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?
This time I’ll be discussing Greg Benford’s Galactic Center Saga, six books he wrote between 1977 and 1996. Benford has a day job as a physics professor at UC Irvine and has done quite a bit of work on astrophysical plasmas, so we know he knows his physics. Although I did have to correct Benford and his twin brother Jim on a paper they wrote on SETI beacon design – they had assumed that receiver noise was independent of frequency, which it isn’t – when Benford wants to do physics correctly, he most certainly can. That makes many of the things that happen in the Galactic Center Saga a little strange to read, since while Benford can make whatever assumptions he wants, there are a lot of internal inconsistencies.
The Beginning of the Saga
I can’t dissect two thousand plus pages completely in this post, so I’ll only touch on a couple of points. In the first book, In the Ocean of Night, an asteroid called “Icarus” is on a near-collision course with Earth and an astronaut has been sent with a large nuke to deflect it. We can excuse the thirty-five-years-outdated deflection technique, but not the name of the asteroid. The near-Earth asteroid 1566 Icarus was discovered in 1949, and it will not hit the Earth for the next several hundred years (on its current orbit, it can get no closer than 0.03485 AU from Earth). Asteroid names are never duplicated, so any future potentially-Earth-impacting asteroid will not be called Icarus. Then things get strange: the astronaut discovers the asteroid is an alien artifact and is largely hollow, so that it does not need to be destroyed or at least not destroyed immediately. NASA orders the bomb to be set off anyway. That does not match with the culture of NASA as I know it. Were things that different in 1977?
Benford’s Inconsistent Aliens
Over the second half of the first book and the bulk of the second book, other alien spacecraft start to appear in the solar system. Apparently, there is a hostile machine civilization that seeks to destroy all other technological civilizations, which is a standard hypothesis to resolve the Fermi Paradox. Watcher spacecraft are assigned to monitor life-bearing planets and destroy civilizations as they develop. The Watcher assigned to Earth apparently crashed on the Moon and was not replaced. It is reverse-engineered to provide a Bussard ramjet for the first human interstellar starship, which is made from what had been a Langrange-point colony (no, the engineering there does not make sense). The destruction of the Watcher in the distant past did not result in additional machines appearing, but our radio emissions do, and the machines want to kill everybody. But their actions don’t make sense: they land small self-replicating units on the Earth and start a ground war. Why not just drop asteroids at very high speeds? And why do they want to destroy all humans in the first place?
Meanwhile, out in space, the human explorers find a civilization that has been nearly destroyed by the machines by orbital bombardment, but still manages to broadcast radio messages that are detectable over interstellar distances (why the still-active Watcher in orbit over their planet has not destroyed them is not adequately explained). In a third planetary system, around Ross 128, there is a Watcher locked in a stalemate with a civilization that lives under the ice shell of a Ganymede/Europa like moon. This makes no sense. The Watcher can make Ross 128 go nova, but not drop a big rock that punches through a 10-kilometer-thick ice shell? The humans heroically disable this last Watcher, destroying their own ship’s engines in the process, and then board it. Somehow, it is still in working order and can be used by humans – why would a machine intelligence bother making corridors a couple of meters wide? And rather than taking this far more capable vessel back to Earth, to help with the machine invasion, and potentially to reverse-engineer the design, the humans head for the galactic center.
The Weird Society of The Families
Between the second and third books, humans reach and colonize several planets near the galactic center, based initially around large orbital habitats called Chandeliers. They develop incredible expertise at genetic engineering, brain-computer interfaces, and using machine technology. At some point, there is a huge social collapse, and the human (more accurately the post-human) population is reduced to a few scattered Families constantly running from machines across the surfaces of their individual planets. They have lost almost all scientific understanding and almost all of their history, despite still having sufficient skill to copy large parts of people’s brains onto computer chips, wire artificial eyes into each others’ nervous systems, and use scavenged machine parts. That mismatch between technical skill and general knowledge confuses me, but maybe it’s not impossible.
The plot of the third book ends with one Family, the Bishops, having found and appropriated an old high-speed starship, which they use to escape the planet they are on for another around a nearby star. In the fourth book, Tides of Light, the Bishops arrive at this new planet, to find that a third intelligent species, the podia, are using a negative-mass cosmic string to mine its core. In the interests of not having an outrageously long post, I’m not going to discuss the rest of the plot of Tides of Light, or of the last two books in the series. Instead, I’ll focus on the problems with one scene involving the cosmic string.
How Not To Mine A Planet
First, this cosmic string has negative mass. Nobody has ever seen negative mass, in this case defined as a form of exotic matter where the gravitational and inertial masses have opposite signs. But it’s good for science fiction stories, because it would have interesting properties, such as repelling normal matter while simultaneously being attracted to it. Benford has the podia using a negative-mass cosmic string, which introduces another thing that nobody has ever seen (Question for somebody more adept at theoretical physics: is a negative-mass cosmic string stable, even if one could be formed in the first place?). Since the cosmic string has effectively zero width and relatively low mass, it can be run through a planet without destroying it (although there are some interesting seismic effects).
Then things get impossible. The podia mine the core of the planet by spinning the string very rapidly, so that one side of the loop flies around the planet at far above orbital speed while the other slices through the planet in an order- 100-m-diameter cylinder aligned with its spin axis. Supposedly, the repulsive effect of the string prevents the rest of the material from falling inward and simultaneously pushes contents of the cylinder out, making a fountain of cooling molten silicates and metal into space above the planet’s pole and leaving an empty hole behind. This would require inconvenient amounts of energy: >100 MJ/kg of material hauled out from the inside of the planet. Are there not any smaller objects in space that the podia can use? And how does the string transfer energy from its own motion to the material next to it?
But that’s merely a difficulty. The impossibility is the idea that the cosmic string can be used to core a 100-m hole through the center of a planet. The pressure at the center of the Earth is 330-360 GPa – call it 10^11.5 Pa to make the math easier (the material along the hole that isn’t at the center will have somewhat lower pressure, but that will turn out to not matter). In order to produce a equivalent outward force, material would need to be very close to the string, which it can’t be all the time, since the string is looping around on a ~300 m long path around the perimeter of the hole. So to figure out the question of how fast the string is moving, we need to know how fast the material will collapse inward. Against 10^11.5 Pa, material strength is irrelevant. We’re considering the explosive collapse of any void space. Give the material along the walls a density of ~10^4 kg/m^3, and consider only the 100 m immediately outside the hole – the rest of the planet would collapse inward as well, but not quite as quickly. So 10^6 kg/m^2 of hole-wall. That material is going to collapse back down to the center with an acceleration of ~300,000 m/s^2 – in other words, absent whatever outward force there is from the string, the entire hole would explode shut in a few hundredths of a second. So the string would need to rotate around much faster than that – traveling 300 m in a millisecond or less. That’s only 0.001 c. But remember the outer part of the string? It’s making a loop around the entire planet, and has to keep pace. That’s impossible – it would mean moving at 100c. And so the entire thing doesn’t work, unless there was FTL, which Benford was careful to avoid in the entire setting.
So: Why Did Benford Include That Cosmic String?
There might be some other way to move a negative-mass string to cut pieces out of planets, if we could find such a thing in the first place. But why did Benford, who knows better, include such an obvious impossibility in his story? It was so Killeen Bishop could be dropped through the inside of the cored-out planet, in a mirror-coated spacesuit, to reenact that favorite harmonic-oscillator problem from freshmen physics textbooks (nicely illustrated by the Wolfram Demonstrations Project here). But I’m afraid the joke doesn’t work for me.