General depiction of GG apocalyptic scenarios misses 2 or 3 important points.
- Not every material can be used for nanomachines construction - bill of materials
- Construction, extraction, etc requires energy, which has to be obtained from the external environment, and no, it can't be magic of sucking heat just cuz those are nanomachines son - no of such activity can break rules we well aware on a macroscopic scale for just because it is some nanomachines.
- This one can be attributed to the inertia of thinking, when people get a hand on the idea of nanomachines they try to solve all and every problem with it, totally missing the fact that they are a good means to create macroscopic constructions - machines, tools, etc - things which we are more familiar with, especially if it better suits for a task. What they really add as a tool, the nanomachines, is the flexibility and ease to do so on all scales from nano to macro.
What that means
Reading currently present answers and it seems everyone missed point 2 and start to consider "a problem" of locomotion, which is not a problem because of point 3.
One answer attempts to assume some degree of cooperation, but let's put few sentences of how it really should be.
You do not do nanomachines for purpose of them procreating, there is no use in them restructuring some materials in themselves for purpose of restructuring. I mean, once you do that for the first time - sure it is cool and all that, but the next step(actually way before you succeed with the first one) is to consider and design things in the way you can practically use. So GG has to have a practical use, and it is not such a bad idea to apply those existing useful uses for planet terraforming as in OP's case.
Idk, let have some simple example(s) what useful things nanomachines can do, what practical things may look like, and at the same time for them to be less like universal matter replicator(which they can't be anyway).
I guess many may be familiar with the terms additive and subtractive manufacturing - additive is your regular 3d printer(and other more factory type approaches like that) processes, subtractive it basically cutting and metal chips production.
Nanomachines can combine both, by being just one subtractive manufacturing process.
- not meaning they can't be additive as well, and it generally, I guess, is the notion about them being just additive ones, but they do not have to be, and there are all sorts of problems for them to be additive ones. But as better and more flexible cutters they really can change things, spare a lot of energy, solve a lot of problems.
Everything which we 3d print today, can be carved out by nanomachines. They do not have to produce a lot of chips to shape blanks into parts, like EDM machine they can do microscopic cuts of any configuration(EDM is good 0.2 mm cuts, but nm can do better 2 orders of magnitude with any configuration of cut surface which EDM can't so as no other tech can). This could be quite a process that could replace all(99%)of current manufacturing processes, not relying on some magical nanoscale to nanoscale interaction of a tool(nanomachines, gg, nm) and the raw materials, blanks, etc.
So there is nothing strange in objectives like carving out stone gears and wheels and making reflectors and Stirling engines early on to address the locomotion problem - I mean mostly there is a myriad of ways to solve that "problem" - airplane, boats, submarines, zeppelins, etc etc. An airplane that can be of a size and mass of typical foam model and be capable to seed a big territory, or it may be a thousand tonnes load zeppelin fleet - like big flat bubbles floating and absorbing energy.
EROEI is a key in every bootstrapping expansion.
EROEI is an important aspect of any expansion. It really meaningless to set a 2minute replication time, and btw quite fast bacterias do that in half-hour, and it is a more reasonable number if you like to handwave the time.
An important question is how much energy it needs to invest in material processing and nanobots construction and how much energy it can bring in return.
Unfortunately, it depends on the technology of those nanobots, and thus unless we get hands-on specs of such technology we are bound to make assumptions. However, those assumptions do not necessarily have to be unreasonable. And it quite helpful to break that box of mentality everything has to be done by nanomachines and adopt the notion that nanomachines are the grease in the processes and can look like or be like macro processes we know.
Doing that(breaking the box) in the right way still lands us in assumption territory, even if we would like to use existing technologies to guesstimate upper limits of the processes, using photovoltaics as an example, because it hard to find real numbers about those and it depends.
But in general, there are numbers like we can increase powerplant energy production, double it in 2-3 years. For multiple reasons, like less waste in cutting wafers(2-3-4 times of improvement here alone), in using less metal, higher utilization of materials laying around, less energy to make brick(just for lack of better words and example) materials with less energy, less energy to make cement-like stuff, better reuse of that cement-like stuff(almost 0 energy to do so), no regulations because of different goals, etc it may be quite reasonable to assume that number of 2-3 years can be significantly lowered, but by how much exactly is unknown, I would guess the order of magnitude easily, would be not surprised by two orders of magnitude.
