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What would birds look like if they did not have to fight gravity to fly?

The question comes from the premise put forward by some that gravity is not real and that what holds us down is pressure caused by density.

Obviously, this is not the case, but if it were, how would it (yes, conceptually) affect the way birds evolve, develop and/or fly?

[EDIT] To conceptualise a little... Let's say we had evolved on Earth and then been transplanted to an artificial planet. One with no gravity but one where the gas we are surrounded by is dense enough to actually cause some amount of pressure and thus keep us on the ground - a ground which has no gravity because it is perhaps only a metre deep, though wide enough to house every country... and then birds had evolved. (Perhaps they had been put there before we were.)

[COMMENT] This is not about whether the birds of a particular type are viable but is asking what they would look like given time enough in a particular environment.

Matt W
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They would be round, because that's the best format for storing gas. They would also probably be drifters with little flight control. So...

A picture displaying Red, the iconic round, wingless bird from the video game franchise Angry Brids

The Square-Cube Law
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    but doesnt current birds already use some kind of little buoyancy i think i remember something like that from biology classes – DreadfulWeather Jan 19 '19 at 01:33
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    @DreadfulWeather They do, but it isn't enough that you'd really call them "buoyant". They're just lighter than land animals their size. – Brilliand Jan 19 '19 at 01:41
  • This is precisely the point of the question, in fact. I'm asking what birds would look like in a world where there were no gravity but that their density caused them to be pushed them (and everything else) down - how would they gain flight? Buoyancy would come into play a lot more during their evolution, I suspect. – Matt W Jan 19 '19 at 07:00
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    I'm seeing flotillas of round non-rubber duckies floating lazily on the breeze.... –  Jan 19 '19 at 10:04
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    Angry Birds just got a backstory :) – val - disappointed in SE Jan 19 '19 at 23:44
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    with no gravity density doesnt cause you to be pushed anywhere but "out". – Josh Vander Hook Jan 20 '19 at 05:16
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    @JoshVanderHook with no gravity there is no push, period. – The Square-Cube Law Jan 20 '19 at 05:30
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    Yep, that is a problem. I don't know what flattards use to justify that. – Matt W Jan 20 '19 at 05:49
  • There's lots of sources of pushes in no gravity, and many of them are based on density gradients. There's just no gravity. – Josh Vander Hook Jan 20 '19 at 05:53
  • @MattW Believers of a flat earth believe that instead of there being such a thing as "gravity" accelerating us to the earth at ~9.8m/s, instead the plane we stand on is actually accelerating upwards at a constant 9.8m/s, causing everything in front of it to "stick" to it with basically the same effect. – James T Jan 21 '19 at 08:28
  • @JamesTrotter Even on this post there are commentators saying that there are other beliefs. One of the problems with the flat earth idea is that the community as a whole has no singularly accepted model - some even claim that no model is required. Please post an answer with the assumption that 'down' is caused by upward motion, though +boodysapie posted in the chat that that would require infinite energy. I make no assumption as to what FE claims induces 'down'. – Matt W Jan 21 '19 at 08:42
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Fish, or possibly penguins

A world where things can fly without needing to fight gravity (I don’t want to think what the method for that is) is a world where the first fish to develop the ability to breathe out of water skips land completely and just goes straight for the air.

Even if it’s something specific to birds, you’d start to see more and more penguin like ‘torpedo’ shapes optimised for slipping through the air rather than pushing against it (hell, in this world penguins might be the most successful bird!)

You’d probably see larger control surfaces (wings/fins) than in the water, as water is denser, but without the need for surfaces to glide/flap with I can’t see why a bird wouldn’t move back to a more energy efficient form of locomotion, namely oscillating the body back and forth and using the limbs purely for control instead of relying on the limbs to do both.

ADDENDUM:

In the comments I added a link to the Festo Air Penguin, a helium filled remote control blimp. I didn’t initially add this as their design of blimp uses fins for locomotive power and only uses the nose and tail of the ‘penguin’ for control, unlike true penguins or fish which also flex their bodies to give better manoeuvrability and power, but it’s a good enough video that I’ve stuck it in the answer!

