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Galactic Life in Context

Does complex life emerge at a gradual, uniform rate? If so, we can come up with one answer to the Fermi paradox: We have not detected signs of extraterrestrial life because the time needed for complex life to appear generally exceeds the life of a star on the main sequence. But the assumption that intelligence appears over time with a gradual inevitability — a key tenet of work by Brandon Carter, Frank Tipler and others in the 1980s, may not in fact be true. Solar system-wide events connect life with its stellar environment, while galaxy-wide events provide yet another context.

Punctuated Evolution Among the Stars

Milan Ćirković (Astronomical Observatory, Belgrade) and colleagues have much to say about this in a new paper in Astrobiology. It’s a rich treatment of our older assumptions and newer thinking about punctuated evolution, the idea that life actually evolves in spasms rather than smooth ascents. Species remain relatively stable for long periods but endure sudden changes that can create evolutionary innovation. The paper relies not only on studying the frequency of extinction events on Earth but also on the idea that a system of stars interacts strongly with its galactic environment.

Here the theories abound. Consider just one of the discontinuities that could affect life on a galaxy-wide level. Gamma-ray bursts (GRBs) caused by merging neutron stars or collapsing, supermassive stars could cause massive extinctions. In one calculation, the collapse of η Carinae could deposit in Earth’s upper atmosphere energy equal to the simultaneous explosion of one kiloton nuclear bombs per square kilometer over the surface of the hemisphere facing the blast. Other calculations show galactic GRBs being fatal to eukaryotes up to a distance of 14,000 parsecs, creating a vast ‘zone of lethality.’

But such events change over time. From the paper:

Since the regulation mechanism exhibits secular evolution, with the rate of catastrophic events decreasing with time, at some point the astrobiological evolution of the Galaxy will experience a change of regime. When the rate of catastrophic events is high, there is a sort of quasi-equilibrium state between the natural tendency of life to spread, diversify, and complexify, and the rate of destruction and extinctions. When the rate becomes lower than some threshold value, intelligent and space-faring species could arise in the interval between the two extinctions and make themselves immune (presumably through space-faring technology) to further extinctions.

A Broader View of Catastrophe and Change

But GRBs are only one possible mechanism illustrating a star system’s dependence on the galaxy around it. Ćirković and team list several others:

  • Cometary impact-causing ‘galactic tides’
  • Neutrino irradiation
  • Clumpy cold dark matter
  • Climate changes induced by spiral arm crossings

All of these have been studied in recent astrophysical literature. And in focusing on them, this paper homes in on a crucial point. We’re beginning to realize that it’s unrealistic to restrict the study of astrobiology to a closed system involving a single star and its planets. The Galactic Habitable Zone (GHZ), first introduced by Guillermo Gonzalez and colleagues in 2001 and followed up by Charles Lineweaver, has come into its own. And as Ćirković notes:

The GHZ constitutes an annular ring several kpc wide, comprising the solar circle at a galactocentric distance of 8.5 kpc; and, although its definition does not mention intelligent beings, it should clearly be the main target of SETI studies. In both astrobiology and Earth sciences, a paradigm shift toward an interconnected, complex view of our planet has already been in place for quite some time in both empirical and theoretical work…

A Galactic Phase Transition

The paper goes on to speculate that the factors above could each play a role in creating an astrobiological phase transition within the galaxy that indicates we may, contrary to common assumption, be living in a galaxy aswarm with intelligence, but most of it at stages of evolution not terribly different from our own. This would be the result of what Ćirković calls ‘galactic neocatastrophism,’ a galactic punctuated evolution. And it leads to the conclusion that there is no reason to reject the existence of extraterrestrial intelligence in the Milky Way.

A rare Earth? Maybe only in time. It’s possible we’re just early for the dance.

The paper is Ćirković et al., “Galactic Punctuated Equilibrium: How to Undermine Carter’s Anthropic Argument in Astrobiology,” Astrobiology Vol. 9, No. 5 (2009), pp. 491-501 (abstract). For more on the GHZ, Ćirković points to the excellent review of recent thinking on the galactic habitable zone in Gonzalez, “Habitable Zones in the Universe,” Origins of Life and Evolution of Biospheres 35 (2005), pp. 555-606.


