Space 16: Building the Tubes / In Your Faces, Romans
(galactic empire, continued from here)
The Colonization Arena
So to recap, we've let loose these self-replicating explorer-cockroaches to visit everything that can possibly be visited, and there will be this sphere expanding at 1/100 lightspeed with us at least vaguely in the middle of it. Everywhere in the interior they're going to be building infrastructure and terraforming whatever they can.
Thus, somewhere within the sphere of Explored Stuff, we'll have the sphere of Terraformed Stuff whose boundary will lag by some distance, be it 20 light-years (i.e., if it's 2000 years before the first terraformings are ready for settlement) or 100 light-years (10,000 years) or more. For the purposes of this discussion it doesn't matter a whole lot which it is.
What matters is that, eventually, we will have new planets coming on line and at a constantly increasing rate. In the 300 years it'll take the radius of the Terraformed sphere to grow from 13 to 16 light years, the number of available planets doubles, and it doubles again in the 400 years after that. (Yes, the doubling rate will be decreasing because this is not exponential growth. It's merely cubic. I don't think anyone will be complaining. Except for the aliens. Meaning in the extremely unlikely event that we encounter them this early in the game, I will, admittedly, be very surprised if they don't have at least a few issues with this plan).
Even if the actual numbers of planets turn out to be depressingly low, say, if, instead of going from 64 to 128 to 256, what I figure is the upper end of plausible, i.e., one planet per system everywhere, we instead go from 2 to 4 to 8, that will still not be a bad outcome. Recall that the main point of this exercise is to get beyond 1. (And, yes, if instead we're going from 0 to 0 to 0, that will indeed suck.)
So let's suppose there will be worlds to settle. Now for the fun part: How do we get there? It being agreed that we need to avoid rockets, what now?
Interlude on Mass Drivers
A mass driver is a method for propelling stuff around, invented by Gerard O'Neill back in the 1970s (i.e., if we're being sufficiently specific about the definition; the basic concept for the railgun, which is really quite similar, goes all the way back to World War I, and catapults go back way further than that, but O'Neill admittedly was most likely the first to consider these things in the context of space colonization).
TL;DR: What matters most for our purposes is that it's a gun, even if it's using electromagnets to propel the payload. Guns are nice in that they allow the payload to be arbitrarily stupid; it won't need engines, fuel or anything else. You just put it in a shell/bucket and pull the trigger.
In O'Neill's version, there's this really long track for the bucket to accelerate along. Then it reaches the end and lets go of the payload, which sails off into infinity. But also you're doing this in a vacuum using magnets that both handle the propulsion and levitate the bucket above the track so that there's no friction. The end result is that virtually all of the energy you're putting in goes towards moving the payload. This is as about as efficient as it gets.
One annoying disadvantage worth mentioning is that if, for whatever reason, you want to do more acceleration (or deceleration) of the payload later on, you will be out of luck, because the payload will not be there anymore.
I propose to solve that problem by having the track extend all the way to the destination.
Yes, you read that right. Suffice it to say, there will be issues:
- O'Neill's version is set up on the moon (or an asteroid, or Mars) because he's trying to solve a different problem: How to get crap off of the moon (or said asteroid, or Mars), which one can reasonably expect is slightly easier than gettng crap to another star system light-years away.
- O'Neill's version is on the order of a few miles long. Well, okay, the length was never really specified. It all depends on how fast you need the payload going, and, in theory, at least, you can make the track as long as you want,… until you run out of moon.
My version will definitely be running out of moon.
Just to be clear about why the moon matters, I'll mention two useful moon attributes that will not be working for us in the stellar scenario:
- Having craploads of mass. When the accelerator pushes on the bucket, conservation of momentum (Newton's 3rd law) requires the accelerator to move in the other direction. An accelerator that has the moon attached to it, is essentially not going to move, so the energy you're applying has no place to go except the bucket.
- Being a rigid body. You may not have realized this, but rigid bodies are actually miraculous things: If, say, I poke something with a 10 foot pole, the pole somehow transmits all of the force I apply without any losses, which really shouldn't be possible, when you think about it (and, to be sure, we lose if we rely on this too much; the pole bends/breaks/whatever).
In case you were wondering, Relativity really hates rigid bodies. (The next time anyone pulls a Relativity problem out of a textbook to try to mystify you and it assumes a rigid body, just remember, "There is no such thing as a rigid body," click your heels together, and you'll have a solution in fairly short order). Meaning even if I were to try to build some 10-light-year long monstrosity out of steel bars bolted together, it will, no matter how tight the bolts are, flap around like the 1940 Tacoma Narrows Bridge. Whack one end of it and the other end won't feel it for another decade. Actual rigidity is quite impossible in this world.
