Fermi Paradox thread; aliens welcome

[Dragar]

One of my friends wrote a paper, with a related blog post, on Bayesian inference - based on our (limited) knowledge, aliens are probably huge.

[ceptimus]

* ceptimus is not convinced by the Bayesian statistics argument but isn't sure why.

[Dragar]

That's the natural reaction to Bayesian statistics!

[JoeP]

Riiight ... I agree with ceptimus.

But beyond the statistics questions, "aliens are likely to be huge" suggests something quite different from their actual conclusion "aliens are likely to be as big as polar bears". Huge to me is blue whales, the First Person Singular entity in the Long Earth series, planet-spanning webs of consciousness. And the subtitle "why we are the ants among intelligent species" also suggests something that much bigger than us:
ant 3mg
human 70kg
ratio ~ 20,000,000
alien 1 million tons

[davidm]

Originally Posted by Vivisectus

This seems to ignore the fact that both science and music are not just made up out of personal achievements. They are part of disciplines that work cumulatively. Both Beethoven and Newton stood on the shoulders of giants. It may not be guaranteed that bursts of genius occur. But because we are an information-transmitting species now, these breakthroughs are now kept and built on.

And it is this information-transmitting between people can be said to have started much earlier than this article stated.

I think that the point the author is making here (it’s a book chapter, not an article; the whole book and his other books are downloadable for free) is that for us and for any intelligent species, ending up with a technological civilization is extremely contingent and defeasible. For most of our history, of course, humans were hunter-gatherers. Maybe there are plenty of intelligent life forms out there but we can’t find the because they are doing something equivalent to our own hunter-gatherer life style?

But I think that his larger and more important point relates to the ant cartoon. These intelligent ants are projecting upon the cosmos their own cognitive, perceptual and conceptual style of intelligence, which means they search for clues to other intelligent life forms by searching for pheromone trails. We do the same by scanning the skies for deliberate radio signals, signs of megastructures, etc; i.e. we project upon the putative aliens our own cognitive, conceptual and perceptual biases. His argument is that aliens my be quite advanced, even quite technologically advanced, without sharing in common with us anything communicable or even recognizable, including maths. Maybe from to time we star aliens in the face without recognizing them, nor they us.

Admittedly this position is open to severe objection, which the author concedes.

[JoeP]

One thing that I sometimes think about (until the aliens suppress it) is timescales of communication and thought. Dragar's link raises the question of aliens being greatly different sizes from us (even sf that allows non-humanoid non-bipedal forms doesn't often venture to much bigger or much smaller scales).

But what if they think and develop and communicate on the microsecond timescale? What if they exist and communicate over thousands of years? We would be much less likely to even spot their communications let alone recognise them as possibly intelligent.

I don't know if it holds as a physical principle, but there's a correlation between speed of living and size, on earth - smaller animals have shorter lives and faster heartbeats. Whale songs are much longer than human songs, I fancy. So maybe massive scale organisms could live for millions of years and communicate over thousands of years (always assuming they lived in an environment where that was adaptive) - and then two-way communication and even travel between stars could be feasible for them.

There are so many ways in which intelligent lifeforms could be completely unlike us.

[Ari]

That's an interesting thought, neurons are pretty slow in the scheme of things and our communication is pretty linear. An alien signal could be super parallel, sending a whole message in a burst of data across a huge spectrum instead of a line of data on a small spectrum.

[mickthinks]

Originally Posted by JoeP

Dragar's link raises the question of aliens being greatly different sizes from us (even sf that allows non-humanoid non-bipedal forms doesn't often venture to much bigger or much smaller scales).

You mean like this?

Hitchhikers Guide To The Galaxy (Aliens attack...

[lpetrich]

Though being big is helpful for getting big brains, large sizes have some difficulties.

I once saw a drawing of a kangaroo rat and a horse jumping vertically, a drawing that showed that they have the same absolute vertical jump height, about 1 m.

I will now explain why that is. It's rather simple physics.

