RIP Frank Drake

[lpetrich]

Now to fc, the fraction that becomes communicative over interstellar distances.

Here also, we have only one example to work from, and here also, we have multiplicity in some parts but not others.

Humanity originated in small-scale societies subsisting by foraging - hunting and gathering - and making tools from a variety of materials: stone, bone/horn/antler, wood, ... humanity was not capable of interstellar communication until about a century ago.

The first step along the way was the invention of agriculture. It enabled much larger-scale societies. For some curious reason, we invented agriculture only late in our species' existence. We as a species have been around for some 100 thousand years, yet all the successful inventions of agriculture are in the Holocene, the last 12,000 years. I say "inventions" because agriculture was invented separately in several different places. From  Neolithic Revolution - "Map of the world showing approximate centers of origin of agriculture and its spread in prehistory: the Fertile Crescent (11,000 BP), the Yangtze and Yellow River basins (9,000 BP) and the Papua New Guinea Highlands (9,000–6,000 BP), Central Mexico (5,000–4,000 BP), Northern South America (5,000–4,000 BP), sub-Saharan Africa (5,000–4,000 BP, exact location unknown), eastern North America (4,000–3,000 BP)" BP = Before Present.

We have domesticated numerous plants, and we eat most plant parts: roots, stems, leaves, seeds (grains, nuts, ...), seed casings (fruits). We also grow lots of plants as ornamental plants, like for their flowers. Looking elsewhere,  List of domesticated animals - the dog is the only one domesticated before agriculture. Domestic animals are used for food (meat, milk, eggs), raw materials (skin as leather, hair as wool, feathers, ...), labor (carrying loads, pulling plows and vehicles), pest control, and companionship. We even have some domestic fungi, like mushrooms and yeast.

We still forage for wood and fish and seafood and seaweed, but even there, we have tree and fish and shrimp and bivalve and seaweed farms.

All these organisms are courtesy of the abundant evolution of our planet's biota.

 

[lpetrich]

More about fc.

Turning to writing, it has the value that information written down can easily outlive its writers, and it does not need the memory cues that are convenient for memorizing large amounts of memory. Rhythmic poetry was a much-valued art, and its rhythms helped to jog the memories of its memorizers. In fact, some people apparently considered writing a bad thing because it would make people’s memories atrophy and make people only appear to be learned, judging from what Plato wrote near the end of his dialogue "Phaedrus".

Writing was invented two or three times, in the Middle East, in Central America, and maybe also in China. But it then spread far and wide, and knowledge of it even provoked the invention of new systems of it: stimulus diffusion.

Theoretical science is also valuable, but developing it was difficult and slow. It started in ancient Greece and Rome, but it was cut off by the Crisis of the Third Century, a period of coups and strife and civil war. It was not restarted until a millennium later in late medieval Europe, with the discovery of the works of those Greco-Roman proto-scientists.

There were curious failures to develop it in the Byzantine Empire and China, though the medieval Islamic world did do a little.

There is a further problem. The social-brain theory of intelligence suggests that members of sentient species may prefer to concern themselves with social relations and gossip and the like; that sometimes seems very evident in our species. That extends to anthropomorphizing nonhuman species like pet species, like making LOLcat pictures.

However, some of us have Asperger’s syndrome, which may help its sufferers understand impersonal things and features and relations. Not many, but enough to be useful for the rest of us. So might other sentient species have Asperger-like variants?

 

[lpetrich]

Now, L, the lifetime of a communicative civilization.

ts value is very conjectural, because there are several factors that can limit such a civilization’s lifetime.

1. Wars
2. Diseases
3. Environmental problems
4. Resource depletion
5. Loss of interest

(1) This was rather obvious from the Cold War. Both the United States and the Soviet Union built enough nuclear bombs to turn each other’s cities into radioactive wastelands, and other nations have tried to join in.

(2) That is a bit farfetched, but not impossible with suitable genetic engineering, like creating a time-bombed microorganism that spreads without causing symptoms, and then starts attacking its hosts.

