RIP Frank Drake

[lpetrich]

Frank Drake was an astronomer and researcher into receiving interstellar broadcasts - he recently died at the age of 92.

•  Frank Drake
• Frank Drake, astronomer famed for contributions to SETI, has died | Ars Technica
• Frank Drake, Who Led Search for Life on Other Planets, Dies at 92 - The New York Times - "He was convinced that human beings would eventually connect with extraterrestrials, and he inspired others to share that belief."

From Wikipedia,

Drake started his career undertaking radio astronomical research on the planets in 1958–63 at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, where he spearheaded the conversion of the Arecibo Observatory to a radio astronomical facility.[5] Using the radio-telescope at Green Bank, Drake mapped the center of the Milky Way Galaxy for the first time, discovered the ionosphere and magnetosphere of the planet Jupiter, and made significant observations of the atmosphere of the planet Venus.[4][5] In April 1959, Drake secured approval from the director of NRAO for Project Ozma to search for extraterrestrial radio communications.[6] Initially, it was agreed to keep the project secret for fear of ridicule. However, Drake was compelled to publicize his work after Cocconi and Morrison published a paper in Nature in September 1959 entitled "Searching for Interstellar Communications"[4][7] Drake started measurements in 1960, using the NRAO 26-meter radio telescope to search for possible signals from the Tau Ceti and Epsilon Eridani star systems. No extraterrestrial signals were detected and the project was terminated in July 1960. However, Project Ozma did lead to the then graduate student Carl Sagan contacting Drake, resulting in a lifelong collaboration between the two scientists.[6][4]

Why listening to interstellar radio broadcasts? Because that's the cheapest sort of thing that one can send across interstellar space.

Cocconi and Morrison 1959: Searching for Interstellar Communications

He led a search effort -  Project Ozma - around 1960. It involved listening for radio broadcasts from two nearby stars, Epsilon Eridani and Tau Ceti, stars that are a little less massive / luminous / hot as the Sun. The listening was done at a radio frequency that FD and others had decided would be a sort of natural signpost: 1420 megahertz, the frequency of radio waves emitted (and absorbed) by a proton's spin flipping as an electron orbits it (H-1 atom). This spectral line is often used as a tracer of neutral hydrogen in our Galaxy.

No radio broadcasts were detected, though to be detectable, they may have to be very strong by Earthling standards. But numerous other SETI efforts - Search for Extraterrestrial Intelligence - have been done in the 60+ years since.

 

[lpetrich]

Back in 1961, Frank Drake contributed a way of estimating how many extraterrestrial civilizations there are in our Galaxy. His famous equation is essentially breaking down that problem into smaller and more manageable subproblems.

N = Rs * fp * ne * fl * fi * fc * L

where

• N = number of communicative civilizations in our Galaxy
• Rs = rate of star formation in our Galaxy
• fp = fraction of stars with planets
• ne = fraction of planets that are Earthlike
• fl = fraction of Earthlike planets where organisms emerge
• fi = fraction of planets with biotas where intelligent life emerges
• fc = fraction of planets with intelligent life where interstellar-communication ability emerges
• L = lifetime of a communicative civilization

Back then, we only had a good handle on Rs. But nowadays, with the numerous discoveries of exoplanets, we have a good idea of fp and a little bit of clue about ne. Though exoplanets are common enough to be present around most stars, many exoplanets have been discovered that are just plain weird by Solar-System standards, exoplanets like "hot Jupiters". So I'm reluctant to claim that we have much of a clue about ne.

The others are still very mysterious, despite numerous discoveries since then.

 

[steve_bank]

Somebody wrote WOW! on the printout.

Wow! signal - Wikipedia

en.wikipedia.org

The Wow! signal was a strong narrowband radio signal detected on August 15, 1977, by Ohio State University's Big Ear radio telescope in the United States, then used to support the search for extraterrestrial intelligence. The signal appeared to come from the direction of the constellation Sagittarius and bore the expected hallmarks of extraterrestrial origin.

Astronomer Jerry R. Ehman discovered the anomaly a few days later while reviewing the recorded data. He was so impressed by the result that he circled on the computer printout the reading of the signal's intensity, "6EQUJ5", and wrote the comment "Wow!" beside it, leading to the event's widely used name.[2]

The entire signal sequence lasted for the full 72-second window during which Big Ear was able to observe it, but has not been detected since, despite several subsequent attempts by Ehman and others. Many hypotheses have been advanced on the origin of the emission, including natural and human-made sources, but none of them adequately explain the signal.

