24th century ships seem to use low level subspace fields to lower their inertial mass, which is what allows them to travel at 1 quarter of lightspeed at minimum - note that higher sublight velocities were stated on screen as well... along with going to Warp. But there's a difference in using subspace fields to benefit your vessel (such as reducing ones inertial mass for high sublight velocities or going to Warp) and entering subspace itself. On-screen, it was never stated that Warp drive allows access to subspace itself. The ship using Warp speeds does NOT seem to leave normal space... which is why Warp fields are susceptible to gravitational fields and why ships at Warp can slam into stationary objects in normal space (aka, risk of colision). If a ship entered subspace and left regular space, then it would technically be capable of passing through a stationary object (and yet, this is not the case). I remember ST: VOY epsode 'Fury' in which VOY supposedly went into a region of space which was occupied by subspace vacuules. It was suggested the ship couldn't even graze one of them... Now, subpsace vacuules obviously exist in regular space... but probably (due to some inherent physics in Trek universe) use subspace fields to make them as dangerous to ordinary matter as they are. The only ship I remember being mentioned of actually going into subspace was the Think Tank vessel from VOY. There were also Houdini mines from DS9 and that ridiculous AR planet episode. Anyway, Houdini mines were actually in subspace and emerged at seemingly random intervals. When it came to FINDING objects in subspace, it was suggested on both VOY and Ds9 that its extremely difficult. To me this would imply that if SF ships physically entered subspace while at Warp, they'd probably have greater/better understanding of it and wouldn't have so many issues finding things in it. That said, not even the Borg TW conduits or Quantum Slipstream seemed to allow a ship to enter actual subspace. They do make elaborate use of subspace fields it seems, but I don't think they allow entrace into subspace.
Transwarp Conduits: That's the only FTL Drive System that goes through Subspace. Quantum SlipStream Drive: Quantum SlipStream is it's own thing.
In conversations about warp drive on these boards, one thing that tends to be overlooked is how much onscreen wriggle room there is to how it works. It really has never been nailed down onscreen. There is no definitive way of how it works because the writers need that wriggle room so it can work whatever way it suits a story. I do favor the idea that a warp field and a subspace field are one and the same, and as such warp drive is a form of subspace propulsion. Other people have different opinions and that's fine--I've never said that their idea was wrong or couldn't work, but I do think because of how loosely defined it is, there's more than one way it can work that can fit with what we see onscreen. The problem for some people might be an idea that a ship enters a totally different dimension while in subspace--perhaps similar to hyperspace in Star Wars--but in Trek, subspace seems to still be a part of space-time.
I would distinguish between "subspace" itself, which I think of as an extra dimension, and "subspace fields" (or warp fields, I agree the terms are synonymous) which are fields that exist in that dimension, and perhaps extend into our ordinary 3D space. Do you think of the terms differently? To clarify on subspace as another dimension, I think of it as similar to the book Flatland, in which we have a 2D universe (a flat plane) that exists within a higher 3D space, and objects from the 3D space can interact with it (in the story an intelligent sphere visits a flatlander, and as his body passes through the 2D plane, the flatlanders see cross-sections of the sphere appearing in their world). So in the same way subspace could be a 4D space, with our 3D space existing like a membrane within it, and subspace fields could be 4D energy fields that might be detectable where they intersect with the 3D membrane. Subspace could also involve more than one extra dimension (like a 5D space) or some unusual topology (like the idea on this page of multiple higher-dimensional "funnels" extending from our 3D space), but either way the basic idea would be the same. As I mentioned the TNG writer/director's guide does say that subspace radio "operates through another dimension of space" so I think this is basically what the creators had in mind. But I take it you are only interested in what was established onscreen, not on behind-the-scenes info that sheds light on what Roddenberry/Okuda/Sternbach were thinking? If so, I think the strongest evidence for subspace as a higher dimension is in the TNG ep "Schisms". In that one, crew members are being kidnapped by beings that seem to be from a sort of parallel universe existing in subspace--at one point Geordi and Data detect tetryon particles that seem to be connected to the incursions those beings are making into our space, and Data says the particles are "emanating from a tertiary subspace manifold", with Geordi adding that "Something from that deep in subspace shouldn't be able to exist in our universe." So, it seems he's defining the manifold to not be part of "our universe". In terms of the idea of subspace as a higher