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60 years of silence - so far

Turning to fi, we can look at the evolution of our planet's biota to get an idea of which steps are common and which are not.

The original source of energy was reactions like what methanogens use: CO2 + 4*H2 -> CH4 + 2*H2O

But that does not get much energy, and it is confined to certain kinds of environments, like hot springs and hydrothermal vents.

A good additional source of energy is sunlight, and capturing it evolved twice. Both times it was added into existing energy metabolism, but added in two different ways:
  • Retinal photosynthesis - membrane pumping - Haloarchaea
  • Chlorophyll photosynthesis - electron transfer - a variety of bacteria, with chloroplasts descended from one of them
The first kind is only an energy source, while the second kind is also involved with biosynthesis, extracting electrons from various electron donors, energizing them, and inserting them into biological molecules, where they attract hydrogen ions from the surrounding water, thus effectively adding hydrogen.

Water is the best-known of photosynthetic electron donors, but it was likely not the first, and some organisms use others, like hydrogen or sulfur or iron or various organic compounds.

Stripping electrons from water gives molecular oxygen and hydrogen ions, and O2 ends up accumulating in the atmosphere and in the ocean. This makes possible much more efficient decomposition of organic material, thus making animal life possible.
 
File my present post under Useless Trivia, or in the Nitpicking Circular File. It was 65 years ago I bested the other 2nd-graders at arithmetic. -- I guess I'm posting this to brag that I haven't completely lost my arithmetic skill!

pood said:
Even on our own planet, if you condensed the entire history of the earth into a single calendar year, with the earth forming on Jan. 1, modern humans didn’t appear until about one-tenth of one second before midnight on the last day of the year, Dec. 31.
Click to expand...

I pride myself on good numeric intuition and this claim seemed very unlikely to me. So I fired up a calculator. Modern man's emergence is usually dated to 200,000 years ago. If that's "one-tenth of one second" in the analogy, then 2 million years is a second, 120 million a minute, 7.2 billion an hour, and there's almost 173 billion years in the analogic day. Even assuming the analogic year is non-leap we must multiply by 365 to get 63 trillion years for "the entire history of the earth", about 14000 times as large as usual estimates for the Earth's age.

Sorry for nitpicking. Next I will look for misplaced commas.
 
Swammerdami said:
File my present post under Useless Trivia, or in the Nitpicking Circular File. It was 65 years ago I bested the other 2nd-graders at arithmetic. -- I guess I'm posting this to brag that I haven't completely lost my arithmetic skill!

pood said:
Even on our own planet, if you condensed the entire history of the earth into a single calendar year, with the earth forming on Jan. 1, modern humans didn’t appear until about one-tenth of one second before midnight on the last day of the year, Dec. 31.
Click to expand...

I pride myself on good numeric intuition and this claim seemed very unlikely to me. So I fired up a calculator. Modern man's emergence is usually dated to 200,000 years ago. If that's "one-tenth of one second" in the analogy, then 2 million years is a second, 120 million a minute, 7.2 billion an hour, and there's almost 173 billion years in the analogic day. Even assuming the analogic year is non-leap we must multiply by 365 to get 63 trillion years for "the entire history of the earth", about 14000 times as large as usual estimates for the Earth's age.

Sorry for nitpicking. Next I will look for misplaced commas.
Click to expand...

No, thanks, I’ll check the math. I saw that on a science documentary once, I think by someone with good credentials. Perhaps I misremembered or misunderstood. I”ll do some reserch.
 
While oxygen-releasing photosynthesis evolved only once, multicellularity evolved several times, though animal-like multicellularity also evolved only once. The other kinds are plantlike, funguslike, and slime-moldlike.

Living on land is necessary for creating interstellar-capable technology, and it evolved several times among animals and once among plants. Among animals, vertebrates colonized land once, arthropods several times (insects, arachnids, myriapods (centipedes, millipedes), pillbugs, land crabs), mollusks once (land snails and slugs), annelids twice (earthworms, leeches).

Another useful feature is grasping organs, what I'd mentioned earlier.

