Portal and Portal 2 are a couple of the best computer games I’ve ever come across. I heartily recommend them, especially if you like puzzles and snark. The basic idea in both is that you’re playing as Chell, a woman trying to escape from a laboratory controlled by a homicidal AI. To aid you in your escape… or scientific “testing”… you get a device that shoots portals onto walls in pairs. If you go in one portal, you come out the other one. Instant wormhole, just add portal gun.
As ever… here, there be spoilers.
Portal Guns Break The Universe
The main physics-breaking part is the Portal Gun itself. It turns out, Larry Niven wrote a very good essay a while ago, The Theory and Practice of Teleportation, which covers a lot of different ideas in great detail. I’ll just hit a couple of the main points about Portal’s portals, and then move on.
The first issue is conversation of energy and momentum. Put a portal on the ceiling, and one on the floor, so that when you go into the one of the floor, you fall out of the ceiling… into the portal on the floor… and this keeps going on indefinitely until something stops you, somehow. The problem here is that you can now accumulate energy until you hit terminal velocity — and then you keep dumping more energy into all the noise and heat you’re making while going that fast. You essentially get to move “up” in Earth’s potential for “free” when you go from floor to ceiling. But all that energy has to go somewhere, and a quick estimate suggests that this would raise the temperature of a room by a few degrees Celsius per second… which would rapidly cook Chell.
The other problem is momentum. Put both portals on the same wall. Throw a ball into one portal, and it comes out of the other with momentum in the opposite direction, without transmitting that momentum into any other object. Oops.
While portals you shoot are limited to light speed travel times, this does suggest a nice way of colonizing other planets. Or sending stuff to other places in our own solar system. I’ll leave the exact details to Niven’s essay, but it includes sending fuel through portals to break the rocket equation… but that’s if only if you have to have an artificial surface to shoot onto. Otherwise… fire, carefully, and wait.
At least there’s an explanation for why all this high-speed portal-hopping doesn’t kill Chell. In fact, this aspect bothered initial testers of the game so much that Valve added Long-Fall Boots. These are specifically designed to perfectly kill the wearer’s inertia so that they don’t go splat on the landing, and also ensure that you land feet down. Nifty, right? Totally doesn’t violate physics…
Your main antagonist in the first game and the first segment of the second is GLaDOS (Genetic Lifeform and Disk Operating System). As you play, it becomes increasingly clear that something is wrong with the AI. This is even more clear after you get shunted from regular testing to the android live-fire test range. And then it gets worse.
How hard is it to program a non-evil AI? With a strong negative weight on harm to human lives? And then, rather than simply cutting off its access to the deadly neurotoxin, they just stick a module on GLaDOS to keep it/her from releasing the deadly neurotoxin. Oy. Why can’t we downweight killing humans? Or apply the Laws of Robotics? That last link is interesting — turns out, there are some real-world guidelines for robot construction. Obviously a robot? Check. Not designed to kill humans [by running them through deadly testing chambers for “science”]? Fail.
The other antagonist you meet is Wheatley. He’s friendly. He’s helpful. He’s also deliberately constructed to be a total moron (intended to keep GLaDOS in check) and undergoes an epic and hostile personality change when you replace GLaDOS with him. Oops. Nice job breaking it, hero.
Cave Johnson Failed Business Planning
Once Wheatley turns on you in the second game, you get to explore the deeply buried history of Aperture Science. Ignoring the problem of the absurdly deep mineshaft that’s storing everything, there’s a more serious problem: Where is Cave Johnson, CEO of Aperture, getting all the money for this?
Millions for moon rocks to make surfaces for portals… which were originally intended to be better shower curtains. Various other nasty things are tested by and on humans, with no regard for anybody’s safety. Cave’s idea of science is building random crap and seeing what happens, which ranges from bouncy repulsion gel (intended as a diet product — the food bounces right out! And does bad things to your stomach…) and things like turning people’s blood into gasoline. Or making them into an army of mantis-men. Among other problems too numerous to mention. WHY HASN’T THE GOVERNMENT SHUT THESE PEOPLE DOWN ALREADY??? Then again, this is all delightfully lampshaded by various signs (such as the one above), and we never see what the exterior used to look like, back in the day. It’s quite possible that there were protestors all the time outside the facility. Maybe even a demonstration dedicated to the missing astronauts…
Meanwhile, the game does demonstrate the increasingly dire financial straights of the company. Since they have trouble marketing their various deadly products. For instance: An ad for the long-fall boots. If you’re bored, you can even try looking for the ad about using turrets for guarding babies. It’s even worse. This is not a good business plan… which explains the lack of funding, but I still wonder how they managed to get any funds in the first place.
