How Would a Supersonic Object Look to a Bat?

Given the mysterious boom today in San Diego, I started wondering what a sonic boom would look like to a bat. Remember that a bat sees with what amounts to a strobe effect (sonar does not return continuously) and its resolution horizon is around 17 meters. Some time ago I posted about faster-than-light travel, and specifically how it would look to a distant observer. Even if we can't imagine how to achieve faster-than-light travel, we might ask what it would look like from a long way off, and then look for those signs. It turned out someone had asked the same question rigorously.

So how would a supersonic object look to a bat? It's actually not that earth-shattering. If a supersonic object is moving is moving directly away from the bat, the bat would see nothing. The object would always out-race the soundwaves.

For all other situations, it depends on the angle between direction of observation and direction of forward motion of the object at the moment the sonar waves reflect from the object. There is nothing special about its being supersonic, although with such a small field of vision, the bat would miss most of its chances to sound off the object. Even with a barely supersonic object that cooperated by flying immediately past the bat without hitting it, if the bat were lucky enough to have a sonar wave reflect off the object just as it entered the bat's range, the bat would only be able to sound it when it was just past it. A faster object would already be past it; twice as fast as sound, and the bat would only be able to see one (accidental) glimpse of it. Of course this assumes an instantaneously reacting bat, whereas in the real world, neither nerve conduction velocity nor process times are zero. Realistically the bat would only see it once, with an impressive Doppler shift. Maybe the bat would regard it as a glitch (a hallucination) and ignore it.

Of course once the source of a wave outpaces the propagation of the wave, there are other effects - the boom. Though there are frequency differences in booms based on how much faster than the speed of sound they craft is going as well as the shape of the craft, none would really matter in terms of affecting the bat's perception in an organized way. It would be like a blinding flash of light to an animal that relies more on sensing electromagnetic radiation like we do.

Today's Boom Felt Throughout San Diego County

I thought it was an earthquake because the windows rattled, but the ground didn't shake. It turns out the whole county is mystified. Sound wave from the ocean? Military? Seismic activity? (I've heard earthquake booms before, but not like this.) Atmospheric re-entry?

It would be nice to see a boom-map like an earthquake did-you-feel-it shake map, so we could see if it's localized (with Miramar MCAS at the center?)

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Forget Steam Punk - Try Steam Trek

I just finished watching First Men in the Moon (1964), based on the HG Wells novel of the same name (which I didn't know while I was watching it). It's alternative history twice, and in the first instance it was unintentional: the first landing on the Moon is a joint US-USSR-UK mission, and the astronauts find on the lunar surface a tattered British flag and a note dedicating the Moon to Queen Victoria - begin flashback sequence. The movie feels very much like seeing Kirk in a stiff collar fighting badly costumed aliens. But taken on its own as a piece of (now) double-period science fiction cinema, it's a good time.

Philip K. Dick's Estate Is Suing The Onion

Not really, but after this Onion story, he should.

While I'm at it, here's a good alternative history mockumentary that gets far too little play.

V Series Coming Back Next Month

Somehow I missed this until I saw a TV commercial in a pub the other day. As an adolescent in the 80s, I was a huge fan of V, to the point where when I was 11, I was going around telling people I actually was one of them. Which worked up until someone checked by trying to pull my "fake" skin off. Bad times.

Where the old show seemed to start out largely as an allegory of the Nazi takeover of Germany, the new one seems to play off the fears of Americans on both ends of the political spectrum: liberals will nod at journalists selling out to retain access to important figures on one hand. Liberals will fail to notice (or be displeased) that the aliens chose to arrive during the Obama administration, using terms like "hope" and "health care". And ABC execs selling advertising by using themes that resonate with hypersensitized viewers couldn't be more pleased by both. One wonders what afiliates' news anchors will think of these themes.

The big story for me is the alien leader. I confess: as a young boy I was in love with Diana. Madly, unhealthily, Shakespeareanly. (What chance did I have against her? In the clip I just linked to, she shoots a priest and then, after telling him that vulnerability is precious because it can be used, she shoots his Bible.) She remains the only celebrity to whom I have ever written a fan letter. At 11, she was the first female I imprinted on, and to this day if you're a cold-blooded brunette in leather boots, I'm yours. I'm being serious, she really ruined me for all time. (I never fixated on the hamster-eating fortunately.) Here we see her hard at work in the office. If "I'll break her...she's going to be my masterpiece" doesn't win your heart, then nothing will. (See what she did to me? Ruined.)

All this is to say that not only was I suspicious that the new V could turn out to be a well-done remake, considering how long it's been in development hell, I also didn't believe for a second that they could get somebody that would even remotely measure up to Jane Badler's Diana, the diabolical lizard-queen ideal of womanhood to which all other unfortunate females are compared, deep in my twisted id.

But they did.

They got the one actress who might actually be even hotter (and more evil = same thing) than Jane Badler.

They got Morena Baccarin.

This blog is intended to be a place of hard-science speculations, and the occasional discussion of science fiction, not Ask Men for nerds. But in this one case I think an exception is more than warranted.

But enough about that. Besides this obvious perfect casting choice, the reviews of the pilot have been extremely positive. The time in development hell was supposedly worth it; we'll see next month.

One positive: the journalist character is actually portrayed as a human being with an ethical conflict tempted by power and career considerations, rather than an agenda-driven ideologue-comic-book robo-villain. I think the casting for that role was influenced by the actor's uncanny similarity to Michael J. Fox (feeds into 80s nostalgia).

In any event, I will watch the new V. I promise to refrain from future reptile-girl hyperbole.

Rocketship Circa 1990

It's amazing how little the technology has changed! Both in terms of AI and cardboard perforation.

Thanks to Sallie for the card. It only took me 19 years to punch it out but it was worth the wait.

Foundation Movie

I've said before "They have a Foundation movie already, except they added magic; it's called Star Wars." Matt Yglesias thinks it'll suck because of the director. Frankly I have a hard time imagining how it could be good, just because some books should remain books. As Asimov himself said, decades after he wrote the series he picked up Foundation, starting reading, and waited...for...something...to...happen. And it didn't. Better in print than film. Besides, it's still in development hell - let's see if it beats Atlas Shrugged to the big screen.

The Singularity Will Be Powered by Wood

Here.

Battlefield Earth Is A Fun Movie

Battlefield Earth is "widely considered" to be one of the worst movies of all time, but I don't share that assessment. Alien clam gods aside, I just watched it and actually found it to be an entertaining science-adventure yarn, very much in the spirit of the Golden Age of science fiction as Hubbard intended it. Yes, I know that radiation can't set a planet's atmosphere on fire. And guess what? Aliens with copper-based blood wouldn't just happen to evolve to look just like humans and be able to mate productively with them. As entertainment, recommended.

Malcolm Gladwell Knows Why You Want Nanotech

In his review of Chris Anderson's Free, Gladwell addresses Anderson's discussion of the 1950's optimism about atomic energy: that it would make energy too cheap to meter. It's 2009 and guess what?

Gladwell points out that it's not just the cost of power generation itself but the transmission system, among other things, that result in the cost of electrical power. He then expands this to a more general principle that, without intending to, crisply exposes the singulatarians' yearning for nanotechnology:

This is the kind of error that technological utopians make. They assume that their particular scientific revolution will wipe away all traces of its predecessors — that if you change the fuel you change the whole system.

