Recently I’ve wrestled with making up a FTL-replacement system for use in a low-tech “space”-fiction setting. The basics go like this:
The Rabbit Hole: spatial travel through a fourth dimension
The purpose of this science fiction setting detail is to provide a simple alternative to faster-than-light travel, by replacing the breaking of relativity and other physical laws we know with a different set of speculative science butchery. In this case, it is specifically so that there are more than three dimensions of space (ignoring the metaphor of time), and that with a sufficiently advanced level of technology it is possible to move on this fourth axis of space.
This fluff-nugget is geared towards a space fiction setting. By its nature, 4d-travel in a setting utilising this will also double as space travel, and there is no possibility of “portals” or “gates”. Travel in the fourth dimension is performed by space craft that have additional technology. Descriptions and motive explanations follow.
1. There is a fourth dimension of space that is similar to our three dimensions of space.
2. But this fourth dimension of space is not uniform. There is a fundamental force, or an attribute of space-time, which causes matter and force to collect in distinct slices limited to the three normal space dimensions. Flatland analogy: imagine that we reduce this 4d-space into a 3d-space, and thus 3d-space becomes a 2d-space. A “slice” in this 3d-space is a flat pancake that is aligned in the same direction as all the other slices in the 3d-space. These slices are arbitrary collections of energy, separated from each other: thus, in 4d-space, though they *are* very thin, they are not infinetisimal. In a similar way, this 4d-space contains many slices of thin 3d-space, i.e. universes.
3. There is empty “space” between these slices, of arbitrary size.
4. A vessel is able to move from its 3d-slice if enough force is applied in the 4th dimension, but because the force mentioned in §2 causes all matter and energy to align in a uniform 3-dimensional frame, this is usually unnatural, or limited to very weak gravity leaks.
5. Gravity is one of the few forces that can act through the slice-force: of the known fundamental physical forces known to us, it is the weakest. Some theories of physics (some string-theory) say that this could be be because part of that gravity is leaked into “other dimensions” (not similar to spatial dimensions used here). In the Rabbit Hole setting, this holds true, and a fraction of the gravity of an object is siphoned off and becomes a 4-dimensional field (separate from the field that is three-dimensional in our universe). Thus, it also weakens at the cube of a distance instead of the square of distance. As you can imagine, 4d-gravity is usually very weak.
6. But not completely weak. Because gravity leaks through, it can affect (and even pull out) stuff in close-by neighbouring dimensions. This effect causes something that is essential to a setting like this: synchronisation of material objects in the 4th dimension. Basically, the gravity of our galaxy has synchronised with other stuff in other universes, and there exist galaxies in other dimensions that occupy the same relative area of space as our own; the same goes for our Sun. This means that when a dimensional ship enters another 3d-slice universe, *there may actually be something there*. This point is here solely to have some kind of explanation why there are planets and star systems accessible to us in the 4th dimension. If there was no synchronisation, all a dimensional ship could find in the 4th dimension would either be empty, empty stretches of space, or, with luck, something that is moving at a humongous speeds relative to the ship itself some light-years away: never a star that is “stationary” relative to the ship.
7. Some slippage due to the force of gravity pulling stuff out of other dimensions probably happens, but it is rare, maybe because all of that happened earlier in the history of the multiverse and we’re in a relative stable period of time.
7b. Insert “dark matter is just stuff in other dimensions fucking with ours” here. The synchronisation of gravity causes strange anomalies in galaxies, and so forth: our own galaxy, for example, has a strange speed of spin.
8. The means of aligning things perpendicular to the ordinary axis forced by the slice-force will have to be quite esoteric, but the result is basically that technology is able to create something that can do work in the 4th dimension, particularly a system of propulsion. This 4-dimensional propulsion system is like any ordinary Newtonian propulsion drive, i.e. it has a means to put energy into a propellant that is shot out in the opposite direction of travel. The details will have to be black-boxed. A dimensional craft will have to carry propellant and fuel for maneuvering in the 4th dimension.
9. The slice force is not an absolute knife’s edge. If there is pressure exerted on it, it will let 3d-space bulge in the 4th dimension. A dimensional drive must have quite high thrust to break through the slice-force barrier, by pressing against it like a dull stick against a rubber sheet. When the “slice barrier” is broken, 3d-space snaps backs, and the vessel that broke through is encased in its own personal 3d-slice bubble.
10. Due to the nature of gravity and the other forces relative to the slice force, exiting a 3d-slice when in any field of gravity or other force takes much more (possibly exponentially more) energy than exiting in a region of space that has less effect of gravity: i.e. you cannot exit the universe when in a gravity well, and the farther away you are from any sources of fields, the easier it becomes.
11. Conversely, *entering* a 3d-slice from the outside is easier when using a gravity well to pull you in, and *harder* when trying to enter an empty stretch of space.
12. Imagine that the slice-force is like a hill. When you’re on top of the hill, you’re inside the 3d-slice. If there is not much gravity, the hill is high, and rolling down it is relatively simple. When there is enough gravity, the hill becomes smaller (and sometimes it becomes a hole in the ground): the gravity is helping you overcome the repulsive forces of the slice force when trying to enter from the outside, but is helping you when it’s pushing you inwards when trying to exit from the inside.
