Further Away From the Lamp-Post

By cjohnson | May 24, 2006 11:44 am

You know the metaphor. Somebody’s looking for something, perhaps their keys, in the dark. There’s a lamp-post somewhere, spreading a circle of light. They confine their search to the circle of light, where they can see. Of course, the keys can be anywhere, not just under the light, and so this search has a seriously limited scope.

Well, a lot of research is like that, somewhat inevitably. Often with the additional limitation that you’re not sure what you’re looking for, either. You’re just hoping you’ll know it when you see it. This happens in all fields.

enceladusOne place where I -as an outsider- always feel somewhat frustrated by the discussions is in the search for life elswhere in the universe. NASA makes some announcement about this sort of thing from time to time and it is always phrased in terms of looking for water. (See e.g., Saturn’s moon, Enceladus (right) where a water geyser was identified recently.) I find that a bit annoying, since they never mention other possibilities. I understand how crucial water can be for life as we know it, but is that really the only sign we should be looking for? And what if it is a red herring?

And what about life as we don’t know it? How do we know we’re not missing huge deposts of life on those objects in the solar system that we’ve ignored because they don’t have water? And one can go to some extremes with this, and have some fun. One thing I especially love to do is speculate about life existing in conditions that are so extremely different than ours that pretty much everything we can imagine about their experiences would be incredibly alien to us. How about creatures that live on the surface of the sun, for example, or in the accretion disc of a black hole, while it sucks the life out of its neighbouring star? What about vast gaseous creatures the size of several star systems, incredibly long-lived and slow to move – but alive, nonetheless. I could go on, but you can have more fun making up your own examples, I’m sure. The problem with most of them is – how would be able to find and recognize them as life?

Coming back to the lamp-post, or at least near it, I was pleased to see an article by Britt Peterson on Seed’s website about two colleagues of mine, USC professors Douglas Capone and Kenneth Nealson. It was about taking a different focus in the current searches for life.

Incidentally, I’d never really known what exactly “Astrobiology” was before this semester, nor met a real practicioner of the craft. Then Douglas approached me at a reception one day and asked me to come and teach a guest lecture to their Astrobiology class on the origins and evolution of the universe, right up to the formation of the Solar System. I was delighted to do it, of course, and in fact invited my colleague from Astronomy, Ed Rhodes, to do the second half of the presentation, focusing on the formation of the solar system, the search for extrasolar planets, etc, topics about which he has much more knowledge than I. We had a great time (and from the questions, possibly the students too), and may well do our double act again next year. (Perhaps even polish it up, get a manager, and take it on the road….)

The article refers to an opinion piece my colleagues wrote in the journal Science. They talk about focusing on Nitrogen. Here are some extracts:

“Water just provides the context for life as we know it,” said Douglas Capone, an environmental biologist at USC and a co-author of the opinion piece, along with Earth scientist Kenneth Nealson. “If we wanted to look for really solid evidence that life had ever existed, organic nitrogen deposits would be good things to look for.”

The article quotes him further…

“They can go all over the place looking for evidence of past water formation, or even finding water deposits maybe down deeper in the geological strata,” he said. “It’s still not going to answer the fundamental question of whether there ever had been life on Mars.”

and…

Nitrogen, on the other hand, is an ingredient of life, one of the building blocks in the protein and nucleic acids from which all life on Earth is made. Its appearance on a planet is difficult to explain if life similar to ours is not also present, the authors said.

“At least on a body that has had a separation of continental and oceanic components, the existence of nitrogen on continents is not easy to explain without special life-supplied chemistry”

Lifting further from the Seed article:

Currently, nitrogen has not been discovered in great quantities on Mars, but evidence suggests it may have been present in the ancient past of the red planet. While Capone said that he is unconvinced that life ever existed on Mars, in the article he and his colleagues argued for a greater emphasis on assessing nitrogen levels there, while still keeping track of the aquatic evidence.

“Our recommended approach might be to search for the nitrogen, characterize and quantify it,” the authors suggest in their paper. If its abundance and chemistry cannot be explained by abiotic processes, do not leave until it is explained and when it comes to sample return, bring back anything that is enriched in nitrogen!”

Actually I find this really very interesting, and would like to know more about the whole Astrobiology field. Are there any among our readers who can tell us a bit about the sorts of signatures that people have thought about? And who gets to call the shots about what instrumentation goes up on the various spacecraft we send to our neighbouring heavenly bodies? There’s a limit to payload sizes, so prioritization must be done. What thoughts go into this? What about ground-based searches? Is spectroscopy, etc, still an active tool in this field, or is it long since had its day?

I like this nitrogen discussion. Of course, it is not really going as far as looking beyond the lamp-post, but rather (to stretch the analogy rather poorly) making sure one is at least using colour vision in one’s search in the circle of light. How one really goes out there and looks for life patterned according to a truly different blueprint is anyone’s guess. It would probably be a very lucky shot in the dark to find and identify something truly different, given our current understanding.

-cvj

CATEGORIZED UNDER: Science
  • Ken Muldrew

    In the 70′s Jim Lovelock suggested to NASA (for whom he was working at the time) that the search for extraterrestrial life should concentrate on looking for planetary atmospheres that were far from equilibrium. The nice thing about this approach (in Lovelock’s view) was that they could search with telescopes rather than spaceships. Apparently NASA wanted to send spaceships and the search for life was justification, so they didn’t really care for what Lovelock had to say on the matter.

  • Rick

    At the risk of sounding like a quack, I’ve often given thought to the “alien life” issue, and NASA’s attempts to send out the occasional contact probe.

    The mere fact that we make serious attempts tells me that at least we humans, as a species, are willing to suspend doubt as to the ‘possibility’ of alternate life forms far enough that we’ll spend money exploring the options.

    I suggest we take that thinking just a tiny, slight step further and assume that, if such a race of aliens exist, why do we always assume that we’re the ‘more developed’ of the two? Perhaps they long ago left coded communications here for us to de-code, perhaps along with our unexplained mysteries of Aztec cultures, pyramids and stonehedge, or perhaps totally and completely unrelated.

    Perhaps language is such a code. Wouldn’t that be the most obvious place to plant the code…somewhere where we all look every day? Somewhere in what we teach our babies before they can even articulate words? What do we teach? The ABC’s.

    I’m not suggesting this actually is the case…just asking ‘why not consider the possibility?’