(3) These include various ways of impairing the habitability of one’s homeworld, like ruining farmland and altering the climate.

(4) This includes running out of fossil fuels, metal ores, and the like, without developing good substitutes. I think that energy resources are especially critical, since without energy, you can’t do anything else. So it is important to learn how to use long-lived energy sources like the light of one’s homeworld’s star.

(5) There are several ways that this can happen.

1. Reversion to a lower level of technology
2. Turning inward
3. Feeling threatened by the possibility of intelligent entities elsewhere in the Universe
4. Deciding that such entities cannot exist
5. Quitting after failing to discover such entities
6. Considering self-advertisement too dangerous or too expensive
7. Considering searches likewise too dangerous or too expensive

There are even examples of similar things happening, like the loss of interest in the Chinese Treasure Fleets and in the Apollo Moon missions.

But if a civilization can overcome these challenges, then it can last as long as the Universe has usable energy to run it.

 

[bilby]

Now, L, the lifetime of a communicative civilization.

ts value is very conjectural, because there are several factors that can limit such a civilization’s lifetime.

1. Wars
2. Diseases
3. Environmental problems
4. Resource depletion
5. Loss of interest

6. Development of closed communication systems

If your communication is typically by non-radio means, and your limited radio communication is encrypted such that it looks like random noise to less developed civilisations, your lifetime as an advanced civilisation could massively exceed your lifetime as a communicative civilisation.

 

[lpetrich]

6. Development of closed communication systems

If your communication is typically by non-radio means, and your limited radio communication is encrypted such that it looks like random noise to less developed civilisations, your lifetime as an advanced civilisation could massively exceed your lifetime as a communicative civilisation.

That's an argument about leakage radiation, not about deliberate attempts to make contact.

ch5.4 - CP-2156 Life In The Universe - Eavesdropping Mode and Radio Leakage from Earth

Discusses what can be learned from analog-TV broadcasts. The broadcasts have a carrier signal with bandwidth 0.1 Hz and a transmitted-information signal with bandwidth 5 MHz - 50 million times larger. The amount of interstellar noise is proportional to the bandwidth, so one needs a MUCH larger antenna for interpreting our TV broadcasts than for picking up our carrier waves. Like a planet-sized antenna.

That page went into detail as to what one would see. One would mainly see transmitters rising and setting, and one may be able to distinguish the two events by when the horizon cutoff happens, ether before or after the peak. One would find the Earth's sidereal (star-relative) day and the longitudes of the transmitters. The latitudes would be more difficult, though one would find that many transmitters are clustered. One would also find that the carrier frequencies are clustered, meaning that we Earthlings share some conventions about which ones to use.

The Earth's motion around the Sun makes a Doppler shift of about 10^-4, and out of 100 MHz (VHF range), that is 10 kHz. That will be easily observable, and one can find the length of the Earth's year and its projected semimajor axis.

Since the observers will be pointing their antennas at the Sun, they will be able to find its luminosity and estimate its mass. That gives the Earth-Sun distance and the Sun's apparent brightness at the Earth, and it will quickly be evident that the Earth is in the Sun's habitable zone.

The Earth's rotation will produce a shift of about 10^-6, or 100 Hz. That will also be observable, and will help to disambiguate transmitter risings and settings. Observing more than one transmitter will help in finding the Earth's equatorial rotation velocity and the latitudes of the transmitters. For n transmitters:

• Observed: 2*n -- interval and shift for each one
• Parameters: n + 2 -- latitude for each one, Earth equatorial velocity, celestial latitude of the observer relative to the Earth

With that rotation velocity and with the Earth's rotation period, one can find its size.

One might even detect the Moon, because it pulls on the Earth. It makes a signal with size about 4*10^-8 or 4 Hz, rather borderline detectable. One would find its orbit period, and from an estimate of the Earth's mass, its projected mass.