E = m*g*h
E = gravitational potential energy
m = mass
g = acceleration of gravity
h = height

Let's rearrange to find the height.

h = (E/m) / g

(E/m) is the energy per unit mass, and that's a function of muscle physiology. Since it's presumably similar across kangaroo rats and horses, that explains their jump heights being roughly the same. That also makes insects' jump distances less than impressive. Though some arthropods can jump many times their body sizes, their jumps' absolute distances are at most comparable to jumps by kangaroo rats or horses -- or by us.

So being very large means not being very agile.

Being large also has further problems, like low population density and requiring a large area.

[fragment]

Originally Posted by JoeP

Riiight ... I agree with ceptimus.

But beyond the statistics questions, "aliens are likely to be huge" suggests something quite different from their actual conclusion "aliens are likely to be as big as polar bears"

The table in the blog post (I haven't read the paper) suggests the posterior probability for size isn't particular constrained. A polar bear might be the median alien mass, but the 95% credible interval looks like it would go from about the mass of a border collie to three times blue whale.

[JoeP]

Another solution to the Fermi paradox suddenly occurs to me - maybe beyond a certain level of development, all intelligent civilisations eventually get trapped in a state of total introspective inertia trying to understand Bayesian probability assertions.

[lpetrich]

I'll now consider the question of what sizes are possible for organisms on other planets, and what that implies for their biology.

Access : The limits to tree height : Nature
Maximum plant height and the biophysical factors that limit it

Maximum tree height likely constrained by gravity. The sap pressure at the base of the tree is (density)*g*(height), where g is the acceleration of gravity. Since the feasible pressure is constrained by the tree's physiology, we have

(height) ~ 1/g

One finds the same equation for the maximum height of a mountain, and the scale height of the atmosphere, and maximum land-animal size is likely given by that equation also.

Maximum mountain height: 10.2 km (Mauna Kea and Mauna Loa, Hawaii, related to the ocean floor) Mars has higher mountains, like Olympus Mons at 21.9 km, but scaled to the Earth's gravity, it's 8.3 km.

Maximum tree height: 112.7 m (California redwoods, Sequoia sempervirens).

Maximum land-animal height: about 7 m (shoulder height of sauropod Brachiosaurus)

The maximum size of an animal has other constraints, like absorption of food and heat rejection. The blue whale is the largest known animal and likely the largest possible. Largest mass: 173 metric tons, longest: 33 m.

So on a planet with low-enough gravity, land animals could grow to blue-whale size. But I'll have to check on how low an Earthlike planet's gravity can get.

[lpetrich]

I've found this: Biology of the Sauropod Dinosaurs: The Evolution of Gigantism — Steinmann-Institut für Geologie, Mineralogie und Paläontologie

Now for stuff on metabolic rates, lifetimes, and the like.

Kleiber's law states that an animal's metabolic rate scales as M^(3/4) where M is its mass.

This is a case of allometry, and an interesting relationship is (heart rate, land-animal breathing) ~ M^(-1/4)

That's roughly (metabolic rate) / M.

Now for something interesting about lifetime, the billion-heartbeat rule.

Similarity in the number of lifespan heartbeats among non-hibernating homeothermic animals. - PubMed - NCBI -- warm-blooded ones.

An investigation was made of the available data on heart rates and maximum lifespan of a number of vertebrates from a variety of sources; only data pertinent to resting adult non-anesthesized homeothermic mammals and birds in a state of thermal neutrality were subsequently analyzed. All known hibernators were excluded because of their extreme, and largely unknown, range of heartbeat from season to season. Plots of heart rate (beats per minute) against reciprocal of lifespan in years showed surprisingly good fits (r = +0.90 for mammals and r = +0.64 for birds). Computation of the total number of heartbeats in the maximum recorded lifespans of the mammalian and avian species involved in this study showed that the mean cumulative heartbeat number for 31 mammalian species was 100 +/- 8 S.E.M. x 10(7) beats and for 23 avian species was 326 +/- 22 S.E.M. x 10(7) beats. This paper documents this analysis, which supports the concept of a close similarity in lifespan heartbeats among mammalian species and among avian species.

Thus, most mammals live about a billion heartbeats. We are long-lived by mammalian standards, at about 3 billion heartbeats, sometimes as much as 4 billion.