Joe Bloggs
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    Fish and penguins are optimized for slipping through the water. There's a lot more resistance in water than there is in air. – Peter Shor Jan 20 '19 at 01:51
  • True, though I think that is handled by the 3rd para. – Matt W Jan 20 '19 at 05:49
  • To be able to float in air, air must be more dense or birds must be a lot larger. In both scenarios, air resistance matters a lot and you are getting penguin-like birds or airship-like birds - and interestingly airships are penguin-like shaped. – Pere Jan 20 '19 at 15:33
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    @Pere In some cases airships are penguins! – Joe Bloggs Jan 20 '19 at 16:14
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    @JoeBloggs - Awesome. This video could be an answer to the question by itself. – Pere Jan 20 '19 at 16:26
  • Choosing this answer because, while all the responses are great, this has a video and has the most upvotes (at time of writing) with detail. – Matt W Jan 22 '19 at 07:58
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I don't see any reason why birds couldn't theoretically achieve buoyancy in the air. It's just probably extremely inefficient for animals the size of normal birds compared to winged, powered flight.

They would need a biological way of producing a gas lighter than the atmosphere, which is much more likely to be hydrogen than helium. Then, they would need a 'bag' of enough volume that its hydrogen offsets the rest of their mass.

Finally, these birds would need some means of controlling their ascent and decent, which means ballast. This is probably going to be water that they can release to fly higher. When they need to drop down, they would release hydrogen instead. They couldn't fly higher again until they had a chance to replenish both, so they would need to be able to float on the water while they produce hydrogen and take in enough water to keep them grounded until they're ready to go.

In short, they would probably look like blimps, but with flapping fins/wings instead of propellers. They would also be much more likely to be large. The square/cube law works the opposite way with things that are lighter than air. The bigger you are, the easier it is to fit in lift gas relative to surface area.

As for 'gravity not being real' and your 'pressure caused by density' alternative, I'm not really sure I understand what you mean, but I don't see how it would make a difference. In both cases, you achieve flight by being less dense than the air.

Pasqueflower
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    I hope this satisfies @ynneadwraith :) – Matt W Jan 18 '19 at 16:44
  • Thanks @Pasqueflower. The gravity not being real thing is apparently part of the flat earth concept - I can't justify it. My understanding of current flight mechanics is that lift does not need require a body which is less dense than air - though maybe I misread your statement? – Matt W Jan 18 '19 at 16:47
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    Lift does not require a body that is lighter than air, it just requires a larger force pointing up than gravity. But that's what buoyancy means. It's the upward force of a liquid or gas that is opposing the weight an immersed object. The magnitude of the force is equivalent to the weight of fluid that the object displaces. And this is different at different altitudes/depths, because pressure increases as you go deeper. Weight = mass x acceleration of gravity. Buoyancy = weight of displaced fluid. Whichever is higher determines which way something goes. – Pasqueflower Jan 18 '19 at 16:54
  • Yep - and that whole concept pretty much destroys the source of this question, hence the messed up nature of the question. – Matt W Jan 18 '19 at 17:06
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    "a gas lighter than the atmosphere, which is much more likely to be hydrogen than helium." - Or possibly just hot air. – 8bittree Jan 18 '19 at 19:46
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    @8bittree - hot air has a pretty high metabolism requirement. A typical hot air balloon is something like 2-3 MW. Or something like 500 food calories / second. You could insulate... but that adds weight and increases the amount of lift you need to generate. Your best bet for hot air might be solar heated... though then the question becomes what you do at night. – TLW Jan 19 '19 at 03:20
  • I've considered this, but many birds don't fly at night anyway. The other side of this problem is that we are all assuming a particular direction given - but in our world that direction is provided by gravity; The question is specifically about a world with no gravity. What makes 'down' in that world, even if the air pressure alone is enough to keep our bodies intact. Shouldn't we all be able to fly simply by controlling air currents and our surfaces? Why would something filled with hot air go 'up'? Fire in space is just a weird (but really cool) -looking ball. – Matt W Jan 19 '19 at 07:03
  • Note that fish are able to regulate their height based on pressure (using inflatable air sacs), but they don't have ballast that can be dropped. I'm only mentioning this because your answer suggests that ballast is needed, which is not the case when you're able to deflate your gaseous balloon (and compress the gas to you can reuse it later). Submarines use the exact same principle as well. – Flater Jan 21 '19 at 07:35
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As there is the physics tag, I feel that the misconceptions should be addressed.

