Do UFOs have to be human-scale in size?

[Edward Current]

One of the things that make me skeptical that any UFOs are non-human or off-world in origin is their size. All UFOs, except perhaps in the most astonishing observer-specific experiences, are firmly stuck inside the scale of humans and their technology. This is epitomized in the classic image from the 1951 film The Day the Earth Stood Still:



While I don't for a moment think that alien beings would be so anthropomorphic, we still have the recurring trope of the bodies of alien "biologics" (as reported by David Grusch and described in this apparent work of creative fiction) being roughly humanoid or at least human-scale. And we have the trope of the spaceship, which according to Grusch and Ross Coulthart can be in some cases so large that a crashed one cannot be moved, but which has been hidden by constructing a building on top or something like that.

Even spacecraft of this size shares a narrow window of scale with the largest "flying" human technology (the ISS, dirigibles, large cargo planes, etc.).

The entire range of scale spans from quarks at <10–19​ meters to galaxy clusters at ~1024​ meters, or 43 orders of magnitude. Yet, "mile-long" Starlink trains notwithstanding, virtually every UFO case falls within a scale of roughly 1 meter to 100 meters, or 2 orders of magnitude.

Obviously alien spaceships, were they to exist, would unlikely be the size of quarks or galaxy clusters. And of course, large and massive objects require more energy to accelerate than small, lightweight objects.

But, is there any particular reason why the size of a highly advanced alien species and/or their technology (e.g., "unmanned" probes) — which have presumably mastered nanotechnology and energetic propulsion — could not be, say, in the range of one micron (10–6​ m) to one centimeter (10–2​ m), or 100 meters to 100 kilometers (105​ m)? Or beyond?

Here’s a bad sketch of the range of scale, with the above estimates overlaid:

To me, the coincidence of UFOs and humans/human tech falling within almost exactly the same window suggests that all UFOs are ultimately the product of human intelligence.

 

[Duke]

One of the things that make me skeptical that any UFOs are non-human or off-world in origin is their size. All UFOs, except perhaps in the most astonishing observer-specific experiences, are firmly stuck inside the scale of humans and their technology. This is epitomized in the classic image from the 1951 film The Day the Earth Stood Still:



While I don't for a moment think that alien beings would be so anthropomorphic, we still have the recurring trope of the bodies of alien "biologics" (as reported by David Grusch and described in this apparent work of creative fiction) being roughly humanoid or at least human-scale. And we have the trope of the spaceship, which according to Grusch and Ross Coulthart can be in some cases so large that a crashed one cannot be moved, but which has been hidden by constructing a building on top or something like that.

Even spacecraft of this size shares a narrow window of scale with the largest "flying" human technology (the ISS, dirigibles, large cargo planes, etc.).

The entire range of scale spans from quarks at <10–19​ meters to galaxy clusters at ~1024​ meters, or 43 orders of magnitude. Yet, "mile-long" Starlink trains notwithstanding, virtually every UFO case falls within a scale of roughly 1 meter to 100 meters, or 2 orders of magnitude.

Obviously alien spaceships, were they to exist, would unlikely be the size of quarks or galaxy clusters. And of course, large and massive objects require more energy to accelerate than small, lightweight objects.

But, is there any particular reason why the size of a highly advanced alien species and/or their technology (e.g., "unmanned" probes) — which have presumably mastered nanotechnology and energetic propulsion — could not be, say, in the range of one micron (10–6​ m) to one centimeter (10–2​ m), or 100 meters to 100 kilometers (105​ m)? Or beyond?

Here’s a bad sketch of the range of scale, with the above estimates overlaid:

To me, the coincidence of UFOs and humans/human tech falling within almost exactly the same window suggests that all UFOs are ultimately the product of human intelligence.

Interesting observations, that never occurred to me before. Out of curiosity, would you include the alleged sightings of mile(s) long craft to be within "human-scale."

 

[Edward Current]

Interesting observations, that never occurred to me before. Out of curiosity, would you include the alleged sightings of mile(s) long craft to be within "human-scale."

No and I noted that in the post. The Moon and Venus have also been reported as UFOs, and neither are they human-scale

I might be tempted to say the observation holds true in the case of "legitimate UFOs," but that is problematic....

 

[jarlrmai]

Presuming we are talking about speculative alien species/craft.

