Navigating Known Space

The Milky Way is a bustling city of four hundred billion stars a hundred thousand lightyears across . The exciting part is downtown, the galactic core where the stars jostle close, packed dense around a black hole the mass of two and a half million suns. The core is a violent place. In the Known Space universe it's is actually exploding, though humanity won't find out about it for twenty thousand years. In the real universe the core is full of dense gas clouds, intense radiation and frequent supernovae. There's no reason that a world in the core couldn't develop life, but it isn't likely to last long before some galactic catastrophe wipes it out again. By contrast, Known Space is an insignificant backwater a few dozen lightyears in diameter. It's located on the unfashionable side of the Orion arm, out in the quiet suburbs of the galaxy. Nothing exciting ever happens out here, which is a good thing from the perspective of complex life.

The stories that encompass involve a few dozen worlds in our stellar neighbourhood. Humanity has colonized a handful, the Kzinti have a substantially larger number. We know of a few other species but have only a vague idea of the number of worlds they might occupy. Known Space is one of the larger and better developed science fiction universes, but the stories have really only scratched the surface of what's out there. Known Space is tiny compared to the galaxy, it's immense on human terms. As part of the background research for Destiny's Forge I downloaded the Gliese Near Star catalog and used an excellent piece of freeware called 3D Starmap to map out just what's out there in our immediate neighbourhood. I discovered there are well over twenty five hundred stars within fifty lightyears of Earth. It now seems that most if not all stars have planets, so, if our system is typical, with ten planets, three of them being at least theoretically capable of supporting life at some point in their history, that puts about eight thousand worlds within our grasp. That's a lot of real estate. Not all of those worlds will be even marginally habitable. However we now enough about life to expect to find it anywhere the minimum requirements are present, liquid water and a source of energy. Even the water may be optional. Life may require only a solvent of some kind, with water being just a common and therefore easy choice. With baseline conditions that easy to meet its clear are hundreds of useful planets and billions of species yet to be discovered out there. Intelligent life will be much rarer, how much so is very difficult to guess. Still, there's a lot out there.. There are places worth getting to in Known Space.

In Niven's universe starships get to those places using the quantum hyperdrive. Hyperdrive is a technology gifted by the Outsiders, deepspace dwelling aliens whose biological working solvent is liquid helium. It works only where space is relatively uncurved, which is to say far, far away from a star's gravity well. It is a natural technology for a deepspace species to develop, and a very difficult one for any civilization that likes to live on planets. The outsiders sold the technology to humans from WeMadeIt in return for the use of icy outer planet moons for their own purposes - a very good deal indeed. We don't know how the technology works, the Outsiders won't sell that information, at least not for a price we can afford. However we do have a few basic facts. We know that it takes you from star to star at a constant rate of three light years a day. We know that entering a gravity well will make the engine vanish - or perhaps more accurately will return the ship to normal space and leave the engine behind in hyperspace. We know that ships travel through hyperspace, whatever that is, exactly. We know that if you look through a window into hyperspace it does funny things to your visual perception, and we know that hyperspace navigation is done with a mass reader, a device that requires a living mind to watch it.

This isn't much to go on, but we can still learn a lot about hyperdrive from it, in outline if not in detail. Einstein's speed-of-light speed limit is absolute for material objects moving through normal space, but the laws of physics as-we-know-them don't automatically rule out wormholes and other exotic solutions to the problem of faster than light travel. Hyperdrive can be taken to exploit this kind of structure in some systematic way. The three-light-year per day absolute rate of travel can then be taken to represent some fundamental characteristic of the structure of hyperspace. On possibility is that there are two opposing forces acting on a ship in hyperspace transit, one which is rate-independent, like gravity and one which is rate-dependent, like air resistance. Force A continually accellerates the ship, and Force B continually decellerates it. As the ship goes faster and faster under the influence of Force A, Force B builds up until it exactly balances Force A, and the ship's speed becomes constant. This is exactly akin to a freefalling parachutist reaching terminal velocity. Exactly what Forces A and B are is another question. The universe as-we-know-it contains only four forces, so we have some serious looking to do. Even without knowing what they might be, we can know about how they will act. Their most important characteristic is that, like gravity, they act on the ship's mass. This ensures that the three-light-year per day limit will be completely independent of anything but the characterstics of the forces themselves. No streamlining or power increase will serve to speed up the ship, which corresponds to what we know. We'll deal with the question of the much faster Quantum II hyperdrive described in Crashlander in a moment.

This reasoning implies that a ship's hyperspace engines have nothing to do with actually moving it through hyperspace, that's taken care of by Force A and Force B. The inevitable conclusion is that their only job can be moving the ship in to hyperspace. It's possible that once in hyperspace a ship will stay there until the drive is used to move it back out again, which makes the intervening journey free and makes all trips take the same amount of power. However we also know that running a hyperdrive ship into a gravity well makes the ship vanish, unless the gravity gradient is very steep, in which case only the engine will vanish. We also know that there must be some constant power requirement to driving a ship through hyperspace, or there would be no reason not to build tremendously huge ships. The Quantum in the name gives us a hint here. Quantum systems are always arranged in discrete energy levels, and some levels are more stable than others. Lasers work by pumping electrons from their rest level to a high, unstable level from whence they immediately fall to an intermediate semi-stable (technically metastable) level, where they sit until a photon of the right frequency comes along to trigger them into the ground state, emitting another, coherent photon in the process.

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