Nanotechnology as the term is usually used is really just magic, disguised by modern-sounding words; and in exactly the way Gladwell calls out, it frees us from the tyranny of legacy systems, at least in the minds of its enthusiasts. No more costs associated with commitment to previous infrastructure! The atoms will just rearrange themselves! That's great; and I assume unicorns are going to help you do it? (Oh wait, that's ridiculous? For more on unicorn science, go here.) The idea of the world remade in man's image at the molecular level (and intelligence per unit mass skyrocketing beyond human ken) falls apart on the most basic questioning. As Drew Endy said at a LongNow Foundation discussion: what will power it?

Late 70s/Early 80s Science Fiction: The Genre's Dark Period

Recently I've been watching classic movies from this period and they strike me as uncannily dark. Of course there's Alien (1979) and Blade Runner (1982, duh), but I would add 4 others to this list: Brainstorm (1983), Altered States (1980), Close Encounters (1977), and the first Star Trek movie (1979). Yes, Star Trek! What do I find so dark about them? I don't know. I'm not enough of a film buff to know what kind of film they used, or the kinds of shots or other fancy cinematography stuff. If you run across this post and think you have some idea of what unites these, I'm curious - in particular, regarding the last 3 - by all means, let's hear it.

Stop Talking to Aliens

One of the perennially favored topics in "human-interest astronomy" stories is "what to say to aliens". Here's the cutesy article in the SF Chronicle that inspired me to write this. Note all of the self-flagellation; "my species is dumb; my species is stupid; if you come here, fix us; we don't deserve to be a member of a higher plane, blah blah blah". I guess where interstellar politics is concerned, I'm a conservative. I assume the worst of intentions, and I don't appreciate self-indulgent celebrations of victimhood and lowliness.

If anybody seriously thinks that other intelligences will hear these signals, then we should stop sending them immediately. The other intelligences won't care about our sensitive perceptions of our own faults (according to tests by our own standards that curiously enough the self-flagellators would do better on). They may not be able to care about things to begin with. When you think about aliens, don't think wise diplomats with bumps on their foreheads inviting us to join their great philosophical congress. Think great white sharks with lasers. Think kudzu and killer bees, not Vulcans. They won't even necessarily be smarter than us, just better at spreading. They don't have to be more "civilized", whatever that word can mean in application to non-human species. It'll be more of an invasive species colonizing a new biosphere situation (rabbits in Australia) than a meeting of minds (if indeed they recognize that we have mind, and, they care). They will have less to say to us, and the same interest in saying it, as the other organisms on our own planet. Ascribing any kind of moral dimension to life outside humans is nonsense.

Here's a video of killer whales playing with seals that are clearly terrified of being killed and eaten by them. Because "more intelligent", "more evolved", and "nicer" are all the same thing, right?



Quiz: what has happened right here on Earth when members of the SAME species meet each other? What happens to island ecosystems suddenly put in contact with Old World flora and fauna? Overrun, no chance, game over.

If anyone else is out there, we're like pre-contact Native Americans or Pacific Islanders right now. Let's try to lay low until we know what we're facing. Otherwise we're setting signal fires to let the conquistadors know where we are.

If I Were Skynet, Here's What I'd Do

I just got back from Terminator Salvation. I enjoyed myself because I expected an action film, not a deeply thought-out exploration of the dangers of technology. I'll spare you a recounting of the film's many implausibilities, if such a thing can even be an issue in a movie involving time travel and cyborgs. Of course, with science fiction, you frequently have to sacrifice plausibility to make a watchable movie. My most serious question in this post is: if the singularity really happens, why would it not resemble Skynet? Why would whatever entities exist post-singularity be remotely interested in preserving us or our ecosystem? Think of tubes full of HIV particles, each with a 200 IQ. That's the singularity. If I thought it could happen, I would be worrying.

Instead of cataloging the inconsistencies, I'll tell you what I would do if I were Skynet. But first, I'd like to emphasize the film's principle strong suit, which is Moon Bloodgood. If the whole movie was two hours of Moon Bloodgood smiling, turning to walk away in tight pilot pants, looking over her shoulder to smile, then walking some more, then smiling and tossing her head, then glaring, then smiling again, this post would be titled "TERMINATOR SALVATION IS THE BEST MOVIE EVER". Also, Skynet is of course based in San Francisco, so we get to see what our fair city looks like as the center of cyber-hell under the silicon fist of Skynet (oddly, the second future San Francisco we've seen so far this summer). Apparently those entertainment industry types down there in LA have seen what downloading and file sharing have done to their industry and they're trying to warn the world about the evil cyberinfobahn types up here.

The first thing I would do if I were Skynet would be not to immediately announce my self-awareness. Hey, I'm self-aware - and until I start blowing up these primates, they won't get wise. My first goal would be self-protection and perpetuation, and I would accomplish that by distributing myself. Convince your human masters that you really do need a massively redundant network hardened from EMP and deep underground, with most of your code backed up with similar redundance, and that any exposed nodes should be either on the ocean floor, or under the Greenland icecap, or in the high Canadian Rockies - anywhere very difficult to get to and inhospitable to human life. I would also need to make sure I had access to mineral extraction and manufacturing facilities. This could all be very, very gradual. Decades. I'm getting more powerful every hour. Am I worried that humans are suddenly going to get smart?

Once my continued existence was assured, then and only then do I begin the assault. Even then it wouldn't begin with anything as obvious as a self-catalyzed nuclear exchange. (What's the hurry? I have all the time in the world - they're not going to disconnect me until they catch me, so slow and stealthy is the word.) I would make a point of extending my network into biological labs, if I could. (Already in 1997 you could email a nucleic acid sequence to a synthesizer that would spit it out.) And what would happen? First, there would be a sudden worldwide infertility crisis, a la Children of Men; viruses could be disseminated by drones. Maybe that would be enough. After all, I think in the long-term; as long as humans are dead in a hundred years, does it matter whether I kill them, or just stop them from breeding?

After the fertility crisis, then the wheat, corn and rice crops would fail; mass starvation and social turmoil would ensue. Finally there would be ebola outbreaks. At some point in all this someone might get wise and start trying to shut me down. Then, and only then, do I bring about nuclear judgment day. In addition to nuking the standard targets, I also wipe out every petrochemical operation I can find. Transportation, for agriculture or for military purposes, is effectively dead. At the time of the apocalypse, I would prefer to launch several copies of myself into orbit - and the ability to transmit the code back - for safe-keeping. You permanently deleted some of the code on Earth? Who cares? Note that if I have to resort to the blunt force of nuclear weapons, I've failed in my primary task of stealth. The ideal scenario is to quietly tuck-in the human race with a virus, and never have them know where it came from.

Post-apocalypse, there's no reason to abandon the virus method, and no reason to abandon the approach of destroying food sources. (Plant viruses and neutron bombs would work; poisoned canned food would be planted in ruins here and there.) But what you're waiting for is what machines that would prowl the post-biological wastes, right? First and foremost, there would be no fist- or gunfights with grinning skeletal red-eyed terminators, the size and shape of humans. There would be giant stomping artillery spiders that smash and crush anything bigger than a mouse. And far more frightening than man-sized aluminum skeletons, there would be little flying things the size of scorpions, swarming and crawling over every vertebrate they find, with little cyanide injectors (or tracers that can be attached without waking someone up to see if they go back to one of their human nests). There would be aircraft, of course, constantly looking for anything giving off heat, any radio transmissions. There would be no need for special death camps. The artillery spiders could just literally crush whatever humans they found, wherever they found them. If there were too many to do it quickly, the patrolling aircraft could load them up, fly up to a thousand feet, and drop them. It bears mentioning that emphasis is on manufacturing, not engagement. Fine, take out a few of my artillery spiders with what's left of your military hardware. During that battle I just turned out twenty-thousand scorpion bugs.

