In the years since Miguel Alcubierre came up with a warp drive solution in 1994, you would occasionally see news headlines saying that warp drives can work. And then a few months later you’ll see that they’ve been ruled out. And then after that you’ll see that warp drives kind of work, but only in limited cases. It seems to constantly go around and around without a clear answer.
What gives?
It’s not that we can’t make up our minds, but it’s that these calculations sit at the very edge of our knowledge and abilities. And if I’m being honest, beyond them. So things are going to shift around quite a bit, and the truth is that we don’t have a firm answer. For example, one set of calculations suggests that quantum fields living at the edge of the warp drive bubble essentially blow up to infinity as soon as you turn the thing on, which would be bad. Very bad.
But other calculations say that only applies in certain limited cases, and that if you ramp up the warp engine slowly enough, you’re going to be fine. Maybe.
Yet more calculations sidestep all this and just look at how much negative energy you actually need to construct your warp drive. As in, assuming you can violate the weak energy condition willy nilly and just have as much negative mass and negative energy shoved into your spaceship as you want, how much are we talking about?
A bucket full? A ton? Let’s say we want to build a single macroscopic bubble, say, a hundred meters across, which would fit a reasonably sized spaceship.
To make a warp drive bubble that big, you need ten times more negative energy than the entire positive energy contained in the entire universe. So you would need ten universes worth of negative energy to power your thing, which doesn’t sound very promising.
However, still other calculations show that this only applies to the traditional warp bubble as defined by Alcubierre. It might be possible to reshape the bubble so that there’s a tiny neck in the front that’s doing the work of compressing space, then it balloons out to an envelope to contain the warp bubble.
This minimizes any weird quantum voodoo so that you only need something like a star’s worth of negative energy. Also that energy has to be compressed into something than an atomic nucleus, which might just give you a black hole.
Even more calculations show that even if you were to get a hold of some negative energy or negative mass, then as soon as you start moving, you’re going to run into problems. Namely, that the negative mass immediately starts flowing out of the edge of the bubble. What ends up happening is that the exotic matter constructing the warp bubble can’t keep pace with the bubble itself, so the bubble stops being a bubble because the whole point is to have negative matter compressed into a very tiny volume to generate this bubble. But then once you start moving, the ship goes one way, and the negative matter goes the other way, and the bubble collapses.
But we don’t know if a warp drive is outright impossible or merely impractical. Ultimately, physicists continue to study warp drives not because we’d like to use them to explore the universe. We’re using warp drives to explore the physics of the universe. If warp drives are possible, then that opens up enormous implications for the nature of quantum gravity and fundamental physics. And that’s a beautiful journey.