13. These “rules” and their explanation was created to remove a small problem from the setting, which was that if there is no energy requirement to move in the 4th dimension, you can basically break conservation of energy by creating a perpetual motion machine that uses a gravity well to power itself: you “teleport” something from the gravity well somewhere higher up in altitude, and let it drop. You need to expend energy in order to even it out. With a teleporter, you could do that by demanding energy when you’re moving stuff from A to B, but with the Rabbit Hole system, you have two parts that are disconnected: you exit a gravity well in A, and then you can move anywhere in the empty, gravity-less space on the other side of the slice barrier, and *then* you move to B. These two events can not be directly connected. Thus, you have to even the energy requirements out in other ways. In this case, the energy required to enter through a slice barrier from the outside into an area of space with the smallest possible gravity field is somehow proportional *to the energy expenditure, the escape velocity required to escape the whole gravity field of the whole universe in question*. Presumably, entering a field with zero gravity (in an absolute sense) would mean infinite energy expenditure, but then again, if there is a zero gravity field, that means there are no other masses of gravity in the whole universe at large… ANYHOW, that’s the explanation, and I worked hard on it. If you have a very small 3d-slice, say, an estranged planet that has been pulled out of its home-slice somehow, then the energy requirements for entering “empty” space would be smaller because the slice itself is nanoscopically small compared to other universes.
14. Well, how do you then pass through other slices? You would either get caught in gravity wells or then hit yourself against the slice barrier. Well, the simplest way is to simply gather enough 4d-velocity, and then plow through. In the case of a gravity-well maneuver, it would be like rolling down a valley and up again the other side, and the acceleration from rolling down the valley is stolen back by the rise. It isthe opposite in the case of gravity-tops (empty space): you need the velocity to “crest” a hill, but after that it’s easy to roll down the hill. This is possible in space because there is no friction, and gravity is very weak in the 4th dimension.
EDIT: Further elaboration on the hill metaphor would be that instead of a hill, you have two hills, or a caldera… Two hills because each 3d-slice has two sides with two different slice barriers. A gravity would effect the depth of the valley between the hills, and also the valleys themselves. A low-gravity environment has only a shallow valley between two high hill peaks. Thus, the 3d-slice sits within on the bottom of a valley, an useful metaphor. If you want to actually stop in a 3d-slice dimension, you would have to end up at the bottom of the valley… Some oscillation would ensue.
15. The explanation for exiting and entering have something to do with bending space. When you exit, you’re pushing 3d space “outwards” into 4d-space, and when you enter, the slice force “catches” you on the fringe and extends 3d-space towards you (while still pushing you outward, awwwwwkward). In any case, something happens with space here, and it seems as though there might be some interesting happening visually. “Stretching” 3d-space locally would probably mean that there should be actual changes in distance. But because the “3d”-space is not moved within its own framework, but in the 4d space, this means that you get more distance while everything stays in the same place. When I think what would happen, I start to imagine that the dimensional craft that is stretching space would actually look like it’s shrinking… But I’m not sure if perspective would work in a situation like this. Well, it sounds a big goofy. Colours should change because you’re causing red or blueshifts in the light that is moving the stretched space.
16. In the Rabbit Hole, you have Stealth In Space due to how easy interdimensional shifting is. This is a bug, not really a feature, and I didn’t set out to make a stealth-in-space setting. Creating kinetic weapons in this setting is horrifyingly easy.
17. If there is gravity in the fourth dimension, could you have a separate 3d-slice orbit a large gravital object in a larger 3d-slice? Short answer: no. You can’t have any kind of regular orbit that intersects the 3d-slice “plane”, because the force of gravity is much stronger inside that slice, and would fuck up your orbit.
18. What happens when you have two objects hit eachother when one of them is moving 4tically? I need to decide on this. If the “space-stretching” analogy from 15 is applicable, the moving object would stretch space and “grow”, so I guess one possible way is that it explodes from inside… except, of course, the two objects would bump into each other, and action and reaction apply. If you have, say, a space-craft entering a 3d-slice within a planet, that craft will probably flatten itself and then bounce back outwards… Etc. The possibilities for mayhem are endless.
19. Presumably the same method that allows you to have 4d-propulsion also gives you a way to send, say, a laser-beam in the same direction. This is only to allow *some* kind of communications capabilities through slice-barriers, otherwise you would need physical capsules for *everything*.
20. Otherwise, the technology level of the setting is low. Space propulsion is relatively, relatively, realistic, (not realistically relativistic), there is some gene-manipulation (for everyone, hurrah!), people live up to, say, 170 years of age or something like that, and we have augmented reality.
21. One idea I had was that breaking the slice barrier causes EMP-like thingies to go boing, and thus you couldn’t do the Reasonable and Safe thing and just send unmanned droid ships out into the ether and have to have humans with really dumb nano-tech or low-tech computers on board, but we’ll see.
22. No rubber forehead aliens. They’ll all be fucking *weird*. Because weird is realistic.
23. I need a term for “delta-v in 4d-space”. Maybe “delta-4” for “delta-(four dimensional velocity)” or “delta-h” for “delta-hyperdimensional velocity” > “delta-hypevelocity”… Heh.
24. Might need to introduce the use of a 5th dimension of space. With 4d-space, you only have a one-dimensional axis for your travels (because you’re still stuck inside a bubble centered on the Sun), but if you add just one more axis the “map” becomes two-dimensional, and you can actually *hide* things there.