dimension, this could be analogous to a 3D space where you have multiple parallel 2D planes at different heights in the 3D space, so flatlanders inhabiting each one would see the other planes as parallel universes. Geordi also says later that "The emissions are coming from a tertiary subspace domain, but subspace has an infinite number of domains. It's like a huge honeycomb with an endless number of cells." And when they put a homing device on Riker, they are able to determine which "subspace energy level" it's coming from. Would you agree that subspace is a higher dimension, but you think the Enterprise actually travels through this dimension rather than being confined to 3D space and manipulating subspace fields to warp our own space? If so that would raise the question of why they couldn't just travel to the "subspace domain" that was messing with them in "Schisms"--instead the only solution Geordi could come up with was that "we can close the rupture by neutralizing the tetryon emissions with a coherent graviton pulse", where the graviton pulse could only be aimed properly once they knew where in subspace it was coming from (hence the need for the homing device). If you do think of subspace as another dimension they travel through, do you imagine they are tethered to ordinary 3D space in some way that prevents them from traveling too far away from it? In "Descent, Part 2" we also have Geordi's line that "The Borg have established several transwarp conduits through subspace", which might suggest that the distinctive feature of transwarp is that it involves travel through subspace, whereas normal warp does not. And there are also the Houdini mines in the DS9 ep "The Siege of AR-558", a piece of Dominion technology which is able to "hide in subspace and appear at random"--if Federation ships were already routinely traveling into subspace, I'd think it would be fairly easy to find mines that were "hiding" in the subspace dimension.
I tend to think of a subspace field as a bubble of subspace a ship sits within. But onscreen material also made me go with that there were levels of subspace, with the connection to normal space becoming more tenuous the deeper one went into it. IMO, conventional warp flight is more towards the lower end of the spectrum, with transwarp conduits being on the higher end. But just starting off (like the smallest fraction of a millicochrane), you'd probably need a tricorder to tell if you've walked through a subspace field because you probably couldn't tell from just looking. That's just one way I think it can go, there are other approaches that can work too.
Do you think of these levels as being situated in a higher-dimensional space like I do (the analogy of multiple 2D planes sitting in a 3D space), or do you just think of subspace as a discrete series of parallel 3D spaces (maybe with the idea that a chunk of one space can displace or merge with a chunk of another, so that a bubble of subspace exists in our space), or neither?
To be totally honest, I think any of the above could apply. I tend to favor the parallel 3D spaces idea myself, but with the caveat that it could become a higher-dimensional space at deeper levels. I think the one consistency about subspace in Trek is its inconsistency.
Yeah, I don't believe that the ship enters subspace when it goes to warp, at least not entirely (maybe the ship kinda bends into subspace somewhat). However, I think it's strongly implied that "subspace radio" waves travel through subspace. What is subspace? Perhaps extra dimensions of the spacetime continuum that are in contact with the usual four. If that's the case, then it's totally believable that even regular matter and energy have components in subspace. But, by the ship "not entering subspace," I think it would be meant that ship's regular components of matter and energy do not vanish entirely from the ordinary spacetime continuum while it's at warp.* And yeah, in that case, collision with a space body such as an asteroid would be possible without navigational deflectors. * - However, this doesn't mean that they're not imaginary or negative instead of normally positive.
There seems to be some movement in that area: https://phys.org/news/2022-06-duality-physics-mystery.html In conventional wisdom, producing a curved space requires distortions, such as bending or stretching a flat space. A team of researchers at Purdue University have discovered a new method to create curved spaces that also solves a mystery in physics. Without any physical distortions of physical systems, the team has designed a scheme using non-Hermiticity, which exists in any systems coupled to environments, to create a hyperbolic surface and a variety of other prototypical curved spaces. As it turns out non-Hermiticity, also has something to do with cloaking--which we think also comes at least from the surface deflector grid: https://phys.org/news/2022-05-transparency-demand-artificial-materials-transparent.html Here though--science might have done better: "Our research provides the recipe for structuring a material in such a way that light beams pass as if neither the material, nor the very region of space it occupies, existed. Not even the fictitious cloaking devices of the Romulans can do that," says co-author Dr. Matthias Heinrich, circling back to the final frontier of Star Trek.