Comments on this entry are closed.

  • James M. Essig July 27, 2009, 10:21

    Hi Paul;

    A one kiloton nuclear blast over every square mile of the surface facing the Eta Carinae! This is equal to about 20o million such blasts.

    A neutron bomb of one kiloton yield has the ability to immeadiately incapacitate all troops within about 800 meters from the point of detonation, in short to kill all such troops within minutes, even those protected within buildings and main battle tanks.

    This lethality boggles the mind, and would make an awesome topic for Hollywood to run with.

    Lethality at up to 14,000 parsecs, that is about 50,000 light years. Talk about a natural death star!

    I can just imagine what a suitably advanced future human or ETI civilization could do by marshalling such power. Science fiction is familiar with the fanciful notions of nuclear bomb produced EMP somehow resulting in space time warps, presumably through some general relativistic effects involving electromagnetism based Einstein Rosen Bridges, the development of temporary multiple space time connectivities and the like mechanisms that might alter the electromagnetic properties of the vacuum of local space. The Planet Of The Apes series involves such a time warp as part of the theme for the human astronauts unknowingly coming back to Earth several centuries after they left only to eventually realize that modern human civilization had done itself in by nuclear war.

    Regardless, the power of such gigantic gamma ray burst should give we humans as well as any ETI within the Milky Way a cause for humility, and to recognize the potential danger posed by Eta Carinae.

    I think we should definately continue with end life stage numerical modeling of blue super-giant and blue-hyper-giant stars, and see if they pose a realistic danger especially given the unstable Eta Carinae. If we could somehow accurately predict its demise over a time resolution of a few hours, we might be able to temporailly relocated deep under ground or on the other side of the planet.

  • Christopher L. Bennett July 27, 2009, 13:59

    I thought current understanding was that Eta Carinae’s radiation would mainly be concentrated in two narrow emission cones, neither of which would intersect Earth. So we’d probably be safe, as would most of the galaxy outside of those two cones. (I wish I could find out what the orientation of those cones is believed to be, and how much of the galactic disk they intersect.)

    I still think astronomers are too quick to assume that environmental catastrophe is a setback for evolution. If anything, it promotes faster evolution. And there’s increasing evidence to suggest that the asteroid impact at the Cretaceous-Tertiary boundary didn’t cause a mass extinction after all, that dinosaurs and other Cretaceous life survived for a considerable time after the impact; at most, it was a contributing element in a more complex process. So a higher cometary bombardment rate might not be as devastating to life as the Rare Earth theorists like to imagine.

    Also, I recall reading somewhere (maybe even this very blog?) of evidence showing that the Solar System was subjected to an intense, nearby supernova fairly early in its history — an event which, according to the Rare Earth theorists, would dissipate a protoplanetary disk and prevent planets from forming at all. Obviously that didn’t happen in our case.

    I also hate the term “Galactic Habitable Zone.” Even its proponents admit that life would merely be less likely beyond that zone, not nonexistent, so frankly I think it’s dishonest to call it a “habitable zone.” “Galactic Temperate Zone” would be a better name.

    (And James, in the original Planet of the Apes film, there was no time warp involved; the crew just travelled in cryogenic sleep for 2000 years. The sequels retconned it into a time warp to justify their stories.)

  • NS July 27, 2009, 15:10

    Charles Lineweaver (mentioned above) supports SETI but is pessimistic about it finding ETI; he believes that high intelligence (e.g. ability to build a radio telescope) is a species-specific feature of humans that probably doesn’t exist anywhere else:


  • philw1776 July 27, 2009, 19:25

    Lineweaver’s paper is excellent. It is our homo centric view of evolution that makes us think that intelligence (the ability to build radios, telescopes, etc.) is a deterministic result of evolution. The evidence from biology says it isn’t. Note the comment that one and only ony phyla in the pre-Cambrian exhibited encephalation, the feature that allowed the ‘smartest’ of Earth’s creatures to survive.