Although the Wow! signal had no detectable modulation—a technique used to transmit information over radio waves—it remains the strongest candidate for an extraterrestrial radio transmission ever detected.[3]

 

[lpetrich]

Turning to fl, the origin of life is still a mystery, though there is some impressive work that approach this origin from both ends.

Turning to prebiotic synthesis, that's possible over a wide range of conditions, though it does not get as far as an organism. Going the other direction, using the evolution of the only biota that we have access to, one also gets close, though one does not quite get there.

Back in 1961, we had a good idea of the evolution of multicelled organisms, though one-celled ones' evolution was very mysterious, and many biologists had given up on that. But with sequencing of proteins, then sequencing of genes, and then sequencing of whole genomes, we acquired a huge mass of data for untangling evolution. Working with it, we confirmed many existing hypotheses and settled some long-time controversies.

In particular, we succeeded in resolving the evolution of one-celled organisms.

There is a big split in cell structure of cellular organisms, between prokaryotes (without cell nuclei) and eukaryotes (with cell nuclei). The ancestral eukaryote turned out to be some prokaryotes that lived together. So we must turn to prokaryotes.

In the 1970's, a very early split in prokaryotes was discovered, between the ordinary bacteria, Bacteria or Eubacteria, and the archaebacteria, Archaea or Archaebacteria. The Bacteria-Archaea split continues to be the oldest known split in the organisms whose genes have been sequenced to any degree.

One can work out what this Last Universal Common Ancestor (LUCA) was like, and it was a full-scale organism with a DNA genome, protein enzymes, and chemoautotrophic metabolism, getting its energy from chemical reactions and making all of its biological molecules from simple precursors.

There are some very LUCA-like present-day organisms: methanogens. They combine hydrogen and carbon dioxide to make water and methane, and they also make all their biological molecules. They are anaerobic and poisoned by oxygen.


Pre-LUCA evolution is more difficult, but a hypothesis has emerged to become the dominant paradigm: the RNA world. In it, RNA served as both information carrier and enzyme. It ended up making proteins, and protein enzymes took over from RNA ones. It also ended up making DNA building blocks from RNA ones, thus having a RNA variant that it exclusively uses for reference copies of genetic information.

The main criticism of the RNA hypothesis that I've seen is that it's hard to make RNA prebiotically. One can make nucleobases prebiotically, but it's very hard to make ribose prebiotically. So RNA may have taken over from something else, but we are not sure what.

Turning to protein building blocks, amino acids, the simpler ones can be made prebiotically, while the more complex ones likely have a later origin. Made prebiotically? That is the likely origin of them in the earliest organisms, since one had to get replication working before one can do a lot of biosynthesis.

 

[steve_bank]

Miller–Urey experiment - Wikipedia

en.wikipedia.org

The Miller–Urey experiment[1] (or Miller experiment[2]) was a chemical experiment that simulated the conditions thought at the time (1952) to be present on the early, prebiotic Earth and tested the chemical origin of life under those conditions. The experiment at the time supported Alexander Oparin's and J. B. S. Haldane's hypothesis that putative conditions on the primitive Earth favored chemical reactions that synthesized more complex organic compounds from simpler inorganic precursors. Considered to be the classic experiment investigating abiogenesis, it was performed in 1952 by Stanley Miller, supervised by Harold Urey at the University of Chicago, and published the following year.[3][4][5]

After Miller's death in 2007, scientists examining sealed vials preserved from the original experiments were able to show that there were actually well over 20 different amino acids produced in Miller's original experiments. That is considerably more than what Miller originally reported, and more than the 20 that naturally occur in the genetic code.[6] More recent evidence suggests that Earth's original atmosphere might have had a composition different from the gas used in the Miller experiment, but prebiotic experiments continue to produce racemic mixtures of simple-to-complex compounds—such as cyanide—under varying conditions.[7]

A lot of hits on electricity and origins of life.

Volcanic undersea vents.

Watched a show on his.

Scientists Build Gun to Mimic Meteorite Crash

Recent lab experiments simulate whether the building blocks of life could have survived the rough arrival on Earth via meteorite impact.

www.space.com

The scientists in this study simulated a meteorite impact by firing cylindrical plugs from a 20 mm gun at a target holding the amino acid-saponite samples in place. Image (Image credit: Marylene Bertrand)

Recreatinghow the seeds of life might have survived aboard an ancient meteorite thatcrashed to Earth is no small feat, but scientists have begun doing just that ina recent lab experiment. The project could help indicate whether life on Earthgot its start from alien organic material that hitched a ride aboard spacerocks.