Why risk your entire battlefleet being swallowed by a small dog..

On the one hand the anthropic bias might influence our creative works and our speculations on alien life, however ultimately the laws of physics are almost certainly universal and likely place some lower/upper limits on the genesis and evolution of intelligent life and civilisations and thus likely some limit on scale.

Planets can only be so big before they are suns, carbon is a great material for life, water is a great solvent to mediate biolofical processes and can only exist as a liquid under certain conditions etc.

 

[Duke]

I've heard the "secret space program" and black a/c program conspiracy theorists talk about "miles long" craft as well. When I've asked them where such craft were built/based/hangered, I've gotten silence. The one exception was the guy who told the answers to all three were in undersea facilities.

 

[LilWabbit]

Here’s a bad sketch of the range of scale, with the above estimates overlaid:

To me, the coincidence of UFOs and humans/human tech falling within almost exactly the same window suggests that all UFOs are ultimately the product of human intelligence.

I guess as long as biological life is defined (yes, by human intelligence) as requiring some kind of a (simple or complex) configuration of aminoacids into some manner of proteins, then there's a limit to the maximum brain size within a certain margin of variability of planetary gravitational pulls that can sustain said life.

We have no choice but to define biological life using humanly accessible intellectual categories that, however, somehow correspond to the world out there which we have explored for the last couple of hundred-thousand years as H. Sapiens Sapiens.

 

[JMartJr]

While I don't for a moment think that alien beings would be so anthropomorphic, we still have the recurring trope of the bodies of alien "biologics" (as reported by David Grusch and described in this apparent work of creative fiction) being roughly humanoid or at least human-scale.

(Edited to add: I have become aware that the emphasis in the OP was more on scale than body plan. This is less responsive to that,but I'll leave it unless folks feel it is a distraction. Examples of humans' capacity to imagine out-of-our-scale aliens would include the tiny aliens in Pratchett's "Bromiliad Trilogy,"the insanely tiny beings living in Ant Man's quantum realm, up to the sentient stars in Herbert's "Whipping Star." )

Oddly, that does not seem to reflect any limitation in humans' ability to imagine aliens. We can imagine some pretty "out there," non-humanoid aliens(see below) but somehow we don't tend to when coming up with UFO aliens. I suppose one could argue that is evidence that they are real and are all humanoids so that's what we see and report. Or it may be that the archetype of the humanoid UFO alien was set back in the pre-CGI days when movies and TV used humans as aliens and maybe glued some pointy ears on them or gave them a big plastic forehead. (Will be interesting to see if the UFO alien evolves to take advantage of the sort of thing modern special effects can give us as "prompts"...)

Anyway, non-humanoid aliens as imagined, but not as typical of UFO stories:

A Puppeteer from Niven's "Known Space" series

An HG Wells Martian

Heinlein's Venerian Dragon,back when Venus was supposed to be a much nicer place to meet an alien.


One of several Trek "energy beings,"as a handy work-around for making aliens without just gluing foreheads and ears on an actor.


Not humanoid nor really human scale,though their ships were in the range of something we might make. From "Arrival."

I'll stop there so as not to clutter up the server more than necessary -- but I'm having a nice time trying to call to mind examples, and look them up. Reliving some fun memories...

 

[Henkka]

While I don't for a moment think that alien beings would be so anthropomorphic, we still have the recurring trope of the bodies of alien "biologics" (as reported by David Grusch and described in this apparent work of creative fiction) being roughly humanoid or at least human-scale. And we have the trope of the spaceship, which according to Grusch and Ross Coulthart can be in some cases so large that a crashed one cannot be moved, but which has been hidden by constructing a building on top or something like that.

This makes sense at first blush, but you could argue that it's possible that biological evolution on a different planet would take a surprisingly similar path as it did on Earth. People always say intelligent aliens wouldn't be humanoid, they could be some totally different crazy shape. But we have all kinds of shapes and sizes of creatures here on Earth, but we're the only ones who developed intelligence (Here meaning capability to do stuff like math, physics, engineering etc). It's possible that being "medium-sized", having hands etc, are why we evolved intelligence and not other animals. So a similar thing could've happened somewhere else, too.