    You conclude your piece with the speculation: “How one really goes out there and looks for life patterned according to a truly different blueprint is anyone’s guess.” Just a big What-If here: What-if our 26-symbol alphabet was the key to such a blueprint, such a pattern? We use a subset of it to describe the complexities and sequences of DNA, and everyone accepts that as science, why not something on a fuller scale?

    There is a uniqueness to each letter of the alphabet when it is considered as falling into one of three categories, or some combination thereof: directional, closed or open system, and complete or incomplete system.

    Something worth pursuing, or complete quackery? I’ve often wondered.

    I enjoy reading your thoughts, thanks for posting them to share.

  • http://eskesthai.blogspot.com/2005/08/entanglement-interpretation-of-black.html Plato

    Lamppost, doesn’t this refer to Lisa Randall?

    Anyway no big annoucement here:)

    Probability 1, by Amir D. Aczel, page 99,

    In 1994 Allan Hills 84001 was handed over to David Mckay of Nasa’s Johnson Space Center. Mckay asembled a nine-memebr team including scientists at NASA, Stanford University, the Universty of Georgia, and McGill University in Montreal. The team endeavored to determine whether there were any signs of life on the meteorite.

    Of course the question is “geological forming in nature.” How we see it?

    Clementine color ratio composite image of Aristarchus Crater on the Moon. This 42 km diameter crater is located on the corner of the Aristarchus plateau, at 24 N, 47 W. Ejecta from the plateau is visible as the blue material at the upper left (northwest), while material excavated from the Oceanus Procellarum area is the reddish color to the lower right (southeast). The colors in this image can be used to ascertain compositional properties of the materials making up the deep strata of these two regions. (Clementine, USGS slide 11)

    http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/clm_usgs_11.html

    Would this be a good example of what you are saying?

  • http://www.amara.com/ Amara

    I am curious how and to which cultures the “key story” has spread, but here it is from the Sufis via a classic Mulla Nasrudin story:

    ##################################################
    On one occasion a neighbor found Nasrudin down on his knees under a lamp across from his house, looking for something.

    “What have you lost, Nasrudin?”

    “My key,” said Nasrudin.

    After a few minutes of searching, the other man said, “Where did you drop it?”

    “At home.”

    “Then why, for heaven’s sake, are you looking here?”

    “There is more light here.”
    ##################################################

    In a nice coincidence I inserted this Nasrudin teaching story in a popular science article about how Earth got its water a couple of years ago and have been trying to get it published; Sky and Telescope wanted me to cut it down too much, and I tried to get it published in Scientific American, but I’m not famous enough for them to want me either, so I have to find another publishing venue. In my (humble) opinion, the best way to answer some of these questions is via geochemistry. Our planet gives us all of data we need: elements, abundances and isotopic ratios of the basalt rocks, interior noble gases, D/H ratio of the oceans, for example. The next step is to collect similar data from asteroids, comets, meteorites, dust particles. One famous application of the geochemistry tool was to learn that comets could not have brought all of the Earth’s water; Why? The D/H ratios are all wrong. But the measurement is not easy, and the comets used were long-period comets, so programs are underway to measure isotopes in short period comets (and hopefully those new main-belt comets too).

    The astrobiology people are extremely well-networked, both in Europe and in the US. I know more from the Europe side, watching a bit from the periphery. If you download this document, you’ll see a sketch of some key scientific questions in astrobiology with suggested approaches to answer those questions. The document was written by the Europlanet working group for exoplanets and astrobiology, to help focus the goals at the same time of networking planetary scientists in Europe. One of the facts that became immediately obvious every time that astrobiology working group talked to us in the the other working groups (I’m co-chair of the small bodies and dust group) was that their field is embedded in all of our fields; it’s truly a multidisciplinary field. The Astronomical Society of the Pacific knows this well too, one of their most popular public outreach modules is the “SETI” module; that is looking for life elsewhere. That module teaches many sub-fields of science in a fun and cohesive way.

    NASA funding for astrobiology is going through a roller coaster these months, they cut it by 50 percent last Spring, and then put most or alot of the funding back in early April.

    Here are a couple more links that might be interesting too:
    http://www.resa.net/nasa/astrobiology.htm
    http://www.astrobiology.com/

  • http://www.amara.com/ Amara

    Thinking more of your (Clifford) questions. The exo/astrobiology people look for water because it played such an important in the development of the biology and life on Earth. So maybe they are a little lazy or egocentric, but it seems like the natural place to start, and water is abundant in our solar system (usually locked up as ice). Andre Brack in his chapter: “Water, the Spring of Life” (_Astrobiology_, Springer) gives reasons why: 1) Water as a diffusion milleu, 2) Water as a Selective Sovent, 3) Water as a Clay Producer, 4) Water Structures the Biopolymers, 5) Water as a Driving Power for Chemistry, 6) Water as a Heat Dissipator.

    And why life based carbon, and not something like silicon? Shulze-Makuch and Irwin (_Life in the Universe_, Springer) answer it is because of its versatility that enable it to form millions of complex polymers, chiral compounds, resonant ring structures, the ease with which it changes from one valence state to another, thereby suiting it well for energy-transferring redox reactions, and its compatibility with water (and ammonia) as a liquid solvent.

    Silicon could be an alternative under more restrictive conditions: 1) little or no oxygen, 2) little or no liquid water, 3) temperatures above 493K (silicones, silicates) or below 274K (silanes), 4) pressures greater than that on the surface of Earth, 5) presence of a solvent such as methane or methanol, and 6) relative lack of available carbon.

  • Elliot

    I may have posted this link on this blog before but it directly to Clifford’s query

    Jerome Rothstein directly addresses the issue in a very interesting speculative essay.

    http://www.bigear.org/vol1no2/life.htm

    Regards,

    Elliot

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Elliot… that was very much a *good read*.

    Thanks,

    -cvj

  • SK

    Clifford’s example of the vast gaseous giants makes me think: how do you define something as “alive”? Just the presence of carbohydrates? Would you call our planet Earth as “alive” because it supports life & its evolution on its surface (just like there are living cells within our body, alive as independent entities, but alive as a whole as well)?
    We form our idea of life by looking around and *trying* to generalise the concept from there. But it’s obviously flawed..

  • http://eskesthai.blogspot.com/2006/05/veneziano-and-theoretical-positions.html Plato

    It is understood Amara, that we have to hold to science and it’s values. Yours is a good read too.:)

    Even good scientists, “philosophize and take theoretical positions?” It can’t always been done by limiting oneself to the “ole ways” of being taught as a cosmologist once was?