Cave’s other major problem seems to be that he thinks of science as throwing together a bunch of random stuff, and then seeing what happens when some poor sucker tries to use it. That’s not science. That’s cruel and unusual. And darkly hilarious for the player.
But anyway, Cave, you don’t know what science is. And don’t get near my house with those combustible lemons.
For the second alien species in the Fermi Problems setting, I decided to go with another take on the “Where are they?” line. Fermi’s argument for a spacefaring civilization spreading across the galaxy in only a few million years depends on a long period of nearly exponential growth and on a travel speed that is a significant fraction of the speed of light. My friend Jacob Haqq-Misra has worked out some limitations on the rapid growth of civilizations, but I decided to design aliens who could not travel at more than 0.001 c. This requires two things: a species that cannot handle high acceleration, and a technology base that does not allow them to either engineer themselves or robotic emissaries to accelerate faster or to build a rocket that can continuously accelerate for a very long time.
The latter is a rather severe limitation. Accelerating at 0.03 m/s^2, a spacecraft would still reach 0.003 c after 1 year and cover ~8 lightyears in a century if the acceleration was sustained. The later is unlikely for something internally powered: fusion rockets are limited to maybe 0.1 c peak speed, given the requirement to stop at the destination. Even a Bussard ramjet is limited to 0.119 c by the exhaust velocity of the fusion rocket. But I have set a speed limit of 0.001 c or 300 km/s. Even a fission-reactor-powered ion engine could in theory reach or somewhat exceed that speed. So, I needed to design a species that cannot handle high acceleration and can travel interstellar distances with nothing fancier than a chemical rocket. Taking some liberties with biochemistry, I propose a biosphere that evolved on small asteroids in the nominal habitable zone of a binary made of two M-dwarf stars. I picked CM Draconis because there is a lot of data available on it, it doesn’t have any large planets in the habitable zone, and it isn’t too close to Earth. Once again, I gave the fictional version of it a name in addition to the catalog listing: Druk. It happens that CM Draconis forms a hierarchical triple system with a white dwarf, GJ 630.1B, several hundred AU away. Continuing the theme, call it Wyvern.
I fully confess that the following is far more improbable than the ursians are, because the reservoirs for biochemistry to develop in are far smaller if for no other reason. With that caveat, I ask for any reasons why it is impossible, rather than merely incredibly improbable (levels of likelihood comparable to large-scale quantum tunneling may be counted as impossible). The improbable makes for good science fiction; the impossible is not allowed.
Consider a water- and carbon-rich asteroid, between a few hundred meters and a few tens of kilometers across. The particular object we are discussing doesn’t exist anymore, but it was like billions of others: a rubble-pile of variously-sized grains. Fine-grained material accumulates in dust ponds on such objects, and on a carbonaceous-chondrite object those grains will be rich in a slew of various relatively complex carbon compounds. This much is well-established.
The fictional assumption that I invoked was for there to be enough chemistry going on in those dust ponds (using monolayers of water molecules on the grain surfaces?) to produce small closed membranes durable enough to keep their contents separated from the surrounding grains and protected from being shaken off a grain into the surrounding vacuum while still being permeable enough to allow new molecules in. From there, chemistry proceeds to something that is self-catalyzing and rather like a protocell – similar to the bubble model of abiogenesis.
Given this admittedly out-there assumption, I can invoke evolutionary processes to expand this strange biosphere. Cells with vacuum-tight walls, high radiation tolerance, and specialized molecule-sized valves survive better; on the surfaces of the ponds as well as beneath them and inside ejected meteoroids. Photosynthesis takes over from feeding on primordial and cosmic-ray-seeded high-energy compounds. Heterotrophs feed on the plants and on each other; larger and better-armored multicellular plants enjoy a survival advantage and larger animals evolve to feed on them. Eventually, three-dimensional forests grow out from small cores – collecting a hundred times as much light as compared to if they only coated the rock. I was inspired to this design by some speculation by Freeman Dyson and possibly by some of the pictures in Le Petit Prince. Conveniently, these forests will be incredibly hard to observe over interstellar distances: they are dark in the visible, and infrared telescopes don’t have the same resolution as optical or radio arrays. Even if they are seen, the forests will be confused for boring dead carbonaceous asteroids until and unless their orbits are known well enough to measure their masses from mutual perturbations – showing how low their densities are.