Buoyancy is the result of gravity. If there is no gravity, there is no buoyancy.

The bigger picture is that a hot air balloon floats upwards because gravity pulls it down less than the same volume of air. So air gets to occupy the lower layer. In the smaller scale buoyancy arises because the pressure is higher in the lower layers. Thus the bottom of an object is pushed upwards more than the top is pushed downwards.

You can not reproduce that by blowing air from above. You might be able to get things staying on the ground, but the pressure gradient will be opposite - for any object the pressure from above will be higher than below. So buoyancy will push everyone downwards and being lighter won't help you. If birds would fly in such circumstances, they would be similar to those on Earth but maybe a bit thinner and having more endurance as they have to constantly fight the wind.

I answered supposing that people are held on ground by blowing wind from above as discussed in the comments. The phrase pressure caused by density by seems to have no real physical meaning, so the question as-is can't really be answered.

Džuris
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  • Oh yes, I completely agree - the entire premise is stupid. As I've said in the comments, this is not my premise but the flat earthers'. I can't justify it and the problems raised because of the logical holes are not the question here. Just this one point. I guess the reasoning would come down to "even if everything about the FE were true, birds would / would not look the same as they do in real life." Then watch the flattards tie themselves in more knots. – Matt W Jan 20 '19 at 05:57
  • @MattW Wait, Flat Earthers don't believe in gravity? That's surprising--especially because an infinite uniform slab does actually have the uniform gravitational field we approximately have on the Earth's surface. So unlike a great many other things, its existence doesn't really contradict their theory. – eyeballfrog Jan 20 '19 at 21:45
  • @MattW: You're "birds fly because of density" argument relies on the lack of aerodynamic friction (because wings don't work the way they do - hence the need for buoyancy); but then you've also defeated how birds are able to move horizontally (left/right/up/down). And if birds are able to flap their wings to move horizontally, why wouldn't they be able to do the same vertically? What if they flap their wings to move at an angle (horizontal+vertical)? – Flater Jan 21 '19 at 07:38
  • @eyeballfrog While I'm asserting that flatters don't believe in gravity because I've seen so many prominent flatters say that it specifically does not exist, it's not really the point to my question. That is to say that my question is not whether gravity does or does not exist or if anyone does or does not say it exists - the question is about how birds would evolve in a gravity-less environment. The specifics of the structure of our world, from the uniform opinion of the flatter movement, is not available because they don't all agree. See the first sentence in this comment, for example. – Matt W Jan 21 '19 at 07:51
  • @Flater No, I have not stated that that friction is a component, merely the evolutionary path. Feel free to post an answer which debates both the lack of or apparent possession of aerodynamic friction. I'm interested in, however, your assertion that "wings don't work they way they do". Surely wings do work the way that wings work? Or do you mean that wings don't work the way wings in a fricitonless environment would work? – Matt W Jan 21 '19 at 07:54
  • @MattW: If wings work the way they do in real life, then birds are not flying through buoyancy and thus have no need for buoancy. If wings don't work the way they do in real life, then buoancy may be a solution to vertical movement, but not horizontal movement. In other words, if you want to disable real bird flight to introduce the alternate method of buoancy, you're also going to have to introduce an alternate method for horizontal movement. – Flater Jan 21 '19 at 08:06
  • @Flater Agreed. An interesting extension to the answers others have put down. Why not post an answer discussing it? – Matt W Jan 21 '19 at 08:09
  • @MattW: Becuase it doesn't really answer the question of what birds would look like assuming they relied on buoyance. All I'm doing is point out the flaws in your alterations and expectations of physics, which Dzuris already summed up better than I would. – Flater Jan 21 '19 at 08:12
  • @Flater I make no alterations, assumptions or expectations of physics for this. Any statement in that direction comes from the flatter community. I'm asking what birds would look like in that environment, that of no gravity, but yes; Dzuris has answered very well :) – Matt W Jan 21 '19 at 08:24
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    Whenever people insist centrifugal force isn’t real I love to tell them that buoyancy isn’t either and watch their heads explode. – Joe Bloggs Jan 22 '19 at 08:08
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If the bird in question needs only to be buoyant then the resulting animal would float at a more-or-less constant height, unable to manoeuver to avoid predators or return to the ground to nest or feed (I will show later that this bird is vegan).