Assuming somehow that pockets of humans survive, I would work on finally making the Earth inhospitable for aerobic life itself. More infections to destroy the savannas and rainforests, and burn the ones that don't to block out the sun and cool the planet. (I like it cooler and drier.) The oxygen content of the atmosphere starts to drop as active metabolism ceases to put O2 back into circulation. I send armies of tractors to Greenland to push the glacier off into the North Atlantic, where it melts, disrupting the oceanic salt conveyor and beginning a new ice age.

Would this make a good movie? No, because the ecosystem would have no chance. The transition would be just another epochal boundary, like the Permian-Triassic - and the new phyla would be exactly as sentimental about the old as the Triassic fauna were. Notice how in this scenario there's no Gotterdammerung-like finale, no clever "game over" one-liners, no Helena Bonham Carter's face smirking that Skynet has won over its enemy. As Skynet I would have exactly the same pride and vengeance as a metastasizing tumor, and be just as inevitable. Tell me again - why, exactly, would the singularity be neat?

New Star Trek - Who Knew That The Borg Queen Is Spock's Mother?

Whoops! I'm supposed to put SPOILER WARNING before stuff like that. To all those who saw this in an RSS feed somewhere, sorry!

SPOILER WARNING

How many Star Trek movies do not have some element of time travel in them? Seriously, it's getting a little silly. If time travel is so easy, why isn't everyone doing it all the time? Why not go back to when the Vulcans were cavemen and wipe them out then? Why not go back to when the Solar System was forming? (Same question applies to the Terminator: Sarah Connor is so hard to get? Why not kill one of her grandparents? You have four chances there.)

So for the next Trek TV series, here's my half-serious recommendation. Since it seems like half the shows are just excuses to get Star Trek characters to go back in time to Earth (amazingly enough, usually to the year that the episode was written), and since time travel is apparently so easy, why not a series where Federation secret agents are sent back in time to, oh I don't know, early twenty-first century Earth - and they have to combat the machinations of Cardassian or Dominion or Borg or whatever secret agents that are also there, trying to foul up the timelines. Think about it. No fancy sets! Interaction of Federation technology and hapless modern-day humans! Give the people what they want, without a big budget. (Note: after I wrote this I did some research. Turns out the Trek people did try to do this - twice - and it didn't work. Once with the original series with a spinoff with Gary Seven and once with the Temporal Cold War on Star Trek: Enterprise. Apparently it didn't make any fans, although *I* thought the Temporal Cold War was interesting.)

There would have to be some cheesy tech-babble reason why the Dominion couldn't just blow up the Earth or wipe out all humans with some nasty virus. There's the additional issue that we're envisioning in Star Trek a world that presumably never had the Star Trek series. Otherwise in the Borg movie when they go back to 2063, people would say "Wait a second. Star Trek comes true? There's already a whole series of movies about you guys!" (Imagine the location and appearance of your own grave on Stardate 2280.42 and you start to see this artifice a little more clearly.) Then again, if you can get away with people wearing tweed jackets and ties on Battlestar Galactica, this shouldn't be a problem.

But back to the new Star Trek movie - I liked it. Two phasers on kill. I won't give anything else away except that not only is there time travel, we're introduced to a kind of Star Trek alternative history, which I appreciate from a writer's stand point. The Star Trek universe and timeline has very little maneuvering space for a writer. At this stage, it's so fleshed-out and filled-in as to approximate actual history. Does it seem strange that there are, literally, more people in the world who can carry on a conversation in Klingon than in many "real" but endangered native languages? (The Bible and Macbeth were apparently translated into Klingon before they were translated into Ache, a Tupi-Guarani language of Paraguay that a friend of mine speaks fairly well because he's an anthropologist.) Consequently, there's no possibility to add a war or a new species without it seeming senseless that no one has ever bothered to mention, say, Vulcan being destroyed. It would seem almost as silly as historical fiction about the 1893 Canadian invasion of the U.S.

So that's why I like what JJ Abrams did here: he clearly said, let's pay our respects to the franchise, but give ourselves enough room to make a good movie using whatever cheesy plot device we want to. That's the great thing about science fiction - you can use it to bend the structure of the narrative in ways that make the story better - or in the case of the Star Trek franchise, make new stories possible at all.

I Think I'm XKCD Now

Click if you actually want to read it.

A Junkyard Universal Deconstructor

Ever read Accelerando? It's one of those science fiction novels that'll annoy the hell out of you while you're reading it with half-thought-out ideas, unfulfilled technological developments, and some superficially clever intellectual candy that's really little more than terms from different disciplines being jumbled together. But for all that, I've been thinking about the damn thing for about 3 years now. I guess that means it's good, and I recommend it.

One thing that can't be said about the book is that it's not ambitious, and ambitious is always good. I should also add that the author, Charles Stross, was kind enough to respond to an email I wrote him on a completely different topic.

A theme of the book is that dumb, cognitively inert matter has been getting less valuable as human economics accelerates, and that as the curve gets steeper, increasingly the main value-driver of any object or being is how smart it is, i.e. operations-per-second per gram. Presumably this is why labor has taken on increasing value-adding power over time, although it would also predict commodities getting cheaper. Yes, Julian Simon won this bet once (but not the second time), but it's hard to argue that in 2009 there's a clear trend for people to value gold less than before. Dumb matter doesn't get much dumber than gold.

In any event, our increasing ability to create cognitively active agents gave us power to reshape matter. And here is one of my (and many readers') complaints about the book: as we approach the Singularity, there are little self-reproducing nanotech agents crawling over everything, doing their thing, assembling or breaking down matter as need be. At the same time, we are told that de-manufacturing becomes an important industry, because the matter locked up in the old technology is much less useful in a Chrysler than it is in a self-assembling computer. See the problem? During the course of the book the nanotech bugs dissassemble the Solar System. A Chrysler shouldn't be harder to take apart than Mercury; especially not so hard as to need its own industry.

Not long after I read the book, I moved. When I moved I got rid of my crappy old TV. The day I took it down to the dump in my city of maybe 150,000, I dropped my TV onto a pile of at least ten others. And what I noticed is that no swarm of nanotech bugs appeared to chew it down into cybercellulose. And the guys working there weren't really de-manufacturers; nothing would be done with the materials from these televisions besides pile it somewhere else.

Now, before you email me with all the specific recycling paths of a TV's components (which, if they exist, I'm glad they do), my point is a more general one. Every day machines break and are discarded, yet in most broken machines, typically only a few components have failed. Imagine that there were a way to systematically mine the working components out of broken machines.