Turning to human-scale intelligence, it is rather difficult to define, but it likely has several parts:
  • Tool making
  • Deliberative reasoning
  • Being very social
  • Theory of mind - what others are thinking
  • Conception of oneself
  • Language
  • Transmission of information down the generations
  • Consciousness (?)
Only a few species come close to us.

Having a conception of oneself can be tested by seeing if one can recognize oneself in a mirror. We can do that starting at some 18 - 24 months of age, but only a few other species have shown evidence of being able to do so. In the absence of that ability, what they see seems like another member of their species.

Human language capability is far in advance of what any other present-day species is capable of, with the possible exception of some cetaceans. I remember being interested in teaching chimpanzees sign language, but while the animals can learn lots of individual signs, they cannot string those signs together very much. The most they have done is two-sign and three-sign phrases.

So it's hard to say whether we were likely to emerge or whether we are the results of some very lucky accidents.
 
200,000 years is 0.005 percent of 4.6 billion, an awfully small amount. Now it just remains to condense 4.6 billion to one calendar year and then do the math ..
 
Anatomically modern humans appear in the last 23 minutes of the last day of the year, if the history of the world were condensed to one calendar year. Thanks for the correction. That’s still an awfully tiny amount of time.
 
lpetrich said:
One finds a similar limit on the height of a tree, because the tree's roots must push their water up to the top of the tree.
Click to expand...
Nitpick - The leaves pull the water up from the roots; If the roots pushed the water up to the leaves, every time you chopped down a tree, you would have a fountain.
 
pood said:
Anatomically modern humans appear in the last 23 minutes of the last day of the year, if the history of the world were condensed to one calendar year. Thanks for the correction. That’s still an awfully tiny amount of time.
Click to expand...
At this scale, writing appeared about 40-50 seconds before midnight on December 31st, and the enlightenment around 2-3 seconds before the end of the year. The entire industrial era has lasted about one second.
 
I look at it in terms of end to end 50 year life spans'

The alleged time of Jesus was about 40 lifespans. The founding of the USA was around.4 lifespans ago.

It would mean somebody born in 1800 would be able to talk with someone born in 1850.
 
bilby said:
lpetrich said:
One finds a similar limit on the height of a tree, because the tree's roots must push their water up to the top of the tree.
Click to expand...
Nitpick - The leaves pull the water up from the roots; If the roots pushed the water up to the leaves, every time you chopped down a tree, you would have a fountain.
Click to expand...

Deciduous trees have an exception to that principle. In midlatitude spring, they have to make new leaves, and to do that, their roots push sap upward. Why does sap rise in spring? - Woodland Trust and What makes sap rise in decidous trees in the Spring? - Quora


How tall can trees grow? | Live Science
noting
The limits to tree height | Nature

From LiveScience:
Two main opposing forces affect a tree's height; one pushes it upward while the other holds it down. By analyzing the interplay between these forces, a team of biologists led by George Koch of Northern Arizona University calculated the theoretical maximum tree height, or the point at which opposing forces balance out and a tree stops growing. This point lies somewhere between 400 and 426 feet (122 and 130 m).

On the one hand, the researchers found, trees in forests "desire" to grow as tall as possible to overtake neighboring trees and reach stronger sunlight. On the other hand, gravity makes it more and more difficult to haul water upwards from the roots to the canopy as the tree grows, and leaves thus become smaller near the top.
Click to expand...
Thus, h ~ 1/g

How Big Can a Land Animal Get? | Science| Smithsonian Magazine
and
How big can animals get? | Live Science
At least in theory, though, there may be a hard limit — enforced by the laws of physics — of about 120 tons (109 metric tons) for land animals, according to Felisa Smith, a professor of paleoecology at the University of New Mexico. "To be bigger than that, on land, your legs would have to be so wide to support your body that you couldn't efficiently walk," she told Live Science in an email.
Click to expand...
I'll be omitting a lot of dimensionless numerical factors here. For density (den) and size L, the force on one's legs is den*L^3*g. The area of one's legs is (f*L)^2. So the pressure on them is den*L*g/f^2.