These are strong evolutionary disadvantages.

Blimps, fish and submarines are buoyant and use fins for steering, so it seems prudent to include these in its design.

Regarding altitude, the bird needs to increase or decrease the amount of lighter-than-air gas in its body. Decreasing is easy, but it needs a method of producing extra gas for lift.

Both hydrogen and methane are lighter than air, and these gases are produced when certain foods cannot be digested but instead ferment in the gut. I would therefore design the bird to have a penchant for beans and pulses, and a method of recycling its own farts.

So in summary it should look like a fish with a tube from its anus to its nostrils.

Boodysaspie
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  • I love it, but you forgot to demonstrate why the bird is vegan (though I think I can see it coming). – JBH Jan 19 '19 at 03:59
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    +JBH Updated. Thanks for your suggestion. – Boodysaspie Jan 19 '19 at 05:33
  • That's awesome. Though don't we all have tubes from our anus to our nostrils? An ex-doctor, ex-boss of mine once described the human body as just a tube surround by meat and bone. I assume the vegan part is because of the density of meat compared to plant material. I don't see why the bird would be essentially stationary - it can still flap to provide momentum from displacement. – Matt W Jan 19 '19 at 07:08
  • Just noticed the edit providing the vegan justification. Yay! – Matt W Jan 19 '19 at 07:10
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    +Matt W We do indeed have a tube from our mouths to our anuses (the gastrointestinal tract). The flow is, luckily, in the opposite direction to my tube mod, burping or vomiting excepted. – Boodysaspie Jan 19 '19 at 15:12
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    Altitude control could be affected by pressurizing the "bag" which contain the "lifting" gas. If the "bag" were surrounded by muscle, tensing and relaxing the muscles would change the density of the gas, thereby changing the lift provided by the "bag". – Julie in Austin Jan 20 '19 at 03:50
  • @boodysapaie Yes, that was my thought, as well. Combined with +julie-in-austin's comment I think we'd have a winner there. It doesn't get over the lack of 'down' but that's not my issue here. – Matt W Jan 20 '19 at 05:52
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    Julie in Austin's bird would easily out-evolve mine. Not only is it more efficient in using, ahem, resources, it can also quickly shrink in size (and have more choice of cover on the ground) when it detects predators - an excellent camouflage technique. My only contribution would be in replenishing the bag, since no system is perpetual. – Boodysaspie Jan 20 '19 at 20:09
  • @julie-in-austin Is that how submarines control their ballast? – Matt W Jan 24 '19 at 16:40
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They would definitely look very silly. They would by default have to have very low momentum in relation to air resistance so they would be much less efficient in their movement than IRL birds and have much less maneuverability. A good comparison is an airplane and a hot air balloon. they would most likely look like big balloons with small propulsive appendages on their back side.

they would certainly be comical to watch go about their days.

https://www.youtube.com/watch?v=yltlJEdSAHw

Sdarb
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    I suspect they'd look very normal to us, given that they'd be 'everyday' things. Having said that, of course, I did state in another comment that they would need to evolve in an environment to which we get transplanted, so maybe they'd look totally nuts. – Matt W Jan 19 '19 at 07:09