Perhapsone of the likeliest building blocks of primordial life on Earth came in theform of aminoacids, which are the basic components of proteins. And so a team of U.S.and European researchers focused on trying to replicate how well amino acids wouldfare when a meteorite slams into the ground.
"Thisstudy is the first which tested amino acid quantities similar to those found inreal meteorites," said Marylene Bertrand, a biophysicist funded by theNational Center for Scientific Research (CNRS) in France and lead author of thework published in the December issue of the journal Astrobiology.

I'd say life in some form is common if planet and solar system formation follow patterns.

 

[bilby]

Back in 1961, Frank Drake contributed a way of estimating how many extraterrestrial civilizations there are in our Galaxy. His famous equation is essentially breaking down that problem into smaller and more manageable subproblems.

N = Rs * fp * ne * fl * fi * fc * L

where

• N = number of communicative civilizations in our Galaxy
• Rs = rate of star formation in our Galaxy
• fp = fraction of stars with planets
• ne = fraction of planets that are Earthlike
• fl = fraction of Earthlike planets where organisms emerge
• fi = fraction of planets with biotas where intelligent life emerges
• fc = fraction of planets with intelligent life where interstellar-communication ability emerges
• L = lifetime of a communicative civilization

Back then, we only had a good handle on Rs. But nowadays, with the numerous discoveries of exoplanets, we have a good idea of fp and a little bit of clue about ne. Though exoplanets are common enough to be present around most stars, many exoplanets have been discovered that are just plain weird by Solar-System standards, exoplanets like "hot Jupiters". So I'm reluctant to claim that we have much of a clue about ne.

The others are still very mysterious, despite numerous discoveries since then.

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

 

[Jimmy Higgins]

Back in 1961, Frank Drake contributed a way of estimating how many extraterrestrial civilizations there are in our Galaxy. His famous equation is essentially breaking down that problem into smaller and more manageable subproblems.

N = Rs * fp * ne * fl * fi * fc * L

where

• N = number of communicative civilizations in our Galaxy
• Rs = rate of star formation in our Galaxy
• fp = fraction of stars with planets
• ne = fraction of planets that are Earthlike
• fl = fraction of Earthlike planets where organisms emerge
• fi = fraction of planets with biotas where intelligent life emerges
• fc = fraction of planets with intelligent life where interstellar-communication ability emerges
• L = lifetime of a communicative civilization

Back then, we only had a good handle on Rs. But nowadays, with the numerous discoveries of exoplanets, we have a good idea of fp and a little bit of clue about ne. Though exoplanets are common enough to be present around most stars, many exoplanets have been discovered that are just plain weird by Solar-System standards, exoplanets like "hot Jupiters". So I'm reluctant to claim that we have much of a clue about ne.

The others are still very mysterious, despite numerous discoveries since then.

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

The larger problem is that at some point, it is possible that as life advances, it actually becomes less detectable and harder to distinguish from nature.

 

[lpetrich]

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

If one is looking for liquid water, then an Earthlike planet is a good place to look. Some outer-planet moons have ice-covered oceans, and those oceans might have some simple organisms in them.

 

[Elixir]

Back in 1961, Frank Drake contributed a way of estimating how many extraterrestrial civilizations there are in our Galaxy. His famous equation is essentially breaking down that problem into smaller and more manageable subproblems.

N = Rs * fp * ne * fl * fi * fc * L

where

• N = number of communicative civilizations in our Galaxy
• Rs = rate of star formation in our Galaxy
• fp = fraction of stars with planets
• ne = fraction of planets that are Earthlike
• fl = fraction of Earthlike planets where organisms emerge
• fi = fraction of planets with biotas where intelligent life emerges
• fc = fraction of planets with intelligent life where interstellar-communication ability emerges
• L = lifetime of a communicative civilization

Back then, we only had a good handle on Rs. But nowadays, with the numerous discoveries of exoplanets, we have a good idea of fp and a little bit of clue about ne. Though exoplanets are common enough to be present around most stars, many exoplanets have been discovered that are just plain weird by Solar-System standards, exoplanets like "hot Jupiters". So I'm reluctant to claim that we have much of a clue about ne.

The others are still very mysterious, despite numerous discoveries since then.

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

The larger problem is that at some point, it is possible that as life advances, it actually becomes less detectable and harder to distinguish from nature.