Rest heart rate and life expectancy. - PubMed - NCBI finds similar results. Living fast, dying when? The link between aging and energetics. - PubMed - NCBI and Body size, energy metabolism and lifespan. - PubMed - NCBI find various complications, like lifetime scaling roughly as M^(0.15 - 0.3).

But the overall trend is clear: the bigger that one can be, the longer-lived one will be. (Lifetime) ~ M / (metabolic rate).

-

Metabolism produces waste heat, and that must be rejected. Let's suppose that it's through the skin. How will it scale? Linear size ~ M^(1/3), and area ~ M^(2/3). Thus heat loss per unit skin area is about M^(1/12). That does not vary very much, even over a wide range of size.

-

Let's turn to how fast one can move.

Using the pendulum equation, we can roughly estimate walk velocity as sqrt(g*l) where l is the leg length. The average adult-human walk speed is about 1.3 m/s (3 mph, 5 km/h), while the average adult-human leg length is about 0.75 m. The equation gives 2.7 m/s. Our walk speed is about half that because of our legs' swing angle as we walk.

[lpetrich]

I'll now consider the range of sizes that will be possible for an Earthlike planet.

At the low end, it will be limited by the planet's ability to hold on to an oxygen-containing atmosphere.

thermodynamics - Can low-gravity planets sustain a breathable atmosphere? - Physics Stack Exchange

The escape velocity at the moon's surface is about 2.4 km/s. The mean speed of oxygen at 293 K is about 0.48 km/s.

A commonly quoted rule of thumb says that the escape velocity needs to be 6 times the gas's mean velocity in order for that gas to remain captive to gravity and the values I quoted are related by a factor of only 5.

Escape velocity = sqrt( (8*pi)/3 * G * (avg density) ) * (radius)
G = Newtonian gravitational constant

If a planet is entirely rocky, it will likely be made of magnesium iron silicates, like the Earth's mantle and stony meteorites. It will have a density of about 3.5 g/cm^3.

Iron has a density of about 7.85 g/cm^3.

For reference, I'll calculate the escape velocities for mixtures of rock and iron, all for a 1000-km planet where I ignore compression.

• 100% rock, density 3.5 g/cm^3 -- EV = 1.4 km/s
• Rock with Iron: 50% radius, 12.5% mass, density 4.0 g/cm^3 -- EV = 1.5 km/s
• Rock with Iron: 78% radius, 50% mass, density 5.7 g/cm^3 -- EV = 1.8 km/s
• 100% iron, density 7.8 g/cm^3 -- EV = 2.1 km/s

The second case is Earthlike, while the third case is Mercurylike.

Using a threshold of 2.9 km/s, I get: 2100 km, 1900 km, 1600 km, and 1400 km, respectively.
{3.67657, 3.14459, 2.9,
Looking in the inner Solar System, the escape velocities and adjusted threshold velocities are

• Mercury 5.0 km/s, 3.7 km/s
• Venus 10.3 km/s, 3.1 km/s
• Earth 11.2 km/s, 2.9 km/s
• Moon 2.4 km/s, 2.9 km/s
• Mars 5.0 km/s, 2.6 km/s

Mercury doesn't have an atmosphere, while Mars does. Mercury is at 1.4 times this threshold and Mars at 1.9 this threshold, so I'll bump up the threshold by 1.5.

That gives radii 3100 km, 2900 km, 2400 km, 2000 km.

-

So about 3000 km is the smallest possible Earthlike planet -- half the Earth's radius.

Using 4 g/cm^3 mean density, its mass is about 0.07 Earth masses, its surface gravity about 3.1 m/s^2, and its escape velocity about 4.3 km/s.

This planet will have about 1/4.5 the Earth's surface area, but about 1/3.2 the Earth's surface gravity, meaning that mountains can be 3.2 times higher, and trees and land animals about 3.2 times taller.

[lpetrich]

Looking in the opposite direction, if a planet is large enough, it will start having lots of hydrogen in its atmosphere. A presently-known planet at that limit is Kepler-10c (Super-Earth, Kepler-10c in Wikipedia).

Mass: 17.2 Earth masses
Radius: 2.35 Earth radii
Density: 7.1 g/cm^3
Surface gravity: 30 m/s^2 -- 3 times the Earth's
Escape velocity: 30 km/s
Surface-satellite velocity: 21 km/s

So the highest mountains and tallest trees and land animals will only be 1/3 as high as on our planet.