Which is why my accelerator will be lots (billions) of pieces moving independently and we'll just have to cope with that. Any of those pieces that don't have moons attached to them (which will necessarily be nearly all of them) are going to move around, and probably a lot, if we don't do something to keep that from happening, which we can deal with, but it costs us.
Note that I might possibly not care about this. If the act of sending a payload shreds the accelerator and scatters it to the four winds, that won't matter so much if I was only ever intending to use the accelerator just that once. However, (1) this does seem kind of wasteful, and (2) an honest accounting would then have the cost of sending include the cost of (re)building the accelerator, and therefore sending won't be as cheap as you might have originally thought it was.
Once we've gotten away from having it be this rigid thing, you probably won't be all that suprised to find out that I'll be using lasers and light-sails rather than electromagnets. (as the QED folks would say, it's all photons anyway…)
In which we one-up the Roman Army Corps of Engineers
So imagine a conceptual tube, however many light-years long. Let's give it a diameter of, say, 100 km. Mainly, we'll want it to be narrow enough so as to be easy to keep clean, i.e., free of large rocks that will ruin the day of any payload trying to be passing along it at near lightspeed, but wide enough to accommodate at least two lanes of traffic, punting for now on the question of how wide "lanes" actually need to be - which I suspect is not going to have much to do with the actual ship/reflector widths, which will be way smaller than the corridor.
The "walls" of the tube will be streams of laser ships all travelling at some low velocity like the 0.018c we were using for the explorer ships - probably six streams in all, 3 going each direction angularly spaced 120° apart for the sake of having the best control over the payload - individual ships in a stream spaced close enough, let's say 600,000 km, that a payload travelling through the tube will always be in range of one of them. Each ship carries enough energy/antimatter to service its share of tube traffic over the course of its own voyage - 1 to 20 kg dribbled out over the course of 600 years in the case of the Tau Ceti tube.
Among other things, this means any changes to the traffic capacity/configuration of tube will need to be arranged no less than 300 years in advance.
The payload ships are really simple: a corner reflector out in front, pulls a bungee cord attached to the rest of the ship, which will just be the payload surrounded with big-ass sphere of (lightweight!) shielding.
Aaaand…, that's all. No engine. No reaction-mass. No windows. No fuel beyond what's needed to keep the lights on and the occupants alive. Well, okay, I suppose we could put in a small engine for those odd, unexpected emergency maneuvers, but every last bit of non-payload extra mass is going to cost us.
(I suppose the bungee is a bit of a splurge, since we could have just mounted the reflector right on the payload, but it should be possible to make the bungee really light and also the passengers will thank us (1) for converting the probably jerky blasts that hit the reflector into a smooth ride - hmm, I'm guessing there's an interesting Control Theory problem there (i.e., we may need a "smart" bungee) - and (2) for not having the lasers aimed directly at them personally - yes, there'll be shielding but the less we stress it, the better, since there's already a whole lot of other crap in interstellar space they'll need protection from)
(Hmm. Let's hope the bungee doesn't break.)
For that matter the laser ships shouldn't be all that complicated either: laser + mirrors + antimatter cell + camera/radar + software. Done.
As soon as the payload ship passes a laser ship, the latter begins firing at the reflector and keeps firing until the payload passes the next ship, with the magnitudes of all of the various bursts carefully calculated so that payload does what it's supposed to.
Every time the laser ship fires at a payload, it also sends out an equal burst in the opposite direction so that its own course and speed don't change. So this will be at least double the energy cost of a moon-based accelerator. One might suppose that 2×(best we can do) is still pretty good, but there's still one more issue:
- O'Neill's version is not attempting to boost anything anywhere near the speed of light…
… the problem being that once the the payload ship gets going fast enough, when various laser blasts catch up, they will have been significantly red-shifted, meaning they will need to have been sent with correspondingly more energy to provide the kick needed. That plus the aforementioned doubling makes the overall cost equivalent to what it would be if the payload ship were self propelled, i.e., we're back to rocket economics. So far, so bad.
But then we get to the midway point, the payload ship flips around. From then on it gets fired at by the laser ships it's approaching rather than the ones behind it.
Which means all of the shots from then on are getting blue-shifted, i.e., amplified by the same ridiculous factor that we were losing in the acceleration phase. Deceleration thus turns out to be incredibly cheap. Which is how we win.
Comparatively cheap, anyway. I suspect there will not be very many people living on AlphaC doing a regular commute to a job on Earth. The vast majority of interstellar commerce will take the form of information flows transmitted relatively cheaply at lightspeed. But now, we at least have a story for what happens when there are actual people and perishable goods that need to get places and not be taking centuries to do it.
(next: counting the beans)
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