    If that is the case, then we should have burned Veneziano at the stake:)

  • http://www.amara.com/ Amara

    Ralph Lorenz has written alot on the entropy aspect of suitable conditions for life:
    http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=105137
    but others such as Charles Lineweaver, discuss that it might not be that simple:
    http://www.mso.anu.edu.au/~charley/papers/LineweaverChap_6.pdf

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Hi Amara,

    I remember the discussion of silicon as an alternative to carbon. Two questions:

    (1) Is the list really only two elements long? Are there other elements besides carbon and silicon which people have given serious thought to? What’s next on the list….. if there is one?

    (2) How much are people actually looking for these alternatives in our explorations? Are there experiments that sit on some of the spacecraft we’ve sent that look for some of these alternative chemistry? We pretty much ony hear about the search for water in the press…I can’t believe that’s all that’s going on, really.

    And what do you mean “not famous enough”? Are you not a regular contributor to CV discussions? How much more famous can you be?!

    -cvj

  • http://eskesthai.blogspot.com/2006/05/edge-of-chaos.html Plato

    More on name, as I wonder.

    Is it possible, to have a “observational status” outside the box?

  • Cynthia

    Amara – I would like to build upon your insightful comment #5. Analogous to carbon, silicon is capable of generating long-chain molecules necessary for complex life. Even though silicon has failed to participate in this evolutionary process of forming complex life, P.W. Atkins (a physical chemist from Oxford) has speculated that silicon might be “a sleeper” in the cosmos. He suggests that as carbon-based life has been on a highly evolutionary pathway, “silicon is lying in waiting.” While carbon-based organisms have been fruitful at enslaving silicon in order to enhance information technology, a dormant reservoir of silicon-based organisms might have a foreseeable edge over carbon-based organisms. Furthermore, as the cosmos gradually approaches greater degrees of entropy, silicon-forms might reign over carbon-forms. Due to their more efficient means of metabolizing and replicating, silicon life – in comparison to carbon life – will have a greater long-term survival rate during this heat death course of the universe. Since silicon does not depend upon sex to reproduce, silicon-based life forms will become ever increasingly correlated with the accelerating universe. Even though carbon-based life has had lots of fun performing sexual operations, their overall survivability will become hindered by this highly inefficient and costly method of reproducing.

  • Elliot

    Cynthia,

    I don’t see why some form of “genetic recombination of software/hardware” would necessarily be precluded in silicon based organisms. While somewhat inefficient and costly, sex allows for favorable traits to be selected into a population.

    Obviously to be determined in the future.

  • Cynthia

    Elliot: I will not deny that sexual reproduction adds diversity as well as adds stronger traits to the gene pool. However, as the universe moves towards higher degrees of entropy, organisms which primarily thrive on sex-free efficiency will gain a greater edge over organisms which thrive on sex and diversity. Admittedly, engaging in sexual operations is key to creating beautiful complexity and vibrant diversity. But – unfortunately – sexual operations are rather messy and inefficient.

  • Elliot

    Perhaps the interaction will “evolve” to self replication with shared “software based genetic” input from “others”. It would preserve diversity and natural selection without the drain on energy resources.

    Who knows? Interesting to speculate on however.

  • http://eskesthai.blogspot.com/2006/05/writing-your-story-of-creation.html Plato

    e- or not to e+ :) There are no “sup” rules here in html, sorry.

    You can give it a “sexual connotation,” if you like?:)Ya know, that Da VInci thingy about the receptacle?

    Afterall, it is a Shakespearean statement about existance?

    But then again, we know what the “symbolism” means, right?

    Maybe this thread was just about funding cuts, and the “question was” about whether it should be here on this topic?

    Yes, transparancy. Money in, money out? :)

  • Cynthia

    Elliot: you make an interesting point… Regardless of future outcomes, it is still fun to speculate!

  • http://eskesthai.blogspot.com/2005/02/shakespearean-quandry.html Plato

    Just in case you wanted to know the source of the picture relayed above.

  • http://alun_clewe.livejournal.com Jeff Nuttall

    Rick–I suggest we take that thinking just a tiny, slight step further and assume that, if such a race of aliens exist, why do we always assume that we’re the ‘more developed’ of the two?

    We don’t. That’s the whole basis behind the SETI program of looking for messages in radio waves coming from distant stars–the assumption is that there may be alien races living on planets orbiting those stars who have the capability to send radio messages (and who developed that capability long enough ago to account for the delay in such messages reaching us from stars thousands of light-years away or more), and who have been sending us messages in the hopes we’d receive them.

    I’m not sure how I feel about the usefulness of the SETI program–not that looking for intelligent life elsewhere in the universe isn’t a valuable endeavor, but I’m not sure the SETI program as it currently exists is really a reasonable way of going about it. AK Dewdney points out some serious flaws he sees in the program’s assumptions in his book Yes, We Have No Neutrons, and I think he makes some good points. Still, though, the existence of the program certainly demonstrates we’re not assuming we’re more developed than any alien race that might exist (even if we may be making other questionable assumptions).

  • Cynthia

    Plato: Please do not misinterpret me… I did not intend to bring up the broad category of sex. I was simply reiterating P.W Atkins’ thoughts on carbon-based life versus silicon-based life. Due their more efficient means of replicating, Atkins speculates that silicon-based organisms (as opposed to carbon-based organisms) might be the “great sleepers” of the cosmos.

  • http://www.amara.com/ Amara

    [Clifford: "(1) Is the list really only two elements long? Are there other elements besides carbon and silicon which people have given serious thought to? What's next on the list..... if there is one?"

    "(2) How much are people actually looking for these alternatives in our explorations? Are there experiments that sit on some of the spacecraft we've sent that look for some of these alternative chemistry? We pretty much ony hear about the search for water in the press...I can't believe that's all that's going on, really."]

    You might not be satisfied with my answer.

    It is one thing to think/speculate/calculate, and another thing to convince funding agencies that your speculation/calculation is worth thousands/millions of dollars/euros/yens/renminbis/rupees/rubles to fund by a government agency for an observation project or a space mission (*). I’m sure you know this, but I’ll say it anyway.

    The water/carbon combination for conditions and signs of life are what planetary scientists and biologists know from Earth’s own example. Even being embedded in it, the scientists do not know exactly what were the conditions under which life emerged here (whether we are ‘intelligent’ is another debateable issue).