  • Ron S July 27, 2009, 20:47

    Lineweaver mainly argues against using Earth’s fossil record (our evolutionary heritage) as an argument for intelligence as a trait subject to convergent evolution (such as, for example, vision). Not being an expert I hesitate to jump into a critique – and the short paper NS referenced is well written – but I find him very unconvincing in his argument against SETI.

    That there are successful organisms on Earth without any notable intelligence, such as plants, is hardly an argument. Going further, he seems to imply that, since man is just one species out of billions, the probability that there is intelligence at all is minute, not subject to convergent evolution (a valuable trait towards which evolution will find many paths), and therefore not a high expectation on other planets that have life.

    There are some specific simplifications that I didn’t much like, such as focusing on encephalization (brain size) as a poor correlating factor for intelligence, yet done without considering what units comprise the brain. He also completely side-stepped the matter of why intelligence evolves at all, as a feature that does indeed confer survival advantages for many species; those that don’t have it typically make up for it with rapid reproduction or other features.

    An entertaining paper, and well worth reading, but unpersuasive to this reader.

  • James M. Essig July 27, 2009, 21:10

    Hi Christopher;

    Thanks for making the clarification about the original Planet Of The Apes film. I had not realized that such was the case.

    The scenes from one of the movies, I do not remember exactly which, touched me down to the core of my conscious psyche in a surreal but mildly disturbing kind of way, especially the scene with a bus partially melted in some underground terminal, and the radioactive twisted, ablated looking steel girders and piles of cintered concrete that was once New York City.

    After I first saw this episode, as a yoong grade school child, I thought about the scenes in the movie with a half disturbed morbid sense of curiosity.

    I guess the point I am trying to make, is that the Hollywood Media can be put to good work. I would like a realistic movie to be done on the possibility of a supernova relatively nearby to Earth. I highly enjoy science fiction space movies from the Star Wars Movies to the latest Star Trek movie.

    The doom and gloom of planetary disaster movies, and even stellar scale disaster movies, especially when mixed with human and ETI drama in terms of the characters, can be very uplifting and motivate people to take on and tackle problems that they have not reflectively pondered in the past.

    Hollywood and art can be a great ally of we practicioners of Tau Zero.

  • Athena Andreadis July 28, 2009, 2:29

    I would not base any theories or predictions on what Guillermo Gonzalez said. As you probably know, he made Intelligent Design the center of his career and was a member of the fundamentalist Discovery Institute without divulging this membership. He acted as a major consultant for Ward and Brownlee’s Rare Earth without disclosing this fact, seriously skewing the conclusions of that book. He also wrote openly creationist articles for evangelical newsletters, although he denied doing so.

    There is more about this issue in David Darling’s book Life Everywhere: The Maverick Science of Astrobiology, which is a very good read in itself… and I’m not saying that because it mentions yours truly! Here are two links that briefly discuss the matter:


    I read a draft of the Ćirković paper — it’s interesting that astrophysicists are borrowing biological terms for a change: Punctuated equilibrium was Steven Jay Gould’s major contribution to biology. The idea that solar systems and their planets are subject to outcomes of galactic events is neither strange nor strictly novel. For example, scientists have previously argued for effects on terrestrial climate and life when 1) our spiral arm crosses another or 2) when the sun crosses the galactic ecliptic.

    I wrote a review of Rare Earth which discusses some of the problems with the assumptions. Here is the link: E. T., Call Springer-Verlag!

  • Adam July 28, 2009, 5:30

    Hi James & Chris

    I was reading “Rare Earth” today and it’s a tad unfair to paint their ideas as dogmatic, as admittedly I’ve done in the past. They devote a chapter to testing their hypothesis after all. As for the current paper Cirkovic makes a strong case that Carter’s argument is refuted by virtue of the biological variables not being independent of astrophysical timescales and his solution to Fermi should give us pause: could we be facing serious danger from the sky? Or are we about to fill the Galaxy?

    Isaac Asimov’s “End of Eternity” presaged Fermi and Cirkovic’s solution back in 1955. In Asimov’s novel our technological evolution is stunted for over ~12,000,000 years by “Eternity” fiddling with the time under its control (28th to 70,000th Centuries.) When humans finally invent hyperdrive and go exploring, the Galaxy is full of aliens. If the Galactic Empire had been allowed to evolve without the delay, then humanity would have filled the Galaxy instead (as per “Foundation” and related books.)

