Edit: I know its long, jump to the bottom for the summary if you don't want the why.
As Exhack pointed out, much of the issue comes round to a lack of understanding of how diffuse nebulas are. The average particle density of a nebula is 1,000
atoms per cubic centimeter (not grams,
ATOMS), compared to roughly 1 atom per cubic centimeter for interstellar space. This is in contrast to air at 1 atm which is ~10^19 atom per cubic centimeter, or ten million, billion times more dense than a nebula.
As you can see, nebulas are very, very diffuse. For example, a starship traveling a
lightyear through your average nebula is going to contact about as much matter as it would traveling
10 centimeters in a planetary atmosphere. The only reason you can
see nebula is because they are
massive things, stretching dozens of lightyears across (actually, most of the nebula in SARP would be pretty dark since many of them don’t have stars in or very near them). If a ship can withstand repeated bombardment by extremely powerful weaponry (AM cannon, megaton-yield lasers, etc) passing through a nebula would not present any significant problem. Just sticking a big magnet on the front would resolve the issue completely.
Beyond this, however, the actual methods of FTL need to be considered.
For hyperdrive, the reigning theory for its operation atm seems to be that it transitions to a coterminous, albeit more compact reality. Every point in real space has a corresponding point in hyperspace. However, due to the peculiarities of this reality the distance between these points is substantially less than in real space. As a result, a ship traveling at speed V for time t covers distance Vt in realspace but while traveling in hyperspace covers distance CVt (where C is some coefficient >>1) as observed from a distant realspace observer.
Gravitational effects in real space have a comparable gravitational “shadow” in hyperspace (and the reverse would hold true). A star would have a similar gravity well (called a “mass shadow”) within hyperspace that it does in real space. Due to the nature of hyperspace this mass shadow will be much more compact and will have a much greater rate-of-change compared to its real space gravity well. Because of this compaction any object crossing through the mass shadow will experience much higher shear than would a ship passing though the same gravity well in realspace.
Think of this as two hills, both 100 meters in height. One has a gentle slope to its bottom, covering a huge area with its base. The other has very steep sides but covers a fairly small area. Both are the same height (intensity) but you are obviously at more risk for injury falling down the steep hill than you are the gentle one.
As a result a ship has to exercise caution when traveling in hyperspace lest they come in contact with one of these gravity wells and suffer damage. Most Anti-FTL systems would disrupt hyperspace travel by generating mass shadows within hyperspace. While not as intense as those created by something like a star, they would be much more numerous and less predictable. This makes a ship traveling through the field either drop out of hyperspace or travel much slower (so the hazards can be navigated) or else risk damage or destruction by running into one of the mass shadows.
However, within this framework for how hyperspace works by necessity
real space matter cannot have a corresponding mass in hyperspace. This is a result of Pauli exclusion principle that states that no two fermions (which include protons, neutrons, etc. ; pretty much all normal matter). If matter where to be carried over from real space to hyperspace as gravity is the compactification of hyperspace would result in particles of matter occupying the same space, which physics does not allow.
Nebula have particles but essentially meaningless gravity. As a result they would have no effect on a ship in hyperspace
For Combined Distortion Drive-vessels how they function needs to also be discussed. CDDs function by generating a field of warped space around the ship and are based (conceptually) on the warp drive posited by Miguel Alcubierre. In the simplest mode of operation these units contract space in front of the ship and expand it behind the ship, creating motion as perceived by a distance observer. Through this method a ship can achieve superluminal (faster than light) speed while not actually traveling faster at or higher than c as observed in its local frame (the space immediately around it). This must be stressed,
the ship is not necessarily moving despite what a distance observer sees.
For a distance observer measuring the speed of the CDD bubble it appears that it is traveling at some speed faster than light. At distance the space-time warpage of the bubble is negligible. As he approaches very close to the bubble perimeter (from the front, as he cannot approach from the rear as it is traveling away from him faster than light) the contracting space becomes noticeable. Any measurement taken in this area of the field over any non-zero time will show a distance much less than expected. On a cursoury inspection one could conclude that this is because the bubble is traveling very fast. However, in actuality it is a result of the contraction of space.
The actual distance you are measuring is less when you finish than when you started. The bubble itself is not moving in relation to its immediate surroundings, it is causing space
itself to move. As a final note, it would be very haza
As a result of this, while it would seem logical that a CDD bubble traveling at high
relative velocity would have a extremely energetic collision with any incident particle this is incorrect. As the particle neared the bubble the
actual measured difference in velocity would be decreasing. When it actually contacted the bubble the relative velocities between the bubble and particle may well be (near to) 0 and this collision would actually be very minor.
The last method of FTL to be discussed is wormholes. These are bridges of space-time connecting two distance points. By their definition they are not crossing the real space between these two points and as a result what is
in that real space is irrelevant to something crossing through a wormhole. It could be a nebula, a blackhole, a star, or a rubber ducky, it doesn’t matter.
So, in summary:
For Hyperspace drives
Hyperspace drives are not affected by nebula because:
-) The matter of a nebula has no physical correspondence in hyperspace.
And
-) The gravitational field of a nebula is essentially 0.
For Combined Distortion Drives (CDD)
CDD drives are not affected by nebula because:
-) As a particle approached the CDD bubble its actual relative velocity approaches 0.
For Wormholes
Wormholes are unaffected by nebula because:
-) They do not cross the space occupied by the nebula. They bypass it entirely.
That is all,
Vesper
Edit 2: Let me note that I am discussing the random particles in the nebula when I am describing how they interact with a CDD drive. A nebula particle bouncing around is not going to hit a CDD bubble with any more force than it would any other random particle in that nebula, despite the appearance of the bubble traveling very fast. A bolt of directed particles (a neutron cannon for example) would still hit and cause damage. Similarly, if a CDD bubble drove into the particle stream coming from a pulsar it would likely take damage as well. It is just that the CDD's
apparent speed doesn't add to the damage from such particle collisions.