The first rock was destroyed in a collision within a few hundred million years of the first appearance of life, scattering microbes across the system in the debris. Objects within the entire habitable zone have things growing on them; going too far out from the star causes un-insulated cells to freeze. On a timescale of a hundred million years, each individual forest is destroyed. On a much much shorter timescale, the plants break off rocket-propelled seed pods and solar-sail leaves that search for other objects using chemical-imaging tracers (the sail leaf idea was used by Larry Niven in the form of the ‘sail seed’). The animals that don’t have sufficient hibernation and delta-v abilities of their own hitch rides.
And, because I said there would be one, an intelligent species of animal evolved in one of the larger forests and then spread. I called them the neari – the etymology is a bit strained, but I hope it sounds cool enough.
Anatomy and Psychology
There is gravity, even on the smallest asteroids, although it can be effectively canceled for fast-spinning objects. The neari can handle continuous loads of ~0.03 m/s^2 without having too many problems, and can do brief jerks up to several gees. But they don’t have a physiologically preferred up and down or left-and-right, since the gravitational pull is a very gradual change as they navigate through the forests. So I designed a body plan for them inspired by sea urchins, crabs, and T4-bacteriophage: an icosahedral central body with limbs (equally validly called ‘arms’ or ‘legs’) at the vertices. The facets have specialized functions, but the neari are equally capable from all directions:
Four facets contain mouths, with airlock throats leading into a central stomach (the mouths are also used to excrete waste). Four are optical eyes, based around fish-eye lenses. Four have lines of infrared-sensitive pits. Four are blank on the surface, and underneath them are the neari’s central brain lobes. And four have organs that work only in near-vacuum: nose-eyes. This is an idea from Hal Clement: in vacuum, molecules travel long distances without collisions. So a pinhole camera can make an image in molecules as easily as in light, as long as whatever it is looking at is putting out any detectable number of particles. Neari exoskeletons and plant leaves smell only faintly. A comet outgassing is detectable tens of thousands of kilometers away – far further along its tail. An simpler version of this organ is used by the rocket pods and sail leaves to home in on objects.
The neari grow by molting and shedding their exoskeleton, a process which also allows the regeneration of limbs. Molting grows less efficient with time, and eventually accumulated damage leads to death – although prompt injuries will kill a neari far more quickly. The other way neari die is by giving birth. To solve the problem of exchanging genetic material and maintaining a pressurized environment for the embryonic neari to grow in, two neari (of any two of the three different sexes) join along the blank faces and slowly fuse together. The embryonic neari grow inside the merged exoskeleton, fed by their parents’ bodies.
These biological differences led to some important psychological differences between neari and humans (or ursians). Neari never know their parents – they are raised by their aunts/uncles and cousins. The individual is generally seen as less important compared to the family than it is for us. And sex is not a recreational activity. I have not fully explored how such different psychology will be reflected in neari culture. There is one other relevant psychological trait, from the hazard of being stuck in space just out of reach of a claw-hold: neari get very anxious if they aren’t holding onto something.
Culture, Technology, and Starships
The neari culture at Druk is in many senses less fragmented than that of the ursians, even though they’re spread out over cubic astronomical units. As of 70,000 BCE, you might have been tempted to call them a bronze-age culture. But there is no such thing as technological levels – real technological developments are spurred by the environment, the available resources and knowledge, and random moments of inspiration. The neari may have not have any metal other than nickel-iron, but they formed it into knives and hooks and parabolic focusing mirrors. Reflecting telescopes and heliographs gave light-speed communication across all of Druk, at least at telegraph-equivalent bandwidth. Writing preserved information. The neari had chemical rocketry, using rocket pods, and solar sails, made by cultivating and then trimming the leaves of sail plants. Travel between the forests, while slow and mass-limited, was inexpensive, and the neari understood Newtonian mechanics, radiation pressure, and numerical integration.
