SciFi Round Two: Benford and the Galactic Center
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 We can excuse the thirty-five-years-outdated deflection technique, but not the name of the asteroid.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.