 

[bilby]

That's an argument about leakage radiation, not about deliberate attempts to make contact.

ch5.4 - CP-2156 Life In The Universe - Eavesdropping Mode and Radio Leakage from Earth

The signals discussed in that paper - the BMEWS radar leakage - were discontinued in the mid-1980s, after around two decades of service. The detector - Arecibo - had five decades of service.

The authors themselves note both that our best detector would have been able to detect those only in a tiny bubble of about 15ly radius, containing around 40 stars.

These signals are chosen because they're the best candidates for extraterrestrial detection, far more visible than any deliberate attempts to broadcast have been.

And they then go on to wild speculation about detectors we never built, in an attempt to rescue the whole idea of radio as a means to detect other civilisations. But ultimately it's a case of very short windows of opportunity (a handful of decades), and very short ranges (at best a couple hundred ly).

The galaxy could be teeming with earth-like civilisations, and we wouldn't have a very good chance of spotting any of them. Adding deliberate attempts to make contact with similar effectiveness to those we ourselves have made doesn't change these odds in any significant way.

 

[Loren Pechtel]

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...

I also pointed out why you can't make a silicon backbone to base your life on.

 

[Loren Pechtel]

Now, L, the lifetime of a communicative civilization.

ts value is very conjectural, because there are several factors that can limit such a civilization’s lifetime.

1. Wars
2. Diseases
3. Environmental problems
4. Resource depletion
5. Loss of interest

6. Development of closed communication systems

If your communication is typically by non-radio means, and your limited radio communication is encrypted such that it looks like random noise to less developed civilisations, your lifetime as an advanced civilisation could massively exceed your lifetime as a communicative civilisation.

But as you get big enough you will leave other signs of your existence even if people can't communicate with you.

 

[lpetrich]

I wouldn't want to dismiss the predicted performance from Project Cyclops as "wild speculation". It's straightforward extrapolation from the known performance of existing radio telescopes.

Here's another paper: Detection of the Earth with the SETI microwave observing system assumed to be operating out in the galaxy - ScienceDirect - PDF of that paper

and  Search for extraterrestrial intelligence have some numbers for the Effective Isotropic Radiated Power (EIRP):

• UHF TV: 6*10⁶ W
• BMEWS: 2*10¹¹ W
• Arecibo: 10¹³ W

These transmitters do not radiate isotropically, of course, meaning that only some of the sky will receive their broadcasts at any one time. For Arecibo, we can estimate with Dawes' limit, using ((wavelength)/(diameter))² and ignoring small factors. For 1 GHz, it's 10^-6 of the sky, meaning a transmitter power of 10 megawatts.

 

[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...

I also pointed out why you can't make a silicon backbone to base your life on.

Silicates.

 

[lpetrich]

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

Then going into a lot of detail. Good discussion.

An alternate way of presenting the elements is to discuss them by columns (groups). Doing it by rows (periods) isn't that informative. That's because element properties are much better correlated for columns than for rows.

• Group 1: Hydrogen, alkali metals - valence 1
• Group 2: alkali-earth metals - valence 2
• Groups 3 - 12: transition metals - variable valence
• Group 13: metalloids: boron, metals: aluminum, gallium, indium, thallium - valence 3
• Group 14: nonmetals: carbon, metalloids: silicon, germanium, metals: tin, lead - valence 4
• Group 15: nonmetals: nitrogen, phosphorus, metalloids: arsenic, antimony, metals: bismuth - valence 3
• Group 16: nonmetals: oxygen, sulfur, selenium, metalloids: tellurium - valence 2
• Group 17: halogens - nonmetals: fluorine, chlorine, bromine, iodine - valence 1
• Group 18: noble gases - nonmetals: helium, neon, argon, krypton, xenon, radon - valence 0

For abundances,  Abundances of the elements (data page) is a good collection. It has abundances in the Earth's crust, the Earth's oceans, and the Solar System.

LP' points out that we need to make long-chain backbones. This means a valence >= 3. If 2, then one gets chains with nothing attached, if 1, two-atom molecules, and if 0, isolated atoms.