    Stephen Baxter, of course, answered Fermi along the lines Cirkovic develops in his novel “Space”, which drew on work by James Annis about one possible astrophysical reset mechanism. Chris, mass extinctions delayed evolution for at least 5-10 million years afterwards and definitely reduced the diversity of the biosphere. Life rebounded in spite of the setbacks, not because of. As for the K/T boundary event/s the geology of the timing is contentious. The researcher who has been most vociferous about an apparent timing mismatch between impact and extinction has been pushing the same barrow for many years now.

  • Michael Spencer July 28, 2009, 6:21


    Another thoughtful summary, and I know that all of your readers are happy you are back.

    One thing that caught my eye: “Climate changes induced by spiral arm crossings”– I don’t see how this would affect climate, perhaps someone could explain or speculate?

  • Administrator July 28, 2009, 8:24

    Michael Spencer writes:

    One thing that caught my eye: “Climate changes induced by spiral arm crossings”– I don’t see how this would affect climate, perhaps someone could explain or speculate?

    Michael, good question. Ćirković points to, most recently, a paper by N.J. Shaviv called “The Spiral Structure of the Milky Way, Cosmic Rays, and Ice Age Epochs on Earth,” in New Astronomy 8, pp. 39-77. It’s not a paper I’ve read, so I can only point to it for more information. If I get the chance, I’ll go through it and write it up here.

  • ljk July 28, 2009, 10:13

    Last month a paper appeared stating that paleontological records do not show
    life changes on Earth corresponding to our planet’s passage through the arms
    of the Milky Way galaxy.

    An article on the subject with a link to the paper are here:


    On a related note, this recent blog post discusses how Western culture went from
    looking at our planet as an organic being to a mechanical device and how the
    Gaia concept may bring us back to the idea of Earth as a large living creature.


  • kurt9 July 28, 2009, 13:08

    Re: Guillermo Gonzalez’s GHZ: Yes, I also read about how he came up with this idea while involved with the Discovery Institute and how he did not disclose his ideological bias to Brownlee and Ward while consulting with them. The GHZ was a credible idea at the time (even though it was ideologically motivated). However, I believe there have been recent discoveries of extra-solar planets around stars of low metallicity since 2000 that suggests that the GHZ at least is unbounded on the outward side.

    For me, the most significant aspect of Rare Earth was the notion that advanced life requires a planet with plate tectonics (the rest has been largely discredited). I think this is still a valid position. At the time, I thought that plate tectonics were rare because they required the large moon-creating giant impact to initiate them. Recent observations of both Mars and Venus suggests that both of these planets had plate tectonics early in their history. If so, that’s a batting average of 3 out of 3, suggesting that plate tectonics are common.

    Kepler will tell us how common Earth-sized planets are in their Goldilocks orbits. Characterization of those planets will require instrumentation more advanced than Kepler.

  • andy July 28, 2009, 13:20

    Firstly, regarding the spiral structure and climate change, might be worth checking this preprint for an opposing view.

    I don’t agree that if you leave a suitable planet to its own devices without springing some kind of cataclysm on it from the stars, intelligence is inevitable. However, while an astronomical cataclysm did provably occur round about the K/T divide, the catastrophe from space hypothesis hasn’t been nearly so successful with the other ones. Furthermore, evolution has not been a steady progression towards higher intelligence, even if we restrict our attention to the tetrapods. The criterion for success in evolution is reproduction, not intelligence.

    Evolution of intelligence would appear to be self-limiting in most cases: I’d guess that the devotion of increasing resources to powering the brain limits things… even the most intelligent non-human animals appear to get on fine without needing to go to runaway technology: diminishing returns for more intelligence?

    May well be that human intelligence is just a weird analogue of various large and elaborate display structures (e.g. peacock tails, antlers on deer, the various crests and horns which adorned various dinosaur species) which evolved as a way to attract potential mates rather than as something driven by the need to get food/avoid predators. That it turned out to be useful for other purposes is probably just a happy coincidence. If so, it implies intelligence is likely to be rare even on planets with macroscopic organisms, as the feathers, antlers, crests, horns, etc. do not require nearly so much constant fuelling as a large brain does.

