevant to the practical utility, except insofar as a backpack happens to be the most efficient location from an aerodynamics perspective.

    If you don't have anything of relevance to the topic to say I will ignore your posts.
    If you recall, the supposed argument was that flying animals would not evolve the ability to carry significant loads over a long distance. I'm giving a direct counterexample to that argument. As I noted, there can be valid evolutionary reasons to nest far away from the source of food, and this provides a valid evolutionary pressure to develop the ability to carry significant loads over long distances.

    Van Rijn already noted the various physical reasons why the flying creatures of Pandora could be possible, and I have noted a plausible evolutionary reason for them.

    By the way, the flying creatures of Pandora seem to share nesting aspects of sea birds. Colonial nesting in isolated cliffs is good for protection against predators, but it also tends to require traveling a long distance for food. Food near an isolated colonial nest tends to get depleted quickly.

  29. #59
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    If a species can live long enough to shape the genetics of a smaller species over time--by rewarding the longest flyers who wear tiny weights over time--that might be enough of a breeding to increase size, especially if the unobtainium ore were mixed in as part of the feed to lessen the effects of gravity still further--in universe.

  30. #60
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    Perhaps certain plants concentrate unobtainium in the pulp of their fruit, to attract flying animals; the seeds or nuts don't contain this fabulous material, and fall to the ground after expulsion. Raptors eat frugivores, and concentrate the ?mineral? even further.

    Flying frugivores can tell which fruits contain unobtainium, because they hang upside-down.

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