So we need to look in groups 13, 14, and 15, especially 14 with valence 4. Carbon, a nonmetal, is what our biota uses as a backbone element, and silicon is an often-discussed alternative. But silicon is not as good as carbon. -Si-Si-Si-Si- is not very feasible, unlike -C-C-C-C-, since the element is a metalloid, so one needs -Si-O-Si-O-Si-.

If prebiotic chemistry is any guide, carbon can form a variety of biological building blocks, while silicon forms refractory metal-silicate crystals: rocks.

Group 13 contains a metalloid, boron, and some metals. Not very good.

Group 15 contains nonmetals nitrogen and phosphorus, but neither is very good at forming backbone molecules.

Group 16 contains some nonmetals: oxygen, sulfur, and selenium, though selenium is very rare.

 

[Jarhyn]

That's an argument about leakage radiation, not about deliberate attempts to make contact.

ch5.4 - CP-2156 Life In The Universe - Eavesdropping Mode and Radio Leakage from Earth

The signals discussed in that paper - the BMEWS radar leakage - were discontinued in the mid-1980s, after around two decades of service. The detector - Arecibo - had five decades of service.

The authors themselves note both that our best detector would have been able to detect those only in a tiny bubble of about 15ly radius, containing around 40 stars.

These signals are chosen because they're the best candidates for extraterrestrial detection, far more visible than any deliberate attempts to broadcast have been.

And they then go on to wild speculation about detectors we never built, in an attempt to rescue the whole idea of radio as a means to detect other civilisations. But ultimately it's a case of very short windows of opportunity (a handful of decades), and very short ranges (at best a couple hundred ly).

The galaxy could be teeming with earth-like civilisations, and we wouldn't have a very good chance of spotting any of them. Adding deliberate attempts to make contact with similar effectiveness to those we ourselves have made doesn't change these odds in any significant way.

I would hazard to bet that if we can catch light refracting through an atmosphere, we are more likely to detect advanced civilizations from their exotic atmospheric pollutants than from their broadcasting.

 

[Loren Pechtel]

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

Then going into a lot of detail. Good discussion.

An alternate way of presenting the elements is to discuss them by columns (groups). Doing it by rows (periods) isn't that informative. That's because element properties are much better correlated for columns than for rows.

Chemically, I agree--but the abundance issue means going farther down in columns 13, 14 or 15 doesn't work, there just isn't enough of it around to support a biosphere. Since I started out looking at more columns than rows organizing by column isn't too useful.

I just realized one other issue I haven't seen addressed with silicon--what's the solvent? CO2 dissolves in water, but in what form does silicon show up? Everything you make your core organism out of must be soluble in whatever solvent you're using.

 

[steve_bank]

Spectral analysis of light from stars indicates the composition of the stars, so we can group stars and assume similarr stars have similar sequences.

For similar stars can we assume similar distribution of elements? Do all nebu;a where stars are froming have the same dstibution of elements?

The periodic table as we know it is now complete! The International Union of Pure and Applied Chemistry (IUPAC) has announced verification of the only elements left; elements 113, 115, 117, and 118. These elements complete the 7th and final row of the periodic table of elements.Oct 11, 2019

If that is true then barring completely new science ETss and their technology will be limited to what we have.

 

[Loren Pechtel]

Spectral analysis of light from stars indicates the composition of the stars, so we can group stars and assume similarr stars have similar sequences.

For similar stars can we assume similar distribution of elements? Do all nebu;a where stars are froming have the same dstibution of elements?

The periodic table as we know it is now complete! The International Union of Pure and Applied Chemistry (IUPAC) has announced verification of the only elements left; elements 113, 115, 117, and 118. These elements complete the 7th and final row of the periodic table of elements.Oct 11, 2019

If that is true then barring completely new science ETss and their technology will be limited to what we have.

The stuff in the right part of the 7th row is only of interest to the atomic physicists in their quest to understand the atom. There is no meaningful chemistry with such atoms, they can't be used for anything other than the quest for knowledge. And the rows are a property of chemistry, not physics, completing the 7th row doesn't mean they won't start the 8th.