That said, the forests were isolated enough from each other that they retained very different cultures. Wars were not unknown, although they were limited in scope by the very low population density. Enough cooperation between forests did happen to organize large scale projects, including starships.
Druk has two red dwarf stars very close to each other, with an orbital velocity of ~75 km/s. With a little extra help from the super-jovian gas giant (>5 AU from the stars) and Wyvern (~500 AU), a careful gravitational slingshot maneuver past both stars can give an escaping interstellar spacecraft a velocity of ~300 km/s. And the neari already have things that can be turned into starships: comets (I got this idea from Greg Benford and David Brin). Several cubic kilometers of comet ice/organics contain ~3e16 J of usable chemical energy, enough to support 100 neari and their life-support system for tens of thousands of years. Select a comet about to make a close flyby of the stars, trim its orbit with high-speed impactors so that it gets ejected at high speed, jump on board before the flyby, afterwards adjust your trajectory using the gas giant or the white dwarf, and then burrow down under the surface for the long ride.
The problem with these comet boats is that they can only be targeted onto a certain range of trajectories, only a small fraction of comets are suitable (e.g. the comet has to be big enough to survive the stellar flybys), and the accuracy of the navigation can only be so good. The comets jet out gas unpredictably at perihelion, and even if the comet boat’s trajectory was known perfectly that of the destination star isn’t. The neari can measure parallax and proper motion accurately, but radial velocity is only approximate. A 0.1 km/s error at launch would be 1000 AU at arrival, and the comet boat can’t change its velocity by too much more than that en route.
For purposes of the setting:
The neari did start to spread across interstellar space ~70000 years ago – launching comet boats every few thousand years. But only one of them got close enough to its target star for the neari then on board to ditch the comet and stop. The others are either confirmed to be dead, still flying through the void, or unknown in status because they stopped replying to heliograph calls centuries ago. So Druk and Wyvern (the latter has a small debris disc out to a few hundredths of an AU from the star that the neari have settled) are still the main place to find the neari and there they are hard to see from parsecs away.
Does all of this work for a species that can travel over interstellar distances without being readily detectable, while simultaneously not spreading so fast that we should expect them to be here already?
The near-Earth asteroid 4179 Toutatis is flying by Earth right now. Closest approach was 0.046 astronomical units (just under 7 million kilometers) on 2012 Dec 11.
This is not unusual. Toutatis’ orbital period is just over 4 years, so it flies by the Earth every 4 years for 24-28 years when the objects are in phase with each other. Toutatis was briefly observed in the 1930s, during the last series of approaches, but formally discovered by Christian Pollas in 1989, just after the 1988 flyby. It has been observed with radar imaging during every flyby since – 1992, 1996, 2000, 2004, 2008, and I’m part of the team observing it right now. There is also a series of optical images from the Chinese Chang’e 2 spacecraft, which flew by Toutatis on Dec 12. So we know quite a bit about the asteroid’s shape, spin, and internal structure. There’s a lot of interesting science there.
We also know Toutatis’ trajectory in space, down to a few hundred meters over the last twenty years. Running the orbit forward, we can say that it will not hit the Earth anytime in the next several hundred years and almost certainly not within the next several thousand (it can’t get closer than ~700,000 km away until the orbit has drifted from the current ellipse).
But despite all of this, there is still considerable nonsense associated with people claiming that Toutatis will hit. This willful ignorance of reality annoys me, so I will defuse my annoyance by discussing asteroid and comet impacts in fiction.
Asteroids do hit Earth. Objects the size of a car fireball in the upper atmosphere every few months. In one case, 2008 TC3, the object doing the fireballing was discovered approximately 30 hours before impact, and a careful search of the predicted impact zone duly found appropriate meteorites. Objects several tens of meters wide hit the Earth every century or so. The most recent one flattened a big expanse of forest in Siberia a bit over a hundred years ago. Objects several kilometers wide hit on a tens-of-millions-of-years timescale. The most famous of those is the Chixulub impactor, which triggered the mass extinction of most of the currently-living dinosaurs (the birds were the exception).