There is also the possibility that there are, at any given time, millions of “advanced” civilizations, but once they become technological they only last a few hundred or maybe a thousand years, so our chances of catching one in the act of existing are negligible.

 

[lpetrich]

About fi, the frequency of intelligence emerging, that is still murky. In our biota, there are several steps along the way that happened several times, and several that happened only once, as far as we are able to determine. So let's look at those steps.

LUCA was a full-scale cellular organism, much like present-day methanogens, but it was very limited in where it could live.

The first step on the way is the evolution of photosynthesis, capturing an additional source of energy. There are actually two kinds: chlorophyll and retinal, from their light-absorbing pigments. The chlorophyll kind evolved in Bacteria, and the retinal kind in Archaea. Retinal photosynthesis is limited to supplying energy, while chlorophyll photosynthesis is also involved in biosynthesis.

The next is water splitting, releasing oxygen. This enabled inhabiting a much wider range of habitats.

Then multicellularity. It emerged several times, though animal-like multicellularity emerged only once. All the other kinds are plantlike, funguslike, and slime-moldlike. It is mostly eukaryotes that have become multicellular, though some prokaryotes have done so also.

If animallike multicellularity is hard to get started, could there be planets with plenty of trees and mushrooms but no macroscopic animal life?

 

[lpetrich]

More about fi.

Living on land has the challenge of avoiding drying out. Nevertheless, the first land organisms were bacteria, branching off rather early. But the first known (multicellular) land plants are much later, in the early Paleozoic, and they were very primitive ones. But animals followed -- several times, and sometimes soon after. Arthropods (insects, arachnids, myriapods, pillbugs, land crabs), annelids (earthworms, leeches), mollusks (land snails, land slugs), vertebrates (fish -> tetrapods).

Helpful for being large on land is having an internal skeleton, but that is not very common. The only examples I know of are vertebrates and woody land plants. External skeletons are common, however. Some kinds of such skeletons are functionally similar to vertebrate ones, like arthropod and echinoderm ones, while others are mostly for protection: shells.

For making tools and using them, it is helpful to have grasping appendages. Jaws can also be used, but they are rather limited. But both jaws and grasping appendages have evolved several times.

About sense organs, vertebrates and cephalopods have independently evolved lens-camera eyes with very similar overall structure, though they differ in a lot of details.

 

[lpetrich]

Still more about fi.

Turning to intelligence proper, I first note that it may not often seem very apparent in our species, but I think that that is because we have very high standards for ourselves by the standards of most of the animal kingdom. Thus, when we see some shoddy reasoning, we often think that its author must be terribly dumb, but that shoddy reasoning is described using linguistic capabilities that are far in advance of what just about every other species on this planet can do. Like a creationist declaring “Evolution can’t be true because dogs don’t give birth to cats.”

Human language is far in advance of what any other species can do, with the possible exception of some cetaceans. Researchers have attempted to teach sign language to chimpanzees and other, but while they can learn a lot of individual signs, they can't get beyond two-sign and three-sign phrases, like "drink fruit" for watermelon and "cry hurt food" for radish. My favorite bit of ape sign language is when ape-language researcher Roger Fouts asked Lucy, a chimp, about some dirty dirty on the floor.

Fouts: What that?
Lucy: What that?
Fouts: You know. What that?
Lucy: Dirty dirty.
Fouts: Whose dirty dirty?
Lucy: Sue. [a reference to Sue Savage-Rumbaugh, a graduate student of Fouts]
Fouts: It not Sue. Whose that?
Lucy: Roger!
Fouts: No! Not mine. Whose?
Lucy: Lucy dirty dirty. Sorry Lucy.

Lying to try to get out of trouble - a rather unflattering sign of intelligence.

That aside, another feature is self-awareness, like being able to recognize oneself in a mirror. Though we learn how to do that early in life, only a few other species are capable of that, like chimps. For instance, to a dog or a cat, the dog or cat in a mirror seems like another one rather than itself.

A big problem with the evolution of intelligence is that big brains are energetically expensive. Large size is useful for processing high-bandwidth sensory information like vision or echolocation, and another theory is anthropologist Robin Dunbar's social-brain hypothesis. He extrapolated from various monkeys' and apes' social-group sizes and brain sizes to find Dunbar's number, about 150.

So human-scale intelligence has evolved only once, in us, though bottlenose dolphins and orcas may come close.

 

[steve_bank]

Civilizations in the Americas did not develop technology comparable to Europe, Asia, and North Africa.

There may be many forms of life even humanoids, but they may not necessarily develop science and technology as we do.