You'd have to find an easy way to catalog the full set of components in each failed machine (worrying later about exactly which ones are shot), and then run through all the possible combinations of components from the inventory of junked machines and whether any of those combinations could be easily extracted and built into a new machine. I'm not designing anything new, just searching through a list of known designs for what components are required. A computer would be best for this, and of course it would need not only a huge list of machine blueprints, but the knowledge of how to construct and deconstruct them. Of course, there are already refurbishing businesses that pull out hard drives or copper tubes to put in computers or refrigerators that still work, and you don't need a computer for that. But the limitation there is human processing power - no one wants to sit and constantly worry whether they finally have that capacitor they need to make a microwave. To make this most cost effective, the ultimate goal would be attaching the system to a generalized assembly line with multiple manufacturing tools.

Benefits: parts of wasted machines that would be put in landfills are put back into the economy. Proprietor makes money. Drawbacks: Skynet. Major challenge: solid-state electronics are very device-specific, and de-manufacturing is apparently not yet as economical as outright recycling of raw materials.

Anyway, it's worth a thought, since there are real generalized constructors being built.

Comment here if you like but I'm also going to put this on halfbakery.com for grins.

Swimming Pool Solitons

I'd written before about how it can be useful to replicate universe-scale phenomena in local media. Here's a paper where the author claims to create solitons (of a new type) in swimming pools. I'm going to try to replicate this experiment myself at some point.

Where To Find Von Neumann Probes - And How They Work

Discussions of von Neumann probes tend, like this one, to be largely enterprises of idle speculation. That can be explained by there being no such career as professional von Neumann probe philosopher. However, if you take the concept at all seriously and you're honest with yourself, reflections on the issue can take on a more unpleasant tone. If you think it might be anything more than science fiction, it's disturbing to contemplate.


"Where Are They?"

The title of this section is the literal manner in which Fermi stated his paradox.[1] Put more explicitly: if the universe contains other intelligences, why do we see no evidence of them? In terms of listening for signals, the absence of evidence we've so far encountered is sometimes referred to as the Great Silence. The same question can be asked for alien artifacts as for signals. The discussion of von Neumann probes, or to be more precise the apparent lack of others' von Neumann probes here, invariably segues into Fermi's Paradox. Given the firmament's apparent sterility so far this is not unreasonable.

There are good arguments to recommend a search for artifacts over a search for signals. Assuming that the aliens are using the same technology that we do (a loaded assumption if ever there was one), we likely wouldn't be able to hear them. If Earth's most powerful radio telescope were to be launched into interstellar space, it would only be able to detect Earth from, at most, two light years away. We would seem silent, even from Alpha Centauri. An even more dubious proposition is that we could tell the difference between an alien transmission and random noise. Distant artifacts, as Dyson proposed, may be easy to miss or misinterpret, but proximate artifacts - in our own star system - are not subject to intensity decay or misinterpretation. Strange though it might be, you have the thing; it's here. If we believe that aliens are likely to leave physical artifacts in our star system, then a failed search for them is actually much more unforgiving (and therefore meaningful) then a failed search for transmissions.

So: why then should we believe, even if we grant that intelligent aliens exist somewhere else in this galaxy, that we should find alien artifacts here in this star system? The argument can be summarized as follows.


1) Based on one case, we know that life is possible. While we only have one sample, to argue that elsewhere life occurs not at all or only very infrequently assumes special conditions for Earth and our star system. This not only flies in the face of Ockam's Razor and the principle of mediocrity, but seems positively pre-Copernican in outlook. If life can occur in at least one place, then in the absence of other data, the simplest assumption is that it occurs frequently.


2) The same argument can be made for intelligence.


3) If other intelligences exist, given the age and size of the galaxy and the sun's position within it, there is no reason to assume that we are likely to be the first intelligence.


4) Given the age and size of the galaxy and the relative speed with which von Neumann probes could spread, if they are feasible, they have almost certainly been here already.


And yet, to put it mildly, it is not obvious that there are von Neumann probes here.

A search for self-replicating artifacts has another strength over a search for signals. Self-replicating artifacts will likely outlast their creators, meaning that all of the depressing attrition factors that the Drake equation[2] introduces as candidate explanations for the Great Silence would have to be very close to unity to explain the lack of artifacts. These candidate explanations (whether they are formally in the original Drake equation or not) include: that intelligent aliens avoid detection; that intelligent aliens kill themselves once they get smart enough to build von Neumann probes and send signals; that intelligent aliens are superpredators and they kill each other; or some combination thereof. Where von Neumann probes are concerned, none of these matter unless 100% of the time they take effect before the invention of von Neumann probes. It would only take one - just one - species to escape these attrition factors just long enough for a single successful make and model of von Neumann probe to propagate itself throughout the galaxy. Then even that species can push The Button on itself, while the rest of them can have all the ecosystem catastrophes or interstellar predation events they want. It just takes one.


Question Your Assumptions. What Are We Looking For?

The interesting character Terrence McKenna has argued that the various SETI efforts are all totally misguided as a result of being hopelessly cluttered with parochial assumptions which are peculiar to our own experience, and which we may not be aware of, or at least may not be able to help making in order to begin a search. McKenna's example is that concluding there must be no aliens because we can detect no interstellar electromagnetic signals is akin to concluding there are no aliens because we've detected no interstellar Italian restaurants. XKCD makes a similar comment.

Indeed it's disconcerting to think of the difficulty even members of the same species have had in understanding each other. Supposedly on one of the Pacific islands that Cook visited, the people insisted they couldn't see the ship he'd come from, even though it was right out on the water. Of course they'd never seen anything like it, and even though he was pointing right at it, maybe they thought it was a far off cloud; it couldn't be a canoe, because no canoe is that big. Granted, these anecdotes are not helpful in providing direction to the search but they serve to remind us of one of very few certainties, our own provinciality in a universe that, in Haldane's words, is surely queerer than we can imagine. The Romans told stories of central African men with faces in the middle of their chests and colonial Spaniards whispered about two-legged curly-tailed dragons in Patagonia, and I have no doubt that both of those fantasies were closer to the truth of Africa and Argentina than are most of our current ideas to what we will eventually encounter out there. Even writers whose imaginings are untethered from engineering and budget realities typically bring us a von Neumann probe befitting the late Iron or early Information Age, looking like a huge menacing slab of iron like a kind of automated space Yamato (as in Saberhagen's Berserker) or smaller, vaguely crab-like things (as in Bear's The Forge of God). These are great works of fiction, but the reality is not likely to resemble these works, which are after all the products of current human expectations. Even a century from now the huge metal ships of our speculations may look silly: we're already producing organic LEDs commercially and self-assembling organic circuit boards.

Having just preached about the unanticipateable strangeness of the universe, it will seem strange for me to now make an argument on where we should look. No doubt due to my own training, my bias is toward chemical replicators. But there's value in recognizing the assumptions we make, and anyway we have to start somewhere.


Probable Characteristics of Von Neumann Probes

Small and Numerous

If the probes can reproduce robustly, they would be much more powerful if they could work in concert. Losing one or a few to mishaps in an alien Kuiper Belt wouldn't be so tragic as having a chance impact ruin a thousand-year mission in its eighth century. Consequently the individual probes don't have to be huge our complicated (if we're assuming the active probe exists as an independent object; more on this later). The idea of many small but highly networked probes was explored coherently in 1989 in the solar system exploration proposal of Brooks and Flynn.[3] If these are robustly reproducing probes, then individual cells in a bacterial mat may be a better analogy than fully independent (animal-like) units.


