It isn’t often that I get a chance to relive my boyhood dreams (fed by equal helpings of Arthur C. Clarke, Star Trek, Buck Rogers and Star Wars) of a future where spacecraft leap from star to star using some sort of engine vaguely described as “warp drive” or “hyper drive.” But that’s exactly what happened when I read this Sept. 20 Wired UK blog post:
NASA scientists now think that the famous warp drive concept is a realistic possibility, and that in the far future humans could regularly travel faster than the speed of light.
Hot dog! Back in 1994, physicist Miguel Alcubierre produced a study showing that warp drive–which would allow a vessel to break Albert Einstein’s law prohibiting travel faster than the speed of light by curving the fabric of space around the ship into its own private wormhole–was at least theoretically possible, though it required the mass-energy of, say, Jupiter to succeed.
But earlier this month, physicist Harold White produced a new paper showing that warp drive didn’t need so much energy. Science writers and sci-fi fans around the world cheered and began making plans to find that one planet with all the hot green-skinned chicks.
Of course, my inner skeptic started to wonder, so I emailed Garrett Lisi, a physicist/surfer who lives on Maui (click here for MauiTime’s 2011 profile of Lisi) and even had his own show on the History Channel. Lisi, as I feared, threw a bucket of very cold water on the whole prospect.
“No wormholes, or warp drives, or time machines,” he emailed me. “Kind of a bummer.”
Tell me about it. Lisi’s reason is that warp drive and wormholes, while looking dynamite on paper, require something called “negative energy density,” which, sadly, doesn’t seem to exist in our universe. But he’s probably better at explaining this than I am:
Einstein figured out an elegant description of how gravity works as a curving spacetime fabric. This opened up all sorts of cool theoretical possibilities, such as black holes, wormholes, and other wild stuff. But Einstein’s equations tightly restrict what can happen, and dictate the relationship between curving spacetime and matter. Some solutions to Einstein’s equations, such as black holes, actually correspond precisely to objects we see. We now have strong evidence that many such black holes exist, especially in regions where there is a lot of matter, such as at the center of our galaxy. But this does not mean that all solutions of Einstein’s equations correspond to objects that physically exist–little details can be important. For example, for a typical wormhole geometry, as a solution to Einstein’s equations, there is a minus sign that appears in the relationship to the matter density. Specifically, matter with a negative energy density is needed to open up the throat of the wormhole, which has a negative spacetime curvature. Well, that’s important, because all the matter that we know exists in our universe has a positive energy density. There is no kind of matter with a negative energy density. (Even antimatter and dark matter have positive energy density.) For that reason, even though wormholes are valid solutions to Einstein’s equations and are interesting mathematical objects to study, they do not physically exist. Unfortunately, the same is true for warp drives. In 1997, Miguel Alcubierre published a solution to Einstein’s equations corresponding to a warp bubble–just like you’d imagine from familiar portrayals in books and movies–which is pretty awesome. It’s a spacetime geometry that allows a bubble of space to travel faster than the speed of light. However, this bubble, essentially a “shell” of spacetime geometry around a quiet interior, necessarily has some negative curvature. This means that for a warp bubble to physically exist in our universe we would need matter to exist that has a negative energy density–just like we’d need for wormholes. And, sadly, there is no such matter in our universe.
But what about White’s new paper? The one that has Wired and apparently NASA buzzing around with starships dancing in their heads? Here’s Lisi again:
I looked at Harold White’s paper, and the slides he presented at a recent conference, and he did make some interesting refinements to Alcubierre’s warp drive solution. Specifically, his solution requires less matter. But it also requires the existence of matter with a negative energy density, which does not exist in our universe. So, as far as we can tell, these cool spacetime geometries cannot be built in our universe, and there is no getting around that.
But Lisi did point out a bright side:
This means a killer robot won’t be traveling back in time to kill your father before you’re born. And it means human kind has some more time to have fun before extraterrestrial aliens detect our presence and eradicate us, since they’re limited by the same light speed limit as we are.
If you take the long view of human achievement and life, that’s a pretty big bright side. But if you’re like me, and pretty much everyone else who grew up asking “What would Captain Kirk do?” it is indeed one galactic bummer.
Photo of Star Trek‘s Enterprise in the Smithsonian: Aatrek/Wikimedia Commons; photo of Lisi: Sean M. Hower