 

[bilby]

Back in 1961, Frank Drake contributed a way of estimating how many extraterrestrial civilizations there are in our Galaxy. His famous equation is essentially breaking down that problem into smaller and more manageable subproblems.

N = Rs * fp * ne * fl * fi * fc * L

where

• N = number of communicative civilizations in our Galaxy
• Rs = rate of star formation in our Galaxy
• fp = fraction of stars with planets
• ne = fraction of planets that are Earthlike
• fl = fraction of Earthlike planets where organisms emerge
• fi = fraction of planets with biotas where intelligent life emerges
• fc = fraction of planets with intelligent life where interstellar-communication ability emerges
• L = lifetime of a communicative civilization

Back then, we only had a good handle on Rs. But nowadays, with the numerous discoveries of exoplanets, we have a good idea of fp and a little bit of clue about ne. Though exoplanets are common enough to be present around most stars, many exoplanets have been discovered that are just plain weird by Solar-System standards, exoplanets like "hot Jupiters". So I'm reluctant to claim that we have much of a clue about ne.

The others are still very mysterious, despite numerous discoveries since then.

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

The larger problem is that at some point, it is possible that as life advances, it actually becomes less detectable and harder to distinguish from nature.

Well we certainly see that here; The high point of radio transmissions escaping into space was the early warning radar transmissions of the Cold War, between 1957 and 1985, which consisted on the NATO side of an array of half megawatt transmitters, mostly pointing into space (albeit at horizon-grazing angles).

From the mid-1980s, these were replaced with vastly more efficient phased array systems operating at around 3% of the power, but able to do a better job of detecting incoming missiles or aircraft.

Other late 20th and early 21st century technological improvements in efficiency have further dramatically reduced the total amount of radio signals escaping into space, such as the use of undersea cables to replace satellite communications for intercontinental telephony, television, and data transfer, and a general move to lower power outputs by more effective noise reduction technologies of various kinds.

So, for the one technologically advanced civilisation we know of, we can assume that even if the lifespan of the civilisation is long, the time spent as a significant emitter of radio signals could be very short. We have been doing it for only about a century, and the rate is rapidly declining, despite our increasing use of technology.

In the early 1960s, when Frank Drake was developing his equation, it would have been reasonable to assume that radio output from Earth would continue to increase rapidly throughout the continuing existence of a technologically advanced humanity. If you had told Drake in 1961 that by 2021 the radio output of our planet would be only a few percent of its 1961 level, he would have assumed that we must have suffered a terrible calamity - probably a global nuclear war.

But it turns out that planetary radio output doesn't increase as technology advances. It has a brief peak, as low efficiency systems become available and popular, followed by a rapid decline as these are replaced by more efficient alternatives.

The number of civilisations detectable from their radio transmissions is therefore Drake's N, but with L modified to the time spent as an inefficient user of radio. In our case, this was from the first big AM transmitters of the 1920s, until the near future, (as the lightspeed delay of satellite communications becomes intolerably long). In round numbers, say a century.

If our technological civilisation lasts a thousand years, or even a million, it's unlikely to ever broadcast radio waves into space anywhere close to as powerfully as we did between 1960 and 1980. And our increasing use of multiplexing, encryption and other technologies means that our signals look less and less like messages, and more like random noise, as time goes on.

 

[Politesse]

Back in 1961, Frank Drake contributed a way of estimating how many extraterrestrial civilizations there are in our Galaxy. His famous equation is essentially breaking down that problem into smaller and more manageable subproblems.

N = Rs * fp * ne * fl * fi * fc * L

where

• N = number of communicative civilizations in our Galaxy
• Rs = rate of star formation in our Galaxy
• fp = fraction of stars with planets
• ne = fraction of planets that are Earthlike
• fl = fraction of Earthlike planets where organisms emerge
• fi = fraction of planets with biotas where intelligent life emerges
• fc = fraction of planets with intelligent life where interstellar-communication ability emerges
• L = lifetime of a communicative civilization

Back then, we only had a good handle on Rs. But nowadays, with the numerous discoveries of exoplanets, we have a good idea of fp and a little bit of clue about ne. Though exoplanets are common enough to be present around most stars, many exoplanets have been discovered that are just plain weird by Solar-System standards, exoplanets like "hot Jupiters". So I'm reluctant to claim that we have much of a clue about ne.

The others are still very mysterious, despite numerous discoveries since then.