Mountain: 3 km
Tree: 40 m
Animal: 2 m

So human-sized would be maximum-sized, and one would have to be built like an elephant or a sauropod: four stocky legs.

Space travel would be *very* difficult. On the Earth, to get into low orbit, one needs a delta-V of 7.8 km/s, with 1.5 - 2 km/s extra for fighting the Earth's gravity and its atmosphere (Delta-v budget - Wikipedia). With typical rocket engines, this requires a mass ratio of around 40: nearly 40 tons of propellant and structure for each ton of payload.

Since that's for going to about 10 km/s, going to 15 km/s requires a mass ratio of 250, and going to 20 km/s requires a mass ratio of 1600.

So our home, the Earth, is close to the largest Earthlike planet that permits us to depart from it.

[lpetrich]

Here on the Earth, our average height is 5'2", 157 cm (female) and 5'6", 168 cm (male) (Wikipedia). I'll use an average of 5'4", 163 cm.

Let's scale our height by the accelerations of gravity of these planets:
Smallest Earthlike planet (1/3 * Earth gravity): 16'1", 489 cm
Largest Earthlike planet (3 * Earth gravity) 1'9", 54 cm

Walk speed: the same (3 mph, 5 km/h), since (size) ~ 1/g.

Longevity, relative to 70 years, using M^(1/4) or (size)^(3/4) scaling:
Smallest planet: 160 years
Largest planet: 31 years

[lpetrich]

The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii For exoplanets whose masses are known or at least estimated, authors Lauren Weiss and Geoffrey Marcy estimate their average densities:

For Rp < 1.5 Re, denp = 2.43 + 3.39 * (Rp/Re) -- avg M = 2.7 Me, chisq = 1.5
For 1.5 Re < Rp < 4 Re, Mp/Me = 2.69 * (Rp/Re)^(0.93) -- avg M = 4.3 Me, chisq = 6.2

They identify a density maximum of 7.6 g/cm^3 at 1.4 Earth radii (Re), or 3.8 Earth masses (Me).

The scatter for masses greater than that is likely due to varying amounts of volatiles. The earlier-mentioned maximum rocky planet has a mass of 17 Earth masses, instead of what one finds from their formula, 5.9 Earth masses.

But let's look at their maximum rocky planet.
Mass = 3.8*Me
Radius = 1.4*Re
Density = 7.6 g/cm^3
Surface gravity = 19 m/s^2 = 2 * Earth's
Escape velocity = 18 km/s
Surface-satellite velocity = 13 km/s

[lpetrich]

Some Earthlike planets may be un-Earthlike in another way: by being covered by oceans several hundred mi/km deep.

A possible such planet is Gliese 1214 b, or GJ 1214 b.

Mass = 6.55 * Earth's
Radius = 2.678 * Earth's
Density = 1.87 g/cm^3
Surface gravity = 24 m/s^2 = 2.4 * Earth's
Escape velocity = 18 km/s
Surface-satellite velocity = 12 km/s

Less than rock. So that planet either has a huge ocean, an atmosphere with lots of hydrogen and helium, or both.

If it is a water planet, its mass would be about 25% rock and 75% water. Ignoring compression, the rock would extend to about 80% of the planet's radius, giving a depth of 3400 km for the ocean. For the bottom of that ocean, I estimate a pressure of 800 kilobar.

This particular one has a surface temperature estimated at 393–555 K (120–282 °C; 248–539 °F) from its distance from its star. So the planet's interior must be even hotter. That surface temperature is at the upper end of what biological molecules can survive before starting to fall apart. What is the maximum temperature an organism can survive in (theoretical) : biology and Hyperthermophile - Wikipedia suggest 130 - 150 C.

Ice - Wikipedia contains a phase diagram of water, including some of the 16 known variations of water's solid phase. At the bottom of the planet's ocean, the pressure is some 800 kilobar, and water is either liquid (if hot enough) or Ice VII (if not).

[JoeP]

A New Theory on Why We Haven’t Found Aliens Yet

It's a clickbait title: the article is good enough that I can tell you the conclusion - they are sleeping, conserving their cooling requirements in a hot universe - and it still be worth reading.

