Warning: a lot of conjecture and questions, mostly an excuse to draw diagrams.
There are different forms of FTL, each of which exists to a varying degree in the MCU. Each one has room for improvement in meaningful and interesting ways, and Mar-Vell might be working on any of them.
The simplest form of FTL is straight line travel. I fly really really really really fast, and pass through every point between A and B. Think warp drive in Star Trek. I don’t think we’ve seen this in the MCU yet – so if that’s what Mar-Well was building, that’s a novel invention that’s interesting to both sides.
This sort of travel allows very free movement – if there’s a clear path between A and B, I can presumably travel between them.
That might make it more difficult to track where I’m going, which is desirable if (say) you’re trying to flee your oppressors from a militant Star Empire.
Mar-Vell was building engines in planes, which travel very fast in straight lines. Maybe she was building this sort of engine?
Another form of pseudo-FTL is wormhole travel. The ship never moves faster than light (relative to the space around it), but a shortcut in space allows you to get from A to B without going through the intermediary points. This is the sort of gate travel that we’ve seen in several of the MCU movies.
So far we don’t know how the gates were created – they exist in fixed spots, but it’s not like the Kree can create gates at their convenience.
If you put your
massive battle cruisers friendly border guards around all the local gates, you get to control who goes in and out. (And honestly, I wouldn’t be so surprised if the Kree do some of this already.)
If Mar-Vell has come up with a way to easily create new gates, to extend the range of existing gate tech, or even better, close them after you’re done – that opens up all sorts of new possibilities.
Or maybe the ship doesn’t move at all… it teleports to its new location. You don’t pass through any points between A and B. Maybe she was knocking on this particular door?
There’s precedence for this sort of travel in the MCU. Remember, the basis for Mar-Vell’s work is
and in Infinity War, we see
Thanos uses the Tesseract to teleport himself and a handful of his cronies around the universe. (The visuals look a bit like a very close-range wormhole, but at some point what’s the difference?)
But that’s the only time we’ve seen it used. Since teleportation is apparently allowed by the rules of MCU physics, finding a way to do it without an Infinity Stone to hand would be very interesting to both sides.
I’ve watched the film a couple of times now, and I’m leaning towards Mar-Vell building an engine for straight-line speed.
When Carol is talking to Yon-Rogg, she says that Mar-Vell has “cracked the secret of light-speed tech” (or similar). Odd choice of words if you already have FTL tech – so they don’t think of the gate network as faster-than-light travel. Maybe because you can’t create gates at will, maybe because there are limits to the distance you can travel in a gate.
Maybe it’s a red herring?
This is my other theory. The FTL drive is a convenient cover story, nothing more.
Mar-Vell wants to get the Skrull refugees out of the reach of the Kree. One way is a really fast ship – another is a slow ship that’s hard to follow, and keep going until you’re clear of Kree territory. As we know, space is big. Really big.
Developing a light speed engine on a backwater planet seems like the sort of thing you could sell to the Kree. If it works, you have a shiny new drive. If it explodes when you switch it on, who cares? (See also: US nuclear tests in the 1950s.)
We also know she has cloaking tech – witness her lab, which Starforce never found. Seems unlikely they never looked for it in six years.
I still lean towards the straight-line engine stuff – possibly similar to Ebony Maw's ship – but I don’t think we can entirely discount this possibility.