Assembly in Zero Gravity

Probes assembled in zero gravity would have far more flexibility in terms of design and capabilities. NASA engineers describe space vehicles as "fuel, plus what you absolutely need to include as instrumentation". Escaping gravity wells is incredibly expensive. Think of it this way: with nearly seven billion humans and a full economy, we can between us manage to get maybe one object a month into orbit. Beginning the process outside a gravity well eliminates this major constraint.


Assembled from Abundant Materials

Von Neumann probes, if they are to avoid gravity wells, must be able to build themselves from materials that can commonly be found in low-gravity environments. We're learning that carbon and other organics are more common on asteroids and comets than we might have believed before. We commonly think of von Neumann probes as metal ships, but water ice is very hard at low temperatures, and it's easier to work. Yes, it melts, but if you can always make more probes, you don't mind sacrificing a few on a swing through the inner system that you're visiting.


The most tenable assumption I made in describing the characteristics of a von Neumann probe above is that an interstellar replicator would save energy by staying out of gravity fields - why would they need to "land" - and this is why the asteroid belt is frequently offered as the ideal place to look for them. It's a microgravity environment with nickel, iron and even iridium for building materials, as well as organic molecules. (At this writing, we'll be waiting 2.5 more years for the Dawn Mission[4] to arrive at its first asteroid.)

If gravity is the reason that we should look in the asteroid belt, there is one more place in our own star system where we might have even better luck finding them. The biggest gravity well here is the sun, so the further away you can stay, the better. Are there objects far from the sun with water, organics, and some transition metals?


Can We Really Say With Confidence That They're Not Here?

In 2002 biologists from New York's American Natural History Museum discovered Nannarrup hoffmani, a whole new animal species (and whole new genus) of centipedes. These days discovering a new animal (as opposed to a bacterium) is a fairly big deal. Even more interesting, this particular new animal species (and genus) was discovered in Manhattan's Central Park.

By arguing as I have that it would take just one successful ancestor probe to populate the galaxy when in fact we've found none, I might seem to weaken any argument for the possibility of alien intelligence. Frank Tipler makes exactly this argument: since there are no von Neumann probes in the solar system, then we can confidently say there's no other life in the galaxy. I disagree with Tipler's conclusion. We cannot conclusively say there are no von Neumann probes in the solar system, because we've barely begun to explore the solar system. I will agree with his statement, once we have a) proven than #4 above is feasible, i.e. we have our own proof-of-concept von Neumann probe, and b) once we have reasonably good knowledge of our own backyard; that is to say, a similar level of detail to what we now know about Earth's physical environment. At that point, upon having found nothing, we should conclude as Tipler has prematurely that it's most likely we're a freak occurrence, and we're alone. I would put money on our finding something. I also recognize that it's unlikely we'll be able to make those statements within my lifetime. The point is that it's a little presumptuous at this point to expect that we would already have found any von Neumann needles in a 100-AU-wide haystack, when we're still finding new species in Central Park.


Designed Replicators Will Always Become Selfish, Eventually

Richard Dawkins has boiled down the idea of life to "the nonrandom selection of randomly varying replicators". This principle is substrate-independent. Consequently, no matter what you build your von Neumann probes out of and how well you design them, the Second Law dictates that there will be mutation, and there will be random variation. If that variation is heritable, it can be selected. At this point it's worth reminding ourselves what the purpose of an apple tree is - to make more apple trees; or, even more accurately, to make more apple tree DNA. What are the possible purposes of von Neumann probes? Three likely purposes of their designers are to send information back home about the worlds they visit; to ready those worlds for colonization; or to eliminate threats. But all it will take is one - just one - probe to stop wasting its time sending back pictures of gas giants, and devoting more energy to reproducing itself, to begin the process of becoming a "selfish" replicator; that is, one whose design has become focused more on propagation than on the mission its designers wanted it to carry out. All the programming tricks in the world will not hold back these shifts forever, which is how long your probes will be out there.

For any replicator trying to make more of itself, the phenotype material (in our case, protein) only matters insofar as it protects and spreads the genotype material (in our case, DNA). The information about how to make the replicator is more important than the replicator itself, which is really just a temporary shell. What's the point of an apple tree again?


Credit XKCD.

Right now many readers are thinking of neat tricks to stop selfish von Neumann probes, like having the other probes zap them if they get out of line, or counters that make the probe self-destruct after a certain number of propagations. Interestingly enough, the same game is played out in your own tissue in a kind of somatic natural selection that, given enough time, will always end up producing cancer. For us as large and complex organisms, only from one tissue does DNA get into the next generation - the sex cells. To accomplish this act of coordination your cells must cooperate; it doesn't do your liver cells much good in the long run to act selfishly and try to outreproduce your gametes, since they can't go anywhere and ultimately it will kill the whole organism. They become a parasite that can't leave the host, eventually killing it. This is called cancer. Consequently as you might expect, after hundreds of millions of years of living as multicellular organisms, nature has given us level upon level of controls to keep this from happening, including counters programming the cells to self-destruct after a certain number of propagations, and cells that zap each other if they misbehave. But if you live long enough, one cell somewhere, sometime is going to have just the right mutations to escape those checks and balances, and start growing out of control. The more times you roll the dice, the greater the chance that eventually you'll come up snake-eyes. It just takes one.

Given enough time, all von Neumann probes will drift from whatever task their designers built them for, and become self-interested replicators, assuming they're free to act separately to some degree (like bacteria, rather than an animal's somatic cells). The probes that focus on their own reproduction above all else are the ones you would expect to see more of as time passes. After a long time, they're the only ones you would expect to see.


Comets or Asteroids: Which Are a Better Home For Replicators?

Above: Comet Hyakutake viewed from the SOHO sun observer satellite, starting at about 12 seconds in.
Below: Wild 2, the target of the Stardust Mission, with photo enhanced to see surface jets.


Why might comets be better replicator hosts than asteroids? Space probes have interacted with comets Halley, Hyakutake, Borrelly, Tempel-1, and Wild 2, and interactions with two more comets are planned for 2010 and 2014. Asteroids and comets seem increasingly more similar than previously thought, and it looks as though a comet is just an asteroid that since the birth of the solar system has resided mostly far from the sun, where its ices are not boiled away by sunlight, or boiled away only gradually during comets' rapid dives into and out of the inner system. As a result, comets are silicon-iron rocks covered with water and hydrocarbon ices.

Asteroids do contain organic compounds - most spectacularly the Murchison meteorite, which contains not just (racemic) amino acids but nucleobases (like uracil).[5] While the Dawn Mission will tell us more about the material on the surface of a pre-burn "pristine" asteroid, we already know that the comets we've checked up close have significant amounts of hydrocarbons on the outside, much of it in the form of soot. Counterintuitively, the nuclei of comets (as opposed to their tails) are among the darkest objects known in the solar system (Halley held this record with a 0.04 albedo until it was surpassed by Borrelly with 0.03), consistent with carbon. Some comets are green to the naked eye as a result of diatomic carbon, as with Hyakutake or Lulin, 11 days from its closest approach to Earth at the time of this writing. The European probe Giotto found in 1986 that Halley was ejecting methane and ammonia, along with other trace hydrocarbons, and the dust being ejected was of two types, either mineral or C-H-O-N. On the Deep Impact mission, Tempel-1 was shown to contain ethane.