Which is not the same as convincing, but as they say it gives us something to think about for our own future.

[JoeP]

If aliens contact humanity, who decides what we do next? | Space | The Guardian

The moment has been imagined a thousand times. As astronomers comb the cosmos with their powerful telescopes, they spot something that makes them gasp. Amid the feeble rays from distant galaxies lies a weak but persistent signal: a message from an advanced civilisation.

It would be a transformative event for humankind, one the world’s nations are surely prepared for. Or are they? “Look at the mess we made when Covid hit. We’d be like headless chickens,” says Dr John Elliott, a computational linguist at the University of St Andrews. “We cannot afford to be ill-prepared, scientifically, socially, and politically rudderless, for an event that could happen at any time and which we cannot afford to mismanage.”

I'm unconvinced. (a) I don't think there's any practical way we can prepare for something we have no idea of the form of ... and (b) it's not going to be urgent. Unless the signals detected are as they arrive at the solar system, we'll have hundreds or thousands of years.

It will be a massive impact on our understanding of the universe, and ourselves. But it won't be communication - the timescales are too great.

[Ensign Steve]

Originally Posted by JoeP

I'm unconvinced. (a) I don't think there's any practical way we can prepare for something we have no idea of the form of ... and (b) it's not going to be urgent. Unless the signals detected are as they arrive at the solar system, we'll have hundreds or thousands of years.

Excellent take.

The far more pressing issue will be when our own AI takes over. Again it's something we cannot prepare for (by definition of the singularity) but it absolutely will be urgent.

And I'm certain we're going to fuck it up. We already are.

[JoeP]

Originally Posted by Ensign Steve

The far more pressing issue will be when our own AI takes over.

The aliens are coming from inside the house.

[Pete Ross Junior]

OK folks. Absence of evidence. Is it evidence of absence?
I certainly take it that way when it comes to belief in God (or the gods).

The application to the Fermi paradox is obvious, but I don't wanna.
But that's an emotional argument.

[LarsMac]

Originally Posted by Pete Ross Junior

OK folks. Absence of evidence. Is it evidence of absence?
I certainly take it that way when it comes to belief in God (or the gods).

Probably not. The fact that we have not seen the evidence, or having seen and had no idea what it meant, or that it was even evidence of something at all, is still a possibility.
Aliens, or God, it's all kind of the same.
We all grew up with some budding notion of what God and Aliens were supposed to be, and spent years learning, and then probably unlearning about God, the Universe and everything, only to reach some point where we conclude it was all bullshit, and learn a whole new set of parameters. And, many continue to run across evidence that trims our sense of self and what it is all about.

It's a big frickin Universe out there, and we've barely made it down the porch steps.

Give it a few more centuries. We might know something, by then.

[Pete Ross Junior]

Maybe we've been looking for the wrong thing.

Jurassic worlds might be easier to spot than modern Earth
https://www.spacedaily.com/reports/J...Earth_999.html

Might a tyrannosaur roam on Trappist-1e, a protoceratops on Proxima Centauri b, or a quetzalcoatlus on Kepler 1047c? Things may not have ended well for dinosaurs on Earth, but Cornell astronomers say the "light fingerprint" of the conditions that enabled them to emerge here - including abundant atmospheric oxygen - provides a crucial missing piece in our search for signs of life on planets orbiting other stars.
Their analysis of the most recent 540 million years of Earth's evolution, known as the Phanerozoic Eon, finds that telescopes could better detect potential chemical signatures of life in the atmosphere of an Earth-like exoplanet more closely resembling the age the dinosaurs inhabited than the one we know today.

Two key biosignature pairs - oxygen and methane, and ozone and methane - appeared stronger in models of Earth roughly 100 million to 300 million years ago, when oxygen levels were significantly higher. The models simulated the transmission spectra, or light fingerprint, generated by an atmosphere that absorbs some colors of starlight and lets others filter through, information scientists use to determine the atmosphere's composition.

...

If not for one very improbable cosmic accident at the end of the Cretaceous (it had to happen somewhere) ...

So no intelligent aliens to swap philosophy with out there, but plenty of Dinosaurs.