Centauri Dreams

It’s hard enough to figure out what dark energy and dark matter are, a task that will occupy physicists for a long time to come. But even if we confine ourselves to ‘normal’ or ‘baryonic’ matter (accounting only for some four or five percent of the universe), we’re still left with a problem. Baryons are heavy subatomic particles like protons and neutrons that experience the strong nuclear force, and the problem is that even these relatively familiar particles are only partially accounted for.

So where is the missing baryonic matter? The answer may lie in a thin haze of hot, low-density gas that connects galactic clusters. Call it WHIM, for warm-hot intergalactic medium. Dutch and German scientists now think they have uncovered a filament of such gas that connects the clusters Abell 222 and Abell 223. The properties of the gas, visible primarily in the far ultraviolet and X-ray bands, fit with simulations in terms of density and temperature. The scientists used the XMM-Newton X-ray observatory to identify the hitherto unobserved filament.

Image: Composite optical and X-ray image of galaxy clusters Abell 222 and Abell 223. The cluster pair is connected by a filament permeated by hot X-ray emitting gas. The optical image was obtained by SuprimeCam at the Subaru telescope, the X-ray image showing the distribution of the diffuse hot gas (yellow to red) was obtained by XMM-Newton. Credits: ESA/ XMM-Newton/ EPIC/ ESO (J. Dietrich)/ SRON (N. Werner)/ MPE (A. Finoguenov).

Norbert Werner (SRON Netherlands Institute for Space Research), who led this work, thinks the team is seeing at least some of the missing baryonic matter. Says Werner, “The hot gas that we see in this bridge or filament is probably the hottest and densest part of the diffuse gas in the cosmic web…”

That last phrase deserves explanation. I’m working through the paper, which likens the structure of the universe to such a web-like structure, with galactic clusters, the largest objects in the universe, congregating at the web’s densest nodes. Let me quote the scientists on this:

According to the standard theory of structure formation, the spatial distribution of matter in the Universe evolved from small perturbations in the primordial density field into a complex structure of sheets and filaments with clusters of galaxies at the intersections of this filamentary structure. The filaments have been identified in optical surveys of galaxies…, but the dominant fraction of their baryons is probably in the form of a low density warm-hot gas emitting predominantly soft X-rays.

Sheets and filaments, with the things we see clustering in the web’s threads and knots. Thirty to forty percent of the baryonic matter in the universe ought to reside in filaments connecting galactic clusters, according to a variety of simulations, but this seems to be the first unambiguous detection (although other candidates have been put forward). And while the observed filament closely tracks at least one previous simulation, we still haven’t seen the largest part of the missing matter:

…according to the simulations… the dominant fraction of the WHIM resides in a lower temperature and density phase, the existence of which still remains to be proven observationally. The detection of the dominant fraction of the WHIM will only be possible with dedicated future instrumentation…

In other words, we’re going to need a more advanced space-based observatory to extend such difficult work, this particular filament being detectable largely because it is along the line of sight from Earth, thus concentrating its emission in a small region of sky. Understanding how matter is distributed in these structures will help us better piece together this web-like structure and the place of baryons within it.

The paper is Werner, et al., “Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223,” Astronomy & Astrophysics Letters, Volume 482-3 (May, 2008), p. L29 (abstract).

By Larry Klaes

Any good news from Arecibo is welcome, and Larry Klaes here delivers it. The observatory, threatened with closure despite its key role in the hunt for Earth-crossing asteroids, may have found at least temporary deliverance. Politics seems to have played a role, as Larry notes, but for once with results that benefit science rather than compromising it. Meanwhile, a new study of the Chixculub impact 65 million years ago tells us that a hail of carbon cenospheres — tiny carbon beads — may have fallen planet-wide following the strike. The more we learn about past impacts, the more we realize how important a role our planetary radars play in forestalling future catastrophe.

What exists on the island of Puerto Rico that is over 1,000 feet across, could hold ten billion bowls of cereal, pick up a cell phone call from the planet Venus, once sent a message to any potential inhabitants of a distant globular star cluster, discovered the first planets around another star, has been a “star” in several major motion pictures, has spent the last two years under the threat of losing its funding, and now may be saved on several political fronts, including one involving a New York senator who has been rather busy these days running for President?