So there is an asteroid impact hazard. It is very well-characterized, because we know the rate of past impacts in the geological record. As the potential effects of a large asteroid hitting the Earth became well known in the late 1980s and early 1990s, the US Congress was persuaded to order and fund – through NASA – efforts to locate at least 90% of all near-Earth objects larger than 1 km in diameter (there are about 1000 of them). That project was known as Spaceguard, and included a number of groups focused on discovering objects, better measuring the orbits of known objects, and understanding the physical properties of asteroids in general. There were relatively few people working on this full-time in the beginning – the comparison was to the staff of a McDonald’s franchise. But Spaceguard did its job. We can now say that no near-Earth asteroid larger than 1 km in diameter will hit the Earth in the next hundred years.
More extensive survey programs now aim to push their completion limits for the near-Earth asteroids down to ~140 m or so. That point is pragmatically defined: for smaller objects, the cost of finding them decades to centuries before they are going to hit is higher than the cost of finding any impactor a few days or weeks before impact and simply evacuating the blast radius. The unknown near-Earth objects are not civilization-ending. We will not all die from asteroid impact.
Should a few-hundred-meter object be found to be on a collision trajectory with decades of warning, there are well-developed plans for deflection. Nukes are not necessary. Kinetic impactors are useful for objects where slower and better-controlled approaches would take too long. If you have enough time, you can just coat the object with a very thin metallic layer (paint or film) and let radiation pressure do the work for you. But the favored method right now is something called a gravity tractor: put a spacecraft next to the asteroid and hover. If you angle the rocket exhaust to miss the asteroid, then momentum goes from the exhaust to the spacecraft to the asteroid and the trajectory is very slowly and precisely adjusted.
So we will not be hit by a large asteroid anytime soon, and we can deal with the smaller ones. There remains a slight impact hazard from long-period comets, which can’t be found more than a couple of years before any potential impact because they are too far away. But that’s a once-per-hundred-million-years event, and there are ways to largely mitigate even an impact like that.
Works That Get Some Things Right
Although it is tangential to the plot, special mention goes to Arthur C. Clarke’s Rendezvous With Rama. In the backstory to the book, an object smashes into the Mediterranean in 2077 and ruins large sections of eastern Italy. There is no such object in reality, but since Clarke was writing in 1973, that’s pardonable. One part of the response to this impact is to set up a survey program called “Spaceguard” to find any other potentially hazardous objects. Several decades later, the survey program discovers the alien spacecraft that the humans label Rama when it is still many months from closest approach to the Sun. Clarke’s Spaceguard is of course the namesake for the real one. His prediction was just 85 years or so behind the times.
As far as impacts themselves go:
In Larry Niven’s fiction, the impact effects are generally fairly well done. In Lucifier’s Hammer, the impactor is a comet rather than an asteroid and a close approach turns into an impact when the comet outgasses unexpectedly. The one problem with that is that that’s a known effect, so any comet coming close enough for outgassing to possibly cause an impact would be dealt with. In Footfall, aliens dropped the snowball as part of an effort to terraform Earth to their liking. In both cases, people and civilization do survive, although the details are unpleasant. The Ringworld has suffered impacts too, punching holes through even its incredibly durable construction. Niven was sneaky enough to specify the failure properties of the ring material so that it would bend enough that impacts breaking through from the outside would deflect the ring surface enough locally that the air would not all leak out. Sufficiently large impacts onto the inside of the ring would still be a problem.
Unfortunately, these are the exceptions.
Those That Get Many Things Wrong
For the earliest impact stories, egregious failures to understand the effects of asteroid impacts can be partially pardoned by the knowledge at the time. But some things can’t use that excuse. In Jules Verne’s 1877 Hector Servadac, the eponymous hero and several dozen others are picked up by a passing comet. Verne clearly didn’t understand what happens to material that goes from zero to roughly 20 km/s nearly instantly. Of course, Verne was the man who proposed launching people to the Moon using a giant cannon with 20,000 g acceleration, so math was not his strong point. By the 1930’s, the public had started to understand the principle of reaction engines. So Balmer & Wylie wrote When Worlds Collide, in which spacecraft at least have rockets. But they got the physics of impacts entirely wrong, as well as requiring various impossibilities in terms of the rogue planet that is doing the impacting.