  • James M. Essig July 28, 2009, 13:27

    Hi Adam;

    Thanks for offering the above insights.

    My hope is that indeed, humanity is about to populate the galaxy.
    Even with a colonization wavefront of about 0.2 C, we could populate throughout the entire galaxy in only about (70,000)/0.2 years or about 350,000 years, which is a relatively small time period on cosmic time scales.

    My guess, is that star and planetary hopping will play a large role here, at least initially until we deveope high gamma factor star ships that can take advantage of some serious time dilation. Faster than light craft would be excellent.

    Luckily, Eta Carinae is the only relatively nearby supermassive star, e.g., of about 100 solar masses plus, that is behaving in an unstable manner that could pose a threat to our civilization and species.

  • T_U_T July 28, 2009, 14:21

    If we consider the fact that our sun has twice the metallicity for a star of its age, the most parsimonical solution to the fermi paradox is that we are simply among the first ones. No catastrophes required.

  • hiro July 28, 2009, 17:05

    “If we could somehow accurately predict its demise over a time resolution of a few hours, we might be able to temporailly relocated deep under ground or on the other side of the planet.”

    To James:
    I think we are very lucky if we can estimate time the event and narrow it down to a few years.
    Anyway, we fail to receive any E.T signal because they are beyond our light-cone. It would be extremely strange that we receive some alien messages just after several decades searching (compare to the life time of our civilization) and I don’t like the idea that we’re the only one in this galaxy or in the Local Group.

  • amphiox July 28, 2009, 18:30

    My understanding was that plate tectonics required water to lubricate the moving plates at their boundaries, but beyond that, any planet large enough should have it. The supposition here is that Venus lost plate tectonics because it lost its water due to its proximity to the sun, and Mars lost plate tectonics because it cooled down due to its small size.

    Even if it takes a big impact to trigger plate tectonics, large impacts are actually pretty common during the formative periods of terrestrial type planets, indeed, such impacts are part of the process of planet formation. Both Mars and Venus must have experienced several large impacts of a scale equivalent to the one that earth experienced that produced the moon. They didn’t get big moons only because the angles of impact were different.

  • Doug M. July 29, 2009, 4:30

    T_U_T, the Sun is unusually metallic, but not extraordinarily so. Compared to other G-type stars, the Sun is around the 90th percentile.

    Figure 5 in this paper — http://www.astro.iag.usp.br/~maciel/research/articles/art70.pdf — shows the distribution of metallicity among G dwarves. The Sun comes in at 0.0, which you can see is near but not at the right end of the graph. Quite a few G-type stars are about as metallic as the Sun, and a handful are even more so.

    Also, the correlation between metallicity and planet formation is still up for grabs. It was thought for a while that it was very straightforward, with increasing metallicity meaning more planets:


    However, several challenges to this have arisen. For instance, one recent paper points out that metallicity and planet formation both scale smoothly with distance from the galactic center


    — raising the possibility that planet formation may be driven by something else, such as the density of the interstellar medium. I’m inclined to the straightforward metallicity -planets correlation myself, but it can’t be viewed as firmly established yet. More data, better models!

    Doug M.

  • T_U_T July 29, 2009, 13:39

    Dough M. : “the Sun is unusually metallic, but not extraordinarily so. Compared to other G-type stars, the Sun is around the 90th percentile.”

    But you should compare not to all G stars, but only to G stars of the same age. Then the sun will look still more extreme. Of course it is not something completely draconian, but the result is dilution of possible present civilizations by one or two orders of magnitude compared to expectations which don’t take metallicity into account.