The answer is the Arecibo Observatory, which has been managed by Cornell University since it began exploring the Universe in 1963. Home to the largest single radio telescope on Earth, Arecibo has made many major discoveries for astronomy. The facility has also been prominent in analyzing planetoids known as Near Earth Objects (NEOs) that could potentially impact our planet and threaten all life upon it.

Despite all these achievements, in 2006 the National Science Foundation (NSF) appointed a senior advisory panel to see where they could get money for new astronomical projects by cutting funds from current projects. Arecibo was one of the larger targets for cuts, with a proposed removal of $2.5 million over the next few years. It became clear that if Arecibo could not find the financial resources from elsewhere, the venerable observatory could close down in 2011. Not only would this be a major loss to astronomy but also a blow to the economy of Puerto Rico and its important contribution to the science education of the population.

On April 14, the governor of Puerto Rico and the director of the National Astronomy and Ionospheric Center (NAIC) signed a $2.3 million agreement between the semi-autonomous United States territory and the agency that Cornell manages Arecibo through for the NSF. The “Inspiration to Science” program will allow tens of thousands of Puerto Rican school children to visit Arecibo annually to see how the observatory scientists work and receive personal instruction from facility staff consonant with their academic curricula. To handle this influx of students, two new teaching scientists and an aide will be hired. The Puerto Rico Department of Education will provide for the resource needs of the students participating in this program.

Image: Arecibo’s observatory appears to have new life ahead, a plus not only for observational science but the search for dangerous near-Earth objects. Credit: Lee Bennett/ATPM.

“For more than forty years, the Arecibo Observatory has been part of Puerto Rico, an icon recognizably identified with the island worldwide,” said NAIC Director Robert Brown at the signing ceremony. “With the agreement signed today, the people of Puerto Rico become fully part of the Arecibo Observatory, cementing a new relationship that will also become a proud heritage of Puerto Rico.”

As the “Inspiration to Science” initiative was inaugurated, another effort to save the Arecibo facility outright was launched thanks to the efforts of New York Democratic senator Hillary Rodham Clinton, who filed a bill to make the NSF reinstate its funding for the observatory.

Some residents noted that though the action by Clinton is welcome, the fact that it is happening less than two months before Puerto Rico’s final Democratic primary elections on June 1 leaves them wondering just how altruistic Clinton’s motivations were.

“Arecibo has been in peril for a while now,” said Andros Lopez to the Orlando Sentinel, an attorney and a co-director of the local campaign to elect rival Democratic candidate Barak Obama. “That she, by chance, finds about it now is an example of the type of old politics that Obama wants to change. The timing is more than suspect.” Lopez did add that he was grateful nevertheless to see that Clinton “finally pays attention to an issue that pertains to us.”

Arecibo Director Robert Kerr was just grateful for the Clinton’s desire to help the observatory, whatever the ultimate motivation.

“I am quite convinced that the excellence of the Arecibo Observatory will prevail,” declared Kerr regarding Clinton’s actions of support.

Senator Clinton’s Senate office published a release about her support for Arecibo, noting that “Cornell University scientists have used the remarkable tools available at Arecibo Observatory to greatly expand our understanding of the Universe. I am proud to support the path-blazing accomplishments of these New Yorkers.”

Regarding the actual stands of the major presidential candidates when it comes to science and space science in particular, Popular Mechanics recently reported on the candidates’ public declarations for national space policy and the reality behind their statements and motivations, which can be read online here.

A recent CNN report quoted experts in the space and military fields expressing the strong hope that the candidates will go beyond their spoken platitudes and address space policy in earnest soon. Not only are there political considerations to contend with in keeping America’s space program at the forefront, but having a robust ability to understand and monitor the Universe with such instruments as the Arecibo radio telescope – one of humanity’s greatest tools for studying and ultimately preventing NEOs that could strike Earth from hitting – is vital both for the United States and the rest of the world.

