How realistic is this? Mainstream sources, to my knowledge, still describe superluminal spaceflight as impossible. And this is the main reason Masters rejects this option.
It's interesting to see the debates about this subject, because nobody argues with the central point: if you create a spacetime metric with the geometry that Alcubierre described mathematically in the language of GR, a craft placed inside the field will be accelerated to any speed - even superluminal speed - as long as that metric is maintained, and it'll arrive at its destination with no time dilation.
So all of the various objections that have been raised pertain to the challenges of creating that spacetime metric and maintaining it. The fundamental physics of the concept itself are proven. So as I see it, we're in a similar position now as we were in around the year 1900 when we would talk about manned spaceflight to the Moon: we could mathematically demonstrate that it is in fact theoretically possible to get a man to the Moon, but if you're looking for reasons that it can't be done, then you could "prove" those mathematically as well (for example, if you don't postulate using a rocket with stages, then the fuel requirements are unrealistically enormous which makes the idea seem impossible).
But the challenges with producing the Alcubierre metric are greater for us at this point because we don't even have a gravitational field technology yet - at least in the year 1900 we had a working combustion technology. A lot of people - even many scientists and engineers, can't foresee the advent of gravitational technology.
But those of us who take UFO/AAV reports seriously have a distinct advantage over those folks, because if any of the UFO cases that we study are legit - such as the Nimitz Tic-Tac case, then we already know that gravitational field propulsion technology is achievable because two of our top fighter pilots and a Navy radar operator who observed the device, reported precisely the unique key signature performance characteristics that our theory of gravitational field propulsion predicts, primarily, lift without any emissions and extremely dramatic accelerations. Theory predicts those characteristics. Reaction propulsion of any kind, on the other hand, cannot explain it.
Wikipedia isn't a good source for theoretical physics because the mainstream community moves pretty slowly, and this subject is advancing pretty quickly. For example, you'll see lots of objections based on exotic matter. 25 years ago, exotic matter (a purely hypothetical form of matter that probably doesn't exist) was the only known solution for producing the negative spacetime curvature (aka antigravity field) that the Alcubierre metric requires. But in 1998 astronomers discovered dark energy, which proved two things: 1.) antigravity is physically real in this universe (so the positive energy theorem doesn't mean that antigravity is impossible after all), and 2.) antigravity doesn't actually require exotic matter - our best model for dark energy today involves a positive energy field causing a negative gravitational field. And in 2016 Manu Paranjape showed that a bubble of matter with sufficient tension could produce a negative gravitational field, per the pressure terms in Einstein's stress-energy tensor.
I'm not saying that it's going to be easy, but these kinds of considerations point in favor of the advent of this kind of technology, eventually. It's certainly far too early in the game to go around calling it "impossible." And if a single case like the Nimitz case is legit, then we've already seen the proof that it can be done, and we just have to figure out how it's done.
"[H]owever, any apparent FTL physical plausibility is speculative." -
Faster-than-light - Wikipedia
"The Alcubierre drive, however, remains a hypothetical concept with seemingly difficult problems." -
Alcubierre drive - Wikipedia
When most people see words like "speculative" and "hypothetical" they seem to think "impossible." But all of our technology today was speculative and hypothetical, before it wasn't. After all we've seen - from manned spaceflight to probe missions to Mars and even beyond the solar system - I think it's stunning that so many people seem to think that "something we haven't figured out how to do yet" means" we'll never figure it out."
How optimistic someone is about FTL travel depends on whether the person thinks it is truly contrary to physical law or is just out of range of present day science/technology.
Well it's definitely not contrary to physical law - the last such objection raised at the theoretical level was the positive energy theorem, and we now know that's inapplicable, because dark energy is real and that's an antigravity effect that exists in our physical universe, so we now know that antigravity is literally all around us. In my opinion that means that one day we'll learn how to produce antigravitational fields technologically. And AAVs seem to prove that others have already figured it out.
Personally, I am hopeful that superluminal spaceflight will be proven possible eventually. But not everyone thinks this way.
More people should think that way, because we're awash in reasons to do that. Imagine how stunned somebody from the 19th century would be, if they could see the world we live in now. That's just a bit more than one century, and technological progress is accelerating. It's literally unimaginable what we might achieve in another century, let alone five centuries, or 500 centuries - and cosmologically, even 500 centuries is right around the corner.
In relation to the above, here's a fun article that ranks alternate Sci-Fi FTL travel technologies in terms of plausibility.
The 5 kinds of sci-fi space travel, ranked by realism
Unfortunately these kinds of ratings systems are only as good as the physicist who writes them - I actually enjoyed that piece, but frankly it's kinda silly to try to place a number on something as radically unpredictable as "plausibility," which is at best limited by the prevailing mainstream viewpoint. I wonder how "plausible" a man walking on the Moon would've seemed to be, to a physicist in 1850.
My point is very simple: as difficult as superluminal spaceflight may seem to be now, we know that time travel into the past will be much more difficult, if it's possible at all. Most time machine concepts, like wormholes, also require intense negative gravitational fields just as warp drive concepts do, and closed timelike curves are on an even more exotic and imo far less theoretically sound foundation. So why on Earth would anyone favor the latter over the former? That makes zero sense to me.
Especially given the ubiquity of habitable worlds in this universe, and the huge and rapidly growing mountain of evidence that the conditions which led to intelligent life on this planet, appear to be common - even mundane - in this universe...and even here in our own galaxy.
So frankly I think we should be
expecting to see all kinds of strange beings dropping by from time to time, like this guy here with those freaky mouth prongs or whatever.