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

The equation is meant to provide a MIN rather than a MAX, I think. One can speculate that one might find life in non-earthlike environments, but we know life can occur in earthlike environments. If there are many circumstances under which life could arise, we'd need to amend the entire equation as fl, fi, and fc would all be different in non-earthlike environments. It is possible that silicate lifeforms might develop for instance, but very improbable that carbon and silicon are exactly equal in their capacity for sustaining intelligent civilizations.

 

[Hermit]

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

If one is looking for liquid water, then an Earthlike planet is a good place to look. Some outer-planet moons have ice-covered oceans, and those oceans might have some simple organisms in them.

fl may not necessary either. Who says intelligent life must be carbon based? A year before the astronomer Fred Hoyle was appointed Plumian Professor of Astronomy and Experimental Philosophy in Cambridge University he wrote a science fiction novel titled The Black Cloud. It features a highly intelligent and technologically advanced non-carbon based life form.

 

[Loren Pechtel]

I am unsure that ne even belongs in the equation at all.

The assumption that only earthlike planets could give rise to technologically advanced life is rather parochial and completely unsupported by any evidence.

I have yet to see anyone present a biochemistry that could function without liquid water.

You need something that makes a long-chain backbone. To do so you need a minimum of three bonding sites--two for the chain, one to stick something on the chain. First row: Boron, Carbon, Nitrogen. Second row: Aluminum, Silicon, Phosphorus. Anything below that is going to be too rare to serve as a basis for life.

Boron--pretty rare. Not good. Nitrogen--molecules with a lot of nitrogen are prone to coming apart producing N triple bond N gas plus debris. Basing your life around something prone to exploding isn't a good idea. Aluminum--nope, metallic bonds don't have discrete molecules. Silicon--unfortunately, the atoms are too big. Si-Si-Si-Si chains don't work very well, in nature we see Si-O-Si-O-Si chains. As a rock, fine, but what happens when you try to stick things on that chain? Look at the most basic chain, all the spare bonding points are occupied by hydrogen. (H6OSi2) Once again we have a molecule that would like to come apart--those H's will look fondly on the O. Phosphorus--I don't know any fundamental problems here but it's rare and reactive.

Note that this leaves carbon-backbone as the only viable option for very complex molecules. Now, we need to stick something to that backbone--something with one bonding point open. Hydrogen, Lithium, Sodium, Potassium. (Going beyond that again gets far too rare.) Note how they grow more and more reactive as we go down the chart? Once again, building life on a molecule prone to decomposition isn't a good idea. Thus hydrogen. (Yes, we stick more complex things to the backbone than just hydrogen, but most of those more complex things have loose bonding points that are capped with hydrogen.)

Now, we need something to react with that. First row, Oxygen, Fluorine. Second row, Sulfur, Chlorine. Fluorine has a big problem--it's way too reactive, it bonds to basically everything in sight. It's also rare--no way you'll have a world with enough Fluorine to actually have it available. Sulfur--how is your critter going to get it's oxidizer? By the time you get warm enough for your critters to breathe sulfur you have temperatures that will tend to decompose your hydrocarbons. It's also got a scarcity problem--bound up as SO2.

Life also needs a solvent. It must be liquid over the temperature range your critter evolves in. It must also be very common. Very few molecules qualify, water is probably the ideal one, nothing exists at high temperatures (anything that will be a liquid up there will be rare), perhaps something like methane or ethane could function on a cold world--but note that we are talking about intelligent life. Reactions proceed much slower in temperatures like that--while such life can't be ruled out there hasn't been nearly enough time for intelligent life to arise on something like Titan.

Thus we are left with C-H-O plus occasional other atoms in H2O chemistry as the basis of any natural alien intelligence. (This does not rule out artificial stuff--say, robots taking over. They aren't really based on a biochemistry in the first place.)

 

[Loren Pechtel]

The larger problem is that at some point, it is possible that as life advances, it actually becomes less detectable and harder to distinguish from nature.

Detected directly, I agree. However, we would see the dyson swarms they construct--they would look like a star that doesn't remotely fit the standard star charts.

 

[steve_bank]

Technology in our reality requires the ability to manipulate tools and materials with hands. Unless there are scifi no corporeal critters whio can affec reality.

There is wag wild ass guess,

There is swag scientific wild ass guess. Subjective estimates based on numbers

The Drake Equation is swag, the variables can be guessed at with some science behind it. It is what I call a 'what if' function. It gives you a way to vary parameters and asses effects.

 

[bilby]

By the time you get warm enough for your critters to breathe sulfur you have temperatures that will tend to decompose your hydrocarbons.

But not your silicates...