 

[LilWabbit]

An additional non-trivial point. The basic building blocks of the universe and its laws have turned out to be remarkably discoverable by the human cognition. If, therefore, these building blocks and laws of nature are roughly true, then human terminology may well be adequate in establishing that the basic parameters of biological life (resting upon the foundation of physical laws) are the same all across the universe.

 

[LilWabbit]

A Puppeteer from Niven's "Known Space" series

Looks like a two-headed three-legged ostrich. Progeny of ostriches that ran away from the Chernobyl zoo mayhap?

 

[Ann K]

Given what we know about physics and chemistry, I think your scale is overly broad. To some extent we know the workings of our own brains, and it's reasonable to assume that there's a lower limit to the size of physical organism required to process complex thought, so no, I don't think we can talk about micron-sized entities.

Physical body size has an upper limit on earth, due to things like food and oxygen requirements, heat transfer, and the need of bone strength to support a large body. That might well be different on a planet with a different gravity or an aquatic environment, but something bigger than, say, a dinosaur, would be unlikely to survive long in the conditions they would find on earth.

 

[Scaramanga]

While I don't for a moment think that alien beings would be so anthropomorphic, we still have the recurring trope of the bodies of alien "biologics" (as reported by David Grusch and described in this apparent work of creative fiction) being roughly humanoid or at least human-scale.

I totally disagree. There are good reasons for supposing that any intelligent aliens that could reach Earth would be humanoid. Something with the form of a cat or dog or other 4 legged creature is simply never going to manage technology...for all manner of exponentially cumulative reasons. How are the ancestors of a 4 legged alien ever going to start a fire ? How are they ever going to manipulate the first primitive technology without an opposable thumb ? The problems merely accumulate. And one can forget about aquatic aliens reaching here....as they'd never be able to smelt metals underwater.

As for scale, that issue sort of answers itself. Too small a planet and the atmosphere would be lost. Too large a planet ( in fact really not that much larger than Earth ) and they'd never be able to take off from it in a rocket. It's doubtful we will ever seen aliens from super Earths. We need a roughly Earth sized alien planet. And then gravity itself plays a huge role in determining creature size....with mega sizes simply not being viable. And when these critters take their first step into space....well, they won't be able to if each one of them weighs 50 tons.

And so on, with brain size also being a factor. Too small a creature simply won't have enough brain neurons to be a rocket scientist. Too large a brain....well that would not be evolutionary advantageous as it would be slow compared with a smaller one.

I'd argue that advanced technological aliens would be roughly humanoid in size and form, for the simple reason that nothing else would work. And I'd throw in convergent evolution, which even here on Earth has thrown up eyes on multiple types of creature, and evolved flight at least four different times.

 

[Edward Current]

While I agree with the comments on limits to biological size given what we know of Earthly evolution, y'all are thinking too small, as it were. To me, an independent alien species almost necessarily takes a radically different evolutionary pathway from the start, which may result in different boundaries on species size. Neurons have been mentioned — do we really think evolution necessarily converges on slow, clunky neurons for information processing? As for general morphology, what if they were more like fungi? There's a fungus colony in Michigan covering almost 40 acres — and that's an Earthly species. Opposable thumbs...come on. There are other ways to mechanically manipulate things than with thumbs. Evolution on Earth is constrained by whatever already exists, so an entirely different pathway could very well result in radically different solutions to physical materials manipulation, tissue support (assuming there even is bulky semisolid tissue), information processing, etc. Conceivably, the bounds on size could be very different, although probably not by many orders of magnitude.

To borrow from a contemporary trope, an advanced species may have replaced its biological bodies with technological bodies that are much more compact and efficient. In this way we might have "organisms" the size of a grain of sand, or on the other end of the scale, lightweight filamentous beings that are miles long and nearly invisible to us — much larger than human-scale along one dimension, much smaller than human-scale along another. Yes, I am wildly making sh*t up here, but that's kind of what you need to do to contemplate a question like this.

"Unmanned" drones open up possibilities further. Information transfer is limited by the speed of light, but then again, so is creature-body transfer. If you can translate bodies in space in some useful manner, either locally or faster than light, you might as well move more-expendable drones instead, whose size is constrained in some ways but presumably by less than the bodies of these beings.

 

[Jeffrey Orling]

My sense is having a "hand" that can manipulate "materials" / things is essentially to "advancement"... and storing knowledge would be essential... brains, books..PCs and so forth.