Still, plenty of guestimations on the road, let's hope OP manages to do that in an interesting way on his own or through more questions on WB.
potential, reasonable scenario, upper limit, and bill of materials
- it sure IMHO, keeping in mind particular technology, 2d-nanomachines, which specs it out of scope.
- also to mention the bill of materials problem, which I forgot to do so earlier
The technology of nanomachines, which I keep in mind, uses mostly carbon nanotubes, and may be doped with some other materials, but nothing exotic like rare earth materials, but more like your typical silicone stuff.
- that notion that nanomachines can be made of any material, engulf or convert a whole planet in nanomachines globe, using 100 percent of it - it has no bearing in reality - a hint? Chemistry.
Based on a space-related estimation, where I used existing technologies, somewhere in my answers, double happens in 3-5 days(without nanotech), upper limit which way too generous is how things are rolling today on the planet a double in 2-3 years.
So it needs to estimate how much energy can 1kg of nanotech worth - chemistry is helpful here idk some random number of 12kWh/kg (on pair with oil) probably is stored in that 1kg.
One of the last things to figure out or assume is the efficiency of extraction of energy from the environment, like sun light, let's say 10-20 percent, and efficiency of use of that energy to extract bill of materials from the environment and use to store/recombine it in nanomachines, let's go with the same 20 percent.
The last thing is how much energy can be produced by that 1kg of nanomachines, quite a question, let's estimate it by silicon wafer 50 microns thick, with 10 percent of efficiency, 2.33 grams/cm3, soo 1kg is enough for about 400mL of the thing, and smeared with a thickness of 0.005cm (why google gets things in g per cm, soo wrong, google use metric units it kg per cubic meter; SpaceX use meters per second) that gives us 80'000 square centimeters and back to the metric world it is 8 sq meters.
- 50 micron is quite a generous assumption, 10 could be enough, but....
That gives us a 12 kWh investment can produce, on sunny days, something like 1kW per hour with 10 percent efficiency and double with 20. With 20 percent efficiency of energy usage, to make bots, it lands us at 30-60 sunny hours which probably, something like 5-10 days for a double.
Taking 10 days, starting with 1 m2, having planet size the earth(water btw isn't a problem for propagation) gets us to about 49 cycles (ln(surface area)/ln(2)) or 490 days. (Or whatever number of days(minutes, hours) per cycle multiplied by 49)
To build a 10's of microns layer on the surface of the planet, with 100 percent coverage. It is not enough for purposes of terraforming, but at this point, the system reaches the bottleneck of energy production. And if there still are sufficient sources of carbon in the air, then it can grow linearly, let's take that 50-micron number, so 5 microns per day. In another year 1.5mm thickness coverage if we do not break the box mentioned in point 3 from the intro, it still can be not enough, otherwise it quite a mighty force.
Under crust, magma can be quite a potent energy source, and nanomachines can be the tool to reach it and work with it.
On Venus-like planets, the atmosphere can be used as quite a potent energy source.
In both of those cases it is nice that it is stored accumulated energy and it basically depends on your abilities to extract it, meaning to lift the bottleneck, but at the same time affecting the whole planet, with desirable(venus) or not desirable effects(earth).
Potential organic matter present on a planet can be used as an energy source.
Uranium and friends can be used but it not so convenient and not worth it in long run, but if one lands on it, yeah why not, at the beginning.
Fusion can be if conditions are right and technology allows it and bill of materials for it accessible enough - there are some interesting fusion approaches that do not use superconductors as an example, nanomachines like fusion, but it depends.
It does not take that long to launch things in space, a city-size structure, with nanomachines grease can do that easily, and then energy bottleneck is basically non-existant in the way it was mention here, limits will have a more complex configuration, still, it will be about energy. Getting in space may be beneficial at cycle 20 or something like that(starting with 1 m2), if double-time is measured in years it can save some time, and it makes sense after cycle 49 with a short double time.