    Taking water as an example, if we don’t know (truly) how Earth got its water, then how can we know what to look for in extrasolar planetary systems? It’s educated guesswork, studying our own system and applying what we have learned to systems outside our solar system. Carbon’s utility for life is special for a number of reasons that I stated above, plus: it is abundant in the universe; found in many types of planetary environments, meteorites, interstellar dust/ISM. Observational evidence and chemical reasoning supports the astrobiologists push for seeking the water/carbon combination.

    With that caveat, yes, people have thought of other, different life forms based on a different chemistry, but I’m not aware of these being implemented _explicitly_ (**) in an observational strategy. SETI is yet a further step assuming an intelligent life and searching for “function”. Since I’ve not participated in specialized astrobiology conferences and don’t read regularly all of the literature, I can just tell you what I have picked up occassionally and read and heard.

    Dirk Schulze-Makuch and Louis Irwin in their text [1] speculate a great deal. The presence of a liquid medium is very useful for atoms and molecules to move freely, encounter reaction partners, dissolve into charged ionic species, chemically react in a reasonable time, transport nutrients, and dispose wastes. However, liquid should not be considered a strict requirement for development of life. On another planetary body if a gas is compressed by gravity or another force to higher density, then one might have similarly advantageous environment, keeping in mind that an effective solvent should match the those characteristics of its environment. Water is the universal solvent for life on Earth, but one can imagine other solvents elsewhere, such as sulfuric acid for Venus and ammonia for Jupiter, as these are available, plentiful, and could lead to prebiotic chemistry. Others might be hydrocyanic acid, hydrofluoric acid, methanol, hydrazine and sulfur-based solvents. The Chapter 6 “”Life and the need for a solvent” in the authors’ text describes in detail the properties of a solvent.

    The molecules for life and solvent must go hand in hand. Shulze-Makuch and Irwin list others as candidates for polymer-based complex chemistry in their Chapter 5: “Building blocks of life”. They say that in order to be a viable alternative, it should be a non-metal and be able to form at least the alkane-equivalent to the hydrocarbons. Silicon is the most promising element to substitute for carbon because it is somewhat similar in physical properties and it is already an important component of living organisms and it already interacts with carbon compounds, perhaps even providing the first templates for carbon-based life. If elements other than carbon constitute the building blocks for any living system on other worlds, they would exist under conditions far different than from those on Earth, including temperatures and pressures where water could not be the solvent. Methane would be a could solvent for a silane-based system, for example, and it has the additional advantage that it stays liquid at low temperatures, so Saturn’s moon Titan fits this particular environment. The authors also list boron, nitrogen, phosphorus, and sulfur as other common non-metallic elements that are known to form heat-resistent polymers. However, I’m not deep into this field to be aware of active chemistry research on other molecules for life. The authors say that the research on polymeric chemistry is carried out overwhelmingly under Earth environmental conditions and many polymers that are stable below the freezing point or above the boiling point of water are unknown.

    So the life searches are basically anthropically-based, given what we know from our own solar system conditions. Other people I know have tried to inject some out-of-the-box thinking into the SETI community, with exotic ideas such as Matrioshka Brains [2], and I know that they’ve been frustrated, so I think it will need time (and of course money) to accept other approaches. But if you are not limited by public money, feel free to explore these other approaches!

    [Clifford: "And what do you mean "not famous enough"? Are you not a regular contributor to CV discussions? How much more famous can you be?!"]

    Well.. not regular, but thanks for the compliment. I would like to know how people get their articles into Scientific American. The translated SciAm (German, Italian) editors are approachable, my colleagues and I have had pieces published in those, but the American Scientific American seems to be “invitation only”. If any of you have a good idea how I might get a SciAm editor to look at my wonderful (!) popular science article about how Earth got its water, I’m all ears.

    Amara

    (*) Especially now with NASA. I was wrong earlier, when I said that NASA put back 50% of the astriobiology funds. My colleague today explained: it is a situation where NASA might let the contracts on the astrobiology “nodes” run out next year, so the field will be asphyxiated quickly. Also the new NASA head is not a supporter of astrobiology projects (quoted in Nature April 2006).

    (**) “Explicitly” is the key word, since I’m sure some creative scientists have found ways to implement their ideas under the disguise of something else. I know of one case in the early 1980s of an infrared astronomer looking for Dyson spheres [3] while his observational program was titled as a search for infrared excess stars. Nowadays Dyson spheres are mainstream enough that even Fermilab can announce publically [4] that they are searching for them. It is good publicity for them, too.

    References
    [1] Dirk Schulze-Makuch and Louis N. Irwin, _Life in the Universe: Expectations and Contraints_, Springer-Verlag (2004).
    [2] http://en.wikipedia.org/wiki/Matrioshka_Brain
    [2] Interview with Robert Bradbury: http://www.nanotech.biz/i.php?id=25_09_01
    [3] http://en.wikipedia.org/wiki/Dyson_sphere
    [4] http://home.fnal.gov/~carrigan/Infrared_Astronomy/Fermilab_search.htm

  • http://www.amara.com/ Amara

    I forgot to give one more reference: Nature 436, 25 August 2005, “Seeking the Solution” by Philip Ball. This two-page article gives an overview of work by biochemists who are studying ‘non-water-solvent’ molecules for life. It might answer more of your questions.

  • http://blogs.discovermagazine.com/cosmicvariance/clifford/ Clifford

    Amara… Wow! Thanks for the very clear and useful explanations!

    -cvj

  • Elliot

    Amara,

    Thank you very much for all the interesting comments/references. I find this area fascinating and am looking forward to digging into some of those sources.

    And although this should probably go on the poetry thread here’s a somewhat relevevant haiku

    no artifacts come
    from casting a thousand twigs
    upon the waters

    e.

  • Elliot

    I have a personal bias that if “life” is in existence elsewhere in the cosmos it will eventually produce lots of Von-Neuman self replicators which may in fact be very small (possibly nano-scale) robots. Any thoughts on how we might detect this type of object/creature?

  • Cynthia

    Elliot

    Sounds like you have just entered the chaotic regime of strange attractors embedded with fractal dimensions and Cantor-set structures. Please spare me the messy details!

  • http://www.amara.com/ Amara

    Elliot: Thanks for the haiku! For anyone to answer your question, I think you need to say something about the goal of the replicators. For example, is their purpose to explore? Or maybe to colonize?

  • http://www.aeiveos.com:8080/~bradbury/ Robert Bradbury

    Amara pointed out this thread on another list and I thought I would add a few comments. The current SETI thinking is almost entirely dominated by the assumption that *all* alien life must be more primitive than us. So either *we* are the privileged first “advanced” technological civilization in the universe or we are thinking about the problem improperly. The way I have thought about the problem (following in the footsteps of Dyson) is to ask *when* will we hit the limits? In Dyson’s argument in Science in 1960 he made it clear that even if growth rates (in energy requirements, presumably due largely to population growth) were only 1% per year we would require only 3000 years until we would need the entire power output of the sun. So going back to Egyption times, assuming ~6000 years for the development of a technological civilizations from the Bronze Age to the age of requiring all stellar power output implies that only 1 millionth of the lifetime of a typical G-class star is spent at our “stage”. The rest of the time is either before our stage (i.e. the primitive stages focused on by astrobiology, etc.) or after our stage (perhaps by millions or billions of years). Work by Lineweaver’s group over the last few years has pointed out that *most* of the “Earths” in the Milky Way are likely to be significantly older than our own — so civilizations from such planets should, on average, be significantly more advanced than ours.

    Some civilizations might have gone extinct (adopting a negative growth rate). In this case astrobiology and SETI transition into the realm of anthropology and archeology. But a natural selection would apply and the only conclusion that one can reach is that the abundant civilizations would most likely be at the natural limits they impose upon themselves and/or are at limits imposed by the laws of physics. Some of these may include concepts such as (a) stars are the most efficient fusion reactors from the perspective of minimal consumption of expensive “metals”; (b) star lifetime can be extended first by star-lifting (converting G-class to M-class); (c) the most efficient use of energy requires that one radiate waste heat at only slightly above the CMB temperature, etc. Think along these lines long enough and one concludes that everything from extrasolar “planets” to dark galaxies to the “missing mass” in the universe stands a better chance of being explained by more advanced technological civilizations than some of the more creative explanations astrophysicists tend to invent (why bother with more advanced civilizations than our own when I can invent new laws of physics instead…). It is the height of hubris to assume “they” don’t know about “us”. Anyone within ~6000 light years is probably actively recording the development of our civilization (if they didn’t set it up as an experiment in the first place). The more advanced civilizations which would prefer to orbit in the cold space around our galaxy have probably predicted our possible development but do not know our actual level of development due to light speed limits. Even if all of this is actively going on you have to resort to very synthetic arguments to explain why KT-II level civilizations consuming 10^26W in solar system sized “brains” constructed using the most advanced nanotechnology would have *any* interest in KT-I level civilizations based on a few billion poorly interconnected inefficient biotechnology based 10W computational units that they claim are “brains”.

    As I said at the Extro III conference in 1997. “We don’t talk to nematodes and ‘they’ don’t talk to us.” (We are far far closer to nematodes than they are to us).

    So we are looking for the carbon or nitrogen that primitive life forms may play around with (astrobiology) rather than looking for the carbon, nitrogen, silicon, etc. that more advanced civilizations may be playing with to serve their own purposes. The more advanced life forms can hide their energy source (e.g. stars) but they cannot hide their size, mass or waste heat, which are respectively those areas which tend to be studied by occultation astronomy, gravitational microlensing and mid-far IR thru microwave astronomy.

    The next time you see an “exoplanet” announcement why not ask yourself, “Why does it have to be a ‘planet’ given that you only probably know its mass and perhaps its orbital period?” (Other than the fact that the universe is presumed to be dead and thus any such objects are probably like those which we currently label as planets.) Could not nearby “exoplanets” just as easily be large collections of Earth monitoring megascale telescopes collecting data on us? (When I speak of megascale telescopes I’m talking about lunar diameter collecting areas… Advanced civilizations can easily build billions of these.)

    As the En Vogue song goes… “Free your mind and the rest will follow.”

  • http://alun_clewe.livejournal.com Jeff Nuttall

    The current SETI thinking is almost entirely dominated by the assumption that *all* alien life must be more primitive than us.

    Huh? What about the best known SETI program of all, the one focused on looking for radio signals being beamed at us from different planets? As I mentioned in a previous comment on this thread, that certainly isn’t assuming that the life there is more primitive than us!

    Anyway, though, there are some possible reasons why we may indeed be among the first intelligent life forms to develop. Sure, there are suns and Earths much older than ours. But they’re also likely to be much poorer in radioactive heavy elements, since those are created in supernovas, and a few cycles of stellar births and deaths may be necessary to build up significant amounts. And since those radioactive elements play a large part in heating the interior of the Earth and, indirectly, in facilitating continental drift, among other factors, their presence may be a necessary condition for the development of complex life. So all those older Earths, which don’t have significant concentrations of those radioactive elements, may never have been in a position to develop complex life anyway. It may be that only on an Earth as young as ours is the development of complex life possible.

    I realize there are a lot of “may”s in the previous paragraph, and I’m certainly not saying that it’s definitely the case that complex life couldn’t have developed on an Earth billions of years older than ours. I’m just saying that there are possible rationales why that may be the case, and why it wouldn’t necessarily be an extraordinary coincidence if we did turn out to be one of the first sentient life forms to have developed in the universe.

  • http://www.amara.com/ Amara

    Jeff: How can you be sure that an ‘advanced’ civilization will broadcast via TV/radio ? Some people on this planet are going to cable, and that number will only increase with time. The SETI program would only detect dummies like us who are broadcasting for less than 100 years.
    (Note: Drake is worried too: http://www.astrobio.net/news/article610.html)

    Plus, I’m not sure exactly when the the first supernovae occurred, but the minimum time is one very giant star lifefime, a few million years, and the s process and r process will begin to generate the heavier elements necessary to form the dust that became planets (and life). I can easily imagine many older civilizations than ours.

  • http://alun_clewe.livejournal.com Jeff Nuttall

    Jeff: How can you be sure that an ‘advanced’ civilization will broadcast via TV/radio ?

    Who says I am? I said in a previous comment in this thread that I thought there were some serious flaws in the assumptions behind the SETI program, and that’s probably the biggest one–I don’t think an advanced civilization is likely to broadcast via radio waves. I agree that the SETI program isn’t likely to detect civilizations much older than ours. (In the book I cited in my earlier comment, AK Dewdney makes pretty much the same points you did, and I think they’re very valid points.) I only brought the program up to dispute the assertion that SETI programs were overwhelmingly assuming alien life would be primitive. That doesn’t mean I don’t think there are other highly questionable assumptions involved in that program.

