Testing Alcubierre's Warp Drive

by Brian Dunning
February 6, 2024

Today I want to talk about something that has penetrated a bit farther into pop culture than I would have ever suspected, and it comes from the comments posted online to my 2023 film The UFO Movie THEY Don't Want You to See. The film talks a lot about the physical laws of the universe, among which is the fact that nothing can travel faster than light. This annoying limitation drastically reduces the possibility that neighboring interstellar civilizations will ever get to visit one another. Many of those who watched the film are among those who believe that aliens already actively visit the Earth, and so one of the frequent comments was that scientists are wrong about the laws of physics. Aliens, they declare, are far smarter than humans; and so have undoubtedly figured out ways around nature's absolutes. As evidence, many of them cited the Alcubierre drive, a hypothetical Star Trek style warp drive, proposed by physicist Miguel Alcubierre in 1994. And so today, we're going to put the Alcubierre drive to the test. Could it ever possibly work, allowing beings to travel the universe as easily as we drive around town?

NASA has put up numerous web pages describing the Alcubierre drive and other exotic and/or speculative propulsion systems, and has even made an elaborate rendering of what such a spaceship might look like; which have led many UFOlogists to conclude that these systems are under active development or that they secretly already exist. As a result, myself and many other science communicators who talk about the reality of space travel, get accused of being paid by the government to lie about it. Like many conspiracy claims, this one is self-contradictory: On the one hand, the US government puts out a NASA website about the Alcubierre Drive including what the spaceship might look like; while simultaneously paying me and countless other science writers to say no such thing is possible. In other words, going to great expense to both say something is real and also that it's not real.

The fundamental problem that the Alcubierre drive seeks to solve is that superluminal travel (meaning faster than light) is not possible according to the fundamental laws of nature. This restricts interstellar travel to impractical lengths of time on the order of thousands of years. So Miguel Alcubierre thought of a way to do it without having to actually travel at all. A spaceship using his drive would sit still in space, but would contract the space ahead of the ship and expand the space behind the ship, thus moving it to its destination without it having to actually move, and thus not breaking any physical laws.

It's really hard to wrap your head around expansion and contraction of space. Think of the fact that the universe is currently expanding. Everything is gradually moving apart from each other. Objects like planets and stars don't change size, but they do actually get farther apart. The Alcubierre drive would do this in reverse to the space between the spaceship and its destination, contracting it; and expanding space itself behind it.

But where NASA should have been more clear is that something like the Alcubierre drive can't and won't ever exist — by any reasonable standard. The two main reasons are:

1. The outrageous energy requirements. We would need to convert all the planets in the solar system from matter into pure energy to have enough energy for even the smallest such drive to move a tiny ship a tiny distance. Other physicists since Alcubierre's original concept have found ways to reduce this, but it's still on an order of magnitude that's simply an absurdity. However, who's to say that in 100,000 years we won't figure out a way to capture this kind of energy.
   
   
2. The real show stopper is that that staggering amount of energy needed is not the kind of real energy that exists in this universe, but a speculative exotic type called negative energy. So when we say all the planets in our solar system, we'd need them to be made of not actual matter, but of matter with negative energy density. We're not talking about antimatter — that's a real thing that exists — we're talking about something purely hypothetical, something that our observations of the universe give us every reason to believe does not exist. Even if the amount needed could be reduced to a manageable amount, one unicorn is no more likely than a million unicorns.

When we say this exotic matter is hypothetical, what exactly does that mean? It's easy to misinterpret that and say "Oh, that means hypothetically we can figure out how to create it," and that leaves us open to the old "Aliens are smarter than us, so they know how to make it." That's not the correct interpretation. Let's take, as an example, a similar thing that most of us are familiar with, and we probably learned it in high school math: the imaginary unit i, representing the square root of -1. i is not an actual quantity that can exist. You will never have a basket with i apples in it. Aliens can be as smart as you can dream, but they will also never have i quantity of something. Yet i is very real — not as a physical thing that exists, but as a well-defined mathematical concept with consistent properties and rules for its use. It's an integral part of complex numbers, which are used extensively in various fields of science and engineering. As a part of mathematics, i makes certain equations possible, and those equations do let us do things in the real world.