 

[Scaramanga]

While I agree with the comments on limits to biological size given what we know of Earthly evolution, y'all are thinking too small, as it were. To me, an independent alien species almost necessarily takes a radically different evolutionary pathway from the start, which may result in different boundaries on species size. Neurons have been mentioned — do we really think evolution necessarily converges on slow, clunky neurons for information processing? As for general morphology, what if they were more like fungi? There's a fungus colony in Michigan covering almost 40 acres — and that's an Earthly species. Opposable thumbs...come on. There are other ways to mechanically manipulate things than with thumbs. Evolution on Earth is constrained by whatever already exists, so an entirely different pathway could very well result in radically different solutions to physical materials manipulation, tissue support (assuming there even is bulky semisolid tissue), information processing, etc. Conceivably, the bounds on size could be very different, although probably not by many orders of magnitude.

But we don't need to invent weird and wonderful hypothetical creatures. We already have 10 million species on Earth that didn't make it into space.

I rest my case.

 

[Henkka]

As for general morphology, what if they were more like fungi? There's a fungus colony in Michigan covering almost 40 acres — and that's an Earthly species.

But why would something like fungi evolve intelligence? Why don't fungi on Earth evolve intelligence?

 

[LilWabbit]

While I agree with the comments on limits to biological size given what we know of Earthly evolution, y'all are thinking too small, as it were. To me, an independent alien species almost necessarily takes a radically different evolutionary pathway from the start,..

You're entering into fallacious territory with this type of reasoning.

The mind of a human is only as sophisticated as it is. If the human mind -- defined as the human capacity to understand and linguistically describe -- can understand and describe something more sophisticated than itself, then the human capacity to understand and describe would be more sophisticated than the human capacity to understand and describe, which violates the law of identity. A mind of any given being can't therefore describe anything more sophisticated than itself. And yet here you are trying to do that.

The only way out of this fallacy is to accept that whatever amazing alien species you imagine (with your human mind), they cannot be more sophisticated than our most sophisticated ideas, but they can be different in their form.

 

[Edward Current]

But why would something like fungi evolve intelligence? Why don't fungi on Earth evolve intelligence?

Maybe in 200 million years, they will. You could have said the same about vertebrates 200 million years ago.

We already have 10 million species on Earth that didn't make it into space.

And 100 years ago, that was 10 million and one species.

Try to think outside our temporal bubble, folks. Live has been around for 3 billion years. Intelligence, a couple million. Space flight....

And that's Earthly life. We are talking about life that didn't start the same way, and may in all likelihood employ different elements, information storage, and energetic mechanisms.

 

[Edward Current]

You're entering into fallacious territory with this type of reasoning.

The mind of a human is only as sophisticated as it is. If the human mind -- defined as the human capacity to understand and linguistically describe -- can understand and describe something more sophisticated than itself, then the human capacity to understand and describe would be more sophisticated than the human capacity to understand and describe, which violates the law of identity. A mind of any given being can't therefore describe anything more sophisticated than itself. And yet here you are trying to do that.

The only way out of this fallacy is to accept that whatever amazing alien species you imagine (with your human mind), they cannot be more sophisticated than our most sophisticated ideas, but they can be different in their form.

I am talking about their form — this thread is specifically and only about their size. I am not trying to understand how they'd think.

 

[Ann K]

Given what we know about physics and chemistry, I think your scale is overly broad. To some extent we know the workings of our own brains, and it's reasonable to assume that there's a lower limit to the size of physical organism required to process complex thought, so no, I don't think we can talk about micron-sized entities.

An addendum to my own post: This lower size limit analysis doesn't hold if a "smart" (bigger-brained) species were to design the craft to send a "dumb" species - say something like a microbe - on an interstellar voyage. I see no reasonable benefit to doing so, but it's the stuff upon which science fiction would thrive.

 

[tinkertailor]

I think about this a lot. Perhaps the reason we see crafts as being scaled to us has to do with the way we perceive scale, which has been discussed in many threads recently.

Basically, I think people see something weird in the sky (or on the ground like a "landing site") and their brains automatically compare and scale it to the other stuff they see in the sky that they know the detentions to: manned craft made for earth humans. Let's say they see a kite with a weird shape and lights on it. They've never seen such a kite, so all they can compare it to is manned human planes. Thus, their brains say "it's the size of a Cessna, X distance away" or "it's the size of a Boeing, X distance away". Because it doesn't look like a plane, they start to think it could be something weird. When it does acrobatic kite stuff, their brains think it's something no earthly big craft can do at the scale and distance their brains perceive it as, so it becomes firmly UFO-adjacent in their minds.