One of the world’s largest impact craters (see below) lies under Mexico’s Yucatan peninsula, evidently a major player in the demise of the dinosaurs. Chicxulub is 180 kilometers in diameter, the subject of continuing research by the man who identified it, Alan Hildebrand (University of Calgary). So you could say Hildebrand has an idea what massive impacts from asteroids can do to the Earth’s surface, having studied the environmental effects caused by this one and mapping the crater’s structure to identify mineral, oil and gas resources. That interest has led Hildebrand into an ongoing asteroid hunt, and explains his current plans to build and launch a space-based observatory designed to look for near-Earth objects.

The scientist currently uses use a retrofitted satellite tracking telescope in NEO work here on Earth. The instrument, based at the University of Calgary’s Rothney Astrophysical Observatory (some 75 kilometers southwest of the city) is an extensive re-build, a Cold War era instrument whose motors were replaced, its mount and optics modified and its electronics brought up to speed several years ago at the cost of $500,000. The telescope has been in asteroid-spotting use ever since.

Image: What we’re all hoping to avoid, an artist’s conception of a near-Earth object heating up as it encounters the upper atmosphere. Credit: Melinda Wenner/Wired Magazine.

Taking the asteroid search into space in the form of the Near Earth Object Surveillance Satellite (NEOSSat), an event that could occur within two years, would create the first space-based asteroid telescope, one to be used not only for identifying potential threats but also for helping us firm up our inventory of asteroids near enough to the Earth for manned missions. Nor is the suitcase-sized microsatellite a costly investment, totalling $10 million. Its position in space should allow the observatory to block sunlight to look for objects between the Earth and the Sun that are otherwise difficult to see.

Because some of these asteroids come close to matching Earth’s orbital speed, a robotic or manned asteroid mission becomes a distinct possibility. That would offer not only useful information about the early Solar System — such asteroids being remnants of same — but would also help us take the measure of the kind of objects we might one day need to push out of Earth-impacting trajectories. Would nukes work? Gravitational tugs? Sooner or later we’ll fly a NEO mission because we need to understand the nature of these asteroids as we assess the various strategies for dealing with them.

NEOSSat has the potential of cataloguing at least 50 percent of the one-kilometer or larger NEOs that orbit largely between Earth and the Sun, as New Scientist reports. Interestingly, the magazine cites Timothy Spahr (Harvard-Smithsonian Center for Astrophysics) as saying that an even better idea (though obviously far more expensive) would be to place a NEO-watching observatory in orbit around Venus, where the inventory of inner system objects could be even more definitively compiled.

Addendum: Although I had original identified Chixculub as the world’s largest impact crater, reader James Davis Nicoll quickly corrected me. Both Vredefort (300 km) and Sudbury (250 km) are larger.

Our recent look at panspermia concepts was largely devoted to the transmission of life via microbes or spores here in our own Solar System. The even richer question of how life might pass from star to star is far more problematic, but as a follow-up to that earlier story, I want to look at work that graduate student Jess Johnson did with Jonathan Langton and advisor Greg Laughlin at the University of California, Santa Cruz. Their work suggests that while life might readily survive an interstellar journey, it is unlikely to wander close enough to seed another system.

Ponder the era here on Earth known as the Late Heavy Bombardment (LHB). After the period of planetary accretion ended some 4.4 billion years ago, life apparently began. But 3.8 to 4 billion years ago, the LHB saw the planet again pummeled, causing debris to be ejected into space. Looking specifically at the mass that is ejected at 16.7 kilometers per second in the direction of the Earth’s motion (this is Solar System escape velocity), Johnson, Langton and Laughlin found that a substantial amount of rock (about 5 X 10²¹ grams) would have been blasted free of the Sun.

Remember, this is a period after life has started, so biological material could presumably be involved in any materials lifted into space. But what could survive the 20,000 g’s the ejecta would have experienced, and then cope with vacuum, radiation, cosmic ray strikes and ultimate re-entry and collision upon arrival? Bacillus subtilis is a common bacteria that needs no oxygen to survive, uses carbon and nitrogen as nutrients and forms spores when it lacks the nutrients to thrive. The dormancy period we’re talking about runs into the tens of millions of years, obviously long enough for an interstellar journey — even our glacially slow (by interstellar standards) Voyager spacecraft could make it to the Centauri stars in 75,000 years or so if they were pointed in that direction.