That's the way to think of exotic matter. The idea of negative energy density is comparable to the idea of the square root of -1; they can't exist in the real world, but they both make certain equations possible, allowing us to work with larger ideas that do have counterparts in the real world. In the case of exotic matter, it gives us certain solutions to the equations of general relativity; and general relativity definitely exists, we all use it every single day.

So saying we're going to build an Alcubierre drive is a bit like saying we're going to bet i dollars at roulette and receive a payout equal to the square of every number on the wheel that didn't come up. It might work as a concept, and you might write a paper explaining how it would work, but it can't happen in the real world — regardless of whether you're human or alien.

(A quick request to all of you physicists out there: Please do not email me to tell me my comparison of i to exotic matter is invalid. I know they can't really be compared; they are very different in many ways. But my job as a science writer often requires me to take super complex topics and present them in a way that's comprehensible to a general audience, and despite its weaknesses, it's the best analogy I could come up with. I will add that the weaknesses in the comparison are only comprehensible to you physicists, and if included in this episode, would send most listeners to the rubber room. It should be a lesson to me to never do episodes involving theoretical physics.)

I have found that efforts to communicate this kind of science to a general audience are often fruitless. Many people still go to "Aliens are smarter than us, so they understand this better than us, therefore they can do it." Many people simply will not accept that fundamental laws of nature exist. They accuse scientists and communicators of arrogance and close mindedness for not accepting that aliens have no limitations. And quite often, they bring up another example that they believe supports their position: spooky action at a distance, aka quantum entanglement.

This is the ability for two particles to be entangled such that a change to one instantly causes a change in the other, no matter how far apart they are. The common belief is that such a system could make superluminal communication possible: I take my entangled particles to my planet, you take yours to your planet; and then we can communicate instantaneously over any distance via some prearranged code of various changes to the quantum states of our entangled particles. This seems like a valid concept, and to nearly any layperson it seems like it should work. But it cannot work; those guardrails on the way the universe functions absolutely prevent superluminal communication.

Here is a simplified explanation of why not. The particles remain entangled only so long as they are in a state of superposition, meaning they are in all states simultaneously until someone measures one of their properties — or any interaction that requires the particle to have some property. That interaction causes the particle to collapse into some state (such as spin up, a high energy state, or vertical polarization), and its entangled partner simultaneously collapses into the opposite state (such as spin down, a low energy state, or horizontal polarization). What those states will be is fundamentally random and unpredictable. So you might suggest "Hey, the specific nature of the change doesn't matter, let's base our communication code on the timing between changes." The problem with that is the recipient cannot know if the sender has made any change: He'd have to check his particle to see if its state has collapsed, and just that checking would cause both particles to collapse, if they hadn't already. There is no way to know if your particle's current state was caused earlier by its entangled partner, or if your checking produced it. And just to make the whole thing even more frustrating, once entangled particles do collapse into any state, the entanglement is broken.

The idea of coming up with a way around this limitation is exactly like trying to have i apples in a basket. It cannot happen in this universe, even if you are an alien a thousand times smarter than a human. Particles in superposition collapse unpredictably with any interaction, and i is not a possible number of apples. And, similarly, we cannot fuel our spaceship with matter having negative energy density, any more than we can with i number of apples.

However, I know from long experience that the reaction to this episode will be accusations of arrogance, and that I'm claiming that scientists already know everything — a ridiculous strawman attack. Our theory of gravity is still incomplete, but we know all the basics of how and why it works, what it does and doesn't do, how we can use it to our advantage, ways to overcome it when we need to. It's not necessary to know everything to understand the basics — and the fundamental laws of nature are about as basic as things get. There are countless things we're going to learn to do better and better over the coming centuries and millennia, accomplishments so amazing that we can't even conceive of them — but casting the laws of physics aside is not going to be among them.

By Brian Dunning
Follow @BrianDunning