    “it can’t be viewed as firmly established yet. ”

    Duh, of course. But compared to other possibilities, this is the most solid ( or, more precisely, the least shaky ) solution of the fermi paradox. ( and the most optimistic to boot ;) )

  • andy July 29, 2009, 13:47

    Planet-metallicity correlation appears to have a stronger effect on Jupiter-type planets: the effect on the lower-mass planets which are more interesting in terms of habitability is not nearly so pronounced. The strongest effect seems to be on hot Jupiters, which are the easiest planets to detect by both radial velocity and transit surveys: going by the Extrasolar Planets Encyclopaedia, there have been no hot Jupiters (for the purposes of getting an easy cutoff, I am defining hot Jupiters as having periods less than 10 days and masses exceeding 1/10 that of Jupiter) discovered below [Fe/H]=−0.3 (roughly half solar metallicity), but there are several planetary systems, including ones containing super-Earths and ones containing multiple Jovian planets below this limit.

  • WLM July 29, 2009, 23:47

    In The Ages of Gaia, James Lovelock details the intricate biochemical changes to the Earth over the last 3.5 billion years that led to the oxygen-rich atmosphere of today. Which has allowed the evolution of large animals and eventually us.

    No oxygen-rich environment, no large animal life, and no intelligent tool-making species.

    I believe in addition to all the other factors a terrestrial planet has to have *just the right* chemical composition in its crust or the whole thing never comes about. This limits the number possibly by another several orders of magnitude. Complex life, in this sense, is like a truffle: everything has to be *exactly* right.

  • Doug M. July 30, 2009, 5:14

    T_U_T, for Population I dwarfs? The correlation between age and metallicity exists, but is surprisingly weak. The median metallicity (Fe/H) of 10-billion-year-old dwarfs is only about -0.4. The median metallicity of new, just joined the main sequence dwarfs is about +0.2. There are G dwarfs that are older than the Sun that are also more metallic.

    Even if we assume that 4.5 billion years is the minimum time for evolving intelligent life, and that no dwarf less metallic than the Sun could produce life — both pretty extreme assumptions IMO, but go with it — about 5%-10% of all G dwarfs would still be eligible.

    Doug M.

  • Doug M. July 30, 2009, 5:21

    — BTW, metallicity correlates weakly with age, but strongly with average distance from the galactic center. Out at the edge of the galaxy, even very young stars are likely to be metal poor. Closer in towards the core, even older stars are likely to have high metallicity.

    This has its own effect on Drake-style calculations. Frex, if we assume a minimum metallicity is needed for life, then the last ten billion years have seen a “metallicity frontier” gradually expanding outwards from the galactic core.

    Doug M.

  • T_U_T July 31, 2009, 2:40

    Dough. The not so strong correlation between G star age and metallicity is to be expected because timescale at which interstellar medium is enriched by heavy elements is much shorter because the most heavy stars live orders of magnitude shorter. It is of course true that if there is a gradient of interstellar matter density the result will be a gradient of new star metallicity slowly moving from thicker places to the thinner , But the exact spatial distribution is irrelevant. It changes nothing about the fact that the sun is out of place either in time or in space thus reducing the number of earth sized planets around our position and time.

    andy, sample of < 5 Mearth explanets( not counting pulsar planets ) we have is almost nil. The lacking effect of metallicity on lower mass gas giants might be an illusion – if in a low metallicity case neptunes form instead of jupiters, we will see no decrease, but there will be still less mass in the inner solar system so we get mars and mercury or ceres instead of habitable earth

  • Torbjörn Larsson, OM August 4, 2009, 4:45

    Late for the party, but it’s good: thanks for the update on this interesting subject! (Ćirković et al have published the main model before, AFAIK.) Also, thanks commenters on treating Rare Earth/metallicity more. I too am somewhat suspicious about GRBs being the synchronizing environmental environment. But the idea of a tie in between biospheres and development and disturbances of “temperate zones” et cetera seems sound.

    Re Gonzalez, remember that also Tipler and Lineweaver are creationists (in the christian sense) and likes to argue to benefit unsubstantiated ideology. Now Lineweaver still can make good models (unfortunately while largely avoiding testing them). But the paper NS linked to is awful, and an old irritant to me.

    I don’t have time for all the specifics. But for example he doesn’t define human-like intelligence, while anthropologists define humans by persistent technology and so have several human (homo) species. Not all with large EQ. Moreover, the relation between EQ and intelligence is unsubstantiated.

    Also specifically there are several convergent examples of multicellularity et cetera convergences, and the RNA work is old and known to be problematic. (Archaeabacteria is close to Eukaryotes, see Cavalier-Smith for this and a good critique of RNA only clock hypothesis trees.)