When Worlds Collide also illustrates one of the two ways that impacts get misinterpreted. Since it was written, there has been a pattern of making impacts far worse than they would plausibly be, either by making the impacting object far too large or making it far too destructive for its size. For example, Balmer and Wylie wanted to destroy the Earth and have the humans evacuate to one of the rogue planets that came in. To do that, they invoke an impactor the size of Neptune. That’s nonsensical overkill – we should refer to Earth hitting it rather than it hitting the Earth. In reality, for a rogue body hitting the Earth, it would only need to have a diameter of ~4000 km (smaller than Mars) to disintegrate the planet entirely. I quote The Impact Effects Calculator. There are also few enough rogue planets that the expected rate of such objects hitting the Earth is far longer than the age of the universe.
Books may often be bad, but movies are the most egregious offenders when it comes to doing impacts wrong. Most particularly are the two impact films of the late 1990s: Armageddon and Deep Impact (no relation to the spacecraft of the same name). Both are horrifically bad in terms of the science.
Armageddon is the worse of the two, since it has an asteroid as large as Ceres be pushed by a comet onto an impact trajectory that is moving about 50 times too fast for anything gravitationally bound to the Sun. That is equivalent to shooting a rhinoceros with a handgun and having the rhino go into Earth orbit. It doesn’t work. Armageddon gets still worse in that a space shuttle is somehow scrambled to launch within two weeks and match that impossible speed, that somebody built a bomb with a yield per mass ten thousand times higher than total conversion, and that Bruce Willis could bury that bomb four hundred kilometers underground. I could go on, but it is So Bad It’s Painful.
Deep Impact has a more reasonably sized comet doing the impacting, and it is discovered with over a year of warning. Good enough. But it still overestimates the effects of the impact. An 10-km comet hitting the ground would cause a mass extinction. But it wouldn’t kill everything. You wouldn’t want to be within 1500-km or so of ground zero, or anywhere on an adjacent coast if it hit in the ocean. But everyone outside of that zone would be relatively okay. A year is time enough to evacuate the zone and stockpile food for the impact winter, if everyone in the world is preparing for it.
That is the problem with Deep Impact: an amateur astronomer finds the comet. That does happen in reality. But then no-one else finds it, and there is an attempted coverup and Masquerade. That’s even more nonsensical than the other examples we gave in our Masquerade post. The comet is quite literally shining in the sky. Anyone can see it. And there is no reason to attempt a coverup and every reason to make it public, so that people can prepare. So the science is better but the sociology is equally absurd.
Thinking through all of this has made it clear to me that the public misunderstanding of the impact hazard is closely tied to popular misrepresentations of it. My normal approach is to try and explain things correctly. But how can I work around the public perception of Bruce Willis?
There’s a planet at Alpha Centauri. And that’s just too cool not to follow up. (For those of you who may want more technical information than the first link, the full Nature article is here… though it may take a subscription to see the whole thing.)
In honor of that discovery, let’s see how well some fictional accounts of Alpha Centauri stack up. It’s a popular system to consider, since the stars at Alpha Centauri are the closest to Earth (other than Sol – aka “The Sun”) at a mere 4.4 light-years. I’ll only be hitting a few examples, but oddly enough, Wikipedia has an extensive listing if you want to see them all…
Alpha Centauri Is More Than One Star
I was shocked to see this mentioned on the aforementioned Wikipedia page, but apparently some authors think that Alpha Centauri is only one star. I have the good fortune to not have read any of these; I would have been very upset by them.
While not obvious to the naked eye (or authors from the northern hemisphere who don’t see it at all), Alpha Centauri is composed of two stars. The larger, Alpha Centauri A, is a spectral type G2V, the same as the sun, has a slightly larger mass and slightly brighter. Alpha Centauri B is type K2V, and is noticeably cooler and dimmer than the Sun, and has about 90% of the Sun’s mass. The two stars are close enough in mass that the system’s center of mass is well between the two stars, not near the center of one or the other. Their orbit is eccentric, with the distance between the two stars varying between roughly 10 and 50 AU. For scale, that closest pass is about the distance between the Sun and Saturn. There’s a nice animation of this, along with other information, which also shows an estimate for where the habitable zones of the stars may lie.