Consider the priming from Hollywood, too: it is sometimes much cheaper to glue weird stuff to a guy and put him in a suit on a cheap set than to design and animate a freaky otherworldly being and ship from scratch. People internalize that scale, and that's how MUFON and NUFORC are filled with reports of human-scaled ships.

 

[Eburacum]

Even on Earth there are many species which have significant problem-solving capabilities. Several bird species, especially corvids, are smart, playful and competent at a wide range of tasks, despite have brains much smaller than humans. So I am reasonably confident that intelligent species do not need to resemble humans in any significant way. The important characteristics that a technologically capable species requires include a controlling processing centre of some kind (a 'brain', or perhaps multiple co-operating processing nodes in close communication). If bird brains can be much more compact than human and mammal brains for comparable abilities, then maybe an intelligent alien species could be as small as a crow. Another highly intelligent animal on Earth is the Blue Whale, and that is much larger than humanity. I'd guess that most technological species are between ten times smaller than humans and ten times larger, but this may be just human bias.

A technological species would also need some kind of manipulative limb system; tentacles, prehensile trunks, tails, or complex mouthparts might work fine, maybe better than our own limbs. But there's no reason to expect that only bipedal animals could have manipulative organs, or that they would be arranged anything like humanoid arms and hands. In short, intelligent species could be significantly smaller or larger than humans, and need not share a recognisably humanoid form.

Technological species that are vaguely humanoid might be reasonably common, but if we ever met such a species we would be acutely aware of the profound differences between such creatures and ourselves, and this may be more disturbing than meeting a species with a completely different bauplan.

 

[Ann K]

To borrow from a contemporary trope, an advanced species may have replaced its biological bodies with technological bodies that are much more compact and efficient.

You'd still need that species to have evolved through some physical form in the past long before it got to that technological phase. In that case brains of some sort, mobility, and appendages capable of manipulating objects are probably likely. There is also a need for communication with others of their species; indeed, it's hard to see how brains could evolve without a means to share knowledge with each other.

(Wild-ass speculation) Your putative "technological bodies", if possible, might be the thing you'd choose to send on a long interstellar voyage, rather than have for "everyday wear".

 

[JMartJr]

We already have 10 million species on Earth that didn't make it into space.

Yet...

 

[LilWabbit]

An addendum to my own post: This lower size limit analysis doesn't hold if a "smart" (bigger-brained) species were to design the craft to send a "dumb" species - say something like a microbe - on an interstellar voyage. I see no reasonable benefit to doing so, but it's the stuff upon which science fiction would thrive.

Everytime I cough I send microbes on a voyage. But it's regrettably never been interstellar.

 

[LilWabbit]

I am talking about their form — this thread is specifically and only about their size. I am not trying to understand how they'd think.

But assuming their form to be wildly outside the parameters of known biology and known physics (such as not having brains at all whilst intelligent) drifts into that fallacious territory of speculating beyond our best currently available intellectual categories. And if we don't, we must accept these parameters for the discussion and the end result is likely not wildly different from us, however different it may be.

 

[Scaramanga]

Maybe in 200 million years, they will. You could have said the same about vertebrates 200 million years ago.

And 100 years ago, that was 10 million and one species.

Try to think outside our temporal bubble, folks. Live has been around for 3 billion years. Intelligence, a couple million. Space flight....

And that's Earthly life. We are talking about life that didn't start the same way, and may in all likelihood employ different elements, information storage, and energetic mechanisms.

I think you sort of missed the point. Our planet has over 10 million species and probably had another 50 million in the past. And out of all of those only ONE ever made it into space. The hominid shaped one with opposable thumbs. Good luck with your expectations that alien equivalents of cats, dogs, hamsters, crows, elephants, dolphins, etc, are ever going to land on the White House lawn.

People always try to argue some other form of life could make it, but they never say what....and there really aren't a huge number of core topological forms of life when you consider that everything has legs from 0 to 8 or so, or fins, or wings. Critters on other planets are going to have the exact same form of morphology or topology.