 

[steve_bank]

My point was any ET will be limited to what we have, barring there is something have not seen or imagined.

 

[bilby]

My point was any ET will be limited to what we have, barring there is something have not seen or imagined.

So it will be exactly as we expect, unless it's something unexpected?

Sharp insight that.

 

[lpetrich]

I just realized one other issue I haven't seen addressed with silicon--what's the solvent? CO2 dissolves in water, but in what form does silicon show up? Everything you make your core organism out of must be soluble in whatever solvent you're using.

On the Potential of Silicon as a Building Block for Life - PMC mentions the solvent issue.

To expand the possibility of plausible silicon biochemistry, we explore silicon’s chemical complexity in diverse solvents found in planetary environments, including water, cryosolvents, and sulfuric acid. In no environment is a life based primarily around silicon chemistry a plausible option. We find that in a water-rich environment silicon’s chemical capacity is highly limited due to ubiquitous silica formation; silicon can likely only be used as a rare and specialized heteroatom. Cryosolvents (e.g., liquid N2) provide extremely low solubility of all molecules, including organosilicons. Sulfuric acid, surprisingly, appears to be able to support a much larger diversity of organosilicon chemistry than water.

First discussing chemical diversity. Carbon can combine with other elements to produce a lot of variety -- variety that we find in our biochemistry.

Carbon with hydrogen alone gives hydrophobic (water-repellent) compounds. Oil and water not mixing is a familiar example of that property. But add oxygen and/or nitrogen, and you get hydrophilic (water-attracting) compounds.

Hydrophilic groups include the likes of -OH hydroxyl, -NH2 amino, -CO carbonyl, -COOH carboxylic acid. Some familiar carboxylic-acid compounds are acetic acid, in vinegar, and citric acid, in citrus fruits.

-COOH acidity is from dissociating into -COO- and H+. But it does so weakly, making it safe from us. "pH" is a measure of the concentration of hydrogen ions. Acid: high, base: low.

Bases -NH2 is a base, attracting hydrogen ions from the surrounding water to make -NH3+

Acid-base chemistry makes some molecules negatively charged, some others positively charged, and yet more others part positive part negative. That makes them stick together more strongly, and stick together in controlled ways.

Life needs a diverse set of chemicals with different functions. On Earth, the diverse set of chemicals are amino acids (to make proteins), sugars, and nitrogenous bases (to make nucleic acids), hydroxyl and keto acids (as core metabolic intermediates), lipids (to make membranes), and more. To build a diverse set of chemicals, life needs a set of elements capable of building molecules composed of many atoms that will provide sufficient biological functionality.

Sufficient chemical diversity needed to build a molecular repertoire suitable for life can only be achieved by a scaffolding element bonded with heteroatom elements. The scaffold atom is one that can join in chains and clusters to construct the skeleton or shape of a molecule, and the heteroatoms provide chemical activity in a molecule. Scaffolds provide the ability to make large molecules, and hence a large number of different molecules (compare the number of stable molecules of the type XnHm that can be formed with X=Nitrogen (three–NH3, N2H4, N2H2) and with X=Carbon (an essentially infinite number of hydrocarbons)). Scaffold elements and heteroatoms tend to be different atoms. The scaffold needs to be relatively stable and unreactive, while at the same time bonding to functional atoms (heteroatoms) that provide chemical functionality and distinctiveness to each molecule (Figure 1).

That figure shows a diagram of a heme molecule, one with carbon, nitrogen, oxygen, hydrogen, and iron. The C's and N's form a ring of rings with the Fe in the center, and the outer parts include H and O.

Of the other common valence-2 and valence-3 elements, sulfur makes pure-sulfur chains and boron makes clusters rather than chains or branching structures, also making them unsuitable as scaffolding or backbone elements. This leaves carbon and silicon.

 

[lpetrich]

Then discussing chemical stability and reactivity, and how one need an intermediate state: not too stable and not too reactive.