Here’s a striking fact: A viable sample of Bacillis has been found in the stomach of a mosquito encased in amber that has been dated at 25 million years old. Moreover, Bacillus passes all the other tests, able to survive impact pressures upon arrival, capable of enduring 33,800 g’s and, shielded by a sufficient outer encasement of rock, more than able to withstand the radiation hazards of the journey. In deriving the amount of ejected materials (the 5 X 10²¹ grams mentioned above), the Santa Cruz team chose only those fragments of rock greater than one metre in diameter to ensure the necessary shielding.

So everything looks promising for interstellar panspermia except the possibility that such life-bearing rocks may make their way to another stellar system. Producing calculations on the odds of capture, the trio found a result discouraging for interstellar panspermia theorists:

The results of our work found that, although there are microrganisms that are easily capable of surviving all of the challenges of interstellar travel, the probability of capture by another planetary system is vanishingly small. It should be noted that this in no way negates the possibilty of transport between worlds in our own system, a situation that seems quite possible.

A poster on this work (though without the later results) can be found here.

Related: An upcoming paper by William Napier and Janaki Wickramasinghe (Cardiff Centre for Astrobiology) in Monthly Notices of the Royal Astronomical Society discusses the Solar System’s movement through the plane of the galaxy, suggesting that the chances of comet collision go up every 35 to 40 million years. The potential for disaster on Earth is obvious, but the paper argues that such impacts help life to spread. Says Chandra Wickramasinghe, the Centre’s director, “This is a seminal paper which places the comet-life interaction on a firm basis, and shows a mechanism by which life can be dispersed on a galactic scale.” Wickramasinghe collaborated with Fred Hoyle in the 1981 book Evolution From Space. More in this news release.

From the first anniversary edition of the Carnival of Space, I’ll send you this week to Brian Wang’s discussion of two propulsion concepts for the near future. VASIMR (variable specific impulse magnetoplasma rocket) is under active development at Franklin Chang Diaz’ Ad Astra Rocket Company, a site to monitor for developments in a technology that offers potential specific impulses from 1,000 to 30,000 seconds.

That’s a major upgrade compared to conventional rocket designs, and one that could conceivably get us to Mars in as little as 39 days. The Finnish solar electric sail concept, which we’ve also looked at here, may be well enough along for a flight test in 2010, assuming the budgetary gods are smiling. Our next step outward depends upon bumping up trip times to relatively nearby destinations like Mars and the asteroids, and these are two of the more promising concepts for making that a reality.

Physicists have recently theorized that the merger of two black holes would create gravitational waves that could eject the resultant object from its galaxy. Now such a black hole event has been observed for the first time. Theory predicted that the gravitational waves would be emitted primarily in one direction, pushing the newly enlarged black hole in the opposite, and that is what we seem to be looking at, according to scientists at the Max Planck Institute for Extraterrestrial Physics (MPE).

We can’t see black holes themselves, nor have we yet directly detected gravitational waves. But we can observe the interactions around black holes, in this case the broad emission lines of gases carried with the recoiling black hole as it exits its galaxy, which contrast with the narrow emission lines of the gases the object left behind. These data allowed the object’s speed — a scorching 2650 kilometers per second — to be measured. The recoil caused by the merger is pushing the black hole, which masses several hundred million times the mass of the Sun, completely out of the galaxy it once called home.

What would cause two enormous black holes to encounter each other? The most likely event is a collision between two galaxies. Early calculations and later simulations of such events predicted that such mergers could produce velocities of up to a few hundred kilometers per second, but working out the numbers for spinning black holes produced much higher velocities, up to the several thousand kilometers per second found by Stefanie Komossa’s team at MPE. With speeds like this, exceeding the escape velocity of even massive elliptical galaxies, we have to ponder the consequences for galaxy evolution absent the central black hole. The work also implies an intergalactic population of black holes.

Finding the first ever candidate for a recoiling black hole, thus verifying theory and simulation, is quite a catch. It’s also noteworthy given the distances involved. Komossa’s team first detected X-ray emissions from the black hole’s accretion disk from a gigantic ten billion light years away. The observation of gravitational waves through experiments like LIGO (Laser Interferometer Gravitational-Wave Observatory) and the space-based LISA (Laser Interferometer Space Antenna) may one day soon provide data that will help us refine our model of such events, as well as other black hole activity. We’ll also find out whether Einstein was right that gravitational waves and light waves travel at the same speed.

See this MPE news release for more. The paper is Komossa et al., “A Recoiling Supermassive Black Hole in the Quasar SDSS J092712.65+294344.0?” Astrophysical Journal 678 (May 10, 2008), pp. L81-L84 (abstract)

Olaf Stapledon’s Last and First Men (1930), amongst other wonders, pictures our descendants millions of years hence moving from world to world as they attempt to save the species. The Moon approaches the Earth, an imminent peril the ‘Fifth Men’ escape by terraforming Venus, unfortunately destroying indigenous life forms there. Later, the Fifth Men move on to Neptune, and when their existence there is endangered, they make an attempt to save themselves as a species by seeding their cells among the stars. Interestingly enough, Francis Crick (famed as a co-discoverer of the structure of DNA) suggested in 1973 that life could have been intentionally sent from elsewhere in the universe with the express purpose of finding a new home, an idea that made the later work of Fred Hoyle and Chandra Wickramasinghe seem positively tame.

We’re talking panspermia, the idea that life can survive long journeys through space to seed other planets (a notion Hoyle addressed in 1982’s Evolution from Space). The apotheosis of the concept is in the realms between the stars, as Stapledon and Hoyle both assumed. We already know that materials, though not necessarily life, can move between planets in our own Solar System, as shown by compelling evidence for Martian meteorites. But interstellar journeys are of another order, the distances so vast that the question of survival dominates the debate.

Yet comets could be interesting places for microbes to thrive, and it’s not beyond the bounds of possibility that an ejected comet might make its way between the stars, finally encountering another stellar companion and making a spectacular arrival upon a warm, rocky planet. There is no way at this juncture to prove whether or not life began on Earth this way, but it’s a concept that demands study. One way to investigate it is to work with microbes in near-Earth orbit, as a Japanese team now proposes to do aboard the International Space Station in an experiment called Tanpopo.

Image: Is panspermia a viable option for moving life not only between planets but also between the stars? Credit: Jess Johnson/UC Santa Cruz.

The discovery of microbes in space would hardly prove the concept of panspermia, for any materials at these altitudes could well have come from Earth. But a positive result could tell us more about how life manages to persist in the most hostile environments. The Tanpopo (’dandelion’) experiment will examine tiny particles captured onto an aerogel, returning them to Earth for study of their makeup and possible microbes. Survival at ISS altitudes would definitely give panspermia advocates a boost while forcing us to contemplate the possibility that life began elsewhere.

After all, infalling material reaching Earth’s surface from space amounts to tens of thousands of tons on a yearly basis. Tanpopo is unlikely to show us any extraterrestrial microbes, but the researchers do plan, as a second part of the experiment, to expose Earth microbes to space on metal plates placed outside the ISS, as this story in New Scientist explains. The microbes will remain outside the station for periods of one to five years. some protected by clay minerals, some openly exposed to the rigors of the vacuum.

The experiment is slated to begin in 2011, offering a useful follow-up to work performed in 2002 by an ESA team using the Russian Foton satellite. Those remote controlled experiments exposed 50 million unprotected spores of the bacterium Bacillus subtilis to space, mixing another set of 50 million with particles of clay, red sandstone and other materials. The unprotected spores died quickly, but the protected ones produced numerous survivors, particularly those in the red sandstone mix. An exchange of biological materials between planets could not be ruled out by this experiment.

So we’ll see what Tanpopo comes up with as it goes to work on a highly resistant microbe called Deinococcus radiodurans, known to fend off ultraviolet and gamma radiation and to survive extreme dryness and vacuum. The project, which was presented at the recent Astrobiology Science Conference in Santa Clara CA, may offer yet more proof of life’s survival in hostile environments, but it will surely leave the question of the origin of that life open to further debate. Backing out to the big picture, it may be a long time before we identify microbes within a comet, but finding such evidence would keep interstellar panspermia in the picture, with obvious repercussions for life’s chances around other stars.

Just how different were things in the early universe? One answer comes from a study of galaxies whose light has taken eleven billion years to reach us. In this early era — the universe would have been less than three billion years old — researchers have found galaxies so unusually compact that they compress a galaxy’s worth of stars into a space only five thousand light years across. Such objects would be able to fit into the central hub of the Milky Way. What’s more, these ultra-dense galaxies may account for as much as half the number of all galaxies of their mass that existed at this time.

“In the Hubble Deep Field, astronomers found that star-forming galaxies are small,” said Marijn Franx of Leiden University, The Netherlands. “However, these galaxies were also very low in mass. They weigh much less than our Milky Way. Our study, which surveyed a much larger area than in the Hubble Deep Field, surprisingly shows that galaxies with the same weight as our Milky Way were also very small in the past. All galaxies look really different in early times, even massive ones that formed their stars early.”

I sometimes try to imagine the scene from within a globular cluster, a sky awash with tightly packed stars. A compact galaxy must have offered any denizens of eleven billion years ago quite a view as well, as in the artist’s concept above, which shows the view from a hypothetical planet in a distant ultradense galaxy, its sky packed with thousands of stars. We would see 200 times more stars here than we see in Earth’s night-time sky. Image credit: NASA, ESA, G. Bacon (STScI), and P. van Dokkum (Yale University).

Today’s far more slowly rotating galaxies have correspondingly slower moving stars. But these early, compact galaxies show stars moving around their cores at 500 kilometers a second. The speculation now turns to just how such galaxies wound up becoming enlarged over the ensuing aeons. Galactic collisions are an obvious possibility, but there is much to learn about other processes that may have been at work, and the role of dark matter in all this may be consequential. The paper on this work goes out of its way to note that the main mechanism for moving from these early galaxies to the size and density of a more mature galaxy like the Milky Way may not have been identified.

Another wild card: The observed densities may be overestimated because of uncertainties in the analysis, about which the authors list several possibilities. Hubble’s Wide Field Camera 3, set for fall of 2008 installation via Servicing Mission 4, may be able to tighten things up. The paper is van Dokkum et al., “Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at Z ~ 1.3: Early-Type Galaxies Did Not form in a Simple Monolithic Collapse,” Astrophysical Journal Letters 677, pp. L5–L8 (April 10, 2008). Abstract available.

Suppose for a moment that life really is rare in the universe. That when we are able to investigate the nearby stars in detail, we not only discover no civilizations but few living things of any kind. If all the elements for producing life are there, is there some kind of filter that prevents it from proceeding into advanced and intelligent stages that use artifacts, write poetry and build von Neumann probes to explore the stars? Nick Bostrom discusses the question in an article in Technology Review, with implications for our understanding of the past and future of civilization.

Choke Points in the Past

Maybe intelligent beings bring about their own downfall, a premise that takes in more than the collapse of a single society. Alaric’s Goths took Rome in 410, hastening the decline of a once great empire, but the devastated period that followed saw Europe gradually re-build into the Renaissance. And as Bostrom notes, while a thousand years may seem like a long time to an individual, it’s not terribly significant in the overall scheme of a civilization, which theoretically might last millions of years. No, a true filter must be something larger, a potential civilization-killer.

Bostrom’s idea of a ‘Great Filter’ comes from Robin Hanson (George Mason University), and consists of the kind of transition that a civilization has to endure to emerge as a space-faring culture. The key question: Is the filter ahead of us or behind? If behind, wonderful — we have already passed the test and can look with some confidence to the future. Recent work, for example, indicates that human beings were reduced to a band of as little as 2000 individuals some 70,000 years ago, near extinction. Yet somehow migrations out of Africa began 60,000 years ago, and all the tools of civilization would emerge in their wake.

Image: The galaxy NGC 6744, a barred spiral about thirty million light years from Earth. Is it possible that such vast congregations of stars may be utterly devoid of life? Credit: Southern African Large Telescope (SALT).

But that’s a filter that still gets intelligent life well on its way, and surely with the number of stars in our galaxy, that would imply at least a few civilizations should have made it through besides ourselves, their presence obvious by now. No, to explain the Fermi paradox, we would like to go further back, making the emergence of complex life of any kind problematic. Making it, in fact, so rare that a galaxy devoid of it (other than here on Earth) is an explicable outcome. That kind of filter gives us hope, because we’ve survived it even though no one else has. The galaxy may be empty of life, but it is also a vast frontier awaiting our expansion.

The Shape of Future Menace

But maybe the filter is still ahead of us. If so, we may be able to see its outline in fairly familiar terms, such as nuclear war, asteroid impact, genetically engineeered disease used as weaponry, and so on. Or maybe, and more likely, it’s something we cannot foresee:

The study of existential risks is an extremely important, albeit rather neglected, field of inquiry. But in order for an existential risk to constitute a plausible Great Filter, it must be of a kind that could destroy virtually any sufficiently advanced civilization. For instance, random natural disasters such as asteroid hits and supervolcanic eruptions are poor Great Filter candidates, because even if they destroyed a significant number of civilizations, we would expect some civilizations to get lucky; and some of these civilizations could then go on to colonize the universe. Perhaps the existential risks that are most likely to constitute a Great Filter are those that arise from technological discovery. It is not far-fetched to imagine some possible technology such that, first, virtually all sufficiently advanced civilizations eventually discover it, and second, its discovery leads almost universally to existential disaster.

Better to have the Great Filter behind us. Then, at least, we know that we are here and that the experience was survivable. And the parameters of the filter have implications for our search for life. Bostrom hopes we find no sign of life elsewhere because such a find would imply that life is commonplace, that the Great Filter kicked in after the point in evolution that that life represents. Well and good if the discovered lifeforms were simple — we could still assume the filter operated early in evolutionary history and that we are past it. But if we found complex life, this would eliminate a larger set of early evolutionary transitions as the filter, and would imply that it is ahead rather than behind us.

Explaining the Great Silence

Remember, we are trying to explain why we are not finding signs of intelligence elsewhere, no von Neumann probes, no artifacts from civilizations that should have had plenty of time to expand through the galaxy. In Bostrom’s view, no news from the stars may actually be good news. It could imply that life itself is improbable, that the Great Filter happened well in our past and we somehow survived it, and that therefore we may be able to make the transition to a higher and better civilization. We are the one species lucky enough to make it this far, and while we cannot rule out the possibilities of other Great Filters lying ahead, we can at least hope we have weathered the worst.

All of which seems to put Earth back into the center of the universe again, a bizarre exception to the overwhelming norm. Bostrom thus has no choice but to explain the observation selection effect, a way to make sense out of our good fortune in being the lucky exception to the rule:

Consider two different hypotheses. One says that the evolution of intelligent life is a fairly straightforward process that happens on a significant fraction of all suitable planets. The other hypothesis says that the evolution of intelligent life is extremely complicated and happens perhaps on only one out of a million billion planets. To evaluate their plausibility in light of your evidence, you must ask yourself, “What do these hypotheses predict I should observe?” If you think about it, both hypotheses clearly predict that you should observe that your civilization originated in places where intelligent life evolved. All observers will share that observation, whether the evolution of intelligent life happened on a large or a small fraction of all planets. An observation-selection effect guarantees that whatever planet we call “ours” was a success story.

