Subspace is, colloquially, the name given to the only known method of faster than light travel. It is a bit of a misnomer; the phrase implies that it is "below" normal space, and in fact, entering it is reffered to as "diving", and"going deeper" is a common phrase. However, these phrases have their origin in a common analogy that while simple to grasp isn't very accurate. Still, centuries after it's discovery, the name has stuck.
Despite it's inaccurate name, subspace travel is ubiquitous among all space-faring civilizations. While, in theory, it's possible to travel long distances via other methods, subspace remains the only known reliable method of travel. Other theoreticals, like hyperspace are all based on the principals of subspace travel.
The basic operating principles of subspace are simple. It is generally accepted that it is impossible to travel faster than the speed of light. This, in fact, is correct. Due to the full derivation of Sepcial Relativity, it's more accurate to say that as you approach the speed of light from either direction, the energy required becomes infinite. So, you could say that you cannot slow a faster than light particle down to the speed of light, and you can not speed up a slower than light particle to the speed of light.
However, General Relativity provides an interesting loophole that subspace takes advantage of. The speed of light is measured in local frame of reference is a fixed constant, but that isn't necessarily true when viewed outside that frame of reference. It is possible to create an enclosed pocket of space-time where everything inside the pocket is traveling well below the local speed of light, however, to an observer outside of that pocket of space-time, they are traveling many orders of magnitude above the speed of light.
The trick to this, however, is closing yourself off from the rest of the universe. You must fully enclose yourself (and anything you want to bring with you) inside your pocket of space-time. This is known as a subspace pocket.
You have to also be able to move relative to the space inside your pocket. This means you have to bring enough fuel, reaction mass, or energy to still move as if you were in normal space.
While it sounds straight forward to generate a pocket of enclosed space-time, it is, in fact, not. The energy requirements to force space to curve so tightly are astronimical. To brute-force this find of curvature would take the energy output of an entire galaxy per second. This was the stumbling block that prevented many races from developing subspace technology much sooner.
The trick, as it turns out, is to cheat. The typical method for curving space-time is to either put a lot of mass in one area, or a lot of energy. Either method causes space-time to curve and by varying the distribution you can get space-time to behave how you desire.
The alternative to this approach is to get space-time to curve as if it were in the presence of a large mass, without actually needing the mass.
In particle physics, a photon is the name of a particle of light. It was long believed that there existed a particle of gravity called a graviton. Unfortunately, experimentation showed that such a particle does not exist, naturally. The breakthrough for subspace travel (and all modern gravity manipulation) is that gravitons can be artificially generated. Or, to be more specific, particles that manipulate space-time can be created briefly as long as they are immediately destroyed.
This does not, in fact, solve the problem on it's own. As much as Quantum Mechanics likes to break rules, rarely does it allow you to get something for nothing. So, in order to generate enough graviton-like-particles to manipulate space-time, you have to capture a faster than light quantum particle and slow it down until it becomes a graviton.
The breakthrough was in understanding that the energy requirements to slow down a faster than light particle until it becomes a graviton is negative compared to the energy requirements to curve spacetime. In fact, it's exactly inverse to the energy requirements for a particular cuve of spacetime. This means that if a way was found to encourage these particles to behave how we want, the energy requirements should, in fact, be zero. Unfortunately, they are not, because work is actually being done and nothing is 100% efficient, but it significantly lower.
As with all things quantum, once the desired effect was known, generating an experiment that would, sometimes, have the desired effect proved managable. Once it was demonstrated that this effect could be caused to happen, the mechanism was simply refined until it could reliably induce the correct quantum state.
"Once, only once, did I watch an entire trip through subspace. It was, and I say this knowing full well that you will laugh and accuse me of converting to the Seekers, a religious experience. Now, before I lose you to laughter, it wasn’t a divine experience. It was terrifying. Whatever it is that rips and knifes over the outer edges of a gravity bubble is something that we should not see. When I have nightmares these days, it’s no longer a scary monster or bad situation. Now, my nightmares are those teal and orange waves ripping through each other. It was like something desperately wanted to get into our pocket of real space.
Something that does not belong. Something that hates our existence.
So no, Marcus, I would not suggest advertising views of subspace as a perk of our new ship model."
—Jameson Emory in an email to Marcus Wright, a lead marketer at Emory Shipwrights.
A ship "dives" subspace by generating a highly shaped gravity well. The tighter the curve around the ship, the "deeper" it is said to be traveling in subspace. The "deeper" a ship is in subspace, the faster it will appear to travel, however, the higher the energy requirement, and the more "bleedthrough" gravitational sheer the hull will experience. Ships can travel faster in subspace by reducing the bleedthrough, increasing their energy output potential, or making their hull stronger. Generally, it's a combination of all three.
Once a ship has entered subspace, it can remain in it's current position, if it chooses to. Due to the nature of the curving of spacetime, it is difficult (but not impossible) to make sense of the sensor readings of things outside of the subspace bubble. Most sensor data is garbled beyond recognition, but with enough computing power or the ability to puncture microscopic holes in their subspace bubble, it's very difficult to know what's going on outside of the bubble.
Ships in subspace are, as a rule, unable to interact. They can both enter the same subspace bubble, or with some coordination, create a larger subspace bubble to bring in a larger area of space with them. Some ships, with incredibly powerful subspace engines are able to "ride the edge" of a subspace bubble, penetrating into it. However, it's very difficult to do and has only been successfully done to ships barely inside of subspace.
Communication in subspace is possible through the use of subspace radio. (In reality, this is just punching a hole into a higher spatial dimension, where the signal only has to travel a much shorter distance.)