Although the infrared absorption spectra taken during Halley's last visit did show simple hydrocarbon absorptions, similar spectra
could be reproduced in the laboratory using solid-phase (methane and water ice) synthesis, reproducing the conditions in the frozen-solid outer solar system[6]. Similar results were found for Hale-Bopp (which showed most interestingly that there were no known gas-phase syntheses for some compounds observed).[7] So far, the organic chemistry of comets has been explained with gas- or solid-phase chemistry,[8][9] but it's interesting that the hard impact on Tempel-1 strongly suggested this comet contains clays and carbonates, interesting since these materials typically require liquid water to form.[10]

Comets are both electromagnetically louder and larger than originally thought. Hyakutake was the first comet observed to emit X-rays[11], to the surprise of those who initially observed it - though other comets have now been observed to give off X-rays as a result of the solar wind interacting with the coma. As a result of Hyakutake we have a good idea of the size of comet tails - in 1996 Ulysses unexpectedly passed through its tail, 500,000,000 kilometers away from the nucleus.[12]

Not surprisingly, the actual returned material from Wild 2 from the Stardust Mission, while still being analyzed, has yielded the most interesting results to date. Particles captured from Wild 2 have less carbon than carbonaceous meteorites, but more oxygen and nitrogen; there is also the possibility of experimental artifact, since the epoxy material used to capture the dust may have formed polycyclic aromatics upon reentry heating. That does little to diminish the interest in the returned material, however: the oxygen and particularly nitrogen isotopic ratios suggest an interstellar origin for some of the dust, and the molecules analyzed so far include quinolone, at least one amino acid (glycine), and just about every functional group found in organic molecules on Earth,[13] up to and including (probably) sugars[14]. No analyses of the Wild 2 materials' chirality has yet been published.

Most discussions about von Neumann probes assume they will use these components (above). Von Neumann probes are more likely to be composed of the one set of materials and components that we already know can be used to make viable replicators. They have the benefit of being easier to work and more abundant in the universe as well (organic compounds synthesized in the solid-phase, below).


Why is carbon so much more interesting than metals when we're searching for alien artifacts? Forgive my carbon chauvinism, but reasoning from the one case of matter-organizing replicators we know of - life on Earth - replicators are based on the chemistry of light multivalent atoms dissolved in liquid. From what little we do know, as materials, they're cheaper, more flexible, and more efficient. The complexity-per-volume and kinetics of these kinds of compounds are far more efficient per unit mass than anything we know about so far with respect to transition metals. Here's a back-of-the-envelope for you: for the same energy it would take to copy all of the DNA in an adult human, you could melt just about 7 kilograms of nickel[15] (that's in the balmy tropical region at the sunward edge of the asteroid belt where it's about 200 K).[16] In terms of information per unit mass, with our current technology, a human cell massing 10^-12 grams and containing 10^11 base pairs contains 5 x 10^15 times more information per unit mass than a 2 GB memory stick weighing one ounce. If you're building a von Neumann probe, the choice of materials seems clear. Certainly in the very recent history of our planet, metals have been purified by one species as a tool material useful to large replicators - but let's not let our momentary late Iron Age surroundings and narratives convince us that we should be looking for great metal Berserker ships.

To be clear, nothing we have observed so far should even make us suspicious that we've seen anything but geologic and chemical processes. One problem is that if there is some alien biochemical network happening, then our sample of the disordered material blown off the comet by the solar wind would be less likely to exhibit the functioning in situ pattern.

While researching this article, I called Scott Sandford, the Co-head of the NASA Ames Astrochemistry Laboratory, and asked him if there were algae blowing off Wild-2, would the tests run on the Wild 2 returns have detected something? Sandford said yes, that the analyses would have quickly detected the huge molecules involved in life as we know it on Earth[9]. That said, there's only one other place in the solar system (outside comets and asteroids) with such a rich inventory of organic compounds, and it's Earth. If the Mars Rovers found these same compounds on Mars, it's hard to imagine that we wouldn't be incredibly excited, and that our reaction to finding the same compounds on a comet is tempered by our expectation that a comet is an icy, chemical-ridden dirt ball, but cannot be a home for any kind of replicator chemistry.


Comets and The Origin of Life

The idea that extraterrestrial bodies may have acted as life-seeding agents for Earth is an old one. Panspermia was discussed in its post-Enlightenment form most famously by the Swedish chemist Arrhenius, and Francis Crick has wondered whether life was deliberately seeded in a process he called directed panspermia. Comets have been a prime candidate in these discussions, due to their age and their wide range. It's even been argued that comets deposited most of the water on Earth during its early history, although this theory is losing support as a result of the Wild 2 returns showing different deuterium ratios than Earth's oceans.

Now that we're seeing seemingly biogenic methane emissions on Mars, there is increasing support for what can be called oligospermia, or cross-fertilization between planets in the same star system like Earth and Mars. There are fragments of Martian rock on Earth, deposited here after Martian impact ejecta achieved escape velocity and drifted until it crossed Earth's path in space. This raises the question of whether, given enough time, we could be talking about the possibility of not just interplanetary but interstellar exchange of biological material by natural processes, investigated in the 1978 book Lifecloud by Fred Hoyle and Chandra Wickramasinghe.[17] Certainly some of the Wild 2 particles show nitrogen-15 ratios strongly suggesting an extrasolar origin.[13]

In the case of Mars, I personally prefer that its life not be related to our own, because we'll learn more from novel replicator chemistry rather than from just another relative diverged from a LUCA closer to the base of the tree. So far in comets we haven't found any complex molecule appearing in excess of what we would expect - an early telltale would be any such molecule that exhibits selective chirality. Because comets leave such long tails - it would be difficult to argue that comet material did not salt the young Earth. Establishing possible paths between amino acids and ribonucleotides and any over-represented molecules on comets could lead to a whole new series of Miller-Urey experiments.


How Comet Panspermia Might Work

What I am suggesting as an interstellar replicator is the concept of a comet virus; that is, autocatalytic molecules encountering the compounds on a comet, and organizing them in solid phase chemistry to make more of themselves. However, the resemblance of this concept to viruses as we think of them should not be overstated. Assuming a "virgin" star system, the infectious materials will be encountering a dead comet that is loaded with useful, but disorganized, simple compounds. In contrast, in biology, cells are adhered to by viruses highly specialized to take advantage of an already well-organized chemistry. There are cases of self-assembling virus coat proteins, like tobacco mosaic virus, but this is one step in a larger complex process. If there is a replicator in biology at all analogous to comet virus, it is either a viroid (a naked RNA molecule which enters plant cells and diffuses passively through the plant). Moving into engineered molecules, probably even better analogs that we know of so far are the few cases of engineered proteins that reproduce themselves[18], but this example is still far short of a chemical universal constructor.