    The good and testable part is the model of divergence time for convergences and its relation to the tree. But one can argue that the tree is larger for convergence of “nervous systems”, from both ends. First, analogous systems are used by plants and even biofilms and cells themselves for with an adaptive measure “intelligent” behavior, on spatial and time scales suiting their niches. Second, brains are tested useful, as not many parasites or other life in simplified niches have lost them.

    (Same argument for convergence of body plans, as bacteria uses mostly exoskeleton membranes and eukaryotes mostly endoskeleton tubulins.)

    With testing Lineweaver’s argument IMHO disappears into thin air.

  • Torbjörn Larsson, OM August 4, 2009, 4:59


    @ Athena: You use a strong version of the anthropic principle without clarifying this.

    The one that I believe Carter uses is the weak, which can be tested and so is, probably, science. (Say, tested by finding all parameters not locked in by other means being highly likely.)

    The weak AP gives a high likelihood for parameters suiting us and, conversely, a high likelihood for other life like us. Not to be confused with the religious anthropic argument akin to the strong AP, from arguing from teleology or in other words erroneously claiming that a posteriori likelihoods are a priori. (“Ie “We are, therefore we had to be.”)

  • ljk August 4, 2009, 9:26

    Torbjörn Larsson, OM said on August 4, 2009 at 4:45:

    “Re Gonzalez, remember that also Tipler and Lineweaver are creationists (in the christian sense) and likes to argue to benefit unsubstantiated ideology. Now Lineweaver still can make good models (unfortunately while largely avoiding testing them). But the paper NS linked to is awful, and an old irritant to me. ”

    I knew about Gonzalez and the nonsense Tipler has been cranking out for
    years makes sense now as to where it is coming from and going, but I did not
    know about Lineweaver, so thanks.

    I keep sensing a negativity about the prospects for alien intelligences in a
    number of papers and comments from certain scientists and groups these
    days, which I often attribute to the fear of being dominated or destroyed
    by beings more powerful than humanity. Now I see that in certain cases
    it has a strong religious bias as well. Too bad these guys don’t just say so
    up front and save both sides a lot of time and bother.

    In the end, it all boils down to science determining whether we are alone
    in the Universe or not. The same with whether our Universe came about
    by deliberate design or happenstance. We cannot allow our own limitations
    and biases to color and corrupt these vitally important questions as much
    as possible. And yes, I know it goes both ways.

  • Adam August 6, 2009, 7:41

    Hi All

    Lineweaver is an atheist NOT a creationist by any stretch of the imagination.

    The Fermi Paradox makes questionable any assumption that we’re in a time of equilibrium for intelligent life in the Universe. Annis/Cirkovic’s suggestion of a phase transition makes more sense.

  • Ronald August 6, 2009, 14:18

    Lineweaver (The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way, 2004) revisited. Very good.
    Paul, could you perhaps give a link to an abstract (or even better, the whole article) of the “excellent review of recent thinking on the galactic habitable zone in Gonzalez, “Habitable Zones in the Universe,” Origins of Life and Evolution of Biospheres 35 (2005), pp. 555-606”.

    I think I understood that GRB are exceedingly rare events, even at a cosmic scale, mainly limited to specific types of galaxies (very rapidly star forming elliptics) in a particular early stage of development. Which is why the observed GRB are from many billions of lightyears away (and hence equal number of years ago). But I will readily be corrected.

    Ordinary supernova explosions are mainly dangerous up to a few tens of lightyears distance.

  • Ronald August 6, 2009, 14:21

    With ref. to rarity or even absence of GRB in our MW type of galaxy:

    Stanek, K. Z. et al (April 6, 2006). “Protecting Life in the Milky Way: Metals Keep the GRBs Away”. Acta Astronomica. arΧiv:astro-ph/0604113v4.

  • Administrator August 6, 2009, 14:38

    Ronald, the Gonzalez paper is here:

  • Ronald August 7, 2009, 9:15

    “The not so strong correlation between G star age and metallicity is to be expected (…) It changes nothing about the fact that the sun is out of place either in time or in space thus reducing the number of earth sized planets around our position and time.