    Plus, I’m not sure exactly when the the first supernovae occurred, but the minimum time is one very giant star lifefime, a few million years, and the s process and r process will begin to generate the heavier elements necessary to form the dust that became planets (and life).

    Begin to, yes, but would they accumulate in sufficient amounts in only one supernova cycle? Maybe, maybe not. Maybe it would take many cycles of star formation and supernovae to build up enough radioactive elements to make complex life possible. I can easily imagine many older civilizations than ours, too. But I can also easily imagine that there aren’t. As I said, I wasn’t saying there are definitely no sentient races much older than us–I was just saying that it’s possible that may be the case, for reasons other than our just coincidentally happening to be the first one to develop.

  • Elliot

    Amara,

    Purpose of replicators. Depends but to borrow from sociobiology maybe their purpose is to replicate. If we made them we might use them to explore colonize and learn…. (or conquer if there were enough natural resources ;) )

    Their purpose might change over time. They might evolve.

    Fun stuff to consider.

    Elliot

  • http://www.amara.com/ Amara

    Dear Elliot, I was asking about the von Neumann machines’ goals because that would probably influence their ‘visibility’ to us. Exploratory von Neumann machines would have no cause to call attention to themselves because they are out just exploring after all. Maybe in their exploration they are watching us, hiding in the nonbaryonic dark matter (?). However colonizing von Neumann machines would be consuming space resources at prodigious rates, so I think it would be hard to miss them. Voids with clearly distinguishable geometric shapes hasn’t caught any of the cosmologists attention in the WMAP data, for example. And if the replicating machines have lost control and are spreading through the universe as a grey goo, one would think it would be even more noticeable. Now if your von Neumann machine’s purpose was simply to replicate, then it’s possible that their programming dictated them to replicate exactly exists in the universe, so then how would we know they are there? You see how much depends on the goals.

    Robin Hanson’s Great Filter essay touches on some of these issues, and goes beyond in order to address the classic Fermi Paradox. Coincidently our “key story” appears once again, near the end.

  • http://www.amara.com/ Amara

    sorry forgot a quote, so the link didn’t appear. The essay is here: http://hanson.gmu.edu/greatfilter.html

  • http://www.amara.com/ Amara

    “Begin to, yes, but would they accumulate in sufficient amounts in only one supernova cycle? Maybe, maybe not.”

    Jeff: This is a straight-forward calculation. People who study the interstellar medium have been calculating this for decades. Stellar nucleosynthesis is the backbone of astrophysics, it has been strongly confirmed by helioseismology, so astronomers do know well the life cycle of stars and their metal production inputs into the interstellar medium. If you start with an initial mass function and a star formation rate you can derive the number of stars with a particular metallicity for a particular epoch time in a galaxy. And extrapolate that to the universe. There are model assumptions, but these are checked by observations and both observations and models are improving all of the time. If you google on “galactic chemical evolution”, then you will see some of this work. The results I read last said that a lot of star formation takes place early in the universe, within about the first 2.5 Gyr, so that is something to consider in the Fermi paradox, too.

  • Elliot

    Amara,

    It is funny/challenging when we try to ascribe motive/purpose to the activities of other intelligent alien civilizations. It is very difficult not to “think” like a human being. I’ll work on it though ;)

    Elliot

  • http://alun_clewe.livejournal.com Jeff Nuttall

    This is a straight-forward calculation….Stellar nucleosynthesis is the backbone of astrophysics, it has been strongly confirmed by helioseismology, so astronomers do know well the life cycle of stars and their metal production inputs into the interstellar medium.

    Of course. But what isn’t known is exactly what threshold concentration of radioactive elements is necessary for the formation of complex life. And without that, it’s still not possible to know how many cycles are necessary to generate it. (Naturally, it’s possible that the threshold is very low, or even nonexistent, and that complex life could form under circumstances different from those on Earth even without any radioactive elements present at all. I’m not trying to argue absolutes here, only raise maybes.)

  • http://www.aeiveos.com:8080/~bradbury/ Robert Bradbury

    For Jeff & Amara… With respect to nucleosynthesis — the radioactive elements that keep the heat warm are primarily uranium and thorium with some question about potassium. The first two at least are produced using r-process nucleosynthesis in supernovas. Estimates involving the observed ratios point to the bulk of the elements coming from a supernova ~6 – 6.5 billion years ago. So the critical factors would not so much be the initial nucleosynthesis (12+ billion years ago) but the size and relative density of supernovas that contributed the element ratios in our solar nebula. One could expect that there may have been a relatively wide distribution of these factors in the Milky Way over billions of years (perhaps over a range of -11 to -4 billion years from today) but you have to resort to extreme handwaving IMO to say the Earth is the first solar system to have produced intelligent life. One thing which has become apparent in the study of extremophile bacteria is that once “life” gets a foothold it is extremely difficult to eliminate it (witness several iceball Earth eras). Then it is only a matter of time before more complex (intelligent) life forms evolve.

    With respect to “replicators” – mindless replication isn’t hard (bacteria do it). But one would presume that “intelligence” is more interested in survival than replication. Intelligence has significant advantages in increasing suvivability vs. replication (intelligent species can develop the means to slow down the aging of stars or develop the means to hop from young star to young star) — “replicators” which lack such capabilities are limited by physical accidents (e.g. impact driven transfer events) to migrate between interplanetary bodies. Migration between interstellar bodies, if possible, is much rarer and much less likely to promote survival.

  • http://alun_clewe.livejournal.com Jeff Nuttall

    Then it is only a matter of time before more complex (intelligent) life forms evolve.

    That strikes me as a very questionable assumption. It seems to me to be quite conceivable that there could be environments that can support simple life forms but don’t have the conditions necessary for complex life forms to develop. I don’t think it’s necessarily a given that the one always leads to the other.