At this point in Star Trek I think it has been demonstrated that in order to survive (at least in deep space or when running into almost any anomaly seen in any show) or go to warp, a deflector is needed. While some ships have no visible dish on screen, there are ways to explain this. The Miranda class starship has three parts, two on the lip between the saucer and the raised shuttle bay hull, and one on top of the torpedo pod, that match parts surrounding the deflector on the Constitution class refit. In several non-canon sources, they generate deflector screens, but a solution that I like is that they are all three miniaturized deflectors that together, protect the ship in the way a normal sized dish would, and they only open at warp, which is why we haven't seen them lit blue. Maybe they have some protective covering at impulse speeds, when they are deemed not necessary, or maybe it is a power saving measure and only opens when needed (at warp or when needing some type of beam). This could be some type of experiment by Starfleet in deflector tech, and since we never saw a Miranda class starship at warp, we can't disprove this theory. The Constellation class starship has two bulges under the saucer section, near the neck connecting the ventral nacelles and the saucer. My theory is that these are both navigational deflectors, which open at warp but are closed when traveling at impulse speeds similar to described above. Perhaps there are two because there are two sets of nacelles, although I admit that holds no water because ships with more than two nacelles (the new Sagan class and the Niagara class) have one deflector. Either way, this accounts for the seeming lack of a dish (or dishes) on the Stargazer and Victory in TNG. The Oberth class has no grill or mesh or any parts for that matter on the secondary that can resemble a deflector. It is possible there is some area that opens, but I find it more likely that the cutout on the front of the saucer section, which looks like a shuttle bay, is the deflector, but for some reason we never see it turned on. Because the Oberth is such a small ship, maybe it takes to much power to run constantly, or the power from the deflector is rerouted to the sensors or weapons when not at warp, because we know the Oberth is primarily a science ship, and no deflector was seen when we saw an Oberth class ship fighting (First Contact and Wolf 359). This idea also makes sense if we consider the numerous non-canon kitbashes of the Oberth class saucer section, where is it missing the original secondary hull. If these kitbashes were to go to warp, they would need a deflector in the saucer section, most likely. The Centaur type is harder to remedy with my theory of dishes only opening at warp, because we saw one at warp... without a visible deflector dish. The most likely spot I can think of it the bulge on the bottom of the torpedo launcher. In my headcanon, it was simply not visible because there was an effects error so it was not lit up, and it, unlike other ships on this list, would be lit constantly, not just at warp, like its cousin, the Excelsior class (I like the thought of the Centaur as the Miranda version of the Excelsior class, not scaled based on the Miranda weapons pod or bridge). I know this doesn't make total sense, but things like that have happened before, so it's not totally unlikely. This is just a list of my thoughts on a few canon ships, and I have thoughts about the Wolf 359 kitbashes as well as the DS9 Frankenstein Fleet and some FASA ships, but this post has gone on for long enough so I'll end here.
The Deflector Shield Emitter doesn't have to be "Dish Shaped". Just look at the USS ProtoStar. It's right above the Cargo Bay doors and is long, wide, slender and Crescent shaped. With modern AESA Radar hidden behind protective covering. they can blend the deflector behind the protective covering / nose cone of aircraft. Since the Deflector Dish originated from the Satellite / Radar dish, it only makes sense to follow IRL Radar development. And AESA style Radar where they use a array of lots of smaller emitters in parallel to do the work is faster to repair, easier to setup, allows flexibility of shape. Heck even the USS Discovery, after it's refit uses that style for it's main deflector. It's beautiful that visual technology in SF is following IRL technology.
I agree, they do seem to evolve over the course of time in the show. I like the idea of a deflector like the Olympic class, where it is just a strip on the bottom of the primary hull, rather than some huge dish, however to me that seems like a new thing as of the 2350s or 2360s, especially given the fact that when first shown, the Olympic class was supposed to be futuristic compared to a ship like the Miranda class or an Oberth class (maybe even the Galaxy class, although not significantly if Mariner served on one before the Cerritos).