 

[Mendel]

Given what we know about physics and chemistry, I think your scale is overly broad. To some extent we know the workings of our own brains, and it's reasonable to assume that there's a lower limit to the size of physical organism required to process complex thought, so no, I don't think we can talk about micron-sized entities.

You seem to be discriminating against hive minds here.

Physical body size has an upper limit on earth, due to things like food and oxygen requirements, heat transfer, and the need of bone strength to support a large body. That might well be different on a planet with a different gravity or an aquatic environment, but something bigger than, say, a dinosaur, would be unlikely to survive long in the conditions they would find on earth.

Article:

A series of air sacs extending from the lungs infiltrated the bones of these dinosaurs, helping to keep them light without sacrificing strength. While an estimated weight of 45 tons for Supersaurus sounds like a lot—especially when translated into about seven African elephants, a traditional unit of measurement for dinosaur size—that’s trim for such an enormous animal.

In a 2012 study, paleontologist Mathew Wedel pointed out that, much like the blue whale, sauropods such as Supersaurus likely had sensory nerves that stretched all the way from their brainstems to the tips of their tails. Such long neurons would have meant relatively long transit times for impulses traveling between brain and tail. A particularly efficient conduction speed along neurons is about 300 feet per second, which means long animals would have pretty slow reaction times. The upper limit on size could be a sign, Wedel writes, “of animals approaching the real physical and chemical limits on living systems.”

Source: https://slate.com/technology/2014/04/whales-or-dinosaurs-what-are-the-biggest-heaviest-longest-animals-ever.html

It's not the food or the heat or the bones—a big body can be quite light—, it's the need to keep it together that puts the constraint on size.

I'm also thinking that a planet could have huge smart aquatic creatures and smaller, human-scale creatures that work together to achieve spaceflight. Though it's true that the aquatic giant would probably not leave the spacecraft, and hence not be seen by human observers.

 

[LilWabbit]

You seem to be discriminating against hive minds here.

A sci-fi idea when assumed to possess consciousness, really, but kinda fun. Like the Eywa from Avatar which is basically a global brain formed by all the species of the biosphere acting as a system with its own global cognitive properties.

Such a being would undoubtedly be in a completely different order of magnitude in terms of size. But violates many basic known parameters of sentient biological life.

 

[FatPhil]

Given what we know about physics and chemistry, I think your scale is overly broad. To some extent we know the workings of our own brains, and it's reasonable to assume that there's a lower limit to the size of physical organism required to process complex thought, so no, I don't think we can talk about micron-sized entities.

Some of that depends on where we're drawing the borders. What is the "individual" in the multineucleate? It makes no sense to say the outer cell wall defines the individual, as they can merge. So you're left with looking at the individual nuclei as the indivuduals, cooperatively riding around in the sac that the big things out there with their so-called microscopes call a "cell wall". OK, slime molds aren't known for their intelligence, but give'em a billion years, and who knows what they could become.
(I often jokingly describe eukaryotes as just vessels that mytocondria use to get around and propagate - why would you draw the line around their spaceship in order to define how big they are?)

Physical body size has an upper limit on earth, due to things like food and oxygen requirements, heat transfer, and the need of bone strength to support a large body. That might well be different on a planet with a different gravity or an aquatic environment, but something bigger than, say, a dinosaur, would be unlikely to survive long in the conditions they would find on earth.

However, once you're in for the long haul through the interstellar medium, you don't need to worry about gravity any more, so the L^2 vs. L^3 scaling considerations don't quite matter as much. The big ones never need to personally head down to the surface, once they've reached a new planet, there's nothing weird about dimorphia to the tune of several orders of magnitude, so no reason to constrain a hypothetical alien to not specialise this way if it's useful.

Likewise the craft themselves - there's no reason to limit alien craft to sizes that are practical to land on planets. A commuter with a Brompton folding bike on an intercity train has no problem biking the last kilometer to work; the trains know their station, and that's good enough. Of course, self gravity means that there will still be limits, but we're well beyond the single-digits in miles stage by then.

 

[FatPhil]

You seem to be discriminating against hive minds here.