    The ideas about the supernovas being necessary for building up the radioactive elements needed, incidentally, I got from the book Rare Earth: Why Complex Life Is Uncommon In the Universe, by Peter Ward and Donald Brownlee. I don’t agree with the central thesis of the book–that there are so many improbable contingencies that led to the Earth’s being habitable that it’s almost certain that intelligent life has not evolved elsewhere. I don’t think all the improbable contigencies they cite necessarily are as improbable as they claim, or are really crucial for the development of intelligent life. However, they do make some interesting arguments, even if I don’t agree with all of them. The supernova/radioactives argument was one of their arguments that struck me as among the strongest (though I still don’t think it’s as ironclad as they make it out to be–as I said, I was only presenting it as a possible limitation on the development of complex life, not a certain one). However, it’s been a long time since I read the book, and I was going from memory in my explication of the argument, and I may have gotten some important details wrong. I’m not at home right now where I can reread the book and refresh my memory, but when I get home I’ll take a look and see if there are any essential points I’m missing. If there is some serious flaw in the argument, I’d be interested in knowing about it–but more so concerning Ward’s and Brownlee’s argument than concerning my own probably incomplete recollection of it.

  • http://alun_clewe.livejournal.com Jeff Nuttall

    (Actually, I think I overstate the case when I say Ward and Brownlee argue that “it’s almost certain that intelligent life has not evolved elsewhere”. They argue for its being extremely rare, but they don’t actually say they think we’re the only intelligent life in the entire universe. Still, I don’t think intelligent life is necessarily quite as rare as they make it out to be–but it’s an interesting book even if I don’t agree with all their conclusions.)

  • http://www.amara.com/ Amara

    Many others don’t agree with their conclusions. Here is One example.

  • http://eskesthai.blogspot.com/2006/05/under-lamppost-mystery.html Plato
  • http://alun_clewe.livejournal.com Jeff Nuttall

    Many others don’t agree with their conclusions. Here is One example.

    Yes, I agree with most of what it says in the review you linked to. (Although I didn’t think the book was as thoroughly sloppy as that review makes it out to be–then again, it was many years ago that I read it; maybe I’d think less of it if I reread it now.) In fact, I find it kind of gratifying to find out that there are so many others who disagree with Ward’s and Brownlee’s conclusions; I’d kind of feared maybe I was just being stubborn in finding fault with their thesis.

    However, that’s not really the issue at hand here. I’m not arguing for Ward’s and Brownlee’s conclusions–which, as I said, I don’t agree with myself. All I said is that I got that one point, about the supernovas and the build-up of radioactive elements, from their book, and that, whatever I thought of their central thesis, that one point struck me as an interesting possibility, if not as the sure thing they made it out to be. Unfortunately, the review you linked to doesn’t address that particular point at all.

    Now, as I said, it’s been a long time since I read the book, and I may be remembering the argument incorrectly. I’ll reread that part when I get home and see if there’s anything I missed.

    But anyway, once more, I want to emphasize I’m not trying to argue (as Ward and Brownlee were) that complex life must be an extremely rare phenomenon. In fact, I’m trying to argue just the opposite. I was bringing up what I thought was a possible explanation for why complex life might not arise until relatively late in the universe–which could be one way of resolving the Fermi paradox. Maybe the reason we haven’t been contacted by any much older races of intelligent life is because the conditions in the universe weren’t right for intelligent life until relatively recently. Again, I’m not convinced that’s the case, but I’m more comfortable with that as an explanation of the Fermi paradox than assuming we must be nearly alone in the universe as Ward and Brownlee do–and as the author does of the Great Filter essay you linked to in a previous post, approaching the issue from a different direction. Obviously, the fact that I’m more comfortable with the explanation doesn’t make it right, and for all I know we are alone in the universe–but I’d like to examine other possibilities.

  • http://alun_clewe.livejournal.com Jeff Nuttall

    Actually, though, on second thought, the exact details of the radioactives/supernovae argument aren’t really important to my main point (though I’m still going to reread the argument when I get home to satisfy my own curiosity about how well I was remembering it). It may have been a mistake to even bring those details into the discussion in the first place, especially since I was relating them secondhand from memory of an argument in a book I mainly disagree with. There are enough unknowns concerning the necessary conditions for the development of complex life that there still could have been something in the early universe precluding it, whether or not it had anything to do with the accumulation of radioactive elements.

    I’ve never seen a resolution of the Fermi paradox that I really found satisfying. Even the Great Filter essay you linked to above focuses on the idea that there must be something about the development of complex life we don’t understand that makes it extremely unlikely. I don’t see it as any more implausible to think that maybe rather than there being something we don’t understand that makes complex life rare and unlikely as a whole, there’s something we don’t understand that just made the development of complex life impossible in our universe (or prohibitively unlikely) until very recently. Either way, there have got to be some issues we don’t understand to explain the Fermi paradox; I just don’t think the development of complex life being extremely unlikely is the only possible conclusion.

    Ah, well. Perhaps there are more things in heaven and Earth than are dreamt of in our philosophy…

  • http://mindstalk.net Damien

    “Either way, there have got to be some issues we don’t understand to explain the Fermi paradox; I just don’t think the development of complex life being extremely unlikely is the only possible conclusion.”

    *Why* have there got to be some issues? Maybe complex (or more to the point, technological) life *is* unlikely. Or maybe we’re first in our light-cone.

    Way I see it, either tech life can spread beyond a solar system or it can’t. If it can, then either we’re first in the local area, or we’re in some well-maintained zoo. This is because I assume spacefaring life would make a visible difference to the universe, if not actively gobble most things up.

    If it can’t, then we get a common SETI vision of lots of old civs locked into their systems, or even their planets. We’ve got company, but don’t expect to meet them, and expect humanity (and descendants) to die with the Sun.

    Basically I ignore the astronomical details and look at my expectations for self-replicating entities.

  • http://alun_clewe.livejournal.com Jeff Nuttall

    This is because I assume spacefaring life would make a visible difference to the universe, if not actively gobble most things up.

    That’s the Fermi paradox in a nutshell, basically.

    *Why* have there got to be some issues? Maybe complex (or more to the point, technological) life *is* unlikely. Or maybe we’re first in our light-cone.

    But then why is it unlikely? See the Great Filter essay–according to our current understanding of the scientific principles involved, it shouldn’t be unlikely. Therefore, if it is unlikely, there are some issues we don’t understand, q.e.d. Similarly, why would we be first in our light-cone? If life is as common as our current understanding of the scientific principles would seem to imply, we shouldn’t be, unless by a really big coincidence.