Then mentioning alternatives to water for a solvent: "sulfuric acid, carbon dioxide, hydrogen cyanide, propane, ammonia, hydrogen sulfide, ethane and methane, nitrogen, and even neon and argon"

In general, protic solvents are chemically aggressive, and their aggressiveness limits the chemistry that can stably dissolve in them. In contrast to protic solvents, aprotic solvents are generally less reactive and stably dissolve a wider range of chemicals than protic solvents.

Solubility of solids in any solvent generally increases with temperature, so cosmically common aprotic solvents such as liquid methane and nitrogen are poor solvents because they are liquid only at very low temperatures. Thus, the nature and temperature of the solvent in which life operates affects both what scaffolds are viable and what heteroatom chemistry is stable in that solvent.

 

[lpetrich]

Then comparing silicon and carbon chemistry.

Silicon is the closest analogue of carbon. Both silicon and carbon are tetravalent atoms that form primarily covalent (non-ionic) compounds. However, there is surprisingly little similarity between them that goes beyond the statement that both elements “can form four covalent bonds”. In this section, we summarize the similarities and differences of silicon and carbon and discuss the consequences of their different reactivities and chemical properties on the formation of complex chemistry.

The covalent radius of a silicon atom is larger than that of carbon which results in generally longer bond lengths and different bond angles. ...

The bonding preferences of silicon are also different than carbon, mainly due to the availability of low-lying 3d orbitals that allow silicon to form compounds that have five- or six-coordinated silicon atoms ...

Silicon is more electropositive as compared to C, N, O, and H. The higher electropositivity of Si creates an electron-deficient center in silicon and results, e.g., in a stronger bond polarization as compared to analogous carbon bonds, or in a reversed bond polarization of the C–H and Si–H bonds ... As a result of those differences, most bonds that silicon forms with non-metals are more strongly polarized than their carbon counterparts and thus more susceptible to electrophilic and nucleophilic attack.

 Electrophile - forms bonds by accepting an electron pair
 Nucleophile - forms bonds by donating an electron pair

Even bonds that are considered to be very stable, like Si−C, have higher reactivity as compared to their carbon analogues. For example, silicon tetrachloride (containing Si–Cl bonds) is hydrolyzed almost instantly in water, whereas carbon tetrachloride (containing analogous C–Cl bonds), which is also thermodynamically unstable to hydrolysis, is stable for years in the presence of water. Silanes (SiH4, Si2H6 etc.) are stable as pure chemicals for many years but are very sensitive to water in the presence of trace alkali, unlike alkanes. Reactions of both disilane (Si2H6) and ethane (C2H6) with oxygen are clearly exothermic, but ethane may be mixed with oxygen at 200 °C without reacting, whereas disilane spontaneously combusts in air at 0 °C [25].

Then discussing strengths of chemical bonds of carbon vs. silicon.

As a result of these differences in individual bond strengths, the chemistry of organic carbon molecules is dominated by C–C polymerization (catenation, the formation of long chains of covalently linked carbon atoms, e.g., in hydrocarbons). Even though silicon is capable of formation of Si–Si catenated structures (e.g., in silanes), they are much more reactive than their C–C counterparts (especially in water). As a consequence of the greater reactivity of the Si–Si bond, the most common stable polymers of silicon are built from Si–O chains, as the Si–O bond is disproportionately stronger than any other Si-containing bond (Table 1). Moreover, the polymerization of silicon often leads to a meshwork of Si–O chains and not linear polymers like for carbon—recall that the formation of long linear polymers is often cited as an absolute, general characteristic of any biochemistry [27]. As a result, Si chemistry in oxygen-rich environments (e.g., water) ultimately leads to silica (SiO2) (which is a refractory solid rather than a gas, with no double bonds to oxygen as in its carbon equivalent CO2) (Figure 3).

So one makes -Si-O-Si-O-Si-O- instead of -Si-Si-Si-Si-Si-Si-.