On top of the bright binary, there’s a third, even dimmer star called Proxima Centauri (or sometimes Alpha Centauri C). It orbits around Alpha Centauri at a distance of about 15,000 AU. Its spectral type is M5.5Ve — at a bit over a tenth of the Sun’s mass, it’s much dimmer, cooler, and redder than the Sun or Alpha Centauri AB. The “e” means it’s an actively flaring star. More on that later.
Sometimes it’s only mentioned in passing, but fictionally speaking, Alpha Cen is a common waypoint or colonization target. It gets mentioned as such, and occasionally featured, in such things as Star Trek, Lost in Space, Buck Rogers, Doctor Who… the list goes on. A couple of books that mention or feature Alpha Centauri are The Songs of Distant Earth by Arthur C. Clark and Foundation and Earth by Isaac Asimov. In the movie Avatar, the moon Pandora orbits a gas giant which in turn orbits Alpha Cen A. Sending a colony ship there is a method of winning the game in Civilization, and it’s the name of the game Alpha Centauri. Typically, these works imply or require the presence of at least one habitable planet orbiting either A or B.
The recently discovered planet is Alpha Centauri Bb. It has a mass somewhat greater than Earth’s, and orbits Bb at a radius of 0.04 AU. Despite the fact that B is dimmer than the Sun, that means this little planet is baked to a surface temperature of at least 1500 K (depending on its albedo and atmosphere). That’s hot enough to melt silicate rocks, and is at least twice as hot as Venus (which averages 735 K). Odds of anything living there are pretty slim.
Because Alpha Centauri is so close, there’s enough data to give good limits on what other planets could be in the system. Anything habitable must be orbiting either A or B relatively closely in order to be warm enough and to avoid having its orbit perturbed too much by the other star — but not too closely. The combination of these two requirements makes it difficult for a planet to stay in Alpha Cen A’s habitable zone — it’s likely to get scattered out by B. This may be a problem for works that put the habitable planet around A, such as Foundation and Earth. B, on the other hand, may have less trouble with this, since its habitable zone is closer in.
Planets far enough out to orbit outside the AB pair would be too cold. C is an unlikely candidate — it’s so small that any planet close enough to be warm enough for life would be close enough to be seriously zapped by radiation from the flares, which would (probably) erode its atmosphere.
Back to Avatar. The moon Pandora orbits a gas giant, which orbits A. Even assuming it gets away with orbiting A, there’s another problem. Despite some false alarms, current limits indicate that there are no gas giants or brown dwarfs anywhere close to A, B or C. Pandora doesn’t exist. (And who names their moon Pandora, anyway? But we’ll cover Avatar another day.)
On the other hand, that leaves plenty of room for speculation — smaller, rocky, habitable planets are possible, and more plausible around B. The stars in the system are also older than the Sun, which means there’s been plenty of time for life to develop…
If the aliens are the technologically primitive (no radio) Na’vi, it’s pretty obvious why we haven’t heard from them yet. Or they could all be a hundred years dead. On the other hand, if we’re talking about aliens like the fithp in Footfall (by Larry Niven and Jerry Pournelle), then we’ve got a problem.
Contact with aliens from Alpha Cen is another common theme, with technologically advanced aliens an option. In Footfall, they’re in the form of an unusually plausible group of alien invaders.
There’s just one problem. We’ve been putting out radio signals for over a century, and relatively strong ones for television broadcasts for over seventy years now. Those signals, while usually not decipherable, are still detectable and clearly artificial for a sphere sixty or seventy light-years in radius and expanding. If we ourselves were sitting at Alpha Centuari, with our current technology, we could unambiguously detect those radio signals and notice both Earth and the fact that it’s inhabited.
By the same token, if Earth with all its radio chatter were orbiting Alpha Cen B in the habitable zone… we’d have heard them by now. At least one of the books listed in Wikipedia (Factory Humanity) seems to get this right — contact with aliens at Alpha Cen is established first via radio. Why we didn’t hear the fithp before they decided to take a detour through our solar system isn’t clearly explained. Maybe they’ve replaced all their radio tech with subspace transponders.
Either that, or they’ve seen us, and are deliberately hiding. Just like the Martians. Insert your conspiracy theory here.
We’ve talked about the contradictions, impossibilities, and failures in worldbuilding of several different stories. But what happens when a novel has been printed, a movie filmed, or a game sold to thousands of players, and then a contradiction comes up? Not even Clement foresaw everything.