Thus, L ~ 1/g
 
Force = Newtons = mass * acceleration = ma.
Pressure = Pascals = Newtons/meter^2
1 Pa = 1 Newton/m^2

For a footed critter
F = mg = m*9.8m^2/s^2, m critter mass in kg
Pressure per foot =( F/n_feet)/area_each_foot

Imagine a heavy machine with four feet of small diameter. To distribute the weight on a floor place the machine on a large stiff steel plate. The total weight of the machine is distributed across the plate.
 
 Allometry discusses body-size scaling in detail, though it does not have much on expected variation with surface gravity. "Principles of legged locomotion identified through allometry" mentions that animals have similar gaits when they walk with similar "Froude numbers" -  Froude number is v/sqrt(L*g) for velocity v, typical size L, and acceleration of gravity g. The Froude number is dimensionless, and compares a velocity to the effects of gravity.

Using body scaling L ~ 1/g, one finds that the Froude number ~ v, meaning the same walk speed for some relative size.
 
Turning to fc, the proportion of intelligent species that invent interstellar communication, though I might add interstellar travel.

Here again, we don't have much to work from.

 Behavioral modernity emerged something around 100 thousand years ago, in some population in eastern and/or southern Africa, and this population became the ancestors of all members of our species. But our ancestors foraged over nearly all that time, hunting and gathering. Canada and northern Europe were iced over for most of that time, and much of the climate was very dry. But this Ice Age ended some 10 thousand years ago, and over the next few thousand years, people in several places of the world invented agriculture, domesticating various plants and animals. That enabled much larger-scale societies that what we had for nearly all of our species's existence.

I once created a thread on why we didn't invent agriculture earlier, and I mentioned the hypothesis that Ice-Age climate was too unstable.

It took a long time from the invention of agriculture to the invention of writing, with the oldest writing being some 5 thousand years old:  History of writing Writing was independently invented a few times, frequently borrowed, and sometimes invented from "stimulus diffusion" - learning about writing, then inventing a writing system.

Abstract science, theoretical science as distinguished from technology, was started some 2,600 years ago, by the Presocratic philosophers, starting with Thales of Miletus (~ 624 - 545 BCE) -- Presocratics | Internet Encyclopedia of Philosophy and in Richard Carrier's hypothesis, continuing to roughly the Roman Empire's Crisis of the Third Century (235 - 284 CE), when it suffered civil war and economic slumping. The Middle East part became a breakaway state under the leadership of Queen Zenobia, and the northwest part became another breakaway state, the Roman Empire of the Gauls. To keep going, the Empire's leadership repeatedly debased its coinage, mixing base metals into the silver for its coins.

As RC tells it, philosophers turned to mystical revelation, notably the Neoplatonists, and early Christian theologians were also like that. Interest in abstract science was killed for roughly a millennium, and it slowly restarted in Western and Central Europe with interest in the works of the likes of Aristotle. It was a rocky road, and it was some time before Aristotle-thumping was generally rejected as a good method of research.

It took some time for the practical value of abstract science to become apparent, and Isaac Asimov proposes as a marker event Benjamin Franklin's invention of the lightning rod in the mid 18th century. He had been experimenting with electricity, and he noticed that lightning seemed like some giant electric spark. He flew a kite in a thunderstorm, and he discovered that the clouds became electrically charged. Since he knew from his researches that metals conduct electricity much better than nonmetals, he proposed a metal rod for conducting lightning away from buildings. He tried one, and it worked. Lightning was successfully tamed.

It took some more development of both science and technology before we could communicate over interstellar distances, and even to travel across interstellar distances, if one counts derelict spacecraft as travel.

As with fi, fc has some single events as well as repeated events.
 
There is a problem with our science and technology. Many people find it hard to understand, especially the more mathematical parts.

A further problem may be from how our species emerged. Robin Dunbar's "social brain" theory says that we got the way we are as a way of navigating relationships with each other. That may explain why many of us have so much difficulty with technical issues. But some of us, those of us with high-functioning autism or Asperger's syndrome, have a much easier time. Could there be some genetic difference? Or something in the way we grow that makes some people flip a switch and become like that? Whatever happens, it may be useful enough for a genetic tendency for that to persist. It's something like Robert Sapolsky’s schizotypal-shaman theory, his theory that schizophrenia persists because mild cases of it are good for acquiring certain types of expertise. In effect, it may be necessary to have some nerds and geeks if one is to build advanced technology, even if most others are social yakety-yaks.
 