    I’ve gotten curious enough I’ve been looking through the scientific literature for a few hours on various takes on the Fermi paradox. Yes, the “zoo” idea is one that has seen print, but it’s not one that strikes me as too probable. There have been closer examinations of the metallic-elements issue, but they don’t all seem to agree. There are other ideas I’ve run across that I hadn’t heard of before. For instance, the abstract to one paper (LK Scheffer, Machine Intelligence, The Cost of Interstellar Travel, and Fermi Paradox, Quarterly Journal of the Royal Astronomical Society 35 (2): 157-175 JUN 1994) asserted the following:

    If machine intelligence is possible, and the computers on which it is based resemble today’s computers in some very fundamental aspects, then interstellar travel can be accomplished by data exchange as opposed to the physical movement of matter. Assuming data exchange by radio, such travel is many orders of magnitude cheaper than physical travel. This low cost provides a huge incentive for an emerging society to join an existing galactic civilization as opposed to physically colonizing the galaxy. It is likely, therefore, that there is at most one advanced civilization per galaxy. This civilization may well have unified goals and objectives, thus removing the strongest underpinning of Fermi’s paradox.

    An interesting idea. Could be. I wouldn’t bet on it, but I can see it as a possibility.

    To me, personally, the possibility that there was something about the conditions in the early universe that prevented complex life from forming long before us strikes me as the most plausible explanation. But it’s certainly not the only one. Maybe you’re right and we are in some kind of “zoo”. Maybe Robin Hanson’s right and there’s some stage in the development of complex life that’s much less probable than we think. We just don’t know enough to know why we haven’t encountered any complex extraterrestrial life; all we can do is state our opinions. And given the amount of debate and disagreement that seems to exist in the published literature on the subject, I doubt we’re going to be able to settle much here. ;)

  • http://mindstalk.net Damien

    Enh, I don’t really like the zoo idea myself, unless we really are in some simulation, in which case no rules really apply. I’d bet on first-in-cone, we’re-likely-to-screw-up, or travel-is-impossible, in descending order.

    Someone has to be first. And it needn’t be a big coincidence: the mediocrity principle can be opposed by the anthropic principle. If spreading is easy then the galaxy is winner-take-all, and there’s only ever one naturally occurring intelligent species per galaxy. The fact that we exist at all tells us we’re it.

    If life is really easy it should have arisen multiple times on Earth, right? Except that the first cell swept the oceans and ruined it for anyone else. (Maybe; not like we know what really happened.)

  • http://mindstalk.net Damien

    I just re-read Robin’s Great Filter essay. One thing he doesn’t mention is mass extinctions, the probability of life getting set back, or even completely wiped out, somewhere along the way; Filter components might be what didn’t happen as much as what did. The Earth wasn’t completely snowballed, nor let loose into runaway greenhouse; asteroids and vulcanism didn’t wipe things out; supernovae didn’t wipe things out; our chemistry and aerospace progressed in sync enough that we could spot the thinning ozone layer, rather than getting wiped out by UV while wondering what the heck was wrong as an atmospheric layer high beyond our ken was eaten away.

    The Sun has been getting warmer; Earth has been getting cooler for tens of millions of years, perhaps because the Himalayas have been scrubbing CO2 out of the atmosphere. What if we had no Himalayas? No ice ages, maybe even greenhouse.

  • http://alun_clewe.livejournal.com Jeff Nuttall

    Someone has to be first. And it needn’t be a big coincidence: the mediocrity principle can be opposed by the anthropic principle.

    Well…I’d say that doesn’t make it any less of a coincidence, just a coincidence that we may have reason to believe happened anyway. But that’s probably just semantics.

    I just re-read Robin’s Great Filter essay. One thing he doesn’t mention is mass extinctions, the probability of life getting set back, or even completely wiped out, somewhere along the way.

    Hansen may not mention that, but Ward and Brownlee do go into some length in Rare Earth about most or all of the factors you list. Then again, as Amara mentioned, their book is considered rather controversial. In fact, though I didn’t know this until I did some websearching yesterday, one of their biggest detractors is one David Darling, who’s written a book of his own, Life Everywhere: The Maverick Science of Astrobiology, in which he counters each of Ward and Brownlee’s arguments, point by point. I haven’t read Darling’s book (yet), so I don’t know what he has to say about those particular arguments.

    Anyway, though, again, I guess at this point it’s just a matter of opinion; even the experts who’ve done substantial research on this subject are strongly divided on their conclusions, so we’re not going to settle the matter here. Doesn’t mean it’s not interesting to talk about, though. ;)

  • http://www.amara.com/ Amara

    Damien: “I just re-read Robin’s Great Filter essay. One thing he doesn’t mention is mass extinctions, the probability of life getting set back, or even completely wiped out, somewhere along the way; Filter components might be what didn’t happen as much as what did. The Earth wasn’t completely snowballed, nor let loose into runaway greenhouse; asteroids and vulcanism didn’t wipe things out; supernovae didn’t wipe things out; our chemistry and aerospace progressed in sync enough that we could spot the thinning ozone layer, rather than getting wiped out by UV while wondering what the heck was wrong as an atmospheric layer high beyond our kin was eaten away.”

    He says something in “The Data Point” section referring to a of hyper-expansion of our descendants going beyond the scale of a single disaster, so it’s there peripherally.

    Your filter components for nonevents is interesting; now I wonder why it’s not there too. Robin tends to formulate his analysis of problems using Bayesian methods, so it seems like these would be natural probabilities (1-p1), (1-p2), … to add to his other filter components.

    In these last 10 years since he wrote the essay (I have a draft version from 1996 which is not very different), astronomers have made large discoveries. The extrasolar planets program was just beginning, the Mars rock (which he does mention) with the proposed evidence for life was under intense analysis, the snowball earth idea was new, automated searches for NEOs was in the beginning stages, for example. You or I could suggest making a re-evaluation or update of that essay (and he could respond reasonably that he doesn’t have time…)

  • http://mindstalk.net Damien

    Or that he’s an economist these days — don’t know how much he’s followed the new science. Planets seem to be common; the big change if any, I’d think would be data on the probability of good planets — modelling of solar system and geological possibilities, plus the chance of nearby astrophysical cataclysms. I’m not sure if anything has substantially changed here.

    In Drake equation terms I guess I’m looking at n_e (planets/star which can support life) and f_i (fraction developing intelligent life, which the emphatic caveat that even if unhampered evolution wandered toward intelligence reliably, which we don’t know, there’s a big question of how often evolution gets to run without giant whacks happening. I suspect we’re still fundamentally ignorant here.

  • http://www.astrobio.net/articles/images/banner3.jpg Plato
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