There are several options here. One is to ignore the contradictions entirely, like the military tactics in Star Trek (trying to fix that is why I’m not supposed to write Star Trek stories anymore – MB). If you can re-write the entire medium, many contradictions can be resolved in the new version, such as in the 2003 Battlestar Galactica miniseries (although the later seasons of the show introduced more problems). The third option is to introduce corrections. If you have a game, introduce a rule patch. If you have a story, add material to a sequel that was either not referred to or contradicts parts of the original. This is a retcon.
Retconning is limited, though. The premise of a story can only be twisted so far before it breaks. This time, we’ll consider this problem in the Known Space series of Larry Niven.
Ringworld And Its Problem
In the extended setting that is Larry Niven’s Known Space universe, the Ringworld is the single largest structure. It is exactly what it sounds like – a ring around a star, a million miles wide, six hundred million miles long, and up to a thousand miles thick (we can pardon Niven for not using metric). Niven specified the implausibly strong materials that would be required for the ring to hold together while spinning fast enough to give the equivalent of about 1 g on the inside of the ring, and to hold together a smaller interior ring of separated squares to give days and nights inside the main ring as they rotated. Whoever built it loaded up the inside surface with a carefully-landscaped Earth-like biosphere.
But Niven missed something critical. As anyone who has studied Newtonian gravity, particularly the shell theorem, will remember, while there is no gravitational field inside of a uniform-density sphere, there is one on the inside of a uniform-density ring. So while a Dyson Swarm can be stable, the ring cannot be. Put the star in the exact center of the ring with any slight relative velocity and it will fall away from it. Out-of-plane perturbations are stable and lead to oscillations, but in-plane perturbations grow. And so the star falls onto the ring; which will destroy the ring either as its orbit around the star changes or as it runs into it.
This is a serious problem, which Niven’s readers were quick to point out to him. And so what did Niven do? He wrote a sequel.
In the sequel, The Ringworld Engineers, Niven reveals to the reader that the Ringworld was not just a Big Dumb Object. The engineers who built it had installed an active control system: Bussard ramjets powered by fusing hydrogen from the star’s stellar wind, which had worked without maintenance for thousands of years (impressive engineering). Then a reader pointed out that the magnetic fields from the ramjets would have fried the nervous systems of the creatures living on the Ring. So a way to fix that shows up in the third book in the series…
The Limits Of Retconning
So Niven has been adept at saving his Ringworld from destruction. But other problems in the setting can’t be retconned away.
In a Known Space story called “Neutron Star”, a human exploration team takes a ship with a (supposedly) indestructible hull and drops it on a near-hyperbola passing a few kilometers from the titular neutron star. But they forgot the tidal forces, and all died, smashed into opposite ends of the hull. This by itself is bad enough. It gets worse when the salvage team doesn’t figure out what happened, and sends a second ship to re-enact the mission (that pilot figures things out just in time). But Niven forgot that in flying by the star, the ships would acquire incredibly fast spins – about 40 revolutions per second. That fast of a spin would kill any human, even if she were sitting at the center of mass of the ship: there would be an effective force gradient of several hundred gravities from one side of his head to the other. Niven has quite sensibly not attempted to retcon that part of the plot, since then the entire story goes bye-bye.
Meanwhile, the Ringworld was inhabited by descendants of The Pak – Human Aliens who were supposedly also the ancestors of all Earth-descended humans, having settled Earth some millions of years ago. This was impossible even when Niven wrote the first Ringworld book in 1970 and the first story featuring The Pak, The Adults; in 1967. Genus homo traces back to the rest of the apes with a divergence time from genus Pan of 4-5 million years. This was known by blood serum albumin work as of 1967. Niven retconned The Pak to have also been ancestors to the chimps, gorillas, and orangutangs; but of course that doesn’t work either. The fossil record and basic biochemical similarity between all Earth-based life rules out any panspermia, directed or otherwise, at least for everything that doesn’t do a lot of horizontal gene transfer. And all that was known in ’67; all of the more sophisticated genetic work that’s been done since then has just been filling in the details.
But if Niven were to erase The Pak from Known Space, then none of his Ringworld-native characters would exist, and that is too big of a perturbation to keep the story within the bounds of what he’d like to tell. And so, again, he doesn’t retcon them away.