    Politely disagree: Doug’s point is (July 30, 2009 at 5:14, 5:21) that since metallicity in solar type stars correllates so weakly with age, younger stars can be more metallic than older ones and vice versa.

    In fact, as Doug also indicates , considerably older (Population II) stars are usually less metallic, Population I stars in the galactic (disc) neighborhood show comparable metallicity to the sun’s in relation to age, as is also shown, besides the first publication mentioned by Doug, in “Dynamical evidence of the age–metallicity relation in the Milky Way disk” (Rocha-Pinto et al., 2006).

    Interesting also is the recent publication “Age and mass of solar twins constrained by lithium abundance” (do Nascimento Jr, May 2009), which focusses om Lithium, but mentions general abundance (metallicity) and mentions that thses are quite comparable for the identified solar twins (HIP 55459, HIP 79672, HIP 56948, HIP 73815, and HIP 100963), see tables 1 and 2 in there.

    “Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics” (Mamajek, Hillenbrand, 2008) also mention that solar-age stars in the galactic neighborhood show within solar range metallicities.

    There is quite a bit on the age-metallicity (or age-abundance) relationship in the astrophysical abstracts. All those I checked indicate that this relationship is quite weak. Solar type stars and solar twins in my own database also don’t show any clear age-metallicity relationship, with both younger and older stars often exhibiting approx. solar metallicity.

  • ljk November 17, 2009, 22:14

    Avatar: A New World That NASA Is Ignoring


    “Avatar”, a film by former NASA Advisory Committee member James Cameron, will debut across the planet on 18 December. Widely hailed as “ground breaking” the film may well push the boundaries of what can be portrayed on the big screen.

    The film centers around humans mining precious materials on a world in the Alpha Centauri star system – and the inevitable conflict that arises with the local (sentient) inhabitants.

    The film delves into a wide range of issues that intersect with what NASA’s Astrobiology Institute and Exobiology Programs have looked into in one way or another. Unparalleled simulations of an extrasolar planet with a whole new ecology – but it would seem that NASA is not really interested in this film.”

  • ljk January 9, 2010, 12:01

    Scientists say dolphins should be treated as non-human persons

    PhysOrg.com Jan. 6, 2010


    Scientists studying dolphin
    behavior have suggested they could
    be the most intelligent creatures on
    Earth after humans, saying the size
    of their brains in relation to body
    size is larger than that of our
    closest relatives, the chimpanzees,
    and their behaviors suggest complex
    intelligence. One scientist said
    they should therefore be treated as…


  • ljk January 26, 2010, 2:18

    Law of probabilities backs hopes for E.T., conference hears

    by Staff Writers

    London (AFP) Jan 25, 2010

    The law of probabilities backs theories that we are not alone in the Universe, although an encounter with an advanced civilisation may shock our species, scientists at a conference said here on Monday.

    “There is no firm evidence that life exists elsewhere, but there is a very firm probability (for it),” said Baruch Blumberg, an astrobiologist at the Fox Chance Cancer Center in Philadelphia.

    “My clear prediction is that living generations have an excellent chance of seeing extra-terrestrial life being detected,” said Martin Dominik, an astronomer at the University of St. Andrews, Scotland.

    Life on Earth may have been kickstarted thanks to carbon molecules and dust that drift through interstellar space, said Pascale Ehrenfreund, an astrochemist at George Washington University, Washington.

    If so, “the basic building blocks of life — at least as recognised on Earth — must be widespread in planetary systems in our Milky Way and other galaxies,” she suggested.

    The two-day conference is being hosted by Britain’s Royal Society, one of the cradles of modern science, as part of a series of discussions on major issues to mark the academy’s 350th anniversary.

    The meeting is not intended to give any conclusion on whether other life exists but give a snapshot of where we are in our quest to find it — and speculate on the impacts of such a discovery on human society.

    Lord Rees, president of the Royal Society, said it was essential to admit to our present ignorance.

    “We don’t even know how life began here on Earth and that being said, we don’t even know how to place our bets on how widespread life is or where to look for it,” he said in an interview.

    Full article here: