The Deadly Thing at 2.4 Kiloparsecs: Are we sharing the galaxy with something large, dangerous and periodic?

Never before online, this highly speculative piece was published in Analog Magazine way back in May of 1984. It won the Analog Award for Best Fact Article for the year. Some of the research may be dated...but the concepts still intrigue...

Note: This article mentions  the work of NYU's Michael Rampino who later on realized that Earth's major extinctions appeared to recur in cycles of roughly 26 to 35 million years.  He has an improved theory for that!  Indeed, despite involving Dark Matter, it is likely better and more plausible than the one I raised in this old Analog piece. Still, my hypothesis was unprecedented and certainly fits the observed facts... a deadly thing may indeed, still be out there, lapping us every... 200 million... or even thirty million or so! Certainly the concept deserves to be posted somewhere and available on the Inter-Tube.

Okay, then. To the way-back machine!

======  Dialing back to 1984... and here we go... =======

Mass extinctions are much in the news these days. Like a scandal long buried and only just being uncovered, the demise of the dinosaurs now seems to be only the uppermost layer of something far more regular – and deadly.

Several recent events have spurred this renewed interest in the ecological holocausts of the past. The most significant of these has been progress in the arcane art of reading the fossil records in ancient sedimentary rocks.

Paleontologists such as Dr. James Valentine of the University of California at Santa Barbara have been reconstructing the family tree of Earth’s living organisms, sorting which orders or phyla ended in extinction, and which branches evolved into new, competitive forms.

Some of the pieces of the puzzle seem, at last, to be falling into place. We now know, for instance, that the fall of the great reptiles – and the associated extinction of many marine forms – was not a unique event. Valentine and others report that there have been at least four, and as many as ten suspected mass dieoffs, in which large portions of the Earth’s biota – whole families, orders, and phyla – declined and then dropped completely from sight. In three of these cases, the evidence is statistically indisputable. These extinctions were indeed catastrophes which enveloped the entire Earthly ecosystem when they occurred.

• At the end of the Cretaceous Period, approximately 65 million years ago
•  At the conclusion of the Permian Period, about 185 million years before that
•  And at the terminus of the Ordovician Period, approximately 210 million years further back in time.

The Great Die-offs

The Earth was far different in appearance each time it happened. Where the Cretaceous featured great reptiles and pre-placental mammals, the Permian was a time of tremendous fern forests and advanced amphibian forms. The Ordovician, on the other hand, featured hardly any life on land at all. But in each case the die-off was sharp and easily distinguished in the geological record. Suddenly, a large fraction of all the species at the time were wiped out.

Now (1984) Andrew Knoll of Harvard and Gonzalo Vidal of Lund University in Sweden report a fourth great extinction, at a time, 650 million years ago, when the highest forms of life were colonies of algae. This is about 200 million years before the Devonian event.

(Take note of the intervals between these major occurrences: 185, 210, and 200 million years. We’ll come back to them shortly.)

The paleontologists aren’t the only ones working on the problem of the past extinctions. A second discovery has received a lot of attention lately, adding another piece to our puzzle.

Led by Louis Alvarez of the University of California at Berkeley, a number of scientists have pointed out that some of the mass deaths are associated with unusual layers of clay – and that the layer representing the catastrophic end of the Cretaceous Period features astonishingly high abundances of certain rare isotopes.

Their conjecture is that a great meteorite struck the Earth, kicking up huge dark clouds and cutting off the sunlight. This supposedly then led to the ecological disaster observed in the fossil layers. Dust contributed by the vaporized meteorite supplied the unusual isotopes Alvarez and his team found in the Cretaceous-Tertiary boundary layer.

But the clay layer by itself is weak evidence for the falling rock conjecture. There are other ways to account for it. The abnormal isotope profile within that narrow layer is what the impact proponents rely upon most heavily.

But there may be another way to explain it.

Theories for Cycles

Veron 2008

Let’s go back to Fact One, the episodic occurrence of ecological disasters in Earth’s history. Would it not be interesting if there were some periodicity to these mass extinctions? If there were some pattern, then we might be able once and for all to assign a culprit … and incidentally know what to watch out for.

Recently two University of Chicago researchers, David Raup and John Sepkowski, have claimed that the four major and six lesser extinctions observed in the sediments seem to be part of a larger pattern that repeats at a rough average interval of 26 million years. They draw the implication that there is some repetitive process which puts the ecosystem of the Earth under stress in a regular pattern.

But even if the pattern they see is real, what sort of process could operate over such vast time scales, repeating reliably at 20 to 30 million year intervals?

Raup and Sepowski are not sure. Along with England’s Martin Whyte, they guess that the culprit may have to do with the interval workings of the Earth itself – with cyclic changes in the planet’s moment of inertia, its magnetic field, or the rate of transfer of heat to the mantle and crust.

It is an intriguing proposal, and it merits further investigation. However, there is a problem. No one can assign a clear-cut mechanism. Nor can anyone explain the dramatic difference between the six lesser and the four great extinctions.

One other potential periodic mechanism, that I discussed in the May 1983 issue of Analog, is the possibility that waves of settlement by starfaring civilizations might be responsible for episodes of extinction, followed by long periods in which the galaxy is empty of intelligent life. The theoretical time scales – 10 to 100 million years – seem to put this idea in the right range to be considered as a candidate, however it still remains pretty vague and hard to pin down. All we can do is catalog the hypothesis and move on.

The Major Extinctions

For the sake of argument, let us look at the four great die-offs alone … the four for which there is no dispute. Remember – 65, 185, 210, and 200 million years? Recall that these are fairly rough numbers. Nonetheless, one quickly sees the outlines of a pattern. If we assume we’re 65 million years into the latest phase of a repeating cycle, we might be tempted to guess that the greater die-offs occur at intervals of approximately –

197 Myr ± 12 Myr. (Myr = one million years.)

The uncertainty of 12 Myr is soft, but it is small enough to leave us encouraged that we may be onto something. It certainly looks like a pattern.

Could something periodic be causing this?

Not many natural processes occur with such regularity at such vast intervals. Only one cycle comes to mind with a periodicity similar to this. It is the revolution of the sun around the center of the galaxy … an orbit that astronomers now estimate to take approximately 238 million years.

 Might we be sharing the Milky Way with something deadly? Something that reaches out to “touch” our planet as we pass near it, roughly every galactic year?

Let’s pause and think about galaxies for a moment.

A spiral galaxy like the Milky Way does not rotate like a solid disk. Instead it is composed of many parts.

The galactic “halo,” like the core, consists of older, metal-poor, possibly planet-less stars of the first generation. In the halo the long, lazy orbits of solitary stars and globular clusters take them far out into the nearly empty territory above and below the spiral plane.

At the opposite extreme, in the galactic core, the crowded stars jostle and occasionally collide. They may even merge into super-compact bodies, giving rise to strange happenings. We shall speak more of these later.

Still, most of the really interesting things seem to be going on in the great, complicated disk of the galactic plane. Here the stars and gas and dust clouds rotate in their nearly circular paths, the inner zones finishing their orbits more quickly than those further out. This “differential rotation” is one of the things that drives the spiral design of our type of galaxy, helping to create the shock fronts where new stars are formed.

The shock fronts, along the concave faces of the spiral arms, are where clouds of gas and dust are compressed into new stellar systems. Some believe that life could not exist without these alternating zones of compression and release around the galactic rim. The sun’s orbit appears to meet one of the galaxy’s great spiral arms about every 110 million years or so. It takes about 10 million years to pass through one, about a million years alone to pass the shock front at the leading edge. We’re emerging from an encounter with the shock front of the galaxy’s Orion Arm right now.

Can one use these spiral fronts to explain the cyclical pattern of the mass extinctions? There are several theories which do make the attempt.

Source: NASA

W.H. McCrea contends that when the solar system moves into a shock region a sudden influx of gas and dust is absorbed by the sun, causing a dramatic increase in luminosity. That, according to the English astronomers Hoyle and Littleton, should increase precipitation on Earth, lowering sea levels and setting off a series of ice ages. 

The history of the last million years – featuring a series of ice ages only recently ended – lends the hypothesis some support. A related idea, by Napier and Clube, is that the galactic shock fronts are crowded with “planetesimals” like asteroids and comets, and that the sun regularly picks up a swarm of these every hundred million years or so, causing the Earth to regularly get “pasted.”

Or maybe the abundance of young, hot stars in the shock-front regions creates an area with a high incidence of supernovae (which would certainly wreak havoc on the Earth if one occurred close enough!)

All three mechanisms sound plausible, at least. Could the solar system’s periodic encounters with the spiral arm shock fronts then explain the major extinctions that have befallen life on Earth?

(2015 aside: Back in 1984 we didn't know the galaxy is "pleated" and that our solar system would rise and then dip through these pleats, several times during every 240 million year galactic orbit.)

Alas, the timing is all wrong.

Our encounter with the Orion Arm may indeed have triggered the ice ages of the ice ages of the Pleistocene, but the cycle of entering and leaving spiral arms clearly doesn’t fit the truly great die-offs of the Pre-Cambrian, Ordovician, Permian, and Cretaceous. The hundred and ten million year interval is over forty percent below the figure we calculated earlier – apparently way too low to apply to the major ecological holocausts of the past.

The Deadly Thing

If we re-examine the numbers just one more time, there does appear to be one more possibility – one more periodicity that no one seems to have covered yet. Our galactic orbital period.

We seem to be hit by something deadly every 195 million years or so. That’s similar to the 230 Myr solar orbit around the galactic center, but it’s clearly not the same. The 15% difference is enough to bother even the most impulsive pop theorist.

Until one realizes that anything truly dangerous floating about in our galaxy would itself have to be in orbit around the galactic center! With differential rotation, every distance from the center has its own unique orbital period, the sun’s happening to be 230 to 250 million years. There may be some “thing” co-orbiting with us – a little further out or closer in – the inner object “catching up” with the outer one at a period a little more rapid than one solar-galactic “year.”

It’s a problem that can be solved – roughly --  using the back of an envelope and a book of astronomical tables.

If the sun has, say, a period of 230 Myr, and we encounter “a thing” about every 197 Myr, then “Its” orbital period is solved by taking the difference of the two reciprocals (orbital frequencies) and dividing one more time.

If we do this, “It” turns out to have an orbital period of about 107 Myr.

We then go to the some of the tables of galactic rotation rates (laboriously collected by diligent astronomers, and published for the benefit of sleuths such as ourselves). The angular frequency versus radial function given in the literature is a little complicated, but when used carefully it gives a pretty clear result.

“It” has to orbit the center of the galaxy at a distance of approximately 2.4 kilo-parsecs, or seven point seven thousand light years. Our system, orbiting at about 10 kilo-parsecs, then has its nearest passage to the thing every 197 million years, as expected.

(2015 aside: note that if the extinctions cycle around 30 million years, that only shifts the orbit of the deadly thing inward, closer to the center of the Galaxy.)

Geological Astronomy

This is “geological astronomy” with a vengeance. We have just used the Earth as a great observatory, reading the sedimentary rocks like ancient photographic plates. Have we deciphered the clues correctly? Is there a Thing out there, which periodically catches up to use and does deadly mischief on our ecosystem with each near passage?

(Like many scientific discoveries or conjectures, this one has a haunting premonition in science fiction. In Poul Anderson’s Brain Wave, the die-off of the dinosaurs was caused by a beam which suppressed brain activity in the entire sector of the galaxy.)

If our Thing exists, it has to be pretty powerful, for according to our calculations we never pass closer to it than seven thousand parsecs. This means that it must somehow be selective, or act over a narrow angle.

The strongest possibility among known or modeled phenomena seems to be a rotating black hole which is emitting a powerful jet of sub-atomic particles.

The central cores of some spiral galaxies are extremely busy places, emitting awesome, energetic beams. SF author Gregory Benford (who is also professor of high-energy physics at the University of California at Irvine) has studied cases in which narrow, self-focused streamers of charged particles seem to be shooting narrowly across tens of thousands of parsecs, carrying as much energy as is being emitted from all the rest of the galaxy!

Clearly nothing like these monsters exists in the Milky Way today. But recent radio surveys have discovered an intriguing object, albeit much, much smaller – perhaps a fair to moderate black hole – very close to our galactic center. Radio-maps indicate a pair of jets several light years in length, spurting outward from the object.

(2015 note: this object has been confirmed to be our galaxy's central black hole, containing more than a million solar masses, and yet quiescent, at present, having long ago sucked in those objects whose orbits might bring them within grasp.)

In terms of modern galactic astronomy, this is small potatoes. But there may be others in the Milky Way, somewhere in between the sizes we’ve mentioned above. And one of these may be our culprit, now hidden behind the dust lanes of the galactic lens.

Source: Popular Science

Benford thinks the best candidate might be a condensed source projecting a beam of positrons and electrons, precessing and sweeping out a disk-like portion of the galactic lens.

An energy source like that would, indeed, be a deadly thing. An interstellar jet, even one barely grazing by the solar system, could explain a lot, such as the anomalous isotopes in those clay layers – if the particle fluxes were high enough to cause elemental transmutation. And it might be no problem for such a beam to overwhelm the ozone layer, causing collapse of the Earthly ecosystem.

Even if the beam passed nearby for only a brief time, it would probably be enough to do great harm. 

(2015 note: Rampino now sees the pattern as being roughly 30 million years.  This would be consistent with an object even closer to galactic center than my earlier hypothesized beast at 2.4 kiloparsecs.  Still, the basic idea here is not disproved.  It belongs on our shelf of possibilities.)

There you have it, still another explanation for a set of mysteries exhumed from under the dust of our ancestors. All the witnesses are long dead, of course. But that doesn’t keep us from sifting through the clues, looking for culprits.

Over the years we’ve heard conjectures of nearby supernovae, wobbling planets, and even colonization from the stars, in order to explain the demise of the dinosaurs and other mass-extinction victims. Though noe of them have rhythmic periodicity.

If the giant-meteorite proponents are right, we might be wise to take some precautions, to keep track of those bits of rock tumbling about the solar system. The other “periodic” solutions, too, each seem to offer their own bogey men to watch out for as well.

Now there’s this new “thing” to worry about, possibly orbiting out there roughly 2.4 kilo-parsecs from the center of the galaxy … just waiting, it would seem, to reach out one more time and get us.

It’s a little unnerving.

Still, one shouldn’t lose too much sleep over it. Dangerous and nasty as the little bugger may be, we probably have another 130 million years to get ready for it. If any money is going to go to chicken little preparations, at this point I think I’d rather spend it on asteroids.

Author’s final (1984) note: Remember where we mentioned TEN recorded ecological holocausts? This paper only dealt with the four greater die-offs, whose apparent regular intervals lead to an interesting conjecture.

But there are six much smaller events in the record as well. Of these, two are “intermediate” in magnitude – one about 80 Myr after the Ordovician disaster, and the other approximately 30 Myr after the Permian.

You can’t do much with two data points, of course. Certainly there’s no way we can imply that each major even is followed by a secondary die-off an average of 55 Myr later, is there?

It is now 65 Myr since the major holocaust of the Cretaceous…

No. The author steadfastly refuses to state that we seem overdue for one of those littler extinctions. That would be stretching things too far.

He hopes.

== ... back to 2015! ==

And there you have it.  A clever -- if somewhat unlikely -- rumination from my younger self.  The article was discussed on the Weird Astronomy page of the Atomic Rockets website: "...just because the assumptions are questionable does not mean that they are wrong."  This "lapping" mechanism has some appeal, whether applied to the thirty or 190 million year cycles.  Still, if wagering, I'd give stronger odds to some version of Mike Rampino's orbital "dipping" process... with or without the recent Dark Matter gloss.

And yet, aren't these marvelous times, as we sift for evidence and plumb the past for mysteries?  Our ancestors, if told of this quest, would be puzzled!

But the best of them -- I think -- would also be proud of us.

You should be too!  Try to get your fellow citizens to realize it, as well.

Galactic Self-Reproducing Probes? (Plus tweet-ready miscellany!)

First a note: my op-editorial, If You Can't Hide From Big Brother, Adapt was syndicated around the world in the New York Times, International Herald Tribune and a longer version, Snooping vs. Privacy: Lessons for an Age of Transparency in the Christian Science Monitor, among other venues, offering a little wisdom about the value of calm pragmatism in the defense of liberty and safety in the Information Age.

Oh, and regarding a matter that's "completely different": In The Immortality Debate, "Decision theorist Eliezer Yudkowsky, biologist PZ Myers, author David Brin and moderator Eneasz Brodski discuss the potential pitfalls and implications of human immortality - both biological and socio-political. Disagreement on the desirability of indefinite lifespans leads to a fascinating conversation."  

== Are alien probes already in our solar system? ==

It's Easier for Aliens to Visit than previously thought: George Dvorsky lately reported - on the ever-brash io9 site - about recent calculations suggesting that "Von Neumann" self-replicating probes, (which I've put into my fiction since 1983) might be able to "fill the galaxy" -- leaving one copy at every star in the Milky Way -- within just 10 million years after the first such probe was launched into space.

That is a mere eyeblink, in comparison to the Milky Way's 10-to-12 billion year age, or its 220 million year rotation rate (at our distance from galactic center.) This new calculation is based on a number of assumptions.  First that Einstein rules. Sorry, no warp drive or even traveling faster than 10% of the speed of light.  In other words, it's a robotic mission.  (Though, as I show in Existence, that doesn't mean it can't be 'human.")

Second, while the traditional Von Neumann probe would decelerate into the next star system, then use asteroidal resources to make and launch its copies, the recent paper by Nicholson and Forgan, offers a unique version, based on a speculative notion: that probes might gather enough raw material (and energy) to copy themselves while hurtling across the cold, black near-emptiness between stars. By building its daughters out of interstellar atoms -- not having to stop and mine asteroids -- a mother probe might drop one into a system that it passes and toss others at varied angles past the new sun, using a slingshot effect to hurl them in varied directions with great efficiency.

I'm rather skeptical of the core assumption -- building robot copies extirely from collected interstellar atoms… but I'm willing to ponder it, maybe in a future story!

In fact, I have been working at this general topic -- both as a scientist and as a science fiction author -- ever since I attended the Los Alamos conference on Interstellar Migration around 1983, when Jones and Finney first analyzed the rate at which such "Von Neumann probes" might be able to set a daughter in place around every star in the galaxy.  By the way, they calculated a figure even lower than Nicholson and Forgan.

Of course, the question of why we haven't yet encountered such probes in our solar system is provocative.  It makes the already puzzling problem of the "Fermi Paradox" or Great Silence even more daunting. Folks can read a dozen reasons why such probes - lurking in our solar system - might choose not to respond, even to earnest entreaties that have been made already, by radio and internet. Or else, read more about SETI.

I chose to make this quandary, in fact, the core element of Existence.

That is less a "plug" than simply a followup and offer to those of you who wish to follow up this teaser with scads and skeins of threads leading in new directions. There are many quirky aspects to this one idea, and the number of possible variations can be mind boggling. Even (especially) to the voraciously curious!

== Amazing Miscellany ==

It can be difficult to know what's real and what's fake when it comes to digital art these days. But don't torture yourself worrying about it now: Here are some of the most photorealistic 3D renderings on the web.

Bites from a voracious tick known as the Lone Star are leaving some suddenly allergic to red meat.  Unlike most food allergies, the symptoms typically set in three to six hours after an affected person eats beef, pork or lamb—often in the middle of the night. The bite that seems to precipitate it may occur weeks or months before, often making it difficult for people to make the link. Geez one could come up with a dozen sci fi scenarios behind this one. Hm. Villainous PETA conspirators… honey, call agent!

Daily closing of India and Pakistan Border crossing -- a terrific show. Almost a Monty Python comedy skit, but actually kind of encouraging as it is choreographed and sure beats use of nuclear weapons!

== TWEETable Highlights! ==

Bowing to the inevitable, I have decided to do my latest dump of amazing miscelklany in handy, tweetable bits, for your convenience! That means both linking and displaying the web URLs.  We'll see if folks like this.

How the Tesla Model S is made -- Behind the scenes: a wonderful video. http://www.youtube.com/watch?v=8_lfxPI5ObM

Rivers and tides of fog and clouds and air, flowing around the bridges of San Francisco.  A gorgeous video homage - Adrift by Simon Christen.  http://vimeo.com/69445362

xkcd offers perspective on the changing pace of modern life. Still… Twitter lobotomizes. http://xkcd.com/1227/

Forty websites that will make you cleverer right now!  http://inktank.fi/40-websites-that-will-make-you-cleverer-right-now/

Gorgeous view of America's waterways...Zoomable map highlights extensive #USGS data http://bit.ly/19h09CZ 

New life for hobbled planet-hunter #Kepler? Microlensing could allow continued search for #exoplanets http://bit.ly/18XSU66 

Quirky Quark Quartet…First particle containing four quarks is confirmed http://bit.ly/19OqZoK 

We need a Fixer not just a #Maker Movement  http://www.wired.com/opinion/2013/06/qq_thompson/

The difference between Geeks & Nerds -- based on Twitter Research: http://www.bitrebels.com/geek/differences-geeks-nerds-twitter-research/

This Robotics Dad makes me feel wholly inadequate as a father! https://www.sparkfun.com/news/1159

Chinese anti-pervert stockings. Girls wear these hairy panty hose to deter men with inappropriate intentions. http://bit.ly/18Fx9EW

Flash Mob - re-enacts Rembrandt's Night Watch.

Ten epochal inventors who did not get rich.  http://www.bbc.co.uk/news/magazine-23179103

Existential Star Wars: If Sartre had co-written StarWars! http://bit.ly/177HG8s

Here's what Elon Musk's new project may look like: http://io9.com/heres-what-elon-musks-secret-project-might-actually-l-793683502

Ten of the scariest aircraft landings caught on video.  

Like Synthpop electronic music?  Try "Drunken Saucer Attack" and others by Talin. https://soundcloud.com/music-by-talin/carpe-noctem

The great Pitch Drop experiment.  Since 1927, only eight droplets of the decade-slow goo have plummeted from funnel to cup.  Finally, caught in the act! http://bit.ly/13lRr0x

All right, this is fascinating. Why are testicles kept in a vulnerable dangling sac? It’s not why you think. http://slate.me/1bZzjmy

or follow more tweets on https://twitter.com/DavidBrin

Are We Alone in the Cosmos, cursed by Fermi's Paradox?

And now the news from Alpha Centauri…

(Oh, I’ve waited for so long to utter those words! News. From Alpha Centauri. Wow!)

After an incredible decade, in which the number of planets known beyond our solar system increased from zero to several hundred, with a couple of thousand potential "hits" still to verify, astronomers have now detected a roughly Earth-sized world orbiting between the two stars nearest to our system, Alpha Centauri A and Alpha Centauri B.  Much too hot to sustain life, it nevertheless will help in narrowing down the search space for others.  Moreover, now we have a target for the first interstellar probes, which are already under discussion.  Indeed, the youngest of you readers may live to see them launched.

Ah, but this raises the perennial question.  If planets are more common than we ever thought, then what about life-worlds? And even alien intelligences?

I have been involved in this topic all my life, having grown up in Southern California, the part of human civilization least rooted in the familiar, traditional or... perhaps... sane.  I am best-known today as an author of novels and stories about our many possible-plausible futures, including some that explore a wide range of possible extraterrestrial civilizations. My scientific career, ranging from optics to astrophysics, led to papers about SETI in the 1980s that include what is still the only full review article in the field, compiling all then public theories for what I called The Great Silence, but that is now more widely known as the Fermi Paradox. 

(See a collection of articles and speculations about the Search for Extraterrestrial Intelligence (SETI).)

Today, we'll dive into the Fermi Paradox, in some detail. But first a little background.

The first time I witnessed the subject of extraterrestrial intelligence brought up in a scientific setting was at a Caltech physics colloquium in 1968, when I was just seventeen. The speaker remarked on the remote possibility that pulsars -- recently discovered radio sources that emitted bursts in perfect rhythm -- might turn out to be beacons of an advanced civilization. They were, after all, several thousand times more regular in their repetitive "beepings" than any other astronomical radio source ever discovered.

The speaker was only partly serious, though pulsars to this day are listed in catalogues with the prefix LGM -- a smiling reference to "Little Green Man." Despite that whimsy,  sides were quickly taken, and it was soon very clear that most of those with tenure didn't like this kind of talk at all.

But attitudes were changing rapidly during that decade -- the exciting era of Apollo moon landings and stunning pop music. A few years later some of those who seemed angriest in 1968 applauded loudly when Carl Sagan unveiled the gold plaque that was to be placed upon Pioneer 10, the first human artifact launched on a trajectory that would take it out of the solar system.

Today that plaque is famous, along with "messages" that followed on Pioneer 11 and the Voyager probes. They depict the nude figures of a woman and a man, an arm raised in greeting, a schematic of the planets of our system, and a rayed pattern of lines and binary dots representing the most prominent pulsars detectable from Earth. The pulsar map should enable any distant beings who recover the spacecraft to trace its point of origin within a light-year in space, and its launch date to within six months.  Oh, and the Voyager probes famously carry disks with recorded sounds and images of Earth. In fact, no scientist expects the messages to be recovered by aliens, though our own speedy descendants may collect Voyager for a museum.

Ever since the 1960s another, related project went through many ups and downs.  SETI programs (Search for Extraterrestrial Intelligence) prospered and withered under public excitement and ridicule, a cycle that continues even today... and that we may discuss another time. But let's stay focused.

Of course other-worlds and their inhabitants had already long been the topic of stories -- some great and others dismal -- on the pages of science fiction pulps of the thirties and forties, then more insightful thought experiments by Arthur C. Clarke, Ray Bradbury Robert Heinlein and others.  A tradition that extends through authors like C.J. Cherryh, Greg Bear and Dan Simmons all the way to more recent speculations about alien thought processes by Ken MacLeod and Iain Banks.

In fact, this tradition goes back much farther, to wanderers' tales like Journey to the West and the Odyssey. The expansion of our horizons of interest may be among the most human of all activities, as we stretch our gaze and curiosity beyond the mere present, the mere town, nation or even planet.  If we ever do encounter the alien, it can be hoped that the vast literature of science fiction gedankenexperiments (thought experiments) about contact will be consulted by our wisest sages, who will be amazed and enlightened by the vast range of possibilities we humans have already imagined.

== The Essential Questions About Alien Life ==

The Fermi Paradox refers to a question posed by the great physicist Enrico Fermi in the 1940s, demanding: "If it seems so likely the universe may host other life forms, how come we haven't seen any signs?"  Not just of radio beacons, but of mighty structures that our own descendants might someday build out there in space. Or leakage from chatty commerce between civilizations.  Or indeed, any trace that the Earth was visited during the 2 billion years that it was "prime real estate" with an oxygen atmosphere, but nothing higher than slime molds to defend it.

It is a fascinating topic... perhaps the fascinating topic.  For it takes you from pondering the birth and death of stars and planets to the dynamics of atmospheres and the potential origins or life... to intelligence (what is it and how many varieties can it come in?)... all the way to the stark possibility that few technological species survive their tense adolescence, attempting to cross a minefield of potentially lethal errors, from nuclear war or designer plagues to ecological devastation or cultural stagnation...

... all of which I talk about in my most recent novel, EXISTENCE.   In ways that I hope readers find both fascinating and thrilling, embedded in an exciting, near-future plot  There's a new idea on almost every page.  And why not? I've been cataloguing the possibilities for what feels like eons.

== The Drake Equation ==

The most common tool that folks use, in appraising the Great Silence is a little gem called the Drake Equation (D.E.), concocted by the early SETI pioneer Frank Drake when he was at the Arecibo National Radio Observatory. It remains the most widely accepted tool for xenological speculation.

Let N = the current number of technological civilizations in the galaxy. Then,

N = R P n(e) f(1) f(i) f(c) L

Here R is the average rate of production of suitable stars since the formation of the galaxy, approximately one per year. (The current rate is slower. R is an average that includes the burst of star creation early in the galaxy's history.)  f(s) is the fraction of stars that are accompanied by stably orbiting planets. Factor n(e) is the average number of planets per system that have the requisite conditions to support life.

The other factors include f(1), the fraction of these congenial planets on which life actually occurs; f(i), the fraction of these on which "intelligence" appears; f(c), the fraction of intelligent species that attain technological civilizations, and L, the average lifespan of each species.

The D.E. certainly seems to line up the varied factors involved in bringing sapient life to prominence in our galaxy.  All the terms on the far left of the D.E. have to do with the prevalence of stable, reliable stars... and then how many have planets. There are plenty of stable, long-lived G-type dwarf stars like the Sun out there... about 6 percent of the galaxy's several hundred billion stars. Are there planets circling many of them? (We astronomers were always sure there were, for reasons of angular momentum that I won't go into here. We've grown a lot more confident in recent years! Though mysteries still abound.)

== The likelihood of life ==

What are the chances of life erupting spontaneously on isolated worlds? It appeared to do so swiftly on Earth, almost as soon as the planet cooled enough for oceans to form. Three scientific discoveries and one useful philosophical tool gave researchers the courage to make crude estimates about the distribution of life among the stars.

The first discovery came when it was found almost ridiculously easy to make amino acids, and other precursors to living matter, from abundant molecules such as methane, ammonia and cyanogen. Stanley Miller subjected a water solution of these substances to electrical discharge and ultraviolet radiation and got an organic "soup" in short order. Leslie Orgel of the Salk Institute accomplished the same thing by a freezing process. The high pressures of ice formation not only gave up amino acids, but the purine adenine as well. (Adenine is one of the four building blocks of DNA, and is the core of ATP, adenosine tri-phosphate, which controls the energy economy of the living cell.

So many mechanisms have been found that can change crude precursors into "biological" molecules that today organic activity seems almost an automatic consequence of the distribution of chemical elements in the universe.

The second major discovery supports this point of view. During the last two decades, radio astronomers -- listening to narrow emission lines from interstellar space -- have discovered great clouds of complex molecules: ethylene, formaldehyde, ethyl alcohol; some even claim evidence for -- you guessed it -- adenine. (Astronomer and science fiction author Sir Fred Hoyle, looking at starlight scattered from interstellar dust, even thought that the dust itself might actually be something akin to bacteria... living cells about a micron in size, in diffuse colonies spanning light-years and outmassing suns. It's an extravagant speculation, but fun to think about.)

It's clear, then, from basic chemistry and radio astronomy, that the basic materials for life are out there. What about the right environments? We have to assume, until we have reason to think otherwise, that complex life must grow and evolve to intelligence on planets orbiting stable stars. Are there other "nursery worlds" like the Earth? Or might ecosystems more likely be found under the ice coverings of "roofed worlds" like Europa and Enceledus, where life-giving heat rises from below and any denizens would never see the stars?

== The Role of Sapience ==

Assuming planets are common and life is not rare, then how do we explain the Fermi Paradox?  Well, some hold that the factor that's small -- that keeps the numbers down --if f(l), the likelihood that a planet will create an intelligent species.  After all, it appears to have happened just once in 4.5 billion years on Earth... though some question whether it has happened yet, on this planet, at all!

What about dolphins, apes, sea lions, crows, parrots... even prairie dogs and octopus, who now show signs of some linguistic ability and problem solving savvy?  They all seem to crowd under a "glass ceiling" that none has ever broken through (except us).  Could that represent some kind of law of nature, and might we be a fluke?

A separate question, that I explore in EXISTENCE and also in my Uplift Series of novels – (now being re-released in the UK in beautiful omnibus editions by Orbit Books) -- is whether we should start to help other species burst through that glass ceiling and join us, as fully sapient fellow citizens of a much broader and more diverse Earthly culture.  The end result, that I portray in Startide Rising and The Uplift War (both won the Hugo Award for best novel), is a much richer and wiser civilization.

But oh, the pain of the two centuries it might take, to get there.  Are we willing – and sure enough of our skill and compassion – to embark on such a journey? Would it be the height of hubris and arrogance? Or would it be the ultimate act of selfishness to reject this challenge? To say to such species “we made it to the level of art and literature and ideas and science… and we refuse to offer anybody else a hand!”

== The Minefield Ahead of Us ==

All of the factors in the Drake Equation that we've discussed so far are ones that might explain the Fermi Paradox by keeping down the numbers of intelligent beings who reach our level.  If any of those factors were responsible for the Great Silence, then that means the Big Filter lies behind us.  We are rare... but the galaxy lies open before us and nothing stands in our way!

Then come the grouches who insist that life and intelligence and good planets and all that must be abundant, but that the Filter lies ahead of us.  Remember the minefield of possible mistakes that a “smart” race might make, from nuclear war to eco-devastation? (I explore a much longer list in EXISTENCE.) With that long litany of potential failure modes in mind, these folks ask how long any technological species can survive that endless expanse of snake pits, quicksand and possible ways to commit suicide. All of which falls into the Drake Equation factor “L” or how long such a species can survive.

(As it turns out, Drake left out several possible factors, but I'll leave it as an exercise for the fanatics among you to read my astrophysical article about this.)

Suffice it to say that these two sides -- those who think the Filter lies behind us and those who cry "look out!" -- are in furious debate to this day.  And it may surprise you that the "grouches" include many in the SETI community, those looking the hardest with radio telescopes, who openly admit that they are searching for the exceptions who do not kill themselves.

Again, there is no topic like this one, so rife with mind-blowing possibilities... and so free of any data about actual alien life! And yet so prone to sudden, premature conclusions, in which smart people declare "I know the answer!" without a shred of supporting evidence.

And why not?  This is, after all, the greatest Rorsach Test... a mirror or ink blot on which we project our personalities and notions and worries about our own species... our own selves. (And if so, what does it say about me, that I am one of the few saying "wait! We don't know enough yet. Don't jettison any of the possibilities too soon. The universe may yet surprise us.)

Perhaps the most painless and entertaining way to learn more is probably on the pages of my novel (I promise).  But I'll supply plenty of other links for those who relish our most precious human gift ---

-- The wonder of curiosity.  The insatiable thirst to know about what we know... and to speculate about what we don't and to explore this vast realm. A topic we all find fascinating... and as-yet we understand so poorly.

David Brin

http://www.davidbrin.com

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