Into a Barren Universe

Bostrom is director of the Future of Humanity Institute at Oxford, a transhumanist philosopher (this is George Dvorsky’s description) who notes that even if the Great Filter were in our past, this would not absolve us from future danger. But this is a man who would like to see all that interesting technology, from nanotech to life extension, kicked in to provide us with a ‘posthuman’ existence whose outline we cannot presently imagine. He’s actively pulling against finding life anywhere else because he’s convinced that life’s rarity implies most organisms run into a buzzsaw before they can colonize space. We survivors, then, may find no one else to talk to, but we should have a fighting chance to use our technologies in a transformative way.

And here is where I truly disagree with Bostrom:

…surely it would be the height of naïveté to think that with the transformative technologies already in sight–genetics, nano­technology, and so on–and with thousands of millennia still ahead of us in which to perfect and apply these technologies and others of which we haven’t yet conceived, human nature and the human condition will remain unchanged. Instead, if we survive and prosper, we will presumably develop some kind of posthuman existence.

I see no evidence in history that the basics of human nature are amenable to change, whether or not such change would be a positive or negative thing. Nor can I go along with those who think we will be able to control our own evolution into some kind of higher lifeform, but long-time readers know my doubts that a genuine ‘transhumanism’ is possible to us. That would be another discussion, though, and I leave this one with the thought that if complex life of any kind is rare, we may have survived only to move outwards into an unexpectedly bleak universe.

Would large amounts of water on the surface provide a glint of light in both the infrared and visible spectrum if we study a distant exoplanet long enough? That’s the premise of an investigation now in progress, one aiming to find Earth-like planets in the habitable zone of a star. Darren Williams (Penn State Erie) and Eric Gaidos (University of Hawaii) have something more in mind than analyzing a planetary atmosphere for signs of water. They want to spot planets with water on the surface.

If the goal sounds chimerical now, bear in mind that various planet-hunting missions like Terrestrial Planet Finder (in its various incarnations) and Darwin are being designed to allow direct observation of planets as small as the Earth. Such observatories, which may be in place within two decades or less, could also examine the visible and infrared light curve of such planets over the course of an entire orbit.

“We are going to look at the planets for a long time,” says Williams. “They reflect one billionth or one ten billionth of their sun. To gain enough light to see a dot requires observation over two weeks with the kinds of telescopes we are imagining. If we stare that long, unless the planet is rotating very slowly, different sides of the planet will come through our field of view. If the planet is a mix of water, we are going to see the mix travel around the planet.”

According to the paper on this work, half of all detected extrasolar planets will have orbital inclinations that make it possible to detect surface oceans. When we looked at this idea back in January, one thought particularly stood out from the team’s paper: “… of all the extremely difficult measurements astronomers hope to make with a TPF-class telescope, time-series photometry and polarimetry that can lead to the identification of specular reflection from surface water might be the easiest.” This Penn State news release catches up with the current thinking of the researchers.

What to do while we wait for a mission capable of making such detections? One idea is to do what Voyager did long ago in the famous ‘Pale Blue Dot’ photograph, looking back at our own planet to study its signature at various wavelengths. Voyager’s view was fascinating largely for what it represented — our world from the outer reaches of the Solar System — but Williams has now enlisted the Mars Express and Venus Express missions to occasionally view the Earth and examine its various phases. That data should provide a useful baseline for the kind of studies that may one day find Earth’s twin.

Image: The famous ‘pale blue dot’ photograph taken by Voyager 1. Looking back at our planet from distant viewpoints will tell scientists much about detecting water on distant worlds. Credit: NASA/JPL.

And if we do find a world in the habitable zone with liquid water on the surface? The odds for life will obviously go up, supplemented by whatever spectrographic data we gather. As to intelligence, we all have our viewpoints on its likelihood, but such observations may prove inconclusive. About the only certainty we’ve had in our explorations of our own Solar System is that we have continued to be surprised, so it’s reasonable to assume we’ll run into more than our share of enigmas around other stars.

The paper is Williams and Gaidos, “Detecting the Glint of Starlight on the Oceans of Distant Planets,” upcoming in Icarus and available online. Let me also recommend Greg Bear’s Queen of Angels for a look at just how tantalizing (and confounding) the detection of exoplanetary life can be, in this case by a probe in the Centauri system.