Article:

In a 2012 study, paleontologist Mathew Wedel pointed out that, much like the blue whale, sauropods such as Supersaurus likely had sensory nerves that stretched all the way from their brainstems to the tips of their tails. Such long neurons would have meant relatively long transit times for impulses traveling between brain and tail. A particularly efficient conduction speed along neurons is about 300 feet per second, which means long animals would have pretty slow reaction times. The upper limit on size could be a sign, Wedel writes, “of animals approaching the real physical and chemical limits on living systems.”

Source: https://slate.com/technology/2014/04/whales-or-dinosaurs-what-are-the-biggest-heaviest-longest-animals-ever.html

Think outside the box, then. Evolve skin that fluoresces as a pain response, and evolve some extra backward-pointing eyes so you can *see* where you've been hit. If there's one thing we've learnt from evolution on earth it's that even the absurd can happen if conditions favour it. (Clearly you don't fluoresce at a frequency that your predators can see in - duh!)

 

[a_robot_kicker]

A very real interstellar probe project called Breakthrough Starshot plans to send gram scale probes called Starchips to other stars. https://www.planetary.org/space-images/starchip

If we end up seeing an alien spacecraft (active or inactive, modern or ancient) I would put money on it being similarly small or even smaller, because of the incredible energy requirements of moving large masses between the stars seems less surmountable than the engineering required to make an intelligent machine very small.

 

[JMartJr]

But why would something like fungi evolve intelligence? Why don't fungi on Earth evolve intelligence?

Intelligence is not some desired end-goal of evolution. Evolution is not "trying"to get to intelligence, evolution is just a process that leads to species better adapted to survive as a species. Intelligence might be useful for that (it also might be a dead end, let's see how the next few decades turn out) but evolution is limited to working with the random variations in the gene pool of the species -- if those don't turn towards brains, and then bigger brains, evolution can't take a species that way.

But given a whole world full of evolving species, it would not be surprising to see some becoming more intelligent. And given enough such worlds, some slime-mold/fungus analog might start eating the fungus next door, creating a condition where being the cleverer of the neighborhood fungi was advantageous. Given enough of THOSE worlds, and giant sentient fungi heading out to the stars looking for the next meal seems at least possible.

 

[Scaramanga]

I might as well throw this question in here, as it is related....and I simply cannot find a complete answer anywhere...

How much gravity does a planet have to have before it is simply impossible to take off from it using a chemical rocket ? This is one where all you maths and tech and rocket science experts can shine. It is totally relevant to whether aliens from a super Earth ( for example ) would ever be able to be space faring.

From what I can see, it looks like a simple question and answer, but isn't, and I cannot find any site that gives a definitive answer.

 

[Jeffrey Orling]

I might as well throw this question in here, as it is related....and I simply cannot find a complete answer anywhere...

How much gravity does a planet have to have before it is simply impossible to take off from it using a chemical rocket ? This is one where all you maths and tech and rocket science experts can shine. It is totally relevant to whether aliens from a super Earth ( for example ) would ever be able to be space faring.

From what I can see, it looks like a simple question and answer, but isn't, and I cannot find any site that gives a definitive answer.

Seems like some engineers could do a calc.
Lower gravity planets seem to be where civilizations could leave to explore "space". And the amount of gravity seems to play a role in development of a civilization... a lot... ain't happenin'

 

[Henkka]

Intelligence is not some desired end-goal of evolution. Evolution is not "trying"to get to intelligence, evolution is just a process that leads to species better adapted to survive as a species. Intelligence might be useful for that (it also might be a dead end, let's see how the next few decades turn out) but evolution is limited to working with the random variations in the gene pool of the species -- if those don't turn towards brains, and then bigger brains, evolution can't take a species that way.

Yeah I totally agree, that's basically what I was implying with the question.

 

[Mauro]

I might as well throw this question in here, as it is related....and I simply cannot find a complete answer anywhere...

How much gravity does a planet have to have before it is simply impossible to take off from it using a chemical rocket ? This is one where all you maths and tech and rocket science experts can shine. It is totally relevant to whether aliens from a super Earth ( for example ) would ever be able to be space faring.

From what I can see, it looks like a simple question and answer, but isn't, and I cannot find any site that gives a definitive answer.

Great question (which never came to my mind before your post)! Googling using your question (in italics above) verbatim, I get two relevant results from stackexchange. It seems the answer is not so easy and depends on a lot of variables, however estimations have been made (I did not check them nor vouch for their correctness):

Because linear increases in delta-v require exponential increases in mass, small changes to the assumptions you make about fuel tank structural mass and engine thrust-to-weight ratio start to make very large changes in the final size of the rocket.