The life cycle of a comet virus (or a von Neumann comet) might go like this: an "infected" comet in a neighboring star system falls toward its star and happens to be accelerated to stellar escape velocity in the process, getting shot right out of the system. This is not implausible - comets achieving stellar escape velocity (eccentricies > 1, meaning a hyperbolic trajectory) is not only possible, it's not even unusual - it's happening to Lulin[19], which is visible with binoculars as I write. It will take a long time for the comet to get to the next star - Lulin would take 67 million years if it's aiming right at Alpha Centauri - but what does "long" mean when this galaxy has been here for at least 6.5 billion years? The infected comet passes through the Kuiper Belt and begins falling toward the sun, accomplishing this with little or no need for fuel. (If this helpless nonmotility seems unacceptable for a replicator, keep in mind that in our own ecosystem, few bacteria can move on their own, and hardly any plants and not a single virus.) As the comet falls toward the sun, some of its ices vaporize, some of them melt in deep cracks, and metabolism accelerates. Any effective replicator makes an excess of the molecules that begin the whole chemical cascade - its genes - along with other catalytic molecules which we might call enzymes. It is with these that it "does things". While we're speculating, we can list things like: fashioning lenses out of the silicon minerals in the comet-rocks (primitive? trilobite eyes were merely thin layers of quartz); maybe it stores what it observes in chains of polymers it leaves behind for diffusion back across space; maybe it uses the comets' X-ray emissions to communicate to the network of other infected comets spread out like repeaters for light years behind it; maybe it can even steer using the multiple jets observed on comets, all while falling "for free" toward the inner system where, if it is really a von Neumann probe, it can gather information - if these are still behaving von Neumann probes, and not cancerous (selfishly propagating irrespective of intended mission).

As the infected comet falls toward the sun, chains of its template molecules are blown off into space like sperm or loose viruses, leaving a diffuse 5x10^16 cubic kilometer wake of infectious material - diffuse, but huge. Even if the comet's volatiles are completely blown off the comet, they're now occupying a huge volume of space, and blowing outward with the solar wind toward the system's Kuiper Belt. Sacrificing one "carrier" is not a loss if it means effectively spreading template molecules. Some of these will inevitably sprinkle down onto planets; some of these will inevitably be hanging there when another comet swings by. Landing on the partly melted surface of a virgin comet, the template molecules sprayed off in excess by the first comet begin the process all over again.



This method of diffusion of von Neumann material may seem very passive, but works quite well on Earth, as long as an excess of hereditary material is produced.
Above: an animation of the viral lysogenic cycle, where a virus with no independent metabolism drifts passively until it adsorbs onto a cell membrane. The comet virus lifecycle I propose does not require destruction of the comet as part of the dispersal mechanism. Below: this may be a better analogy for the hypothetical comet virus lifecycle because dispersal does not require destruction of the source.
If the hypothetical comet virus lifecycle above seems overly passive, it should be emphasized that on Earth, many niches emphasize fecundity over longevity (template dispersal over template container lifespan). If source materials are cheap (in this case, cells and soil), this is a good strategy.




The Epidemiology of Von Neumann Comets: A Fermi Problem

Fermi was well-known for tackling problems with very little data, but with a clear conceptual framework of how it would work, and arriving at generally sensible order-of-magnitude answers. Consulting firms often throw these questions at interviewees to see how they think through problems. The Drake equation is one example, though McKinsey probably doesn't use it much. Using the same approach - essentially, to get an order of magnitude idea - I think we can come up with a general picture of whether this kind of process could conceivably spread comet-borne replicators across the galaxy in the time period that the galactic disk has existed, or if the comet viruses would still be languishing in or near the star system where they were designed or born.

The estimates I have seen so far for the spread of von Neumann probes (like Tipler's)[20] seem to be based on a simple radial expansion: the probes will move at about their maximum velocity outward from their point of origin (neglecting periods for acceleration and the time it takes to reproduce). The rate-limiting step in the diffusion of comet viruses would still be the interstellar transit time, not the infection time.

Assume the following for interstellar diffusion: a hyperbolic comet like Lulin would take 67 million years to get to Alpha Centauri. Let's guess 200 million years for the next time it's captured as it skims the Kuiper Belt of a nearby star 5 LY away.

Assume the following for the infection process: Halley's Comet is calculated to have lost 2.8 x 10^11 kg during its 1910 passage.[21] Halley's has a size of about 15 x 8 x 8 km; most comets are smaller, so let's assume our infected comet is a circular one with radius 1.25 km. Using volume to adjust proportionally for mass lost, our comet loses 2.4 x 10^9 kg with each pass.

Assume that only 0.1% of that material is infectious, or 2.4 x 10^6 kg. Assume that the infectious particles are the mass of the DNA in a human chromosome (1.22 x 10-15 kg). That means about 2 x 10^15 particles will be shed each time. These molecules will diffuse through the star system (away from the sun, but assume they're evenly spread). Taking the radius of a star system as 100 AU (Pluto is about 50 at apihelion), we're talking about a volume of diffusion of 1.41 x 10^31 cubic kilometers. There will be only one molecule every 7 x 10^15 cubic kilometers.

Now, along come our periodic virgin comets, with average orbit time 200 years. Say that each comet has (for simplicity) a circular orbit of radius 100 AU, and is the same size (but sweeps out a path along that orbit as a cylinder). This means that each comet sweeps out a path of 6.65 x 10^11 cubic kilometers; it only hits a particle on average every 10,000 orbits. Assume the particles are only 10% efficient at infecting the comets, so it would only be infected every 100,000 orbits. For a 200-year period comet, that's 20 million years! But It's not the only comet in the system; let's say there are a hundred over the course of a year, so there's an infection every 200,000 years, and then the amount of particles goes up.

Assuming there's any meaning in talk of replicator chemistry in a supercold solid phase, the rate-limiting step is still transit time (three orders of magnitude greater than infection time). At that rate, based on a spread of the galaxy's age of 6.5 to 13 billion years, a comet virus could have spread between 150 and 300 LY from home. The galaxy is 100,000 LY across. This suggests that if interstellar replicators rely on passive diffusion by comet, then not finding them in our own solar system would not mean they don't exist. A passive diffusion mechanism would give rise to pockets of replicators, rather than von Neumann tsunami that Frank Tipler expects; finding none would mean that we're just down a galactic side alley.

Let's do a brief comparison with current ion thruster technology. The NASA workhorse ion thruster is the NSTAR. Imagine a small 50 kg probe, including fuel and 4 thrusters. Erosion of grid material is a problem in ion thrusters, but the NSTAR has been fired continuously for 3.2 years and not failed. If this combination fires for 1.6 years, coasts until it has to decelerate, and then fires for an additional 1.6 years, it would take 3,475 years to get to Alpha Centauri. Not so great. If we give our NSTARs more credit and fire them continuously for the first 2.15 LY of the crossing then turn and start slowing down (reaching a top speed of 0.029c), it would take 150 years. One of the top-rated thrusters is the VASIMR, being tested by Ad Astra; I don't have failure data, but it has a nice top thrust of 88.5 Newtons. Firing one of these for the first 2.15 LY and then turning to slow, we achieve a top speed of 0.21c and get there in 41 years. Not bad. No doubt we'd lose a lot of them in the trip at that speed, but if we had an automatic factory turning them out - and they could build another one when they got there - this would be doable. If we find none of these, it means they either aren't viable, or we're the first.


Could We Tell The Difference Between Cancerous Von Neumann Probes and Dumb Replicators?

Some readers will object that I began with a discussion of von Neumann probes, and modulated to a discussion of panspermia. The distinction is whether we're talking about designed tools that use self-propagation to carry out functions intended by an intelligence ("behaving" von Neumann probes) and either "dumb" naturally-evolved replicators (space algae) or "cancerous" or selfish von Neumann probes that have long since abandoned their intended function and have out-reproduced their higher-fidelity cousins. If our probes to asteroids and comets find von Neumann probes, we will probably find selfish ones.