Fl*Fi*Fc can be estimated based on the single datum we have for these probabilities on an earthlike planet - planet Earth.

We have been capable of interstellar communication (to a very limited degree) for about a century; And our planet has existed for about 4,500 billion years.

So to date, our one earthlike planet has had life, and intelligence, and has been using radio, for about 100/4,500,000,000 = 1/45,000,000 of its existence.

The question is, how much longer will this ability persist? If the answer is "less than several orders of magnitude as long as it has existed so far", then the probability of detecting another intelligent life form must be very low; We would expect maybe a thousand in the entire galaxy, and the chances of any being within the 100ly or so necessary to detect us (and therefore to be detectable by us) is tiny.

And radio is already way past its peak. It's rapidly becoming a rare, niche, technology, with few powerful signals being sent (deliberately or accidentally) into space. The vast majority of our communications is moving to landlines or very short range, low-power, radio.

The answer to the Fermi paradox is likely that radio broadcasting life is very uncommon. After all, radio broadcasting life occupies a minuscule fraction of the timeline of the one planet that we know for certain has life capable of producing those signals.
 
Finally, the lifetime of a communicating civilization, L. There are numerous factors that can limit it:
  • War
  • Environmental Problems
  • Resource Depletion
  • Disease
  • Social / Political / Economic Collapse
  • Technological Failure
  • Loss of Interest
I'm not very willing to speculate about them.

But I note that the rarity solution of the Fermi paradox is sometimes called some  Great Filter -- something or other makes the emergence of communicative civilizations *very* rare.

In the evolution of life on our planet, there are several events that happened only once, like the origin of life, the origin of oxygenic photosynthesis, and the origin of animallike multicellularity. If these events are very improbable, then we would be very lucky to come into existence.
 
lpetrich said:
Finally, the lifetime of a communicating civilization, L. There are numerous factors that can limit it:
  • War
  • Environmental Problems
  • Resource Depletion
  • Disease
  • Social / Political / Economic Collapse
  • Technological Failure
  • Loss of Interest
I'm not very willing to speculate about them.
Click to expand...
That's far from an exhaustive list of reasons why a civilisation might 'go silent'.

For example, technological success is another. Radiating signals into space is hugely inefficient, if those signals are intended for local receivers.

To make an analogy, fifty years ago, you could tell which way the nearest city was, even if it was below the horizon, by its smoke. The loss of that "signal" doesn't represent technological failure; It's not due to cities becoming smaller, or less productive, or less prosperous.

Obviously this doesn't apply to deliberate broadcasts intended to be seen by aliens. But how many such signals have we broadcast, and how likely are we to do significantly more of that kind of thing? What is the reduction in probability of our signal being detected when it's directed at a specific nearby star which we don't even know has a life-bearing, much less a civilisation bearing, planet?

I suspect that the whole idea that we should expect to ever detect another civilisation, even if such civilisations are fairly common in our galaxy is deeply flawed. Space is too big, light speed is too slow, and wastefulness is too short-lived.

A wasteful society, that accidentally sends a lot of energy into space as fugitive radio signals, is likely to cease to do that fairly quickly, whether or not it collapses. Fibre optic cables are simply a better solution for on-planet communications than radio is - once you reach the technological level that allows mass production and installation of such cables at intercontinental scales.
 
The only way communicating makes sense is if we find a place to communicate with. A portion of the issue is if we detect possible advanced life on another planet, that planet actually needs to be close (won't be) enough to establish viable two-way communication. Anything past 15 light-years and who knows who is talking back and forth. Heck, we'll likely destroy ourselves in an attempt to one up other nations to build ties first.
 
bilby said:
Fl*Fi*Fc can be estimated based on the single datum we have for these probabilities on an earthlike planet - planet Earth.
Click to expand...
Except that our existence is consistent with a very low value of that product -- we would then be very lucky to exist.
 
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