For example, if you're getting off a 3.6g planet with a 7-stage rocket, the difference between 88% fuel fraction and 92% fuel fraction yields about a 10:1 difference in the total mass of the rocket.


So I don't think it's really reasonable to talk about ultimate theoretical limits; too many engineering factors are involved.


Locking down a lot of variables, I can tell you what kind of rocket you'd need for a given surface g, though. Let's make these assumptions:

• We are placing 1 ton of payload into low planetary orbit.
• Required delta-v to reach orbit, including atmospheric and gravity losses, is 10,000m/s per surface g. Seems to hold for Earth, Mars, and the "Earthtoo" which was discussed in another Q/A.
• We can build rocket stages of arbitrary size, with a tankage propellant fraction of 90%; the rocket stage mass is the tank mass plus the engine mass -- ullage rockets, interstage, etc. is all handwaved out.
• We have an infinite supply of Apollo-era rocket engines: RL-10, J-2, M-1, H-1, and F-1.
• First-stage TWR at ignition must be at least 1.2 (relative to local gravity)
• Middle-stage TWR at ignition must be at least 0.8
• Final-stage TWR at ignition must be at least 0.5

Given those assumptions, here is a table of surface gravity, stage count, first-stage engines, and total rocket mass.

Surface First Total Saturn V
Gravity Stages Stage Mass, t Equivalent
0.5 2 1x RL-10 4.5
1.0 3 1x H-1 49.4 0.02
1.5 3 1x F-1 249.2 0.1
2.0 4 5x F-1 1329.0 0.5
2.5 5 40x F-1 8500.9 3
3.0 6 274x F-1 50722.2 17
3.5 7 2069x F-1 331430.9 100
4.0 8 20422x F-1 2836598.4 950
4.5 8 392098x F-1 47 million 15000
5.0 9 3.5 million F-1 391 million 130000
6.0 11 400 million F-1 38 billion millions
10.0 18 2.88e19 F-1 1.65e21 quadrillions


Up above 10g, something really interesting happens that is kind of a theoretical limit. The mass of the rocket reaches a measurable fraction of the mass of the entire planet it's launching from. [more follows]

https://space.stackexchange.com/que...th-be-before-rockets-wouldnt-work/17576#17576
https://worldbuilding.stackexchange...-a-planet-that-a-chemical-rocket-can-leave-it

Not sure if this was what you were looking for!

Edit: the space.stackexchange article links to a (semi-humorous) arXiv article:


https://arxiv.org/pdf/1803.11384.pdf

 

[Edward Current]

If we end up seeing an alien spacecraft (active or inactive, modern or ancient) I would put money on it being similarly small or even smaller, because of the incredible energy requirements of moving large masses between the stars seems less surmountable than the engineering required to make an intelligent machine very small.

Very insightful.

A similar kind of thing could be posited about biology: Due to the physical constraints of size regarding energy requirements, tissue support, tissue nourishment, etc., vs. the physical constraints of size regarding information processing, it's probably more likely that an intelligent/crafty/tool-making species would evolve that's x orders of magnitude smaller than humans vs. x orders of magnitude larger than humans.

Like this guy:

 

[Jeffrey Orling]

So this raises the questions related to the "amount" of gravity and the "development" of life... into creatures with the body types-features to interact and modify their environment... develop tools and so forth... and an environment with "raw materials" for tool making and so forth.

 

[Ann K]

A similar kind of thing could be posited about biology: Due to the physical constraints of size regarding energy requirements, tissue support, tissue nourishment, etc., vs. the physical constraints of size regarding information processing, it's probably more likely that an intelligent/crafty/tool-making species would evolve that's x orders of magnitude smaller than humans vs. x orders of magnitude larger than humans.

Ask yourself why we didn't develop according to those specs. Warfare - competition - is probably one reason. No developed species would evolve as a monoculture, and your postulated small guys would require no competing species. Don't forget, they'd have to survive through eons of time before they reached that intelligent tool-making stage. Sheer strength is another reason to grow, if they had to survive through the equivalent of our tree-felling, rock-piling ancestors.

Unless we write the sci-fi novel and have them outclassed and outnumbered and taking to space to flee...