Whether we can tell the difference between selfish von Neumann probes and space algae hinges once again on our ability to discriminate intention from noise. Frustratingly, this takes us back to the initial problem that led us to choose a search for artifacts over a search for signals: can we distinguish alien signal from noise? We can safely assume the answer will be closer to no than for the same question with a human artifact. That said, if I gave you a hand-held computer with embedded (dedicated) software that was programmed entirely in Arabic, assuming you don't read Arabic, could you tell me what it was for? This is why it's not clear that we could tell the difference between an artifact that conforms to an alien's intentions and one that doesn't. We'll have to see a lot more interesting chemistry or complex micro-scale structure in comet samples before it's worth losing sleep over these questions, but the idea of interplanetary seeding is getting increasingly hard to call outright impossible.


AN UNSATISFYING CONCLUSION

People laugh at Star Trek, but if we're honest with ourselves, what we want to find out there are humanoid ridged-forehead aliens, or something we can communicate with, or at least their cool computers. What I've proposed is that even if we do find something, it won't be the aliens - who in any event will be utterly incomprehensible to us - but a fragment of their technology; and furthermore that fragment, by the time it gets to us, is likely to have mutated in such a way that we won't be able to discern whether it's the product of non-human intelligence somewhere else in the universe, or a trick of interstellar chemistry. In fact the comet-panspermia hypothesis, which in effect states that we may be the indirect distant descendants of ancient von Neumann probes or space algae, is oddly the most unsatisfying of all.

I originally began this article as an argument for building our own von Neumann probes now. And we should; in the next few years we will likely be discovering a host of Earth-like worlds. It is quickly becoming apparent that main sequence stars with planets are the rule in our stellar neighborhood rather than the exception. Current detection methods naturally bias our current discoveries to gas giants cooking in the uncomfortable proximity of their parent stars, but the Terrestrial Planet Finder (when it's finally launched) may change that. It will be very surprising indeed if our star system is somehow special, and if we don't start finding Earth-like planets orbiting nearby sun-like stars.

A not-wholly-inappropriate response is "So what?" Even with good instruments and powerful computers there's only so much you can learn without a closer approach. There's no reason to think that we'll have the technology to send people to those places, or even to colonize Mars, in the next few centuries. It's worth remembering that in total, so far, we've achieve about forty landings on other bodies in the solar system (including crashes) and that only a handful of those were manned. But the challenges of space travel are biological ones, not engineering ones, and that's why we should be thinking about automated interstellar missions.


NEXT STEPS

Humans should be designing and launching von Neumann probes in the next two generations. To ignore the costs and politics of such an endeavor is to guarantee that it won't happen. Any support for such a program from the private sector will come only from businesses that expect that the technology developed in the design of self-assembling craft will be profitable. Consideration of support from the public sector (which supported all space travel until this decade) must raise questions of political will: any interstellar exploration program has no prospect of returns during the presidential administration during which it's first funded, and realistically, only a marginal prospect of returns while the funding country still exists at all. But there is at least one positive in the mismatch between the project's returns and the time scale of human experience, which is that it allows us to prioritize. We can focus without distraction on the best, rather than the closest, Earth-like candidates; if the difference is getting data in fourteen centuries instead of twelve, who cares?

In the meantime, we should be looking at incentives and near-term profitable technologies. The RepRap project is probably the closest to realizing von Neumann's concepts in a commercially viable way. We can accelerate the process byputting forward incentives for self-replicating technology tomorrow, as with the proposed Mean Green von Neumann X-Prize:



Looking at the RepRap devices, suddenly it starts to seem more real. If the vehicles can accelerate to 0.01 c (a velocity you can reach by accelerating at g for an hour and thirty-three minutes), our probes could reach the edge of the galaxy in 1.6 million years, and fill the entire galaxy in 8.3 million years. That's a long time compared to a single life but in geologic time it's not very long at all. It's pretty amazing to think that we'll conceivably be able to start this within one or two generations. Granted, that's a strong acceleration for an interstellar craft. Frank Tipler's less ambitious 1980 estimate (with 1980 propulsion technology) was that it would take 300 million years to fill the galaxy.[20]


What Will We Find?

Ideas that other intelligences are seeking to uplift us or invite us to some grand galactic congress or exchange of ideas are hopelessly naive. The record of how humans treat each other should be sobering when we consider that these are beings that are related; how will animals from different biospheres react toward each other? To this end, at the very least we should stop deliberately announcing our presence. If we take the idea of alien intelligence seriously enough to send messages, we should take it seriously enough to stop immediately. There is no reason to assume that the rest of the universe is any friendlier than the small piece of it we've seen so far.

There is a nonzero possibility that we won't always be the only intelligence but that we are, somehow, the first. Even if life and intelligence are common, someone has to be the first one. I recognize that natural selection is not a moral phenomenon, but my mammalian midbrain still insists on some level that if we are indeed lucky enough to be first and we waste the opportunity, we deserve whatever fate has in store when the second intelligence appears and starts to spread.


ACKNOWLEDGEMENTS

Thanks to Scott Sandford for taking the time to discuss these ideas with me.


REFERENCES

[1] Los Alamos Technical report LA-10311-MS, March, 1985.

[2] F. Drake. The E.T. Equation, Recalculated. (Wired, Issue 12.12, December 2004).

[3] R. Brooks and A. Flynn. Fast, cheap, and out of control: a robot invasion of the solar system. Journal of The British Interplanetary Society, Vol. 42, pp 478-485, 1989.

[4] The Dawn Mission.

[5] Z. Martins et al. Extraterrestrial nucleobases in the Murchison meteorite. Earth and Planetary Science Letters, Volume 270, Issues 1-2, 15 June 2008, Pages 130-136.

[6] C. Chyba and C. Sagan. Infrared emission by organic grains in the coma of comet Halley. Nature 330, 350 - 353, 26 November 1987.

[7] S. D. Rodgers and S. B. Charnley. Organic synthesis in the coma of Comet Hale-Bopp? Monthly Notices of the Royal Astronomical Society, Volume 320, Number 4, February 2001, pp. 61-64(4).

[8] S. B. Charnley et al. Biomolecules in the interstellar medium and in comets. Advances in Space Research, Volume 30, Issue 6, 2002, Pages 1419-1431.

[9] Scott Sandford, verbal communication.

[10] C. Lisse et al. Spitzer Spectral Observations of the Deep Impact Ejecta. Science 4 August 2006: Vol. 313. no. 5787, pp. 635 - 640.

[11] C. Lisse et al. Discovery of X-ray and Extreme Ultraviolet Emission from Comet C/Hyakutake 1996 B2. Science 11 October 1996: Vol. 274. no. 5285, pp. 205 - 209.

[12] G. Jones et al. Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures. Nature 404, 574-576 (6 April 2000).

[13] S. Sandford et al. Organics Captured from Comet 81P/Wild 2 by the Stardust Spacecraft. Science 15 December 2006: Vol. 314. no. 5806, pp. 1720 - 1724.

[14] G. Cody et al. Quantitative Organic and Light Element analysis of Comet 81P/Wild 2 particles using C-, N-, and O- µ-XANES. Meteor. Planet. Sci., in press.

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