One of the off-shoots of subspace technology is the ability to create gavitational sheers around your ship, either deflecting or crushing incoming weapons fire. Missiles are especially susceptible to strong gravitational sheers. Fighters, or more advanced gravity drive enabled missiles can penetrate less powerful sheers, but larger ships with stronger subspace drives can generate stronger ones to deflect even those.
It's often a game of brinksmanship too find the right level of defense screens. If you completely enclose yourself, you end up just going into subspace. But worse, you lose all ability to tell what's going on around you as well as any ability to fire back. Keep too many gaps in your screens and missiles can ride through those gaps and land on target. Firing out of weaker screens can be compensated for by most weapons systems, but weaker screens can be penetrated, and you've spend some of your weapon's potency (either in flight time for missiles, or just over all destructive potential) on penetrating your own screens.
One of the largest issues with subspace travel is that "mass shadows", the curvature of space-time caused by large masses, must be avoided. The act of curving space-time tight enough to allow for subspace travel is inherently destructve to things nearby. (Otherwise, defensive screens wouldn't work.) While "diving" and "surfacing" from subspace can be controlled in such a way to minimize damage to things near by, the control required is fine grain enough that it's never considered a pilot's first choice. Exiting too close to a larger ship, or a planet can cause just as much damage to your ship as to others.
In the same vein, traveling past planets is something one has to take care with when traveling in subspace. Most ships have mass shadow detectors and will automatically avoid entering a large object's mass shadow.
It should be noted that the deeper you travel in subspace, the less noticable mass shadows are, which generally allows for more direct travel.
Subspace drives (or "subspace engines") are just highly specialized computers hooked to powerful graviton generators. There's nothing exotic about them, at least not compared to any technology that involved gravity manipulation (including artificial gravity). Technically with a pad and the generator from a grav sled you have all the things you need to build a subspace drive. That is, assuming you only want a very small object to enter subspace for a very brief period of time, and you have a lot of time on your hands to write the software required. And at least four advanced degrees.
A majority of ships use gravity drives for at least some portion of their propulsion. These drives are, themselves, just subspace drives run at lower power. Some of these are specialized enough that they could never be powerful enough to run as a subspace drive, even for a short jump, but most gravity drives are, in fact, just special configurations of the ship's existing subspace drives.
Subspace drives are often refered to as having a 'strength' associated with them. What this is really referring to is how much power can be run through them before damaging their components. A "stronger" drive is able to have more power run through it without burning itself out. Also, stronger drives tend to be more energy efficient; a "weak" drive might require twice as much power for the same graviton output as a "stronger" drive. But, "stronger" drives will almost always be larger (and more expensive). Oversizing their subspace drives is a trick many Freelancers do to both allow them an unexpected ability to run away, as well as simply defend themselves.
Subspace Travel Time is too long! We need to do a blanket
x10to all the subspace speed ranges to make the travel time/distances make more sense and be reasonable.
How fast, relative to the galactic center, it takes to reach a location is dependant on several things. First, how deep you are in subspace is a direct multiplier to how fast you are going. As a rule, the deeper you are, the faster you're traveling, and the harder you are to detect.
Subspace depths are referred to as "bands". This allows for a more direct comparison of "how fast" a ship is, without getting into complicated space-time field equations. The bands are, as follows:
| Depth | Band | Speed Range | Used By |
|---|---|---|---|
| Upper (Shallower) | Alpha | 0c - 5,000c | Light craft, intra-system traffic |
| Beta | 5,000c - 9,999c | Light craft, Military fighters | |
| │ | Delta | 10,000c - 24,999c | Commercial traffic, Military fighters |
| │ | Gamma | 25,000c - 49,999c | Commercial traffic, Military traffic |
| │ | Epsilon | 50,000c - 74,999c | Military traffic |
| │ | Zeta | 75,000c - 124,999c | Advanced Capital Ships |
| │ | Eta | 125,000c - 199,999c | Advanced Capital Ships, Archigos Class |
| Theta | 200,000c - 299,000c | Dante Class, Sooni Ships, Grey Ships | |
| Lower (Deeper) | Iota | 300,000c+ | - |
Note: All speeds listed are in multiples of the speed of light. I.e. 5,000c is read as "five thousand times the speed of light".
To put this in perspective, the distance between Sol and Calysto is 4,500 light years. This means the trip from Sol to Calysto would take long-haul frieghter capable of just hitting the Gamma band roughly 65 days. It would take your average military vessel capable of just hitting the Zeta band about 22 days. An Archigos Class could make the trip in about 11, but a Dante Class could make it in under 7.
Here's the math for the above:
So, we have:
Distance = 4,500 ly * (9,500,000,000,000,000 m/ly) = 42,750,000,000,000,000,000 m
Speed = 150,000 * 300,000,000 m/s = 45,000,000,000,000 m/s
Time (seconds) = 42,750,000,000,000,000,000 m / 45,000,000,000,000 m/s = 950,000 s
Time (days) = 950,000s / 60 s/min / 60 min/hr / 24 hr/day = 10.9953703704 days
Here is a simple calculator widget:
All of the travel times listed above assume subspace is uniform. It, in fact, is not. Besides simple mass shadows, there are naturally occuring gravitational eddies in the upper bands. These eddies can cause massive hull strain on ships attempting to travel against them are often are avoided instead of traveled through. (They can be used as shortcuts by the daring, but 'ridding the eddies' is a good way to get yourself killed, as they can be unpredictable.)
In addition to things that slow down travel, subspace corridors can occasionally exist between one large body and another. These are natural "wrinkles" in space-time that make travel between two points significantly easier, allowing ships to travel in much lower bands of subspace than they would normally otherwise be able to.