The Culture is known for having huge starships that are capable of sustaining millions of people and traveling faster than light.
Do the novels attempt to explain the power source for these ships ?
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Sources of energy for the Culture are fusion, anti-matter (used for small powerful devices like hand guns), and the Energy Grid, the latter for starships, being the most plentiful and complete technobabble, which Banks was always happy to admit. From his essay A Few Notes on the Culture:
Lastly, something of the totally fake cosmology that underpins the shakily credible stardrives mentioned in the Culture stories. (...) We accept that the three dimensions of space we live in are curved, that space-time describes a hypersphere (...) Within our universe, our hypersphere, there are whole layers of younger, smaller hyperspheres. And we are not the very outer-most skin of that expanding onion, either; there are older, larger universes beyond ours, too. Between each universe there is something called the Energy Grid (I said this was all fake); I have no idea what this is, but it's what the Culture starships run on.
Accordingly Banks never went into much detail beyond this. The Grid is both a source of energy and a means of traction: Culture ships use field engines/grid motors to push against the grid and thus propel themselves. From Consider Phlebas, chapter 9:
Imagine a vast and glittering ocean seen from a great height. It stretches to the clear curved limit of every angle of horizon, the sun burning on a billion tiny wavelets. Now imagine a smooth blanket of cloud above the ocean, a shell of black velvet suspended high above the water and also extending to the horizon, but keep the sparkle of the sea despite the lack of sun. Add to the cloud many sharp and tiny lights, scattered on the base of the inky overcast like glinting eyes: singly, in pairs, or in larger groups, each positioned far, far away from any other set.
That is the view a ship has in hyperspace as it flies like a microscopic insect, free between the energy grid and real space.
The small, sharp lights on the undersurface of the cloud cover are stars; the waves on the sea are the irregularities of the Grid on which a ship traveling in hyperspace finds traction with its engine fields, while that sparkle is its source of energy. (...)
The two “Killer” class Rapid Offensive Units Trade Surplus and Revisionist raced through the hyperspace (...) Their engines were each a focus of energy almost beyond imagining, packing sufficient power within their two hundred meters to equal perhaps one percent of the energy produced by a small sun, flinging the two vessels across the four-dimensional void at an equivalent speed in real space of rather less than ten light-years per hour. At the time, this was considered particularly fast.
The Grid is said to be more "tenuous" in the empty space outside the galaxies, making space travel much slower there.
It’s stated in the books that they use something called the “energy grid“, which seems to be basically the zero-point energy inherent in the vacuum.
Physicists Richard Feynman and John Wheeler calculated the zero-point radiation of the vacuum to be an order of magnitude greater than nuclear energy, with a single light bulb containing enough energy to boil all the world's oceans.
For example, from Excession:
The big GSV was already moving, rolling and twisting to point directly upwards out of the system. Its engine fields reconnected with the energy grid.
Consider Phlebas is set centuries before the other Culture books, and they seem not to have mastered the technology for powering ships at that time:
That line of light was part of the grid itself, the fabric of pure energy which lay underneath the entire universe, separating this one from the slightly younger, slightly smaller antimatter universe beneath. The Culture, like the Idirans, could now partially control that awesome power, at least sufficiently to use it for the purposes of destruction. A line of that energy, plucked from nowhere and sliced across the face of the three-dimensional universe, was down there: on and inside the Orbital.
The appendices of 'Consider Phlebas' has information on Culture energy production when describing the motivation, or lack thereof, for the Culture-Idiran war:
And energy was, if anything, even more generally available (than matter) through fusion, annihilation, the grid itself, or from stars, taken either indirectly as radiation absorbed in space or directly tapped at the stellar core.
So The methods would be:
I don't recall a description in any of the books of a Culture ship running out of fuel, so my guess is that they are able to get all the energy they need from the grid.
Ships do take time to produce munitions such as CAM (compressed antimatter) warheads. My guess is that they are able to use energy from the grid to create antimatter.
In surface detail there is an armada of ships from a level or two lower in the technological hierarch from the culture where it is mentioned that they will work on fusion but be much more powerful if they have time to create their own antimatter, so a higher civilization provides them with some antimatter, but importantly not from their flagship. So fusion is apparently enough for space travel.
The Energy Grid
The cosmological model in the culture universe is explained in 'Excession', the first book from the series that I read. The universe we see is like the surface of an expanded balloon. This balloon resides inside another balloon, which resides inside another balloon, etc. Inside our balloon is another balloon, and inside that is another balloon, and on until supposedly there is some universe creation engine creating these expanding universes like Russian nesting dolls. The three dimensional space and matter in our universe exists on the surface of one balloon. Between it and the next outer balloon is a region of hyperspace called ultraspace. Between it and the next inner balloon is a region of hyperspace called infraspace.
Seperating those areas from their counterparts of the other balloons is something called the 'energy grid'. It is a chaotic energy field. Ships propel themselves through hyperspace by using the fields created by their engines to 'push' against the grid in hyperspace. At one point a ships is described as diving from infraspace through normal space to ultraspace and back again. I imagine the engines are pushing so hard off the lower grid in infraspace that it pushes them to ultraspace where it has to push off the higher grid to increase or maintain velocity in what would look like a sine wave with real space in the middle, hopping back and forth between infraspace and ultraspace.
The energy grid can be used as a weapon, something that happened in the Idiran war was using those engine fields to distort the grid and cause ripples large enough to penetrate real space where they would annihilate anything they came in contact with.
I assume getting energy from the grid entails those finely tuned engine fields somehow.