What rules actually prohibit us from building a warp drive?

In 1994, Miguel Alcubierre was able to construct a valid solution to the equations of general relativity that enable a warp drive. But now we need to tackle the rest of relativity: How do we arrange matter and energy to make that particular configuration of spacetime possible?

Unfortunately for warp drives, that’s when we start running into trouble. In fact, right away, we run into three problems. And these three problems are called the energy conditions. Now, before I describe the energy conditions, I need to make a disclaimer. What I’m about to say are not iron laws of physics.

They are instead reasonable guesses as to how nature makes sense. General relativity is a machine. You put in various configurations of spacetime, various arrangements of matter and energy. You turn the handle and you learn how gravity works. General relativity on its own doesn’t tell you what’s real and what’s not.

You have to look at other things. General relativity is a machine that produces answers, and we need some sort of filter to decide which answers are good and reasonable and which are stupid. These filters can change. These are not set in stone. These do not come from physical theory. They’re just statements that seem to make sense in our universe that we think are true, even though we have no direct proof except a lifetime of experience and observation that shows absolutely no violation of them.

There are many of these so-called energy conditions, and what they do is they place limits on what energy is allowed to do in the universe. The three that we care about when it comes to Alcubierre and his warp drive are called the strong energy condition, the dominant energy condition, and the weak energy condition.

The strong energy condition states generally that matter must gravitate toward matter. The dominant energy conditions states that energy must not flow faster than the speed of light. And then the weak energy condition is that your local energy density must always be positive. This means that negative mass and negative energy, aka exotic matter, don’t exist.

Once again, these are not iron laws of the universe. These are just observations we have made after a century of playing around with Einstein’s relativity and looking out in the wider cosmos. And the warp drive solution of Alcubierre appears to violate all three.

It violates the strong energy condition because we have a situation where local gravity is repulsive. It violates the dominant energy condition because we have energy flowing faster than the speed of light, and it appears to violate the weak energy condition because to set up a warp bubble you need to use a ball of negative mass, of negative energy density. That’s the only way to make that warp drive geometry solution of spacetime work.

Now, you could just say, hey, maybe these energy conditions are wrong, our intuitions are off, and nature is more clever than us. That’s fine, but I dare you to try to find any example anywhere in the natural universe where these conditions are violated. You’re going to have a tough time finding places where matter doesn’t gravitate toward matter, where local gravity isn’t attractive. You’re going to have a tough time finding places where energy flows faster than the speed of light. So as long as these conditions appear to hold, the warp drive is out.

What about that last one? You know, that the local energy density must always be positive. Don’t we have the Casimir effect? Yes, the Casimir effect is this weird quantum interaction where local energy density can be negative, and in a sense the weak energy condition is violated. But that’s only in the region between the plates—once you average out the area surrounding the plates, the weak energy condition holds true.

But maybe some clever technology and advances in physics could exploit this to create a large-scale loophole to the weak energy condition. Unfortunately, an answer to that question lives in the land of quantum gravity. That’s a theory of nature that we do not yet have access to. So we’re going to have to hold off on any plans to build a warp drive—at least for now.